pharmacology of the fluoroquinolonas

7/25/2019 Pharmacology of the Fluoroquinolonas

1/19

Review

Pharmacology of the fluoroquinolones:A perspective for the use in domestic animals

Marilyn Martinez a, Patrick McDermott b, Robert Walker b,*

a US Food and Drug Administration, Center for Veterinary Medicine, Office of New Animal Drug Evaluation, Rockville, MD 20855, USAb US Food and Drug Administration, Center for Veterinary Medicine, Division of Animal and Food Microbiology,

Office of Research, Laurel, MD 20708, USA

Abstract

The fluoroquinolones are a class of compounds that comprise a large and expanding group of synthetic antimicrobial agents.Structurally, all fluoroquinolones contain a fluorine molecule at the 6-position of the basic quinolone nucleus. Despite the basic sim-ilarity in the core structure of these molecules, their physicochemical properties, pharmacokinetic characteristics and microbialactivities can vary markedly across compounds. The first of the fluoroquinolones approved for use in animals, enrofloxacin, wasapproved in the late 1980s. Since then, five other fluoroquinolones have been marketed for use in animals in the United States, withothers currently under investigation.

This review focuses on the use of fluoroquinolones within veterinary medicine, providing an overview of the structureactivityrelationship of the various members of the group, the clinical uses of fluoroquinolones in veterinary medicine, their pharmacokinet-ics and potential interspecies differences, an overview of the current understanding of the pharmacokinetic/pharmacodynamic rela-tionships associated with fluoroquinolones, a summary of toxicities that have been associated with this class of compounds, their usein both in human and veterinary species, mechanisms associated with the development of microbial resistance to the fluoroquino-lones, and a discussion of fluoroquinolone dose optimization. Although the review contains a large body of basic research informa-tion, it is intended that the contents of this review have relevance to both the research scientist and the veterinary medicalpractitioner.Published by Elsevier Ltd.

Keywords: Fluoroquinolones; Veterinary; Pharmacokinetics; Pharmacodynamics; Resistance

1. Introduction

Quinolones, also referred to as 4-quinolones, quino-

lone carboxylic acids and fluoroquinolones, comprise alarge and expanding group of synthetic antimicrobialagents. The first drug of this class, nalidixic acid, was dis-covered in 1962 and was a modification of a compoundisolated during the production of the anti-malaria drug,chloroquine. Nalidixic acid was first discovered as a by-

product of anti-malarial research in 1962 (Lesher et al.,1962) and was approved for clinical use in 1965. How-ever, its antibacterial spectrum of activity was restricted

to the Enterobacteriaceae and, because of limitations inabsorption and distribution, the drug was effective solelyfor the treatment of urinary tract infections. In the 1980s,the addition both of a fluorine molecule at the 6-positionof the basic quinolone structure and a piperazine substi-tution at the 7-position was found to enhance quinoloneantibacterial activity, gaining effectiveness against suchorganisms as Pseudomonas aeruginosa and Gram-posi-tive cocci, and to increase the extent of oral drug absorp-tion and tissue distribution (Ball, 2000).

1090-0233/$ - see front matter. Published by Elsevier Ltd.

doi:10.1016/j.tvjl.2005.07.010

* Corresponding author. Tel.: +1 301 210 4187; fax: +1 301 2104685.

E-mail address:[email protected](R. Walker).

www.elsevier.com/locate/tvjl

The Veterinary Journal 172 (2006) 1028

TheVeterinary Journal
mailto:[email protected]:[email protected]


2/19

Products possessing this fluorine molecule are knownas the fluoroquinolones. The first fluoroquinoloneapproved for use in clinical medicine was norfloxacin,followed shortly thereafter by ciprofloxacin in the mid1980s. The first of the fluoroquinolones approved foruse in animals, enrofloxacin, was approved in the late

1980s. Since then, five other fluoroquinolones have beenmarketed for use in animals in the USA, with others cur-rently under investigation.

Generally speaking, quinolones may be classified intoone of three generations (although some authors havesuggested that these compounds can be divided into fourgenerations, e.g., Van Bambeke et al., 2005). The dis-tinctions between these generations are not clearly delin-eated, especially for those drugs that may be classified assecond and third generation compounds. The first gen-eration comprises the original quinolone compoundssuch as nalidixic acid, oxolinic acid, pipemidic acidand cinoxacin. These molecules were associated with

poor oral bioavailability, limited distribution into sys-temic tissues, and a spectrum of activity limited to Esch-erichia coliand several other Gram-negative organisms.The second generation drugs, the first of the fluoroquin-olones, were developed in the 1980s, exhibited increasedantibacterial activity against the Enterobacteriaceae andother Gram-negative bacteria (such as P. aeruginosa),and had some activity against certain Gram-positivecocci. Structural changes associated with the secondgeneration increased their oral bioavailability and sys-temic distribution. Quinolones fitting into this categoryinclude entities such as norfloxacin, ciprofloxacin, enro-

