Bioavailability

The in- vitro methods of evaluating dosage forms provide only indirect evidence of the therapeutic utility of the drug in a given dosage form.It would be more desirable to employ a direct means of ascertaining the utility of a particular dosage form.

In order to this in-vivo methods are needed which will reflect the true safety and efficiency of the dosage forms.

The estimation of the bioavailability of a drug in a given dosage form is direct evidence of the efficiency with which a dosage form performs its intended function.

The bioavailability of a drug is controlled by three principle factors:

1.The rate and extent of release of the drug from the dosage form.2.The subsequent absorption from solution state3.The biotransformation during the process of absorption.

Plasma concentration following the oral administration of a drug assumes four sequential phases:

Absorption >eliminationAbsorption = eliminationAbsorption<eliminationAbsorption = 0, elimination> 0

Figure 1

The estimation of the bioavailability from plasma concentration profiles requires a thorough understanding of the nature of these profiles.

Graphically bioavailability of a drug is portrayed by conc-time curve of administered drug in appropriate tissue system, e.g. plasma.

Bioavailability data are used to determine :1.The amount or proportion of drug absorbed from a formulation or dosage form.2.The rate at which the drug was absorbed3.The duration of the drug presence in the biologic fluid or tissue, and when correlated with patient response.

4-The relationship between drug blood levels and clinical efficacy and toxicity.

Blood (or serum or plasma) concentration time curve:Following oral administration of medication if blood samples are drawn from the patient at specific time intervals and analyzed for drug content, the resulting data may be plotted on ordinary graph paper to yield the type of blood level curve presented in the figure.

Figure 2

When the drug is first administered the blood concentration of the drug should be zero.As the drug pass into the stomach or intestine, it released from the dosage form and absorbed.As the sampling and analysis continue the blood sample increasing concentration of drug until the maximum (peak) conc. (Cmax) is reached.

Then the blood level of the drug decrease until reach to zero.The diminished blood level of drug after peak indicates the rate of the drug elimination, and it is greater than the drug absorption.The drug absorption does not terminate after the peak blood level reached, but may continue for some time.

Similarly the process of drug elimination is continuous one. It begins as soon as the drug first appear in the blood stream and continuous until all the drug has been eliminated.

A urine analysis for drug or its metabolites may be used to indicate the extent of drug absorption and/or rate of drug elimination from the body.

Parameters for assessment and comparison of bioavailability from blood level curvesThe parameters used for comparison of oral administration of single doses of two formulations of the same drug entity are the following:1.The peak height concentration (Cmax)2.The time of peak concentration (Tmax)3.The area under blood concentration – time curve (AUC)

Peak height concentration (Cmax)It the maximum drug conc. observed in blood plasma following a dose of the drug

For conventional dosage forms, as tablets and capsules, the maximum will usually occur at only single time point is Tmax

Figure 3

This figure represent conc- time curve showing different peak height conc. for equal amounts of drug from two different formulations A,B following oral administration.

The horizontal line represent minimum effective conc (MEC) for drug substance (4 ug/ml)

It is apparent from the figure that formulation A will achieve the required blood level to produce pharmacologic effect, while formula B will not.

If the minimum toxic conc. (MTC) is 4 ug/ml) and MEC is 2 ug/ml ,then the formula A will result in toxic effects while formula B give desired effect. The objective in individual dosing of patient is to achieve the MEC but not MTC.

The size of the dose administered influence the blood level conc. and Cmax for the drug substance.

Figure4

In this example it is assumed that all doses are completely absorbed and eliminated at the same rates.

It is evident that as the dose increases, the Cmax is higher and AUC increases. The peak time Tmax is the same for each dose.

Time of peak (Tmax)

The second parameter of importance in assessing the comparative bioavailability of two formulations is time required to achieve the maximum level of drug in blood (Tmax)

This parameter reflects the rate of drug absorption from a formulation.

It is the rate of drug absorption that determine the time needed for MEC to be reached and thus for the initiation of the desired pharmacologic effect.

The rate of drug absorption also influence the period of time over which the drug enters the blood stream, and therefore affect the duration of time looking at figure, formulation A allows the drug to reach the MEC within 30 min., and a peak in 1 hour.

Figure 5

Formulation B has slower rate of drug release, it reached MEC 2hour, and its peak conc. after 4 hours.

Thus formulation A permits the greater rate of drug absorption, it allows drug to reach both MEC, and its peak height sooner than drug formulations B.

