03_chapter 2.pdf - shodhganga

Chapter 2 Review Literature

Pharmaceutical Medicine 7 Jamia Hamdard

2.0 Review Literature

2.1. Pregabalin

2.1.1. History and place in therapy:

Developed by Pfizer, Pregabalin, marketed under the brand name Lyrica. It is a 3-

substituted analogue of gamma-amino butyric acid (GABA). It is a compound related to

Pfizer's hugely successful antiepileptic drug gabapentin (Neurontin).

In July 2004, Pfizer secured Europe-wide approval for Pregabalin for use in the

management of peripheral neuropathic pain as well as an adjunctive therapy in the

treatment of partial epileptic seizures.

Subsequently, in December 2004 the company gained US Food and Drug Administration

(FDA) approval for use of Pregabalin in neuropathic pain associated with diabetic

peripheral neuropathy and postherpetic neuralgia; making it the first FDA-approved

treatment for these neuropathic pain states.

Pregabalin was also reviewed by the FDA as an adjunctive treatment for partial epileptic

seizures in adults. In June 2005, the FDA granted approval to market Pregabalin for

adjunctive treatment of partial epileptic seizures in adults. In June 2007, Pregabalin

became the first drug to be approved by the FDA for the treatment of fibromyalgia.

First marketed in 1983, gabapentin (Neurontin) has been one of Pfizer's top performing

drugs. Pregabalin is seen as an important successor now that gabapentin is facing the

threat of generic competition.

Pregabalin is mechanistically similar to gabapentin and shares similar advantages, such as

a lack of pharmacokinetic interactions with other medications or enzyme induction. But,

there are several differences between the two drugs. According to preclinical studies,

Pregabalin has an increased binding affinity for the α2-δ protein subunit of voltage-gated

calcium channels, which is associated with analgesic and anticonvulsant activity, and has

shown greater analgesic activity compared with gabapentin [Frampton et al 2004;



Sabatowski et al. 2004; Frampton et al. 2005]. Despite these preclinical data, it is unclear

if Pregabalin has a clinical advantage over gabapentin. As the two drugs have not been

compared in clinical trials. Unlike gabapentin, Pregabalin exhibits linear

pharmocokinetics after oral administration, with low intersubject variability [Frampton et

al 2004; Sabatowski et al. 2004; Frampton et al. 2005]. This provides a more predictable

dose- response relationship, as plasma concentrations increase linearly with increasing

dose. Gabapentin on the other hand requires disproportionately larger dosage increases to

achieve increases in plasma concentrations. The large dosages required for some patients

receiving gabapentin could worsen dose-dependent adverse effects, such as dizziness and

somnolence. Thus, the linear pharmacokinetics of Pregabalin impart a better-defined

effective dosage range and may provide the basis for the efficacy of either fixed- or

flexible-dosage regimens. This property also accounts for the defined dose-dependent

adverse effects and benefits reported in clinical trials. Pregabalin's linear

pharmacokinetics and low intersubject variability allow it to be initiated at or adjusted to

the target dosage more rapidly. Whereas, gabapentin requires a long, slow adjustment to

the effective dosage [Bloomel ML, Bloomel AL 2007].

Pregabalin is comparable to gabapentin in terms of cost for all but the lowest doses of

Gabapentin. Pregabalin appears to be well tolerated; however, it does not have the proven

long term safety profile as gabapentin, which has been used in large numbers of patients

during years of clinical practice. Another limitation of Pregabalin is the potential for

abuse and dependence, necessitating the monitoring of patients for signs of Pregabalin

abuse [Bloomel ML, Bloomel AL 2007].

Despite the lack of long-term safety and efficacy information and the abuse potential for

Pregabalin, the initial clinical results, the comparable medication cost (especially at

higher doses), and the favorable pharmacokinetic profile of Pregabalin support the use of

Pregabalin as an alternative to gabapentin for Pregabalin's FDA-approved indications. As

Pregabalin and gabapentin have a similar mechanism of action, it is not expected that

Pregabalin would benefit a patient when used concomitantly with gabapentin; however,

these drugs could be given concomitantly with other medications used for diabetic



peripheral neuropathy, postherpetic neuralgia, or seizures in refractory patients [Bloomel

ML, Bloomel AL 2007].

Other medications commonly used as first-line treatment of diabetic peripheral

neuropathy and postherpetic neuralgia include tricyclic antidepressants, opioids, and

topical lidocaine. The costs of tricyclic antidepressants and opioid analgesics are lower

than the cost of Pregabalin. Clinical trials comparing the efficacy of tricyclic

antidepressants and other clinical options with Pregabalin are lacking. A potentially

limiting adverse-effect profile (tricyclic antidepressants and opioids), lack of suitability

for long-term use (lidocaine), and slow onset of analgesic action (tricyclic

antidepressants) are some limitations of using these therapies for the treatment of diabetic

peripheral neuropathy and postherpetic neuralgia [Frampton et al. 2004; Frampton et al.

2005]. Until the results of comparison trials are available for Pregabalin and the other

accepted treatment options, Pregabalin should be considered as an alternative to tricyclic

antidepressants and opioids for the treatment of diabetic peripheral neuropathy or

postherpetic neuralgia in patients not responding to or tolerating the current treatment

regimen or in those patients who would not be suitable candidates for therapy with

tricyclic antidepressants or opioids.

2.1.2. Therapeutic indications and safety and efficacy of Pregabalin in various

indications:

2.1.2.1. Neuropathic pain

The International association for the study of pain defines neuropathic pain as “initiated

or caused by a primary lesion or dysfunction in the nervous system” and due to

disordered peripheral or central nerves. [Merskey et al.1994] This disorder can be caused

by compression, transection, infiltration, ischemia, or metabolic injury to neuronal cell

bodies, or in combination.

Neuropathic pain may be classified as either peripheral or central in origin [Dworkin

2002] Examples of the former include diabetic peripheral neuropathy (DPN),

postherpetic neuralgia (PHN), antineoplastic therapy–induced or HIV-induced sensory

neuropathy, tumor infiltration neuropathy, phantom limb pain, postmastectomy pain,



complex regional pain syndromes (reflex sympathetic dystrophy), and trigeminal

neuralgia. Central neuropathic pain include multiple sclerosis, spinal cord injury, central

poststroke pain, and parkinson disease.

The prevalence of neuropathic pain is estimated to be about 1%. Well-known neuropathic

pain syndromes are diabetic neuropathy and post-herpetic neuralgia.

A. Diabetic neuropathy

Diabetic peripheral neuropathy (DPN) occurs in approximately 20% of all diabetics

[Schmader 2002] and among persons who have had diabetes >25 years, its prevalence is

about 50%.

Pregabalin is a nonopiate that is well tolerated and relieves painful symptoms of distal

symmetrical polyneuropathy with minimal risk of dependence or impact on patients'

diabetes control [Frampton et al. 2004]. Pregabalin has consistently proved an effective

treatment for DPN and postherpetic neuralgia (PHN) in its extensive clinical trial

program [Dworkin et al. 2003; Lesser et al. 2004; Rosenstock et al. 2004; Sabatowski et

al. 2004; Freynhagen et al. 2005; Richter et al. 2005; Van Seventer et al. 2006; Tolle et

al. 2007]. It is among the agents recommended by the american academy of neurology as

a Group 1 treatment for PHN [Dubinsky et al. 2004], and as a first-line treatment for

painful polyneuropathy by the european federation of neurological societies [Attal et al.

2006]. Recent consensus guidelines have identified Pregabalin as one of the first-tier

treatments for painful DPN [ Argoff et al. 2006 (suppl 4); Argoff et al. 2006 (suppl 6)].

B. Postherpetic neuralgia

It may be considered a complication of herpes zoster. Post herpetic neuralgia (PHN) is

defined as pain persisting for more than 3 months after resolution of the rash. [Stacey et

al. 2003; Argoff et al. 2004; Dubinsky et al. 2004]

It is estimated that about 9-14% of patients with herpes zoster develops postherpetic

neuralgia.



PHN pain is often severe, unrelenting, and exhausting. As a result, PHN can dramatically

affect a patient’s quality of life and functional status. It is estimated that more than 50 %

of patients with PHN have sleep disturbances, and about 25 % report a decrease in

socialization. Eventually a patient with PHN may lose the ability for self-care, leading to

depression and social isolation [Engberg et al. 1995 ; Stacey et al. 2003; Sabatowski et

al. 2004; Bader et al. 2005].

Successful management of PHN can be complicated and challenging, especially with the

fact that there is no definitive treatment algorithm specifically for patients with PHN. In

recent years, there have been a number of published guidelines proposed for the treatment

of neuropathic pain in general. [Dworkin et al. 2003; Moulin et al. 2003; Dubinsky et al.

2004; Attal et al. 2006; Gore et al. 2007] These recommendations are essentially based

on evidence of efficacy from randomized controlled trials (RCTs) of pharmacologic

therapies; there is a lack of clinical trials directly comparing efficacy and safety of one

pharmacotherapy versus another [Finnerup et al. 2007; Gore et al. 2007] These

guidelines uniformly recommend tricyclic antidepressants (TCAs), opioids, and

anticonvulsants as first-line therapeutic options for treating neuropathic pain.

Both gabapentin and Pregabalin (PGB) are approved for the management of PHN. They

are both recommended as first-line therapeutic choices for neuropathic pain based on

several RCTs. [Dworkin et al. 2003; Moulin et al. 2003; Dubinsky et al. 2004; Attal et al.

2006; Gore et al. 2007] Although there have been no had-to-head RCTs between these 2

agents in patients with PHN, both have significantly reduced pain (p _ 0.01) and

improved sleep (p _ 0.01). [Rowbotham et al. 1998; Rice et al. 2001; Bockbrader et al.

2002; Sabatowski et al. 2004; Van Seventer et al. 2006]

Neuropathic pain has been shown to be therapy resistant. Medications used to treat

neuropathic pain include over-the-counter analgesics, anticonvulsants, tricyclic

antidepressants (TCAs), and selective serotonin-norepinephrine reuptake inhibitors

(SSNRIs), topical anesthetic agents, nonsteroidal anti-inflammatory drugs (NSAIDs),

antiarrhythmics, nonnarcotic analgesics, and opioids [Bowsher et al. 1999; Dworkin

2002; Namaka et al. 2004]



However, there is no acknowledged standard treatment for the neuropathic pain in the

EU. In several european countries carbamazepine and amitriptyline are used off-label for

this indication. In some Member states Pregabalin and gabapentin has been approved for

the treatment of neuropathic pain.

C. Efficacy and safety of Pregabalin in neuropathic pain [SPC of Lyrica]

Efficacy has been shown in studies in diabetic neuropathy and post herpetic neuralgia.

Efficacy has not been studied in other models of neuropathic pain.