floxacin, danofloxacin, difloxacin and marbofloxacin.The third generation drugs maintained the favorable

characteristics of the second generation drugs whileexhibiting increased activity against Gram-positive bac-teria, anaerobes and mycobacteria. These compoundsalso exhibited excellent oral bioavailability and wereassociated with a prolonged terminal elimination half-life. The third generation fluoroquinolones have lowercentral nervous system toxicities and exhibit fewer inter-actions with the cytochrome P450 (CYP 450) system(Ball, 2000).

With regard to quinolones, specific compounds asso-ciated with the four generations (some not approved foruse within the USA) are provided below (Owens andAmbrose, 2002a):

1. First generation: nalidixic acid, oxolinic acid andcinoxacin.

2. Second generation: ciprofloxacin, enrofloxacin, mar-bofloxacin, danofloxacin, difloxacin, norfloxacinand enoxacin.

3. Third generation: orbifloxacin, levofloxacin, sparflox-acin and grepafloxacin.

4. Fourth generation: trovafloxacin, gatifloxacin, moxi-floxacin, gemifloxacin and sitafloxacin.

An update on the perspective relating to the clinicaluse of fluoroquinolones in human patients has beenrecently published elsewhere (Van Bambeke et al.,2005).

While in some cases, principles concerning the physi-cochemical properties of the fluoroquinolones as a drug

class will be based upon information generated withcompounds approved for human use, the purpose of thisreview is to focus primarily on the fluoroquinolones thathave been marketed for use in animals. Discussions in-clude the differences in chemical structure relative toantibacterial activity, pharmacokinetic and pharmaco-dynamic parameters, the application of these parametersin dose determination, the clinical use of the molecules,their potential toxicities, and a synopsis of mechanismsassociated with the development of bacteria resistantto these compounds.

2. Mechanism of action

In multiple species of bacteria, early biochemical evi-dence indicated that fluoroquinolones damage bacterialDNA and lead to defects in negative supercoiling (Gell-ert et al., 1977). This effect was linked to inhibition ofDNA gyrase activity, an enzyme found in all bacteria.In concert with other proteins, gyrase catalyzes changesin the degree of double-stranded DNA supercoiling. Inthis capacity, it plays a vital role in DNA packing, rep-lication and transcription.

The active holoenzyme is a heterotetramer composed

of two subunits each of gyrA and gyrB (A2B2). GyrAbinds to DNA and mediates strand breakage and rejoin-ing activity, whereas gyrB contains the ATP bindingsite. GyrA activity involves cleavage of both DNAstrands, (mediated by an enzyme-DNA covalent inter-mediate), passage of DNA through the break and re-ligation of the strand. In vivo, this process results intwo negative supercoils and the hydrolysis of ATP.The topoisomerase IV enzyme, encoded by parC/parE,is a secondary fluoroquinolone target. This enzyme isalso a multimeric protein composed of two parC sub-units and two parE subunits, which exhibit sequencehomology to gyrA and gyrB, respectively. This enzymemediates relaxation of duplex DNA and the unlinkingof daughter chromosomes following replication (Zech-iedrich and Cozzarelli, 1995).

When susceptible DNA gyrase is exposed to aquinolone, the drug interacts at the surface of an al-pha-helical domain of the enzyme involved in DNAcleavage and re-ligation, termed the quinolone resis-tance determining region (see below). The toxic effectsresult from the irreversible formation of a trappedintermediate consisting of quinolone, gyrase andcleaved DNA (Gellert et al., 1977). This preventsprogression of replication forks and transcription

M. Martinez et al. / The Veterinary Journal 172 (2006) 1028 11


3/19

complexes (Willmott et al., 1994), leading to fragmenta-tion of the chromosome and cell death. Because quino-lones mediate DNA damage by binding to susceptibleenzymes, fluoroquinolone-resistance mutations arerecessive. The result of this is that for topoisomerase-mediated fluoroquinolone resistance to be transferred

horizontally, an acquired mutated gene would have tosupplant the wild-type gene.The effect of fluoroquinolones on bacterial prolifera-

tion suggests three mechanisms of cell killing (Guthrieet al., 2004; Maxwell and Critchlow, 1998):

1. Mechanism A: common to all quinolones. Thisrequires RNA and protein synthesis and is only effec-tive against dividing bacteria. Mechanism A appearsto involve the blocking of replication by the gyrasequinolone complex on DNA.