On the other hand, formulation B provides greater duration of action, and drug concentration maintained above MEC. 8 hours from (2-10) to 5.5 (30-6h) for formulation A.

Thus if rapid onset of action is desired a formulation similar to A would be preferred ,but if longer duration action is desired than rapid onset , a formulation B would be preferred.

In sum, changes in the rate of drug absorption will result in changes in the values of both Cmax,Tmax.Each product has its own characteristics rate of absorption.When rate of absorption decreased the Cmax is lowered and Tmax occur at later time.If the doses of the drug are the same and completely absorbed, the AUC for each is essentially the same.

For example: a higher or earlier peak does not necessarily mean greater overall absorption than from a product giving a smaller or delayed peak.

The total absorption of drugs is proportional not only to the plasma conc. achieved but also to the length of time these conc. persist in the blood or plasma.

Area under the serum conc. –time curve

The AUC is considered representative of total amount of drug absorbed into circulation flowing the administration of a single dose of that drug.

Equivalent doses of a drug when fully absorbed into the circulation would produce the same AUC.

Thus two curves much unalike in terms of Cmax andOne parameter that characterizes this aspect is the area under the plasma conc. vs time profile.Many methods are available for the measuring of AUCs.Three of these are described as follows.

The trapezoidal rule

This is the simplest of all the method and involves the breaking up of the plasma conc. vs time profile into several trapezoids, calculating the area of individual trapezoids, and the adding up these areas to arrive at a cumulative AUC:

AUC1 = (Co + C1) (t1-t0) + C1+C2 (t2-t1) +

2 2-----------+ (Cn-1+Cn) (tn- tn-1)

= Σ (Ci-1+ Ci) (ti – ti-1) 2

The units for AUC are : conc × time e.g. µg.hr /ml or mg.min/liter.ExampleTime(hrs) C,µg/ml trapezoidal area cumulative area0 0 - _1 2 1 12 15 17 183 5 20 384 1 6 445

The AUC is proportional to the dose absorbed only when the calculation are extended to the point where the plasma conc. approaches zero.

2-Integration MethodThe rate of change of plasma conc (c ) is described asdc/dt = rate of absorption – rate of elimination = KaXa – KXWhere Ka and K are absorption and elimination rate constantsXa and X are the amounts of drug in GIT and body respectively

An integration of this equation between limits of time for which the drug remains in the body, as reflected by plasma conc. gives 

C= A ( e-kt - e-kat)And the total area under the curve (AUC), for which the total integral between time zero and infinity is given by:

AUC = A{ 1/K -1/Ka}

3-Physical methodsA variety of methods which utilize physical properties can be used to calculate AUCsFor example, if plasma conc. profiles are plotted on smooth paper, these can be cut out and weighed on an electronic balance.

Area under the serum conc- time curve

The area under the curve (AUC) is considered representative of total amount of drug absorbed into circulation following the administration of a single dose of that drug

Equivalent doses of a drug when fully absorbed into the circulation would produce the same AUCThus two curves much unalike in terms of Cmax, Tmax, but may be much alike in AUC and thus in the amount of drug absorbed.From the last figure, formula A with AUC = 34.4, and formula B with AUC = 34.2 are essentially the same

If equivelent doses of drug in different formulations produce different AUC values.The difference exist in the extent of absorption between the formulations.

F = (AUC)oral / (AUC)IV

F : is the fraction of dose absorbed In practice, it would rare for a drug to be completely absorbed in the circulation followed oral administration.

As noted earlier many drugs undergo the first- pass effect lead to some metabolic degradation. In addition factors of drug product formulation, drug dissolution, chemical and physical interactions with the GI contents, gastric emptying time, intestinal motility and others contribute the incomplete absorption of administered dose of a drug.The absolute bioavailability following oral dosing is generally compared to IV dose.

Urinary excretion dataCumulative urinary drug excretion

curves

Measurement of the conc. of intact drug and/or its metabolites in the urine can also be used to assess bioavailability

The assessment of bioavailability by urinary excretion is based on the assumption that the appearance of drug and its metabolites in the urine is a function of the rate and the extent of absorption.

This assumption is only valid when the drug and / or its metabolites are extensively excreted in the urine, and where the rate of urinary excretion is proportional to the conc. of the intact drug in the blood plasma. This proportionally is not hold if:

The drug and/or metabolites are excreted by active transport process into distal kidney tubule.