Pregabalin has been studied in 10 controlled clinical studies of up to 13 weeks with twice

a day dosing (BID) and up to 8 weeks with three times a day (TID) dosing. Overall, the

safety and efficacy profiles for BID and TID dosing regimens were similar.

In controlled clinical trials in peripheral neuropathic pain 35% of the Pregabalin treated

patients and 18% of the patients on placebo had a 50% improvement in pain score. For

patients not experiencing somnolence, such an improvement was observed in 33% of

patients treated with Pregabalin and 18% of patients on placebo. For patients who

experienced somnolence the responder rates were 48% on Pregabalin and 16% on

placebo.

2.1.2.2. Epilepsy

Epilepsy is a common neurological disorder. It has a worldwide estimated prevalence of

50 million. Despite antiepileptic drug (AED) treatment, up to one third of patients

continue to experience seizures [Kwan et al. 2000].

The classification of epileptic seizures according to the International classification of

epileptic seizures (ICES) depends upon clinical symptoms and signs during the seizure,

and the age of the patient at onset. Both aetiology (idiopathic, symptomatic and

cryptogenic) and localization (partial vs generalised) are considered crucial prerequisites

for an adequate approach of epileptic disorders.

In approximately 70% of patients, monotherapy will satisfy. Whereas in another 10% of

patients treatment with more than one compound is necessary. Still, up to 30% of patients



remain refractory to conventional treatment. For this reason, over the past decade, several

new antiepileptic drugs were developed and marketed (a/o. felbamate, gabapentin,

lamotrigine, topiramate, levetiracetam), in order to optimise the therapeutic spectrum and

risk/benefit profile.

A. Efficacy and safety of Pregabalin in partial seizures:

Gabapentin is approved worldwide for adjunctive treatment of patients with partial

epilepsy. Because it is not metabolised (and so does not alter the pharmacokinetics of co-

administrated drugs) it is a good candidate for use in combination with other antiepileptic

medications [SPC].

Pregabalin (PGB) is the latest compound that joins the list of approved "new" AEDs.

Clinical studies with oral Lyrica (Pregabalin) suggest it is at least as effective as

gabapentin as adjunctive therapy in patients refractory to one or more conventional

antiepileptic drugs [SPC].

PGB has been evaluated in three pivotal fixed-dose randomised, double-blind, placebo-

controlled, multicentre trials involving patients at least 12 years of age with refractory

partial seizures. To enter the trials, patients must have failed at least one or two previous

AEDs and must be on one to three concurrent AEDs. After a 6- or 8- week baseline

phase, patients enter a 12-week double blind treatment phase.

In the largest trial conducted in the US and canada, 453 patients were enrolled [French et

al. 2003]. Patients were randomly assigned to placebo, PGB 50, 150, 300 or 600 mg/d

administered twice daily. Seizure frequency reduction from baseline for was 7%, 12%,

34%, 44% and 54%, respectively.

In the second trial conducted in europe, south africa and australia, 287 patients were

randomised to placebo, PGB 150 or 600 mg/d given three times daily [Arroyo et al.

2004] Both doses of PGB were significantly more effective than placebo in reducing

seizure frequency and had higher responder rates (defined as reduction in seizure

frequency of 50% or more).



In the third trial involving 312 patients recruited from centres in US and Canada, patients

were randomized to receive placebo or one of two regimens of 600 mg/d PGB as two or

three divided doses. [Beydoun et al. 2005] Both regimens were similarly effective in

reducing seizure frequency (twice daily, 44%; thrice daily, 53%; placebo, 1% increase).

A separate ad hoc analysis on an intent-to-treat (ITT) patient population showed that

PGB doses of 300 or 600 mg/d were able to achieve complete freedom from seizure in

7% and 19% of patients respectively over a 12-week period [Brodie et al. 2004].

Considering the refractory nature of the trial patients, the efficacy data may be viewed as

highly encouraging.

In summary, data from these pivotal trials demonstrate that PGB doses in the range 150

to 600 mg/d, administered two or three times daily, are effective as adjunctive therapy for

partial-onset seizures.

2.1.2.3. Generalized Anxiety disorder

Generalized anxiety disorder (GAD) is characterized by excessive and inappropriate

worrying that persists (lasting 6 months or more) and is not restricted to particular

circumstances. DSM-IV-TR diagnostic criteria for GAD (APA 2000) require that anxiety

and worry are accompanied by at least 3 of 6 key symptoms (restlessness, fatigue,

difficulty concentrating, irritability, muscle tension, and disturbed sleep).

GAD and major depression have a common genetic basis, and that environmental factors

influence their manifestation [Kendler et al. 1992].

Current treatment approaches in GAD

In acute treatment, systematic reviews and randomized placebo-controlled trials indicate

that selective serotonin reuptake inhibitors (SSRIs) (escitalopram, paroxetine, and

sertraline), serotonin noradrenaline reuptake inhibitors (SNRIs) (duloxetine and

venlafaxine), benzodiazepines (alprazolam and diazepam), the 5-HT1A partial agonist

buspirone, the antipsychotic trifluoperazine, and the antihistamine hydroxyzine are all

efficacious [Baldwin et al. 2005]. Most comparator-controlled studies reveal no

differences in efficacy between active compounds [Mitte 2005], although escitalopram



appeared superior to paroxetine on some outcome measures in a recent large multi-centre

placebo controlled study [Baldwin et al. 2006], and psychological symptoms of anxiety

are traditionally thought to respond better to antidepressant drugs than to benzodiazepines

[Baldwin and Polkinghorn 2005].

In longer-term treatment, some randomized controlled trials indicate that continuing an

SSRI or SNRI is associated with an increase in overall response rates, up to 24 weeks

[Montgomery et al. 2002; Bielski et al. 2005]; and placebo controlled relapse prevention

studies reveal an advantage for staying on SSRI treatment, after initial response, for up to

6 months [Stocchi et al. 2003; Allgulander et al. 2006].

Little is known about the management of patients with GAD who do not respond to first-

line treatment, although the second generation antipsychotic drugs olanzapine and

risperidone have both been found efficacious, in small placebo controlled SSRI

augmentation studies [Brawman-Mintzer et al. 2005; Pollack et al. 2006].

There is still much room for improvement in the treatment of GAD, as the “ideal”

anxiolytic drug does not yet exist.

For example, SSRIs and the SNRI venlafaxine have proven efficacy in acute and long-

term treatment of GAD, but treatment-emergent adverse effects such as sexual

dysfunction are common,

And discontinuation symptoms can be troublesome with paroxetine and venlafaxine.

Benzodiazepines may promptly reduce symptom severity, but their limited efficacy in

treating depressive symptoms and associated risks such as drowsiness and the

development of dependence in predisposed individuals lead to recommendations that they

are restricted to patients who have not responded to other approaches [Bandelow et al.

2002; Baldwin et al. 2005].

Efficacy and safety of Pregabalin in generalized anxiety disorder [SPC of Lyrica]

Analyses of the comparator controlled studies involving alprazolam or venlafaxine

indicate that Pregabalin (across all doses) is associated with a significantly (p<0.01)

greater reduction in symptom severity, when compared with placebo, after 1 week of

double-blind treatment; this finding is similar to that with alprazolam, whereas it was

seen with venlafaxine after only 2 weeks of treatment [Montgomery et al. 2003].



Pregabalin is also significantly superior to placebo in relieving both the psychic and the

somatic symptom clusters, as shown through analysis of pooled data from the five

“positive” acute efficacy studies [Lydiard et al. 2003]. This result is in contrast to the

lack of efficacy of some benzodiazepines in relieving psychic symptoms, seen in some

studies, and to the relative lack of efficacy of certain antidepressants in relieving somatic

symptoms, seen in others.

Pregabalin has been studied in 6 controlled studies of 4-6 week duration, an elderly study

of 8 week duration and a long-term relapse prevention study with a double blind relapse

prevention phase of 6 months duration.

Relief of the symptoms of GAD as reflected by the hamilton anxiety rating scale (HAM-

A) was observed by Week 1.

In controlled clinical trials (4-8 week duration) 52% of the Pregabalin treated patients and

38% of the patients on placebo had at least a 50% improvement in HAM-A total score

from baseline to endpoint.

2.1.2.4. Fibromyalgia

Fibromyalgia syndrome (FMS) affects _3–6 million people in the US, with a prevalence

in the general population estimated at 2% and an increased frequency among women

[Wolfe et al. 1995]. A characteristic symptom complex of chronic widespread

musculoskeletal pain, disordered sleep, and fatigue associated with a lowered pain

threshold is shared among those patients meeting the american college of rheumatology

(ACR) classification criteria for FMS [Wolfe et al. 1990].

The etiology and pathogenesis of FMS are not well understood, but they are probably

multifactorial [Crofford et al. 2002]. Available evidence points toward dysregulation of

neurotransmitter function and central pain sensitization as fundamental mechanisms

[Clauw et al. 2003].

The symptoms of FMS overlap considerably with those of other chronic illnesses, such as

chronic fatigue syndrome, irritable bowel syndrome, temporomandibular disorder, and

chronic headache syndromes [Goldenberg et al.1993]. The lifetime prevalence of anxiety



and depression is higher among patients with FMS than it is in the normal population

[Epstein et al. 1999]. However, the presence of psychiatric comorbidity is neither

necessary nor sufficient for the diagnosis of FMS [McBeth et al. 2001].

At present, treatment of FMS is symptom based, aiming to alleviate pain, increase

restorative sleep, and improve physical function.

nonpharmacologic therapies include education, psychological or cognitive-based

therapies, and exercise-based treatments [Richards et al. 2000].

Pharmacologic treatments include medications that have a neuromodulatory function,

such as tricyclic compounds, selective serotonin reuptake inhibitor and

serotonin/norepinephrine reuptake inhibitor antidepressants, analgesics, muscle relaxants,

and hypnotics [Richards et al. 2000, Barkhuizen 2002]. No single agent has demonstrated

consistent efficacy across all symptom domains [Barkhuizen 2002]. While some

interventions offer benefits for some patients, additional treatment options are needed for

patients with FMS in whom currently available treatments are either ineffective or poorly

tolerated.

A. Efficacy and safety of Pregabalin in fibromyalgia [USPI 2006]

The efficacy of Pregabalin for management of fibromyalgia was established in one 14-

week, double-blind, placebo-controlled, multicenter study (F1) and one six month,

randomized withdrawal study (F2). Studies F1 and F2 enrolled patients with a diagnosis

of fibromyalgia using the american college of rheumatology (ACR) criteria (history of

widespread pain for 3 months, and pain present at 11 or more of the 18 specific tender

point sites). The studies showed a reduction in pain by visual analog scale. In addition,

improvement was demonstrated based on a patient global assessment (PGIC), and on the

fibromyalgia impact questionnaire (FIQ).