2. Mechanism B: does not require RNA and proteinsynthesis and can act on bacteria that are unable to

multiply. Mechanism B (chloramphenicol insensitive)can be correlated with dislocation of the gyrase sub-units that constrain the ternary complex.

3. Mechanism C: requires RNA and protein synthesisbut does not require cell division. Mechanism Cmay correlate with trapping of topo IV complexeson DNA.

The fluoroquinolones, as with the penicillins, exhi-bit a paradoxical effect. Survival curves show thatwhen the fluoroquinolone concentration is near theminimal inhibitory concentrations (MIC) of the bacte-

rium, the drug has a static effect on bacterial growth(bacteriostatic). As the drug concentration increasesrelative to the MIC of the bacterium, bacterial killingincreases up to a certain drug concentration (termedthe optimum bactericidal concentration). As concen-trations exceed the optimum bactericidal concentra-tion, further increases in drug concentration can leadto a decrease in bacterial killing. Initially, these con-centration-related differences in drug effect may beassociated with the difference between concentrationsneeded to inhibit DNA supercoiling versus thoseneeded to inhibit bacterial growth. In general, it ap-pears that the supercoiling reaction of gyrase is lesssensitive to the drugs than is bacterial growth byone or two orders of magnitude (Guthrie et al.,2004; Maxwell and Critchlow, 1998).

It is believed that the inhibition of RNA and proteinsynthesis may account for this third phase of bacterialeffect (decreased killing). This implies that protein syn-thesis may be required for quinolone-mediated celldeath. In this regard, protein synthesis and inhibitorssuch as chloramphenicol, and RNA synthesis inhibitors(such as rifampin) reduce fluoroquinolone effectivenessin bacterial killing (Guthrie et al., 2004; Maxwell andCritchlow, 1998).

3. Structureactivity relationship

Nalidixic acid lacks several characteristics associatedwith modern 4-quinolones. For example, nalidixic acidcontains two nitrogen atoms in the basic nucleus, mak-ing it a naphthyridine rather than a quinolone, and nali-

dixic acid is not halogenated like other quinolones suchas chloroquine (Fig. 1). In addition, nalidixic acid exhib-its modest serum and tissue concentrations, and suscep-tible bacteria have relatively high MIC values.

Following the discovery of the therapeutic effects ofnalidixic acid, chemists began to probe every positionwithin the quinolone nucleus in an attempt to improvepotency, broaden the spectrum of antibacterial activ-ity, and reduce recognized side effects. To date, severalthousand related compounds have been synthesized.Studies have shown that there are four componentsof the 4-quinolone nucleus that, when manipulated,can enhance the antibacterial activity of the quinolone

nucleus. These include an ethyl group at the N-1position, a carboxylic group at C-3, an oxygen atomat C-4 and a fluorine atom at C-6. Of these, it wasthe addition of a fluorine atom to the 4-quinolone ringat the 6-position that has substantially widened theantibacterial activity spectrum of the quinolones(Wright et al., 2000). This modification also enhancedthe quinolones oral bioavailability and tissue penetra-tion. As a result of this discovery, all of the 4-quino-lones marketed for clinical use today are halogenatedat the 6-position. Some are also halogenated at the8-position.

The first compound with a fluoro group at position 6was flumequine which was patented in 1973 (Appel-baum and Hunter, 2000). Difloxacin has two fluorineatoms. While dual substitution does not significantly in-crease its potency relative to products containing a sin-gle fluorine atom, the addition of a second fluorinemolecule at C-8 enhances difloxacins oral bioavailabil-ity and increases its terminal elimination half-life. How-

Fig. 1. Nalidixic acid.

12 M. Martinez et al. / The Veterinary Journal 172 (2006) 1028


4/19

ever, this change also increases the potential for photo-toxicity. The addition of a piperazinyl side chain at po-sition 7 improves the ability of the drug to penetrate thebacterial cell wall, thereby improving its activity againstGram-negative organisms and providing some degree ofGram-positive activity (Appelbaum and Hunter, 2000).

Norfloxacin (patented in 1978) was the first compoundto combine a piperazinyl side chain in position 7 and afluoro group in position 6. Although the addition of apiperazinyl ring at the 7-position resulted in increasedactivity against bacteria such as P. aeruginosa andGram-positive cocci, norfloxacin continued to sufferfrom poor bioavailability.

Subsequent modifications resulted in significantimprovements in oral bioavailability, as well as an in-crease in the spectrum of antimicrobial activity. Variousother substitutions, such as the inclusion of the ethylgroup at N-1, have enhanced both Gram-positive andGram-negative antimicrobial activity. Other examples

of modifications at the N-1 position include the cyclo-propyl group found with ciprofloxacin and enrofloxacinand the phenyl ring of difloxacin (Appelbaum and Hun-ter, 2000).