The intact drug and/or its metabolites are weakly acidic or weakly basic ( i.e. excretion is dependent on urine pH).

The excretion rate depends on urine flow.

The important parameters in urinary excretion studies are: the cumulative amount of intact drug and its metabolite and the rate at which this excretion takes place.

FigureThe segment (x-y) reflect the absorption phase

The slope of this segment related to the rate of absorption of the drug.

The total amount of intact drug or its metabolite excreted in the urine at point Z.

The total amount of drug excreted at point Z may be quite different from the total amount of drug administered (dose), either because of incomplete absorption or because of drug being eliminated by process other than urinary excretion.

Absolute and relative bioavailability

Absolute bioavailability

It is the fraction (or percentage) of the administered dose which is absorbed intact into systemic circulation.

•Absolute bioavailability may be calculated by comparing the total amount of intact drug that reaches the systemic circulation after administration of a known dose of the dosage form via a route of administration with total amount that reaches the systemic circulation after the administration of an equivalent dose of drug in the form of IV injection bolus.

•IV injection bolus is used as a reference to compare the systemic availability of the drug administered via different routes, because when a drug is delivered IV the entire administration dose is introduced directly into systemic circulation, i.e. it has no absorption barrier to cross and it is considered to be totally bioavailable.

•The absolute bioavailability of a given drug using plasma data may be calculated by comparing the total AUC following the administration of equivalent doses of the drug via absorption site and via IV route in the same subject on different occasions.

For equivalent doses of administered drug: ( AUC) absAbsolute bioavailability = ------------- (AUC ) IV(AUC)abs : area under the curve following administration of single dose via absorption site.(AUC)Iv : area under the curve following administration of single dose via rapid IV injection.

FigureIf different doses of the drug are administered by both routes, a correction for the sizes of the doses can be made as follows ( AUC) abs/ Dabs

Absolute bioavailability = ----------- (AUC ) IV/ DIV

Where Dabs is the size of single dose of the drug taken via absorption siteDIV : is the size of single dose of the drug taken via IV injection 

Absolute bioavailability using urinary excretion data may be determined by comparing the cumulative amount of unchanged drug excreted in urine following administration of drug via absorption site and that of IV route, respectively on different occasions to the same subject.

For equivalent doses of administered drug ( Xu) absAbsolute bioavailability = ------------ (Xu) IV (Xu) abs, (Xu)Iv are the total cumulative amounts of unchanged drug excreted in the urine after administration of equivalent single dose via absorption site and IV route.

For equivalent doses of administered drug ( Xu) absAbsolute bioavailability = ------------ (XU) IV (Xu) abs, (Xu)iv are the total cumulative amounts of unchanged drug excreted in the urine after administration of equivalent single dose via absorption site and IV route.

( Xu) abs/Dabs

Absolute bioavailability = ---------- (XU) IV / DIv

It should be noted that the value calculated for absolute bioavailability will be only valid if the drug kinetics of elimination and distribution independent of the route and time of administration and also the dose of size administered.

Relative bioavailability

In the case of drugs that cannot be administered by IV bolus injection, the relative bioavailability is determined rather than absolute bioavailability.

In this case the bioavailability of a given drug from a (test) dosage form is compared to that of the same drug administered in a standards dosage form, which is either orally administered solution or established commercial preparations of proven clinical effectiveness.

Hence the relative bioavailability is a measure of the fraction of a given drug that is absorbed intact into the systemic circulation from a dosage form relative to the recognized (i.e. clinically proven) standards dosage form of the drug.

The Relative bioavailability of a given drug administered as equal doses of a test dosage form and a recognized standard dosage form the corresponding plasma conc-time curve as follows ( AUCT) testRelative bioavailability = ----------- (AUC T ) stand

When different doses of the test and standard dosage form are administered a correction for the size of dose is made as follows: ( AUCT) test/ D

test

Relative bioavailability = ------------- (AUC T )stand/ D stand

 

where D test and D stand and the sizes of the single doses of the test and standard dosage form respectively .

urinary excretion data may also be used to measure relative bioavailability (Xu) testRelative bioavailability = ----------- (Xu ) standard

where Xu test and Xu stand and are the total cumulative amounts of unchanged drug excreted in the urine for test and stand drugwhen different doses are used from test and standard (Xu) test / D test

Relative bioavailability = ----------------- (Xu ) stand/ D stand

Relative bioavailability measurements are often used to determine the effects of dosage

form differences on systemic bioavailability of a given drug.