2.1.2.5 Pregabalin in acute pain settings: Sensitization of neurons of dorsal horns has

been demonstrated in acute pain models also [Lascelles et al. 1995; Woolf, Chong 1993]

and the persistence of this mechanism may be responsible for increasingly recognised

problem of chronic pain after surgery [Aasvang, Kehlet 2005; Perkins, Kehlet 2000].

Pregabalin may also be beneficial in post operative acute pain settings. Several studies

have reported usefulness of Pregabalin in perioperative settings resulting in reduced post



operative pain, post operative analgesic requirement, side effects, prolongation of

analgesia and higher patient satisfaction [Tiippana et al. 2007; Rorarius et al. 2004; Al-

Mujadi et al. 2006; Turan et al. 2006]. After the assembly of evidence-base of sufficient

size and quality to be sure of its efficacy and safety, addition of Pregabalin to pre-

operative pain medication may very well be the gold standard for managing acute post

operative pain and also it will minimise the long term complications and occurring of

chronic pain syndromes within weeks or months after surgery. In all these studies,

Pregabalin has been used as a single dose premedication given prior to incision. Use of

Pregabalin as an analgesic in settings of acute pain after affliction of trauma is still to be

studied as the present literature lacks any such data where it has been used in his kind of

settings.

2.2. Rationale of this study

Pregabalin is available as an immediate release (IR) formulation in capsules and is

administered to patients two- or three- times daily (BID or TID).

Many patients receiving Pregabalin or other drugs which are administered two or more

times daily would likely to benefit from once daily dosing. The convenience of OD

dosing generally:



Improves patient compliance:

Especially for elderly patients and for patients taking multiple medications due to less

frequent drug administration:

Neuropathic pain is a chronic disorder, requires regular long term therapy. Poor

compliance is a recognised factor related to the inadequate control. Numerous studies

have demonstrated that poor medication compliance poses a significant impediment to

the effective treatment of a wide variety of illnesses.

Compliance improves as prescribed dose frequency decreases [Brun et al. 1994; Paes et

al. 1997]. Health care providers can improve compliance by selecting medications that

permit the minimum daily dose frequency.

Reduction in fluctuation in steady state levels:

An extended release formulation is expected to cause fewer fluctuations in drug

plasma levels that may lead to a better response as seen by indirect evidence in

Pregabalin clinical trials.

In epilepsy trials, it has been seen that the responder rate with thrice daily doses was

numerically better than the twice-daily doses as compared to placebo though not

statistically significant. [USPI 2006]

In diabetic neuropathy trial the proportion of responders (= 50% improvement) with

Pregabalin 300 or 600 mg/day three times daily was 39 to 48% which was more than

double that for placebo (15-18%); p=0.0001 [Rosenstock et al. 2004].

In trials with post herpetic neuralgia, the pain was relieved with 150, 300 and 600-mg/day

twice-daily dose (p=0.01 vs placebo). The studies with three times daily doses of 150 mg

and 300 mg/day relieved the pain significantly (p=0.0002 vs placebo) and with 600

mg/day thrice daily response improved further in comparison to placebo (p=0.0001 vs

placebo).

The results of these trials show that thrice daily regimen as compared to the twice-daily

regimen over placebo in equal doses shows better response.

The reason is that the plateau level achieved with thrice-daily regimens may be higher

than the twice daily regimen and also may archive steady state at a faster rate than twice

daily dosing..



It is expected that relatively flatter plasma levels with an extended release formulations

will be translated into better therapeutic effect.

Reduction in fluctuation in steady state levels and therefore better control of

disease condition and reduce intensity of local or systemic side effects:

Adverse events with Pregabalin are shown to be dose dependant. Common adverse

events like dizziness, somnolence and peripheral edema are the common causes of

discontinuation of treatment. However, it is not known whether these adverse events are

related to peak plasma concentration or total systemic exposure.

An extended release formulation that produces a relatively flatter blood concentration

profile may be expected to minimise these adverse events.

Once daily dosing of Pregabalin, however presents numerous challenges. Conventional

extended release compositions are problematic for OD dosing. Clinical studies indicate

that Pregabalin is absorbed in the small intestine and the ascending colon in humans, but

is poorly absorbed beyond the hepatic flexure. This suggests that the mean absorption

window for Pregabalin is, on average, about six hours or less- any drug release from a

conventional ER dosage form beyond six hours would thus be wasted because the dosage

form has traveled beyond the hepatic flexure as per the patent filed by Pfizer “solid oral

pharmaceutical compositions for once daily dosing containing Pregabalin, a matrix

forming agent and a swelling agent.”

The development and subsequent validation of an in vitro-in vivo correlation (IVIVC) is

an increasingly important component of extended release dosage form optimization. The

USP (United States Pharmacopoeia) defines IVIVC as the establishment of a relationship

between a biological property (Cmax, Tmax or AUC) produced by a dosage form and a

physicochemical property (in vitro dissolution profile) of the same dosage form [USP].

The recent In vitro/In vivo correlation guidance developed by the FDA states that the

main objective of developing and evaluating an IVIVC is to enable the dissolution test to

serve as a surrogate for in vivo bioavailability studies. This may reduce the number of

bioequivalence studies required for approval.



According to the biopharmaceutics classification system, Pregabalin is a “Class I” drug,

i.e. high solubility and permeability [Amidon G., 1995]. In addition, its relatively short

half life suggests that it is a suitable candidate for an extended release formulation. The

availability of a meaningful IVIVC of high quality and predictability for an extended

release Pregabalin formulation should provide a sound foundation for product

optimization.

2.3. Drug Delivery system [Brahmankar et al. 1999]:

2.3.1 Definitions:

Immediate release dosage form (Conventional Release dosage form) [EMEA 1999]:

Preparations showing a release of the active ingredient which is not deliberately modified

by special formulation and/or manufacturing method. In case of a solid dosage form, the

dissolution profile of the active ingredient depends essentially on the intrinsic properties

of the active ingredient.

Modified release dosage forms [EMEA 1999]: Preparations where the rate and/or place

of release of the active ingredient (s) is different from that of the conventional dosage

form administered by the same route. This deliberate modification is achieved by special

formulation design and/ or manufacturing method. Modified release dosage forms

include prolonged release, extended release (controlled release), sustained release,

delayed release, pulsatile release and accelerated release dosage forms.

2.3.2 Advantages of extended release over conventional dosage form are:

1. Improved patient convenience and compliance due to less frequent drug

administration.

2. Reduction in fluctuation in steady state levels and therefore better control of

disease condition and reduce intensity of local or systemic side effects.

3. Increase safety margin of high potency drugs due to better control plasma levels.

4. Maximum utilization of drug enabling reduction in total amount of dose

administered.



5. Reduction in health care cost through improved therapy, shorter treatment period,

less frequency of dosing and reduction in personnel time to dispense, administer

and monitor patients.

2.3.3 Disadvantages of extended release over conventional dosage form are:

1. Decrease systemic availability in comparison to immediate release conventional

dosage forms; this may be due to incomplete release, increase first pass

metabolism, increase instability, insufficient residence time or complete release,

site specific absorption, pH-dependent solubility, etc.

2. Poor in vitro-in vivo correlation.

3. Possibility of dose dumping due to food, physiologic or formulation variables or

chewing or grinding of oral formulation by the patient and thus, increase risk of

toxicity.

4. Retrieval of drug is difficult in case of toxicity, poisoning or hypersensitive

reactions.

5. Reduced potential for dosage adjustment of drugs normally administered in

varying strengths.

6. Higher cost of formulation.

2.4 Design of controlled drug delivery systems:

The basic rationale of a controlled drug delivery system is to optimize the

biopharmaceutic, pharmacokinetic and pharmacodynamic properties of a drug. It is done

in such a way that its utility is maximize through reduction in side effects and cure or

control of condition in the shortest possible time by using smallest quantity of drug

administered by the most suitable route.

2.4.1 Biopharmaceutic characteristics of the drug

The performance of a drug presented as a controlled release system depends upon its:

1. Release from the formulation.

2. Movement within the body during its passage to the site of action.

The former depends upon the fabrication of the formulation and the physicochemical

properties of drug while the latter element is dependent upon pharmacokinetics of the

drug. In comparison to conventional dosage form where the rate-limiting step in drug



availability is usually absorption through the biomembrane, the rate-determining step in

the availability of a drug from controlled delivery system is the rate of release of drug

from the dosage form which is much smaller than the intrinsic absorption rate for the

drug.

The desired biopharmaceutic properties of a drug to be used in controlled drug delivery

systems are:

A. Molecular weight of the drug: Drugs with large molecular size are poor candidate

for oral controlled release systems for e.g. peptides and proteins.

B. Aqueous solubility of the drug: A drug with good aqueous solubility, especially if

Ph- independent, serves as a good candidate for controlled release dosage forms.

C. Apparent partition coefficient of the drug: Greater the Apparent partition

coefficient of the drug greater is its rate and extent of absorption.

D. Drug pKa and Ionization at Physiologic pH: Drugs existing largely in ionized forms

are poor candidates for controlled delievery e.g. hexamethonium.

E. Drug stability: Drugs unstable in GI environment cannot be administered as oral

controlled release formulation because of bioavailability problems e.g. nitroglycerine.

F. Mechanism and site of Absorption: Drugs absorbed by carrier-mediated transport

processes and those absorbed through a window are poor candidates for controlled

release systems e.g. several B vitamins.

Biopharmaceutic Aspects of route of administration: Oral and parenteral (i.m) routes

are the most popular followed by transdermal application.

a) Oral route: For a drug to be successful as oral release formulation, it must get

absorbed through the entire length of GIT.

b) Intramuscular/ Subcutaneous routes: These routes are suitable when the

duration of action is to be prolonged from 24 hours to 12 months.

c) Transdermal Route: Low dose drugs like nitroglycerine can be administered

by this route. The route is best suited for drugs showing extensive first-pass

metabolism upon oral administration.



2.4.2 Pharmacokinetic Characteristics of the drug:

A. Absorption Rate: For a drug to be administered as controlled release formulation, its

absorption must be efficient since the desired rate-limiting step is rate of drug release. A

drug with slow release will result in a pool of unabsorbed drug e.g. iron.

B. Elimination half-life: Smaller the t1/2, larger amount of drug to be incorporated in the

controlled release dosage form. Drugs with half-life in the range 2 to 4 hours make good

candidates for such a system e.g. propranolol.

C. Rate of Metabolism: A drug which is extensively metabolized is suitable for

controlled release system as long as the rate of metabolism is not too rapid.

D. Dosage form index: Since the goal of controlled release formulation is to improve

therapy by reducing the dosage form index while maintaining the plasma levels within

the therapeutic window, ideally its value should be as close to one as possible.

2.4.3 Pharmacodynamic Characteristics of a drug:

A. Therapeutic Range: A candidate drug for controlled delivery system should have a

therapeutic range wide enough such that variations in the release rate do not result in a

concentration beyond this level.