The pharmaceutical industry has produced a largenumber of derivatives and analogues of the 4-quinolonestructure. With these products, there seems to be an in-verse relationship between a compounds Gram-positiveand Gram-negative activity such that enhanced activityagainst one often accompanies reduced activity againstthe other. Another drawback to increased activity isthe potential for increased host toxicity. For example,

several broad-spectrum fluoroquinolones approved foruse in human medicine, such as temafloxacin (FederalRegister, 2004) and grepafloxacin (Glaxo Wellcome,2004), have been voluntarily withdrawn from clinicaluse as a result of emerging safety concerns.

Structural modifications in quinolone compoundsover time have led to changes in pharmacokinetic prop-erties, tolerability and antibacterial potency (Appel-baum and Hunter, 2000; Peterson, 2001). There is noevidence to date, however, suggesting a relationship be-tween potency and the likelihood of selecting for resis-tant isolates.

Varying the substitutions placed on the quinolone nu-cleus does affect the likelihood of drugdrug interactions(Mizuki et al., 1996). For example, those fluoroquino-lones containing a bulky substituent at position 8, orwith a 4 0-nitrogen atom in the 7-piperazinyl group, areless prone to interact with theophylline than are thosewithout an 8-substituent. These substitutions tend tomake the molecule less planar, and therefore less likelyto be associated with drugdrug interactions involvingthe CYP 450 system of enzymes. Examples of two com-pounds with bulky substituents that are approved forhuman use are levofloxacin and sparfloxacin. With re-gard to veterinary examples, the bulky nature of the po-

sition 7 substituent for danofloxacin results in itsnegligible interaction with the CYP 450 enzymes (referto the pharmacokinetic section for further discussionof veterinary elimination pathways).

Structural modification may also affect the interac-tion of the fluoroquinolone with divalent cations. When

there are fewer substitutions on the quinolone nucleus oron the piperazinyl group (e.g., norfloxacin, ciprofloxacinand enoxacin), there is a greater reduction in fluoroqui-nolone bioavailability in the presence of divalent cat-ions. In particular, substitution at the 5-positiondiminishes the interaction with divalent cations, suggest-ing that the 5-substituent may affect the formation and/or stability of non-absorbable chelated complexes(Appelbaum and Hunter, 2000; Peterson, 2001). Itshould be noted that sucralfate has an effect similar tothat of divalent cations on the bioavailability of thesecompounds (Polk, 1989; Radandt et al., 1992).

Structureactivity relationships have been identified

for certain adverse effects. For example, phototoxicityis more commonly associated with a fluorine or chlorinesubstitution at the 8-position. Central nervous systemeffects are more commonly associated with the unsubsti-tuted 7-piperazine derivatives. However, in other cases,such as QTc prolongation, no clear cut structureactiv-ity relationship is observed (Ball, 2000).

Because of their chemical structure, fluoroquinolonesare zwitterions, containing two ionizable moieties. Thelowest pKavalue is associated with the acidic carboxylicacid moiety while the second is attributable to the basictertiary amine. Examples of pKa values (using varied

methods) are provided in Table 1 (Barbosa et al.,1999; Barron et al., 2001; Dalhoff, 1989; Escribanoet al., 1997; Mitscher et al., 1993). It should be notedthat pKa estimates can vary, depending upon the buffersystem used and the experimental conditions (Barbosaet al., 1999, 2001; Barron et al., 2001; Sanz-Nebotet al., 2001).

Table 1pKa values associated with various fluoroquinolones

Compound pKa1 pKa2 pKa1a pKa2

a

Ciprofloxacin 6.0 8.8 5.86 8.24Danofloxacin 6.07 8.56

Difloxacin 6.1 7.6 5.66 7.24Enoxacin 6.0 8.5Enrofloxacin 6.0 8.7 5.88 7.74Fleroxacin 5.7 8.0Lomefloxacin 5.8 9.3Marbofloxacin 5.69 8.02Norfloxacin 6.4 8.7 5.94 8.22Ofloxacin 6.1 8.2Pefloxacin 6.3 7.6Sarafloxacin 5.62 8.18Sparfloxacin 6.2 8.6Temafloxacin 5.6 8.8

a Based upon a liquid chromatographic procedure using a 0% (w/wmethanol) in aqueous potassium hydrogenphthalate buffer.