Numerous dosage form factors can influence the bioavailability of a drug.

These factors include 1-the type of dosage form (tablets, capsules, solution, and suspension), differences in the formulation of a particular type of dosage form, 2- and manufacturing variables employed the production of a particular type of dosage form.

Bioequivalence of drug productsIt has become well established that the rate and extent to which a drug in a dosage form become available for biologic absorption or utilization depends on great measure upon the materials utilized in the formulation and also on the method of manufacture.

Thus the same drug when formulated in different dosage forms may be found to possess different bioavailability characteristics and hence exhibit different clinical effectiveness.Further two identical or equivalent products, of the same drug in the same dosage form type but differing in formulation materials or method manufacture may vary widely in bioavailability and other single or multiple dose.

According to USP, significant bioavailability and bioequivalence problems may be revealed through dissolution testing and are generally the result of one or more of the following casual factors:

1. The drug particle size2. Excessive amount of the

lubricant mg.stearate in the formulation

3. Coating materials especially shellac

4. Inadequate amounts of tablets and capsule disintegrants

Pharmaceutical equivalentsAre drug product that contain identical amounts of identical active drug ingredient i.e. the same salt or ester of the same therapeutic moiety in identical dosage forms but not necessarily containing inactive ingredients.

They meet the identical compendia or other applicable standard of identity, strength quality and purity including potency and where applicable content uniformity, disintegration times and/or dissolution rates.

Pharmaceutical alternativesAre drug product that contain the identical moiety or its precursor, but not necessarily in the same amount or dosage form or as the same salt or ester.Each such drug product individually meets either the identical or its own respective compendia standard (as above).

Bioequivalent drug productsAre pharmaceutical equivalents or pharmaceutical alternatives whose rate and extent of absorption do not show a significant differences when administered at the same molar dose of the therapeutic moiety under similar experimental conditions

Some pharmaceutical equivalents or pharmaceutical alternatives may be equivalent in the extent of their absorption but not in the rate of absorptionTherapeutic equivalents has been used to indicate pharmaceutical equivelents which when administered to the same individuals in the same dosage regimens, will provide essentially the same therapeutic effect.

The most common experimental plan to compare the bioavailability of two drug products in the simple crossover design study.

In this method each of the 12-24 individuals in the group of carefully matched subjects (usually healthy) adult males between 18-40 years of age of similar height and weight.

Once the first assigned product is administered samples of blood or plasma are drawn from the subjects at predetermined times and analyzed for active drug moiety and its metabolites as a function of time.

The same procedure is then repeated (crossover) with the second product after an appropriate interval of time i.e. a washout period to ensure that there is no residual amount of the first drug.After that the patient population data tabulated and parameters as Cmax, Tmax and AUC, are then analyzed with statistical procedures.

Absolute BioequivelenceIt is rarely occur between drug products Such absolute equivelency would yield serum concentration time curves for the products involved that would be exactly superimposable.

This simply is not expected of products which are made at different times, different batches ,or by different manufactures.

In most studies of bioavailability, the originally marketed product known as pioneer or innovator is recognized as the established product of the drug and is utilized as the standard for bioavailability comparative study.

According to the FDA, a generic drug is considered bioequivalent if the rate and extent of absorption do not show a significant difference from that of the pioneer drug when administered at the same molar dose of the therapeutic ingredient under the same experimental conditions

To gain FDA approval a generic drug product must:1- Contain the same active ingredients as the pioneer drug

2- Be identical in strength, dosage form and route of administration

3- Have the same indications and precautions for use and other labeling instructions

4- Be bioequivalent

5- Meet the same batch-to-batch requirements for identity, strength, purity, and quality.

6- Be manufactured under the same strict standards of FDA’s for pioneer products

In-vitro demonstration of bioavailability:Dissolution testing:Dissolution testing the most important way to study, under in-vitro conditions, the release of drug from solid dosage form, and thus represents an important test to assess factors that affect the bioavailability of a drug from solid preparation

Dissolution studies are carried out for several reasonsTo evaluate the potential effect of formulations and process variables on the bioavailability of a drugTo ensure that preparations comply with product specificationsTo indicate the performance of the preparation under in-vivo conditions

•This last point requires that in-vitro dissolution data correlate with the in-vivo performance of the dosage form, which must be experimentally varied

•The term in-vitro/in-vivo correlation in this context is related to the correlation between in-vitro dissolution and the release or uptake of the drug in-vivo