B. Therapeutic Index: The release rate of a drug with narrow therapeutic index should

be such that the plasma concentration attained is within the therapeutically safe and

effective range. This is necessary because such drugs have toxic concentration nearer to

their therapeutic range.

C. Plasma Concentration-Response Relationship: Drugs such as reserpine whose

pharmacologic activity is independent of its concentration are poor candidates for

controlled release systems.

2.5 Oral Controlled release systems:

Oral route has been the most popular and successfully used for controlled delivery of

drugs because of convenience and ease of administration, greater flexibility in dosage

form design (possible because of versatility of GI anatomy and physiology) and ease of

production and low cost of such a system. Depending upon the manner of drug release,

these systems are classified as follows:



2.5.1 Continuous release systems: These systems release the drug for a prolonged

period of time along the entire length of GIT (especially upto the terminal region of small

intestine) with normal transit of the dosage form. The various systems under this category

are:

1. Dissolution controlled release systems

2. Diffusion controlled release systems

3. Dissolution and diffusion controlled release systems

4. Ion-exchange resin-drug complexes

5. Slow dissolving salts and complexes

6. Ph-dependent formulations

7. Osmotic pressure controlled systems

8. Hydrodynamic pressure controlled systems

2.5.2 Delayed release systems: The design of such systems involve release of drug

only at a specific site in the GIT. The drugs contained in such a system are those that are:

a. Destroyed in the stomach or by intestinal enzymes

b. Known to cause gastric distress

c. Absorbed from a specific intestinal site, or

d. Meant to exert local effect at a specific GI site.

The two types of delayed release systems are:

1. Intestinal release systems

2. Colonic release systems

2.5.3 Delayed transit and continuous release systems (gastroretentive drug

delivery system): The gastric emptying time (GET) in humans is normally 2-3 h through

the major absorption zone, i.e., stomach and upper part of the intestine. It can result in

incomplete drug release from the drug delivery system leading to reduced efficacy of the

administered dose [Rouge et al. 1996]. Therefore, control of placement of a drug delivery

system (DDS) in a specific region of the GI tract offers advantages for a variety of

important drugs [Singh et al. 2000]. The advantages of gastroretentive drug delivery

systems are:

1. Enhanced bioavailability

2. Enhanced first-pass biotransformation



3. Sustained drug delivery/reduced frequency of dosing

4. Targeted therapy for local ailments in the upper GIT

5. Reduced fluctuations of drug concentration

6. Improved selectivity in receptor activation

7. Extended time over critical (effective) concentration

8. Minimized adverse activity at the colon

9. Site specific drug delivery

2.5.3.1 Suitable drug candidates for gastro retention:

In general, appropriate candidates for CRGRDF are molecules that have poor colonic

absorption but are characterized by better absorption properties at the upper parts of the

GIT:

• Narrow absorption window in GI tract, e.g., riboflavin and levodopa

• Primarily absorbed from stomach and upper part of GI tract, e.g., calcium supplements,

chlordiazepoxide and cinnarazine

• Drugs that act locally in the stomach, e.g., antacids and misoprostol

• Drugs that degrade in the colon, e.g., ranitidine HCl and metronidazole

• Drugs that disturb normal colonic bacteria, e.g., amoxicillin trihydrate

2.5.3.2 Factors controlling gastric retention of dosage forms

The gastric retention time (GRT) of dosage forms is controlled by several factors:

2.5.3.2.1 Density of dosage form: Dosage forms having a density lower than that of

gastric fluid experience floating behavior and hence gastric retention. A density of <1.0

gm/cm3 is required to exhibit floating property.

2.5.3.2.2 Size of dosage form: In most cases, the larger the size of the dosage form, the

greater will be the gastric retention time [El-Kamel et al. 2001] because the larger size

would not allow the dosage form to quickly pass through the pyloric antrum into the

intestine.

2.5.3.2.3 Food intake and nature of food: Usually, the presence of food increases the

GRT of the dosage form and increases drug absorption by allowing it to stay at the

absorption site for a longer time.

2.5.3.2.4 Effect of gender, posture and age: A study [Mojaverian et al. 1988] found

that the gastric emptying in women was slower than in men. The authors also studied the



effect of posture on GRT, and found no significant difference in the mean GRT for

individuals in upright, ambulatory and supine state. On the other hand, in a comparative

study in humans by [Gansbeke 1991], the floating and non-floating systems behaved

differently. In the upright position, the floating systems floated to the top of the gastric

contents and remained for a longer time, showing prolonged GRT. However, in supine

position, the floating units are emptied faster than non-floating units of similar size

[Timmermans 1994]

2.5.3.3 Types of gastroretentive dosage forms

2.5.3.3.1. Floating drug delivery systems

Floating drug delivery systems (FDDS) have a bulk density less than gastric fluids. And

so remain buoyant in the stomach without affecting gastric emptying rate for a prolonged

period of time. While the system is floating on the gastric contents, the drug is released

slowly at the desired rate from the system. After release of drug, the residual system is

emptied from the stomach. This results in an increased GRT and a better control of the

fluctuations in plasma drug concentration. FDDS can be divided into non-effervescent

and gas-generating system:

(a) Non-effervescent systems

This type of system, after swallowing, swells unrestrained via imbibition of gastric fluid.

Thus, it prevents their exit from the stomach. One of the formulation methods of such

dosage forms involves the mixing of the drug with a gel. Gel swells in contact with

gastric fluid after oral administration. And maintains a relative integrity of shape, a bulk

density of less than one within the outer gelatinous barrier [Hilton et al. 1992]. The air

trapped by the swollen polymer confers buoyancy to these dosage forms. Excipients used

most commonly in these systems include hydroxypropyl methyl cellulose (HPMC),

polyacrylate polymers, polyvinyl acetate, carbopol, agar, sodium alginate, calcium

chloride, polyethylene oxide and polycarbonates.

This system can be further divided into four sub-types:

(i) Colloidal gel barrier system

Seth and Tossounian first designated this ‘hydrodynamically balanced system’ [Seth et

al. 1984]. Such a system contains drug with gel-forming hydrocolloids meant to remain

buoyant on the stomach content.



(ii) Microporous compartment system

This technology is based on the encapsulation of a drug reservoir inside a microporous

compartment with pores along its top and bottom walls [Harrigan et al. 1977].

(iii) Alginate beads

Multi-unit floating dosage forms have been developed from freeze-dried calcium alginate

[Whitehead et al. 1996].

(iv) Hollow microspheres / Microballons

Hollow microspheres loaded with drug in their outer polymer shelf were prepared by a

novel emulsion solvent diffusion method [Kawashima et al. 1992].

(b) Gas-generating (Effervescent) systems

These buoyant systems utilize matrices prepared with swellable polymers such as

methocel, polysaccharides (e.g., chitosan), effervescent components (e.g., sodium

bicarbonate, citric acid or tartaric acid) [Rubinstein et al. 1994]. The system is so

prepared that upon arrival in the stomach, carbon dioxide is released, causing the

formulation to float in the stomach.

2.5.3.3.2. Expandable systems: Expandable gastroretentive dosage forms (GRDFs) have

been designed over the past 3 decades. They were originally created for possible

veterinary use but later the design was modified for enhanced drug therapy in humans.

These GRDFs are easily swallowed and reach a significantly larger size in the stomach

due to swelling or unfolding processes that prolong their GRT. After drug release, their

dimensions are minimized with subsequent evacuation from the stomach [Klausner EA et

al 2002]. Gastroretentivity is enhanced by the combination of substantial dimensions with

high rigidity of the dosage form to withstand the peristalsis and mechanical contractility

of the stomach. Positive results were obtained in preclinical and clinical studies

evaluating the GRT of expandable GRDFs. Narrow absorption window drugs

compounded in such systems have improved in vivo absorption properties.

2.5.3.3.3 Bio/Muco-adhesive systems: Bioadhesive drug delivery systems (BDDS) are

used as a delivery device within the lumen to enhance drug absorption in a site specific

manner. This approach involves the use of bioadhesive polymers, which can adhere to the

epithelial surface in the stomach [Moes AJ 1993]. Gastric mucoadhesion does not tend to

be strong enough to impart to dosage forms the ability to resist the strong propulsion



forces of the stomach wall. The continuous production of mucous by the gastric mucosa

to replace the mucous that is lost through peristaltic contractions and the dilution of the

stomach content also seem to limit the potential of mucoadhesion as a gastroretentive

force. Some of the most promising excipients that have been used commonly in these

systems include polycarbophil, carbopol, lectins, chitosan and gliadin, etc.

2.5.3.3.4 High-density systems: Sedimentation has been employed as a retention

mechanism for pellets that are small enough to be retained in the rugae or folds of the

stomach body near the pyloric region, which is the part of the organ with the lowest

position in an upright posture. Dense pellets (approximately 3g/cm-3) trapped in rugae

also tend to withstand the peristaltic movements of the stomach wall. With pellets, the GI

transit time can be extended from an average of 5.8–25 hours, depending more on density

than on the diameter of the pellets [Bechgaard H and Ladefoged K 1978]. Commonly

used excipients are barium sulphate, zinc oxide, titanium dioxide and iron powder, etc.

These materials increase density by up to 1.5–2.4g/cm-3.

2.5.3.4 Works on gastroretentive dosage form:

Basak et al. [2007] designed floatable gastroetentive tablet of metformin hydrochloride

using a gas-generating agent and gel-forming hydrophilic polymer. The formulation was

optimized on the basis of floating ability and in vitro drug release. The in vitro drug

release test of these tablets indicated controlled sustained release of metformin

hydrochloride and 96-99% released at the end of 8 h.

Jaimini et al. [2007] prepared floating tablets of famotidine employing two different

grades of methocel K100 (HPMC K100) and methocel K15 (HPMC K15) by an

effervescent technique. These grades were evaluated for their gel-forming properties. The

tablets with methocel K100 were found to float for a longer duration compared with the

formulation containing methocel K15M. Decrease in the citric acid level increased the

floating lag time. The drug release from the tablets was sufficiently sustained and non-

Fickian transport of the drug from tablets was confirmed.

Badve et al. [2007] developed hollow calcium pectinate beads for floating-pulsatile

release of diclofenac sodium intended for chronopharmacotherapy. Floating pulsatile

concept was applied to increase the gastric residence of the dosage form having lag phase

followed by a burst release. This approach suggested the use of hollow calcium pectinate



microparticles as promising floating pulsatile drug delivery system for site- and time-

specific release of drugs for chronotherapy of diseases.

Chavanpatil et al. [2006] developed a new gastroretentive sustained release delivery

system of ofloxacin with floating, swellable and bioadhesive properties. Various release

retarding polymers such as psyllium husk, HPMC K100M and a swelling agent,

crosspovidone, in combinations were tried and optimized to obtain release profile over 24

h. The in vitro drug release followed Higuchi kinetics and the drug release mechanism

was found to be non-Fickian.