5/19

When pH values are lower than the pKa1, quinoloneshave a net positive charge. These lower pKavalues deter-mine the pH range within which the drug is soluble inaqueous fluids. This is particularly important for disso-lution of suspensions and tablets. However, in concen-trated acidic urine, such as may be found in dogs and

cats, some quinolones form needle-shaped crystals(Merck Veterinary Manual, 1998).With few exceptions, fluoroquinolones exhibit poor

water solubility between pH 6 and 8. At pH values be-tween the pKa1 and pKa2, these zwitterions have a netneutral charge. Accordingly, they can freely diffuseacross biological membranes, but exhibit very poor sol-ubility in aqueous fluids. For this reason, liquid formu-lations of various quinolones for oral or parenteraladministration usually contain freely soluble salts in sta-ble aqueous solutions. Solid formulations (e.g., tablets,capsules or boluses) contain the active ingredient eitherin its betaine form or, occasionally, as the hydrochloride

salt (Merck Veterinary Manual, 1998). At pH valuesexceeding pKa2, the quinolones have a net negativecharge.

Passive diffusion across biological membranes is afunction of fluoroquinolone lipophilicity relative tothe pKa values of the two ionizable moieties. Two flu-oroquinolones, enrofloxacin and ofloxacin, can be usedas examples of the impact of pKa on product bioavail-ability. With enrofloxacin, maximum aqueous solubil-ity occurs at pH 5.02, with low aqueous solubilityoccurring within the pH range of pH 6.08.0. However,it is also within this pH range that enrofloxacin has its

greatest lipid solubility. This lipophilicity facilitates itsdiffusion into biological tissues, including bacterialcells. Maximal transfer of drug from the aqueous tothe lipid phase (octanol/water partitioning) occurs at

pH 7.0 (Lizondo et al., 1997). Similarly, maximalabsorption of ofloxacin occurs at neutral pH, whichcorresponds to the pH value when ofloxacin liposolu-bility is at its maximum. Above and below pH 7.0,ofloxacin absorption declines, with pH having a greatereffect on the R(+) as compared to S() enantiomer

(Rabbaa et al., 1997).The importance of understanding the relationship be-tween pKa, pH and drug diffusion is exemplified by therelationship between antimicrobial activity and prostati-tis (Wagenlehner and Naber, 2003). Depending upon thepKaof the compound, the presence of a pH gradient be-tween the infected tissue and blood can result in iontrapping. In these situations, ionized drug is trapped atthe site of the infection, resulting in higher tissue drugconcentrations as compared to the blood. Interspeciesdifferences in the pH of prostatic fluid (acidic in dog,alkaline in humans) can lead to interspecies differencesin the extent of ion trapping. However, the higher con-

centrations in dogs may not result in higher antimicro-bial activity as the ionized form of the drug maypoorly penetrate the bacterial wall.

4. Clinical use of fluoroquinolones in veterinary medicine

There are currently six fluoroquinolones that havebeen approved for use in animals within the US. Thestructures are shown inFig. 2. These include enrofloxa-cin, difloxacin, danofloxacin, marbofloxacin, orbifloxa-cin and sarafloxacin. However, effective April 30, 2001,

the US Food and Drug Administrations Center for Vet-

erinary Medicine (CVM) proposed withdrawing theapprovals of two new animal drug applications (NA-DAs) sponsored by Abbott Laboratories (Federal

Fig. 2. Chemical structures of the fluoroquinolones that have been used for the treatment of veterinary bacterial infections within the U.S.



6/19

Register, 2001). The NADAs provide for use ofsarafloxacin formulations to treat colibacillosis in poul-try. One is NADA 141-017 for SaraFlox (sarafloxacinhydrochloride) WSP, a water-soluble powder used inthe drinking water for control of mortality associatedwith E. coli in broiler chickens and E. coliand Pasteu-rella multocida in turkeys. The other is NADA 141-018for SaraFlox (sarafloxacin hydrochloride) injectable

solution used in 18-day embryonated broiler eggs andday-old broiler chickens for control of early chick mor-tality associated with E. coli. Concerns focused on thepotential selection of fluoroquinolone-resistant food-borne zoonotic pathogens when these products areadministered to poultry. For similar reasons, the with-drawal of the US approval of enrofloxacin for adminis-tration to poultry has been proposed and is still inlitigation (Federal Register, 2000).

Products approved for marketing within the US,summary information, along with the correspondingCode of Federal Regulations (CFR) citation, are pro-vided inTable 2.

5. Pharmacokinetics

It may be particularly important to factor a com-pounds pharmacokinetic and physicochemical proper-ties into the drug and dose selection process when:

1. The animal has modified renal or hepatic function. Inthis case, clearance can be compromised, leading tohigher systemic drug concentrations that may affectproduct safety.

2. The infection is associated with a purulent discharge(which can affect protein binding and drug diffusionat the site of infection) or is in a region that maynot be highly accessible to the compound.

3. The susceptibility of the pathogen is unknown or thepathogen susceptibility is classified as Intermediatebased upon Clinical Laboratory Standards Institute,CLSI, criteria, (CLSI is the new name of the former

National Committee for Clinical Laboratory Stan-dards, NCCLS).

4. Dosing conditions (e.g., food effects) can affect drugbioavailability and therefore, the level of systemicdrug exposure.

5. There is concomitant drug use, resulting in potentialdrugdrug interactions.

6. The animal is infected with a highly virulent bacterialstrain or the host presents with compromised immunefunction (e.g., geriatric animals or animals on corti-costeroid therapy), rendering dose optimization veryimportant.

7. When developing prudent use practices to limit thelikelihood of resistance development.

Although this section on pharmacokinetics is in-tended to be applicable to veterinary drug products,very little information on basic principles has been gen-erated using compounds approved for veterinary use.Consequently, much of the following discussion is takenfrom studies employing compounds used in humans.Nevertheless, the findings from these studies reflect gen-eralizations that are relevant to the entire fluoroquino-lone class of drugs, and can be applied to understandthe veterinary-specific compounds.

Table 2Compounds approved within the US for use in veterinary species as of November 2003

Drug 21 CFRcitation

Dosage form Route Species Dose(mg/kg)

Instructions Withdrawal information

Enrofloxacin(520.812)

Tablet PO Dog, cat 520 To be given as single or divided(b.i.d.) daily doses. Administerfor 23 days beyond cessation of

clinical signs, for a maximum of30 days

NA


Injectable solution SC Dog 2.5 As an initial dose, followed bytablets

NA


Injectable solution SC Cattle 2.55 Q 24 h for 35 days 28 days7.512.5 Administered once

Danofloxacin(522.522)

Injectable solution SC Cattle 6.0 Repeat 48 h after first injection 4 days

Difloxacin(520.645)

Tablet PO Dog 510 Administer once daily for 23days beyond cessation of clinicalsigns

NA

Marbofloxacin(520.13)

Tablet PO Dog, cat 2.755.0 Q 24 h NA

Orbifloxacin(520.1616)

Tablet PO Dog, cat 2.5 to 7.5 Administer for 23 days aftercessation of clinical signs, for a

maximum of 30 days

NA

Summary is based upon information contained within 21 CFR, April 2003.



7/19

In cases where there are active metabolites (e.g., enro-floxacin (Tyczkowska et al., 1989) and ibafloxacin(Coulet et al., 2005)), when there is stereospecific phar-macokinetics and antimicrobial activity (e.g., ibafloxa-cin [Coulet et al., 2005]), or when there is high proteinbinding (discussed later in this review), the analysis of

plasma and urine samples via microbiological assaysversus high-performance liquid chromatography(HPLC) methods may lead to different pharmacokineticresults. Therefore, although the merits of the respectivemethods can be debated, for the purpose of this review,all pharmacokinetic information is based upon data col-lected using HPLC methods.

5.1. Bioavailability

Although there is considerable individual variationamong the different compounds and in the different ani-mal species, as a rule, the fluoroquinolones are rapidly

absorbed following oral administration in monogastricspecies. With ruminants, systemic concentrations of or-ally administered fluoroquinolones are below therapeu-tic levels. For example, the oral bioavailability ofenrofloxacin is only 10% in adult ruminants as com-pared to greater than 80% oral bioavailability in mono-gastric species (Greene and Budsberg, 1993; Vancutsemet al., 1990).

Generally, unless administered with foods containinga high concentration of divalent cations, the postpran-dial oral administration of fluoroquinolones does not re-sult in clinically significant decreases in bioavailability

(F). A slight delay in time to peak and a decrease in peakplasma concentrations may occur as a consequence ofthe delay in gastric emptying (Blondeau, 1999). Accord-ingly, these drugs can generally be administered withoutconcern for prandial state (Allen et al., 2000; Efthymio-poulos et al., 1997; Gajjar et al., 2002; Johnson et al.,1999). However, for lipophilic compounds, food can sig-nificantly enhance oral bioavailability and therefore in-crease systemic drug concentrations. For example, foodsignificantly increases the AUC andCmaxof ibafloxacinwhen administered to cats (Coulet et al., 2005). Withthe advent of various taste-masking techniques such asion exchange resins (Agarwal et al., 2000), film coatings(Chopra et al., 2002), and micro-encapsulation (Al Om-ran et al., 2002), food could potentially influence the rateand/or extent of drug release. Therefore, the FDA oftenasks that food-effect study results be submitted to sup-port the approval of products employing novel formula-tions for veterinary use.