Rahman et al. [2006] established a bilayer-floating tablet (BFT) for captopril using direct

compression technology. HPMC K-grade and effervescent mixture of citric acid and

sodium bicarbonate formed the floating layer. The release layer contained captopril and

various polymers such as HPMC-K15M, PVP-K30 and carbopol 934, alone or in

combination with the drug. The formulation followed the Higuchi release model and

showed no significant change in physical appearance, drug content, floatability or in vitro

dissolution pattern after storage at 45 °C/75% RH for three months.

Xiaoqiang et al. [2006] developed a sustained release tablet for phenoporlamine

hydrochloride because of its short biological half life. Three floating matrix tablets based

on a gas-forming agent were prepared. HPMC K4M and carbopol 971P were used in

formulating the hydrogel system. Incorporation of sodium bicarbonate into the matrix

resulted in the tablets floating over simulated gastric fluid for more than 6 hours. The

dissolution profile of all the tablets showed non-fickian diffusion in simulated gastric

fluid.

There are several commercial products with floating drug delivery system available

in the market as shown in table LR 1:

Table LR 1 showing marketed preparations of floating drug delivery systems [Wu et al.

1997]

S. no

Product Active Ingredient Reference No.

1 Madopar Levodopa and benserzide

[Erni et al. 1987]

2 Valrelease Diazepam [Sheth et al. 1984] 3 Topalkan Aluminum [Degtiareva et al. 1994]



Magnesium antacid 4 Almagate

flatcoat Antacid [Fabregas et al. 1994]

5 Liquid Gavison Alginic acid and sodium bicarbonate

[Washingtn et al. 1986]

2.6. In Vitro In Vivo Correlation:

The development and subsequent validation of an in vitro-in vivo correlation (IVIVC) is

an increasingly important component of extended release dosage form optimization.

Various definitions of in vitro–in vivo correlation have been proposed by the

International pharmaceutical federation (FIP), the USP working group [USP 2002], and

regulatory authorities such as the FDA or EMEA [CDER 1997; Guidance for nonsterile

semisolid dosage forms, 1997; EMEA guidance, 1998; EMEA guidance on modified

release, 1999]. The FDA [CDER 1997] defines IVIVC as “a predictive mathematical

model describing the relationship between an in vitro property of an extended release

dosage form (usually the rate or extent of drug dissolution or release) and a relevant in

vivo response, e.g., plasma drug concentration or amount of drug absorbed.”

2.6.1. Purpose of IVIVC:

1. IVIVC is established to enable a dissolution test to be used as a surrogate of the

bioavailability study.

2. It supports and/or validates the use of dissolution methods and specifications; and

3. It assists in QC during manufacturing and selecting appropriate formulations. [CDER

1997; Young et al. 1997]

2.6.2. Fundamentals of IVIVC [Welling 2006; Mathiowitz 1999; Venkateshwarlu

2004; Brahmankar et al. 2006; Leon Shargel et al. 1999]

USP defined five levels of correlation each of which denotes the ability to predict in vivo

response of a dosage form from its in vitro property. Higher the level better is the

correlation. The level of correlation is categorised as:

2.6.2.1. Level A correlation

Among all the level of correlation defined, level A is of prime importance. It is defined as

a hypothetical model describing the relationship between a fraction of drug absorbed and



fraction of drug dissolved. In order to develop a correlation between two parameters one

variable should be common between them. The data available is in vitro dissolution

profile and in vivo plasma drug concentration profile whose direct comparison is not

possible. To have a comparison between these two data, data transformation is required.

The in vitro properties like percent drug dissolved or fraction of drug dissolved can be

used while in vivo properties like percent drug absorbed or fraction of drug absorbed can

be used respectively. It is considered as a predictive model for relationship between the

entire in vitro release time courses. Most commonly a linear correlation exists but

sometimes non-linear In vitro- in vivo correlation may prove appropriate.

However, no formal guidance for non-linear IVIVC has been established. When in vitro

curve and in vivo curve are super imposable, it is said to be 1:1 relationship, while if

scaling factor is required to make the curve super imposable, then the relationship is

called point-to-point relationship. Level A correlation is the highest level of correlation

and most preferred to achieve; since it allows bio waiver for changes in manufacturing

site, raw material suppliers, and minor changes in formulation.

2.6.2.2. Level B correlation

Here the mean in vitro dissolution time (MDT) is compared with either the mean in vivo

residence time (MRT) or mean in vivo dissolution time derived by using principle of

statistical moment analysis. Though it utilizes all in vitro and in vivo data, it is not

considered as point-to-point correlation since number of in vivo curves can produce

similar residence time value. Hence, it becomes least useful for regulatory purposes.

2.6.2.3. Level C correlation

It is referred as single point correlation which is established in between one dissolution

parameter (t50%) and one of the pharmacokinetic parameter (Tmax, Cmax or AUC).

However, it does not reflect the complete shape of plasma drug concentration time curve,

which is the critical factor that defines the performance of a drug product. Level C

correlation is helpful in early stages of development when pilot formulations are being

selected.

2.6.2.4. Multiple Level C correlation

It refers to the relationship between one or several pharmacokinetic parameters of interest

and amount of drug dissolved at several time point of dissolution profile. It should be



based on at least three dissolution time points that includes early, middle and late stage of

dissolution profile.

2.6.2.5. Level D correlation

It is a semi quantitative and rank order correlation and is not considered useful for

regulatory purpose.

2.6.3. Predictability of correlation [Antal et al, 1975; De Muth 1999]

It can be calculated by prediction error that is the error in prediction of in vivo property

from in vitro property of drug product. Based on therapeutic index of the drug and

application of IVIVC, evaluation of prediction error internally or externally may be

appropriate. Internal error provides a basis for acceptability of model while external

validation is superior and affords greater confidence in model. The % prediction error can

be calculated by the following equation:

% Prediction error (P.E) = (Cmax observed – Cmax predicted) × 100/

Cmax observed

2.6.3.1. Internal predictability

The bioavailability (Cmax, Tmax/AUC) of formulation that is used in development of

IVIVC is predicted from its in vitro property using IVIVC. Comparison between

predicted bioavailability and observed bioavailability is done and % P.E is calculated.

According to FDA guidelines, the average absolute % P.E should be below 10% and %

P.E for individual formulation should be below 15% for establishment of IVIVC.

2.6.3.2. External predictability

The predicted bioavailability is compared with known bioavailability and % P.E is

calculated. The prediction error for external validation should be below 10% whereas

prediction error between 10-20% indicates inconclusive predictability and need of further

study using additional data set. Drugs with narrow therapeutic index, external validation

is required.

2.6.4. Reasons for poor in vitro-in vivo correlation [Aoyagi 1982]

A. Fundamentals – When in vivo dissolution is not the rate limiting pharmacokinetic

stage, and when no in vitro test can simulate the drug dissolution along the

gastrointestinal tract.

B. Study design – With inappropriate in vitro test conditions.



C. Dosage form – When the drug release is not controlled by the dosage form or is

strongly affected by the stirring of synthetic liquid.

D. Drug substance – With a non- linear pharmacokinetics, for e.g, first - pass hepatic

effect, an absorption window, a chemical degradation and a large inter or intra subject

variability. All these factors are of vital concern and should be kept in mind, especially

the inter variability of patients’ response to a drug.

2.6.5. Biopharmaceutics classification system (BCS) [Mattok et al. 1972; Shaw et al.

1973; Chasseaud et al. 1983 ; Mathiowitz 1999; Dressman 2005]

Biopharmaceutics classification system is based on solubility, intestinal permeability and

dissolution rate, all of which governs the rate and extent of oral absorption from

immediate release solid oral dosage form. Based on solubility and permeability, there are

four classes of BCS as shown in table 2 solubility criteria defined in present regulatory

guidance for classifying an active pharmaceutical ingredients (API) as “highly soluble”

requires the highest strength to be soluble in 250 ml of water over the pH range of 1-7.5

at 37°C, otherwise it is considered as poorly soluble. The FDA and also EMEA Guidance

define “highly permeable” as having a fraction dose absorbed of not less than 90%. The

recently adopted WHO guidelines set a limit of not less than 85% of the fraction dose

absorbed, otherwise it is considered to be poorly permeable.



Table LR 2: IVIVC expectations for immediate release products based on BCS

Class Solubility Permeability IVIVC expectations

I High High IVIVC expected, if dissolution rate is slower than gastric

emptying rate, otherwise limited or no correlations.

II Low High IVIVC expected, if in vitro dissolution rate is similar

to in vivo dissolution rate.

III High Low Absorption (permeability) is rate determining and limited

or no IVIVC with dissolution.

IV Low Low Limited or no IVIVC is expected.

A. Biowaiver for BCS Class I

On the basis of FDA guidelines, sponsor can request biowaiver for BCS Class I in

immediate release solid oral dosage form, if the drug is stable in GIT and having narrow

therapeutic index with no excipient interaction affecting absorption of drug in the oral

cavity. Once a drug enters in stomach; it gets solubilised in gastric fluid rapidly before

gastric emptying and the rate and extent of absorption is independent of drug dissolution

as in case of solution. Hence, the goal of biowaiver is achieved.

B. Biowaiver Extension Potential for BCS Class II

The rate and extent of absorption of BCS Class II drug depends on in vivo dissolution

behaviour of immediate release products. If in vivo dissolution can be predicted from in

vitro dissolution studies, in vivo bioequivalence study can be waived. In vitro dissolution

methods can mimic in vivo dissolution behaviour of BCS Class II drug and are appealing

but experimental methods can be difficult to design and validate because of number of

processes involved.

C. Biowaiver Extension for BCS Class III

If excipient used in two pharmaceutically equivalent solid oral immediate release product

does not affect the drug absorption and the products dissolves very rapidly (>85% in 15

min.) in all relevant pH ranges, there is no reason to believe that these products would not

be bioequivalent.

2.6.6. Approaches for Development of Correlation [Sullivan et al. 1974; Di Santo et

al. 1975; Zaman et al. 1983; De Muth 1999]



Basically, two methods are available for the development of correlations

2.6.6.1. Two stage deconvolution approach: This involve estimation of in vivo

absorption profile from plasma drug concentration - time profile using wagner nelson or

looe-riegelman method, subsequently the relationship with in vitro data is evaluated.

2.6.6.2 One stage convolution approach: It computes the in vivo absorption and

simultaneously models the in vitro – in vivo data.

Two stage methods allows for systematic model development while one stage obviates

the need for administration of an intravenous, oral solution or IV bolus dose. Mostly

IVIVC models developed are simple linear equation between in vitro drug released and in

vivo drug absorbed. But sometimes these data can be better fitted by using nonlinear

models like Sigmoid, Weibull, Higuchi or Hixon-crowell.