While the lipophilicity of these molecules promotesabsorption by passive diffusion, non-passive mecha-nisms for intestinal uptake also may exist (Dautreyet al., 1999; Rabbaa et al., 1997). Carrier-mediatedtransport across the apical membrane, as is seen withsparfloxacin, levofloxacin, ciprofloxacin, and norfloxa-

cin, has been reported (Griffiths et al., 1994; Rabbaaet al., 1997). Whenever carrier-mediated transport is in-volved in intestinal absorption, it is critical that theproduct is fully dissolved by the time that the dosageform reaches the absorbing segment of the small intes-tine. If this is not the case, poor oral bioavailability

and lower than expected systemic drug concentrationsmay occur. In addition, transporter saturation and com-petitive transporter binding (by other molecules) mayoccur (Griffiths et al., 1994).

Intestinal transporters may also be an importantmechanism of drug elimination, resulting in the activesecretion of drug from the blood back into the intestine.Such transporter mechanisms have been described for avariety of compounds, including ciprofloxacin, norflox-acin and perfloxacin (Dautrey et al., 1999; Griffithset al., 1994). Interestingly, this intestinal secretion mayinvolve different transporters for different fluoroquino-lones. For example, while sparfloxacin is a P-glycopro-

tein substrate, ciprofloxacin transport may bemediated by organic anion and/or cation transporters(Dautrey et al., 1999). Given concerns regarding the po-tential for changes in the susceptibility of the intestinalmicroflora when these compounds are administered tofood producing animals, this secretion process may alsohave consequences that affect human food safety (Food& Drug Administration, 2004).

5.2. Protein binding and tissue distribution

Protein binding is a function of the number of bind-

ing sites on the protein (n), the protein concentration (P)and the affinity constant defining the strength of binding(KA) (Bergogne-Berezin, 2002). The in vivo activity ofan antimicrobial agent, as well as the drug s ability totransfer from blood to tissue, is dependent on the freeconcentration of the drug. Therefore, when using phar-macokinetic and pharmacodynamic principles to evalu-ate an appropriate fluoroquinolone dose, it is criticalthat assessments be made on the basis of free rather thantotal drug concentrations (Bergogne-Berezin, 2002;Craig and Ebert, 1989; Derendorf, 2003; Drusano,2002). In this regard, it is also important to note thatin vitro MIC values are determined on the basis of free(unbound) drug concentrations. Unfortunately, withinthe veterinary literature, there is little consideration gi-ven to free drug concentrations, and most pharmacoki-netic studies report exposure metrics (e.g., area underthe curve, AUC, and maximum observed concentration,Cmax) for total drug concentrations. Toutain et al.(2002)provides an excellent overview of why free ratherthan total drug concentrations should be considered. Asummary of protein binding (based upon human data) isprovided inTable 3.

Equilibrium exists between the free drug concentra-tions in blood and the free drug concentrations in



8/19

tissues. Therefore, free fluoroquinolone concentrationsin serum generally reflect concentrations within theextracellular fluids, where the majority of infections oc-cur. However, potential barriers may influence diffusionof the drug to the site of infection, leading to discrepan-cies between free serum concentrations and bacterialdrug exposure. Examples of such barriers includeformed abscesses, the bloodbrain barrier, bacterial bio-film and inflammation debris (Costerton et al., 1999;Toutain et al., 2002). For this reason, understandingthe host response to a particular type of infectious agentwill greatly facilitate efforts to develop and use pharma-

cokinetic/pharmacodynamic relationships for dose opti-mization. Similarly, understanding the bindingcharacteristics of the drug and its ability to diffuse acrossbiological barriers (e.g., as affected by charge, pKa val-ues and its partitioning behaviour) will help cliniciansdefine an appropriate therapeutic moiety.

Fluoroquinolones are characteristically associatedwith volumes of distribution well in excess of 1.0L/kg (Aminimanizani et al., 2001; Lode et al., 1998;Rodvold and Neuhauser, 2001). When consideringthese values, it is important to recognize that compart-mental fluid volumes are actually 0.05 L/kg forplasma, 0.2 L/kg for extracellular fluids and 0.7 L/kgfor total body water (Wamberg et al., 2002). Volumesin excess of 0.7 L/kg indicate that a drug is bindingpreferentially to tissues, may or may not be availablefor activity, and may have been sequestered into cells.For example, when testing the ability of human poly-morphonuclear leucocytes to concentrate various fluo-roquinolones (in vitro test), ratios of intracellularversus extracellular concentrations ranged from 3.5 (fle-roxacin) to about 7.0 for ofloxacin (R and S forms)and lomefloxacin (Butts, 1994). These high concentra-tions reflect active transport of drug into the neutro-phils (Walters et al., 1999). Nevertheless, when

considering free interstitial drug concentrations, inves-tigators confirmed (based upon the use of microdialysismethods) that unbound concentrations of fluoroquino-lones in interstitial fluids are comparable to the freedrug concentrations in venous plasma (Araki et al.,1997). Therefore, excluding those situations associated

with ion trapping, since equilibrium is established be-tween blood and tissue free drug concentrations (Mul-ler et al., 1999), tissue free drug concentrations can bepredicted on the basis of free drug concentrations inthe plasma, even in cases where there is nonlinear pro-tein binding (Kovar et al., 1997).