2.6.7. Parameters to be considered while developing IVIVC [Sullivan et al. 1974;

Dietrich et al. 1988; Vergnaud et al. 2005]

2.6.7.1. Metabolic factors

A drug must pass sequentially from the gastrointestinal lumen, through the gut wall, and

the liver, before entering in the systemic circulation. This sequence is an anatomic

requirement because blood perfusion virtually all gastrointestinal tissues drain into the

liver via the hepatic portal vein. Drug loss may occur in the GIT due to the instability of

the drug in the GIT and/or due to complexation of drug with the components of the GI

fluids, food, formulation excipients or other co-administered drugs. In addition, the drug

may undergo destruction within the walls of the GIT and/or liver.

2.6.7.2. Drug loss in GIT

Any reaction that completes with the absorption of a drug may reduce oral bioavailability

of a drug. Reaction can be both enzymatic and non-enzymatic. Acid hydrolysis is a

common non-enzymatic reaction. Enzymes in the intestinal epithelium and within the

intestinal microflora, which normally reside in the large bowel, metabolize some drug.

The reaction products are often inactive or less potent than the large molecule.

2.6.7.3. Stereochemistry

When one enantiomer has higher affinity towards receptors than other, the phenomenon

is termed as stereo selectivity which results in pharmacokinetics or pharmacodynamics. If

such stereoisomers in the form of racemate are administered orally, one form may have



higher bioavailability than the other. Obviously use of in vitro dissolution data of

racemate will not be useful in the development of IVIVC and hence prediction of in vivo

availability of active enantiomer. So consideration of stereoisomerism in the development

of IVIVC may provide more meaningful relationship.

2.6.8. Parameters studied for IVIVC

Earlier disintegration was considered as the most important pertinent in vitro parameter

but recently, dissolution rate has been used as a manufacturing process standard and is

generally considered to be the in vitro parameter most likely to correlate with in vivo

bioavailability. In vivo bioavailability is described in terms of the rate and extent of drug

absorption. Rate of absorption is reflected in peak drug concentrations in plasma (Cmax)

and the terms at which they occur (Tmax). Other methods may be used to describe

absorption rate profile, for example, deconvolution and statistical moment theory.

However use of these approaches does not detract from the basic relationships between

absorption rate, Cmax and tmax. Extent of absorption is reflected in Cmax and also the

area under the plasma drug curve (AUC).

2.6.9. Attempts to establish in vitro – in vivo correlation [Benidikt et al. 1988; De

Muth 1999; Welling et al. 2006; Vergnaud et al. 2005]

Many attempts have been made to establish IVIVC for a variety of drugs. Some of these

are summarized in the table LR 3 which describes studies on a variety of dosage forms

for a broad spectrum of therapeutic indications, and provides a brief comment on the

results obtained.

Table LR 3: Investigations of In vitro Dissolution and In vivo Bioavailability Relationship

Drug Test Formulation Comments

Steroids and Hormones

Prednisolone 5mg tablets Products were bioequivalent despite difference in in vitro dissolution. Dissolution test modified to agree with in vivo data.

Prednisone 5mg and 50mg tablets In vitro dissolution rate not predictive of overall bioavailability



Anti-inflammatory and analgesic agents

Aspirin Four 300mg tablets No IVIVC correlation

Ketoprofen 50mg conventional capsules and two 200mg sustained release capsules

Slower absorption and reduced systemic bioavailability from slower dissolving SR capsule.

Indomethacin Four Indomethacin preparations

All preparations were bioequivalent despite different dissolution rate of one preparation.

Respiratory Tract

Theophylline Four experimental controlled release formulations.

Correlations obtained between in vitro and in vivo data.

Central Nervous system Drugs

Promethazine Two 50mg tablets, one 25mg tablet and a solution.

No discrimination. No significant differences among products in in vitro or in vivo data.

Antibacterial and antifungal

Griseofulvin Four 100mg capsules compared in dog and humans

Good in vitro-in vivo correlation using specific sink condition dissolution method.

Doxycycline Three 100mg capsules compared with a suspension and a solution

Rank order correlation between dissolution rates and absorption rate constants, but no statistical significant difference in bioavailability of the three capsules products.

Nitrofurantoin Nineteen 100mg products Neither disintegration nor dissolution accurately reflected absorption.

Hypoglycaemic Agents Glyburide Four marketed preparations Two dissolution tests yielded

different rank orders of dissolution rates. Neither test correlated with in vivo data.

Cardiovascular Isosorbide dinitrate Two experimental 40mg

tablets Products were bioequivalent despite different in vitro release



rates. Digoxin Seven 0.25mg tablets Close correlation between

dissolution rate and bioavailability.

2.6.10. Applications [Chalk et al. 1986 ; Leon Shargel et al. 1999; Mathiowitz 1999;

Vergnaud et al. 2005]

The most vital application of IVIVC is to use in vitro dissolution study in lieu of human

bioequivalence studies which will reduce the number of human bioequivalence studies

during initial approval process as well as certain scale up and post approval changes.

2.6.10.1. Manufacturing Control

The extended release products are distinguished through their input rate to the absorption

site. Therefore, the rate of drug release from these products is an important feature and

should be carefully controlled and evaluated. The in vitro dissolution/release test is

meaningful only when the test results are correlated to the products’ in vivo

performances.

2.6.10.2. Process Change Assurance

The manufacturing processes of approved products are regulated by the regulatory

agencies. The manufacturers are required to demonstrate that kind of change, even an

engineering improvement, does not cause changes in the finished product’s in vivo

performance. Consequently, many changes have to be supported by a bioequivalence

study. With the availability of an in vitro test with one-to-one correlation to the product’s

in vivo performance, a bioequivalence study should no longer be necessary. In such

cases,

the scientists and regulatory agencies may consider a pilot pharmacokinetic study as an

assurance that the new excipient does not inadvertently affect the absorption.

2.6.10.3. Dissolution/Release Rate Specifications

Without a correlation, the specifications of an in vitro test can be established only

empirically. This approach is data driven but is valid only if all the batches have been

extensively evaluated in clinical trials; furthermore, it probably can detect only relatively

large differences between different batches. It is therefore more precise to set up the

specification using the correlation to evaluate the in vivo consequences of the range.



Clearly, the pharmacokinetic consequences alone are not sufficient to set up the

specifications. The pharmacodynamic knowledge is the key to make the specification

clinically meaningful. In the absence of the information, some scientists may be willing

to rely on the empirical bioequivalence range of ±20% as the first guidance. In case of a

one-to-one correlation, this automatically translates in a dissolution rate change of ±20%.

It is empirically derived dissolution range is much wider than ±20%, and then the

companies invariably believe that the products have been punished by the presence of

one-to-one correlation.

2.6.10.4. Early development of Drug Product and Optimization

In the early stages of drug product development drug products are characterised by some

in vitro systems and some in vivo studies in animal models to find out toxicity and

efficacy issues.

2.6.10.5. Biowaiver for Minor Formulation and Process Changes

After the evaluation of critical manufacturing variables and in vitro dissolution rate for

controlled release formulation an IVIVC has been established. In vitro dissolution data is

used to justify minor formulation and process changes. The changes may include minor

change in shape, size, amount and composition of materials, colours, flavours, procedure,

and coating, source of inactive and active ingredients, equipment or site of

manufacturing.

2.7 Bioavailability

2.7.1. Definition: As per US-FDA, Bioavailability is defined as the rate and extent to

which the active ingredient or active moiety is absorbed from a drug product and

becomes available at the site of action. For drug products that are not intended to be

absorbed into the bloodstream, bioavailability may be assessed by measurements

intended to reflect the rate and extent to which the active ingredient or active moiety

becomes available at the site of action [Bioavailability and Bioequivalence Studies for

Orally Administered Drug Products--General Considerations. CDER, 2003].

The EMEA guidance defines Bioavailability as the rate and extent to which the active

substance or active moiety is absorbed from a pharmaceutical form and becomes



available at the site of action [Note for guidance on the investigation of bioavailability

and bioequivalence, EMEA].

The CDSCO, India, defines Bioavailability as the relative amount of drug from an

administered dosage form which enters the systemic circulation and the rate at which the

drug appears in the systemic circulation [Guidelines for bioavailability & bioequivalence

studies, CDSCO, 2005].

2.7.2. The need for Bioavailability and Bioequivalence Studies:

1. Bioavailability studies provide an estimate of the fraction of the orally

administered dose that is absorbed into the systemic circulation when compared to

the bioavailability for a solution, suspension, or intravenous dosage form that is

completely available.

2. Bioavailability studies provide other useful information that is important to

establish dosage regimens and to support drug labeling, such as distribution and

elimination characteristics of the drug.

3. Bioavailability studies provide indirect information regarding the presystemic and

systemic metabolism of the drug and the role of transporters such as p-

glycoproteins.

4. Bioavailability studies designed to study the food effect provide information on

the effect of food and other nutrients on the absorption of the drug substance.

5. Such studies when designed appropriately provide information on the linearity or

nonlinearity in the pharmacokinetics of the drug and the dose proportionality.

6. Bioavailability studies provide information regarding the performance of the

formulation and subsequently are a means to document product quality.

7. Bioequivalence studies provide a link between the pivotal and early clinical trial

formulation, a link between formulations used in the pivotal clinical trial and the

stability studies, the pivotal clinical trial and the to-be-marketed drug product, and

other comparisons as appropriate.

8. Bioequivalence studies are the basis for determination of the therapeutic

equivalence between a pharmaceutically equivalent generic drug product and a



corresponding reference listed drug. This list is provided in the orange book

[Approved drug products with therapeutic equivalence evaluations, 1999].

9. Bioequivalence studies provide information on product quality and performance

when there are changes in components, composition, and method of manufacture

after approval of the drug product. The FDA has provided guidance for the

industry, such as SUPAC-IR [Scale up and post approval changes: Immediate

release forms: FDA guidance, 1995] and SUPAC-MR [Scale up and post approval

changes: Modified release forms: FDA guidance, 1997], to determine when

changes in components and composition and/or method of manufacture of the

drug product suggests a need to perform further in vitro/in vivo studies.

2.7.3. Bioavailability and Bioequivalence testing recommended by the FDA:

Some of the situations when bioavailability and bioequivalence testing is essential for a

drug are mentioned below:

• For all new molecular entities.

• For new formulations of active drug ingredients.

• For a new dosage form of a drug.

• For a new dosage strength or dosage regimen.

• For a new salt or ester of a drug.

• For a new indication

• For the administration in special patient populations, e.g., Pediatrics.

• For a change in the manufacturing process of the drug or the drug product

that produces variabilities beyond the specifications of approved

applications.

2.7.4. Bioavailability assessment methods:

Bioavailability is the measurement of the rate and extent of drug that is systemically

available. Hence, pharmacokinetic parameters that give information on the amount of

drug reaching the systemic circulation (extent) and the time taken to reach the systemic

circulation (rate) are used as measures for assessing bioavailability. Bioavailability can be

measured by direct and indirect methods mentioned below.



2.7.4.1. Direct measures of Bioavailability

1. Based on plasma drug concentrations:

Drug concentrations in the blood and plasma are the most direct methods of determining

the systemic availability of a drug. The pharmacokinetic parameters that describe the rate

and extent of absorption and systemic exposure based on plasma drug concentration data

are summarized below.

a.) The area under the plasma drug concentration and time curve (AUCt, AUC∞, or AUCtau) (units = ng.h/ml): AUC is the measure of the extent of drug bioavailability. This gives a measure of the total systemic exposure. AUC can be obtained by a numerical integration method such as the trapezoidal rule. A recent recommendation by the FDA has been the use of early exposure as a measure of rate of systemic exposure [Chen, 1992]. This can be calculated as a partial AUC, where the area can be truncated at the population median of the tmax values. Measurement of early exposure may be useful when rapid onset of action is desirable (e.g., an analgesic effect) or if a slow input is required to achieve efficacy or safety. The FDA has recently proposed a shift away from the focus on rate and extent of absorption to the measurement of systemic exposure, which can be determined as total, peak, or early exposure (if needed). This is based on the understanding that these measures better reflect the rate and extent of absorption [Chen 1992; Bois et al.1994; Tozer et al. 1996]. b.) The peak plasma drug concentration (Cmax) (units = ng/ml): The Cmax is also a measure of the extent of bioavailability or peak exposure and indicates concentration required for a therapeutic or toxic response. It relates to peak exposure of the drug. Cmax is obtained directly from the plasma concentration time profile. c.) The time to peak plasma drug concentration (tmax) (units=hours, minutes, etc.): The

tmax is a measure of the rate of drug absorption and is the time required to reach the

maximum drug concentration after drug administration. The tmax is obtained directly

from the plasma concentration time profile.

2.7.4.2. Indirect measures of Bioavailability:

1. Based on urinary excretion data

This method can be used only if urinary excretion of unchanged drug is the main

mechanism of elimination of the drug and urine samples have been collected in intervals

as short as possible to measure the rate and amount of excretion as accurately as possible.



Limitations of using urinary data:

• There is a high degree of variability associated with the cumulative amount of

drug excreted in the urine, and the method is less reliable compared with the

estimation of bioavailability from plasma concentration time profiles.

• Urinary data should be collected for a period of time equal to five times the half

life corresponding to the terminal phase of the drug concentration- time profile to

achieve 97% recovery after a single dose.

• Urinary data are valid only if the excretion of the drug or metabolite is related to

the bioavailable dose of the drug.

• Urinary data cannot be reliably used to determine bioequivalence, Cmax, tmax,

absorption rate, and duration. Theoretically data could be used for determination

of bioequivalence, but practically they will not be reliable because of the high

degree of variability that could be associated with the parameter estimation.

2. Based on acute pharmacodynamic effect: This approach may be applicable when the

drug is not intended to be delivered into the bloodstream for systemic availability. It is an

indirect measure of bioavailability in cases where the analytical method for assessing

drug concentrations in the plasma or other biological fluids cannot be developed. In such

cases a dose-response relationship must be established. This method can be used only if

the method is sensitive, accurate, and reproducible. The pharmacodynamic parameters

evaluated to assess bioavailability are the following:

a. Total area under the pharmacodynamic effect-time curve

b. Peak pharmacodynamic effect

c. Time to peak pharmacodynamic effect

3. From Well-Controlled Clinical Trials: Well-controlled clinical trials that establish

safety and efficacy of a drug product, for purposes of establishing bioavailability can be

used. However, this approach is the least accurate, sensitive, sensitive, and reproducible

approach. This approach can be used when analytical methods cannot be developed for a

particular drug.



4. From dissolution Studies: In vitro dissolution studies are used to assess product

quality. In ideal circumstances in vitro dissolution rate should correlate with in vivo

bioavailability. A dosage form with a rapid dissolution rate is likely to have a rapid rate

of drug bioavailability in vivo. However, bioavailability is not only dependent on the

dissolution of the drug product, but also the permeability and solubility of the drug

substance. When an in vitro-in vivo correlation is available, the in vitro test can serve as

an indicator of how the product will perform in vivo.

2.7.5. Absolute and Relative Bioavailability

2.7.5.1. Absolute Bioavailability: Absolute bioavailability of a drug is the systemic

availability of the drug after extravascular administration of the drug and is measured by

comparing the area under the drug concentration-time curve after extravascular

administration to that after IV administration, provided the Kel and Vd are independent

of the route of administration. Extravascular administration of the drug comprises routes

such as oral, rectal, subcutaneous, transdermal, nasal, etc.

Absolute bioavailability is denoted as F, which is also the fraction of the dose that is

absorbed. After IV administration, the entire dose is placed into systemic circulation;

therefore, the fraction of the dose absorbed (f) or the absolute bioavailability is equal to

unity. For routes other than IV administration F≤1, absolute bioavailability is most

commonly expressed as a percentage, where an F of 1 is 100% bioavailable or an F of 0.8

is 80% bioavailable.

Absolute bioavailability can be calculated from the following equations:

Absolute bioavailability = AUC extravascular x dose i.v. / AUC i.v. x dose extravascular

2.7.5.2. Relative bioavailability: The relative bioavailability is the systemic availability

of a drug from one drug product (A) compared to another drug product (B). Relative

bioavailbilty can be calculated from the following equations:

Relative bioavailability = AUC of A / AUC of B

2.7.6. Factors affecting Bioavailability

Some of the important factors that affect bioavailability are outlined as follows:



2.7.6.1. Gastric emptying: Although not true in all cases, increased gastric emptying

generally enhances bioavailability of orally administered drugs. Gastric emptying

depends on the following factors:

• Volume of liquid intake

• Volume of solid food intake and its fat content

• Viscosity of stomach content

• pH of the stomach

• Intake of other drugs

• Age and weight of the patients

• Physical activity of the patients taking drug

• Emotional state of the patient

• Various disease states

The variability seen in the absorption of orally administered drugs is mainly due to

different rates of gastric emptying, which are affected by the various factors listed above.

Hence, to minimize variability, bioavailability studies may be conducted under controlled

conditions, such as healthy individuals of controlled weight and age under fasted

conditions or with a controlled diet. The use of healthy subjects minimizes both inter and

intrasubject variability.

2.7.6.2. Presystemic and systemic metabolism: Presystemic metabolism, which occurs

during first- pass metabolism are commonly seen:

• First-pass metabolism: First – pass metabolism occurs when an absorbed drug

passes directly through the liver before reaching systemic circulation after oral

administration.

• Intestinal metabolism: Drug metabolizes in the intestine itself or during the

passage through the intestinal wall.

• Hydrolysis of the drug in the stomach fluids.

• Transporters such as p-glycoprotein may influence the bioavailability of a

drug.



2.7.6.3. Complexation with other agents in the gastrointestinal tract: Formulation

factors, such as may occur with inert ingredients, the manufacturing process and /or use

of surfactants, etc.

2.7.7. Design of Bioavailability and bioequivalence studies

Both bioavailability and bioequivalence focus on measuring the absorption of the drug

into systemic circulation; hence, similar study design approaches are used to establish

bioavailability of a drug or to assess bioequivalence. Bioavailability is a comparison of

the drug product to an intravenous formulation, a solution, or a suspension, whereas

bioequivalence is a more formal comparative test that uses specified criteria for

comparisons with predetermined bioequivalence limits for evaluation.

The study design for bioequivalence mainly depends on the criteria for evaluation. Since

July 1992, the center for drug evaluation and research (CDER) has recommended the use

of the average bioequivalence criterion as published in the guidance [Statistical

procedures for bioequivalence using a standard two-treatment crossover design: FDA

guidance, 1992]. This criterion calls for a conventional nonreplicate crossover study for

evaluating bioequivalence. Recently, two new approaches have been described for

evaluating bioequivalence, which are termed the individual Bioequivalence criterion and

the population Bioequivalence criterion. The individual Bioequivalence criterion calls for

a replicate study design, wheraeas the population Bioequivalence criterion does not

involve a replicate study design, but a replicate cross-over design or parallel design,

which can also be used for this criterion [Statistical Approaches to establishing

bioequivalence, FDA guidance, 2001]. A replicate study design is one in which both the

test and the reference drug products are administered to the same individuals on two

separate occasions. The general study design considerations for conducting

bioavailability or bioequivalence studies are as follows:

• An initial pilot study with a smaller number of subjects to assess variability,

optimize sample collection time (as suitable for the immediate release and

modified release dosage forms), and other useful information.

• A conventional two-formulation, two-period, two-sequence non replicate

crossover design. This design is used for the average bioequivalence criterion. It



is also used if a population criterion is chosen for bioequivalence comparisons.

This study is recommended for a single-dose study. A single-dose bioequivalence

study is generally more sensitive in assessing release of the drug substance from

the drug product into systemic circulation for both conventional and modified

release products. The non-replicate design is recommended b the FDA for most

orally administered immediate-release dosage forms [Bioavailability and

Bioequivalence studies of orally administered drug products-general

considerations, FDA guidance, 2000].

• A replicate-crossover study design with four periods, two sequences, and two

formulations. The FDA recommends the use of the average bioequivalence

criterion for this study design as well, [Bioavailability and Bioequivalence studies

of orally administered drug products-general considerations, FDA guidance,

2000] although this study design is not necessary when an average approach is

used to establish bioequivalence. Replicate crossover designs allow for estimation

of within-subject variances for the test (T) and Reference (R) measures and the

subject-by-formulation interaction component. The same lot of the test and

reference formulation should be used for the replicated administration. This

design is desirable for modified-release dosage forms or highly variable drug

products, and is suitable for an individual bioequivalence approach.

• A parallel design could also be used under special circumstances, for example, a

drug with a long half-life.

• The reference standard in a bioequivalence study is a formulation currently

marketed with an approved full NDA, for which there are valid scientific safety

and efficacy data. The list of reference products is provided in the orange book

[Approved drug products with therapeutic equivalence evaluations, 1999].

• The reference product is usually the innovator’s brand-name product. The total

content of the active drug substance in the test product must be within 5% of the

reference product. Usually similar routes of administration are used for the test

and reference products unless an alternative route is needed to answer specific

pharmacokinetic questions. In some cases the reference material could be a



solution, suspension, IV product, or the clinical trial material containing the same

quantity of active drug ingredient.

• Healthy subjects are preferred as the study population and should be ≥18 years of

age. In some cases it may be useful to conduct the study in patients. A

heterogenous population would be preferable that includes males and females,

young and elderly people, and subjects from different racial groups or the targeted

age and gender if the drug product is to be specifically used in those populations.

• An adequate number of subjects should be enrolled to allow for dropouts. It is not

desirable to replace dropouts. At least 12 subjects should be included in a study.

• The highest marketed strength should be used for evaluating bioequivalence.

• The test and reference product should be administered with 240 ml of water.

• The test and reference drug products should be administered under fasting

conditions (overnight) and the fast should continue for up to 4 h after dosing.

Subjects should abstain from alcohol for 48 h prior to each study period and until

after the last sample from each period is collected. Subjects can be allowed water

as desired except for 1 h before and after drug administration.

• An adequate washout period should separate each treatment.

• Plasma and blood samples are preferred over urine and other tissue samples for

evaluating drug/ metabolite concentrations. An adequate number of samples

should be taken to characterize the absorption, distribution, and elimination

phases of the drug/metabolite accurately.

• For bioavailability studies, the parent compound or the active moiety and the

active metabolites should be measured if analytically feasible. For bioequivalence

studies, the measurement of the parent compound is desirable, unless the parent

drug levels in the plasma or serum are too low to allow reliable measurements. In

addition to measuring the parent, the measurement of the metabolite is important

when it contributes to either safety or efficacy of the drug product. The

bioequivalence criterion is applied to the parent with supportive evidence from

the metabolite measurements. Similarly, measurement of enanatiomers or

racemate may be necessary as appropriate.



2.7.8. Pharmacokinetic information for evaluation of Bioequivalence studies: The

following pharmacokinetic information for the drug should be obtained for the

evaluation of bioequivalence studies:

• Area under the plasma concentration-time curve from zero to time t and its

log transformation.

• Area under the plasma concentration-time curve from zero to time infinity

and its log transformation.

• Peak drug concentration and its log transformation and time to peak drug

concentration.

• Elimination rate constant and half-life of the drug.

• Cmin, Cavg, and degree of fluctuation, swing and evidence of attainment of

steady state, if steady state studies are used.

2.7.9. Bioequivalence evaluation criteria

In the past 20 years the evaluation criteria for bioequivalence studies recommended

by the FDA have evolved and been revised several times. For bioequivalence

comparisons the new formulation or method of manufacture is the test product (T)

and the prior formulation or method of manufacture is the reference (R) product. To

establish bioequivalence, the difference between the bioavailability of the test product

and the reference product must be within the prespecified bioequivalence limit as

governed by the approach taken to assess bioequivalence.

The first approach that was used by the FDA to evaluate bioequivalence was the

75/75/125 Rule, which required that a test and reference ratio for 75% of the subjects

should fall between the interval of 75 to 125%. In subsequent years this approach was

replaced by the power approach, which utilized a standard t-test for testing

equivalence. The power approach consisted of testing the hypothesis of no difference

at a 0.05 level with an estimated power of 0.80 to detect a 20% difference in the

means of the test and reference.

The current evaluation criteria are based on the two one-sided test approach, also

commonly referred to as the confidence interval approach or average bioequivalence,

which determines whether the average values for the pharmacokinetic parameters



measured after the administration of test and reference products are comparable. This

approach involves the calculation of a 90% confidence interval about the ratio of the

averages of T and R products for AUC and Cmax values. To establish

bioequivalence, the AUC and Cmax of the T product should not be less than 0.80

(80%) or greater than 1.25 (125%) of the R product based on log transformed data

(i.e., a bioequivalence limit of 80 to 125%). For some time prior to the use of log-

transformed data, the non transformed data were used to assess bioequivalence. In

1989, it was realized that log transformation of the data enables a comparison based

on the ratio of the two averages rather than the difference between the averages in an

additive manner [Schuirmann, 1989]. Moreover, most biological data correspond to a

log-normal distribution rather than to a normal distribution.

More recent proposals discussed for evaluating bioequivalence are based on

approaches termed individual Bioequivalence and population Bioequivalence. The

average bioequivalence approach focuses only on the comparison of population

averages (µT, µR) of a bioequivalence metric of interest and not on the variances of

the metric for the T and R products. The individual bioequivalence approach not only

compares the population averages (µT, µR), but also assess the within-subject

variability (σ2WT, σ2

WR) as well as the subject-by-formulation interaction (σ2D). The

population bioequivalence approach is designed to assess the total variability i.e.,

within- and between- subject variability (σ2TT, σ2

TR) of the pharmacokinetic

parameter (metric) in the population. The individual and population bioequivalence

approach allow the use of mixed scaling, which takes into account the variability of

the R product (termed as reference scaling). Reference scaling is used when the R

product is highly variable; otherwise constant scaling is used.

The bioequivalence or the evaluation of bioequivalence (BE) based on the average,

individual, and population approaches are given in Equations 1 through 3. This

criteria should be ≤ BE limit (θA, θI, θP for average, individual, and populaton

approaches, respectively) for each approach:

Average BE

(µT - µR)2≤ θA2 (1)



Individual BE

[(µT - µR)2+ σ2D + (σ2

WT - σ2WR) ]/ σ2

WR ≤ -θI (2)

Population BE

[(µT - µR)2 + (σ2TT - σ2

TR)]/ σ2TR ≤θP (3)

2.7.10. Statistical models to assess bioequivalance

Log-transformed data are used for comparisons to represent a normal distribution of

the data. For AUC and Cmax, the log of ratio (In T/R or log T/R) between the test and

reference are used for comparisons. The arithmetic mean for the test and reference

products, geometric means, means of the logs, standard deviation of he logs, or

coefficients of variation should be calculated for individual bioequivalence approach

the subject-by –formulation interaction variance and the within-subject variance for

the T and R product should be determined

General linear model or mixed effect model procedures are performed on the

pharmacokinetics parameters AUC and Cmax to test the data for difference within

and between test and reference groups. For a general linear model, the statistical

model should include factors accounting for various sources of variability, such as

sequence, subjects, study period, and treatment or formulation depending on the study

design.

For the average bioequivalence approach, two one sided test of hypothesis at the 5%

level of significance are carried to construct 90% confidence intervals. For the

population and individual bioequivalence approach, an upper 95% confidence bound

for the population or individual criterion is estimated, which should be less than or

equal to the bioequivalence limit (i.e. θI, θP ).

2.7.11. Criteria for waiver of evidence of in vivo bioavailability or bioequivalence

studies

Under the following circumstances at the applicants request, the FDA [CFR Vol.21,

part 320; 2000] may wave the requirement for in vivo bioavailability studies for a

drug product if the drug product meets any of the following provisions:



The following drug products that also meet the condition of containing he same active

and inactive ingredients in the same concentrations as a drug product that is the

subject of a full approved NDA can receive a waiver for in vivo evidence of

demonstrating bioavailability of the drug product:

• The drug product is a parenteral solution intended solely for administration by

injection.

• The drug product is an ophthalmic or otic solution.

The following drug products that also meet the condition of containing the same

active ingredients in the same dosage form as a drug product that is the subject of a

full approved NDA can receive a waiver for in vivo evidence of demonstrating

bioavailability of the drug product:

• The drug product is administered by inhalation as a gas, e.g., a medicinal or

inhalation anesthetic.

• The drug product is a solution for application to the skin.

• The drug product is an oral solution, elixir, syrup, tincture, or a similar solubilized

form. These products should not contain any inactive ingredient that is known to

significantly affect absorption of the active drug ingredient.

If the drug product is in the same dosage form, but in a different lower strength and

the following conditions have been met:

• The drug product is proportionally similar in its active and inactive ingredients to

another product for which the same manufacturer has obtained approval by

meeting the bioavailability requirements for a submission.

• Both drug products meet an appropriate in vitro test approved by the FDA.

• An in vivo study has been conducted on the highest strength.

These criteria could be used for immediate release tablets or capsules.

a. If the drug product is in the same dosage form, but in a higher strength, the waiver for

in vivo bioavailability will depend on:

• Clinical safety or efficacy data.

• Linear elimination kinetics over the dose range.

• Higher strength being proportionally similar to the lower strength.

• Similar dissolution profiles.



These criteria could be used for immediate release tablets or capsules.

b. If the drug product is a modified-release dosage form, a lower strength could be

waived if the following conditions are met:

• For beaded capsules, the difference should only be in the amount of beads present

and the lower-strength capsules should have similar dissolution profiles.

• For tablets, the lower-strength tablet should be compositionally proportional to

the higher strength both should have the same drug release mechanism and similar

dissolution profiles.

c. The drug product shows an in vitro-in vivo correlation.

d. The drug product is a reformulated product that is identical, except for a different

color, flavor, or preservative that could not affect the bioavailability of the reformulated

drug product, to another drug product for which the same manufacture has demonstrated

bioavailability and obtained approval and that has an FDA approved in vitro test.

e. In vivo bioavailability requirements may be waived for a good cause that is compatible

with the protection of public health.

f. For a drug product that was approved prior to 1962 and is determined to be effective in

at least one indication in a drug efficacy study implementation (DESI) notice and is listed

not to have a potential bioequivalence problem.

g. Recently the FDA has proposed the waiver of bioequivalence studies for immediate-

release solid oral dosage forms for a class I drug substance based on the

Biopharmaceutics Classification System (highly soluble and highly permeable) and for a

rapidly dissolving product [Waiver of in vivo bioequivalence studies for immediate

release forms based on BCS, FDA guidance, 2000].

2.7.12. Limitations of Bioavailability and Bioequivalence studies

1. A crossover design may be difficult for drugs with a long elimination half-life. Three

to four elimination half-lives may extensively prolong the duration of the study in a

crossover design. In this situation a parallel design can be used for bioequivalence

studies.



2. Highly variable drugs may require a far greater number of subjects to meet the FDA

bioequivalence criteria [Shah et al. 1996]. The variability seen in the performance of

certain drug products may be due to the inherent characteristics of the drug or due to the

drug formulation or both.

3. Certain characteristics in the biotransformation of drugs make it difficult to evaluate

the bioequivalence of such drugs. For example, for drugs that are stereoisomers with a

different rate of biotransformation and a different pharmacodynamic response, the

measurement of independent isomers may be difficult for analytical reasons.

4. Drugs that are administered by routes other than the oral route or drugs/dosage forms

that are intended for local effects have minimal systemic bioavailability. Some examples

of such drug classes are the ophthalmics, dermals, intranasal, and inhltion drug products.

Bioequivalence assessment of drugs that are insignificantly absorbed into the systemic

circulation are difficult. In some cases, for such drugs a biological marker has been

established for the assessment of bioequivalence. Examples of biological markers used

are skin blanching in the case of hydrocorticosteroids and neutralization of stomach acid

for antacids. For certain cases a pharmacodynamic end point may be more appropriate for

the assessment of bioequivalence.

03_chapter 2.pdf - shodhganga

Documents