5.3. Clearance and elimination

Fluoroquinolones are frequently categorized by theirprimary pathway of elimination. These include elimina-tion via renal mechanisms (e.g., enrofloxacin, orbifloxa-cin, ofloxacin, temafloxacin and lometfloxacin), hepatic

metabolism (e.g., difloxacin and perfloxacin) or both re-nal and hepatic mechanisms (marbofloxacin, danofloxa-cin norfloxacin, ciprofloxacin and enoxacin (Karablutand Drusano, 1993)). The extent to which the fluoro-quinolones undergo hepatic metabolism varies greatlybetween molecules and animal species, with a corre-sponding wide range in terminal elimination half-life(Greene and Budsberg, 1993; Nix and Schentag, 1988;Vancutsem et al., 1990). Fluoroquinolone metabolicpathways include glucuronidation (cinafloxain,grepafloxacin, sparfloxacin and moxifloxacin) andN-oxidation and desmethylation (levofloxacin and spar-

floxacin). Generally, metabolism involves the CYP 450system (Bergogne-Berezin, 2002).

As previously stated, one mechanism of fluoroquino-lone elimination is via active drug secretion across theintestinal membrane. Intestinal concentrations can alsobe a function of biliary secretion. The presence of enter-ohepatic recirculation can increase the residence time ofa compound. For example, in beagle dogs, 80% of anintravenous dose of difloxacin is eliminated in the feces(Federal Register, 1998). This was largely attributable tobiliary secretion. Approximately 7280% of the drug inthe bile was the ester glucuronide, with only 69% beingthe parent compound. The glucuronide appears to behydrolyzed in the gut, thereby restoring the parent com-pound which was subsequently available for re-absorp-tion. The terminal elimination half-life of difloxacinfollowing oral administration to dogs is approximately9.4 h.

The extent of renal elimination varies across the fluo-roquinolones. Levofloxacin and gatifloxacin are elimi-nated primarily by the kidney, with the renal clearanceof levofloxacin exceeding creatinine clearance byapproximately 60%. This suggests the involvement ofboth glomerular filtration and tubular section (Okazakiet al., 1991). The latter was confirmed by a 2435%

Table 3Fluoroquinolone elimination mechanism, bioavailability and proteinbinding characteristics

Compound F(%) % Renal % Protein binding

Cinoxacin 97 62Ciprofloxacin 5080 4060 30Enoxacin 44 20

Fleroxacin 50 32Gatifloxacin 8990 20Gemifloxacin 70 3040Levofloxacin >95 6087 31Moxifloxacin 8590 20 50Norfloxacin 20Ofloxacin 75 2030Sparfloxacin >90 10 45Trovofloxacin 88 6 76

For consistency, all data reflect information derived from humansubjects. Similar properties can be anticipated for veterinary species.Based upon information fromAminimanizani et al. (2001), Bergogne-Berezin (2002) and Wright et al. (2000).



9/19

decrease in renal clearance following doses of probene-cid or cimetidine (Aminimanizani et al., 2001). Proben-ecid can also prolong the terminal elimination half-lifeof gatifloxacin and ciprofloxacin. A summary of themetabolism and elimination of fluoroquinolones usedin veterinary species is provided in Table 4.

5.4. Interspecies differences

There can be marked interspecies differences in fluor-oquinolone pharmacokinetics. For example, the sys-temic clearance (CL) of difloxacin is substantiallylower in pigs (0.16 L/kg/h) as compared to chickens(0.72 L/kg/h). However, chickens have a substantiallylarger steady-state volume of distribution (Vss=3.06 L/kg) as compared to pigs (1.7 L/kg) (Inui et al.,1998). Nevertheless, owing to its more rapid CL, the ter-minal elimination half-life following an intravenousdose in chickens (4.1 h) is more rapid than that associ-

ated with swine (T1/2= 7.92 h). After oral administra-tion, difloxacin bioavailability was similar across thetwo species: 93.7% and 86.9% for swine and chickens,respectively 6.

Moxifloxacin pharmacokinetics was compared acrosssix mammalian species: dog, mini-pig, mouse, monkey,human and rat (Siefert et al., 1999). Vss values werefound to be highly correlated to body weight (BW).When using all five species, the resulting allometricequations were:

CL 1.185 L=h BW0.529r 0.96;

Vss 3.73 L BW0.918

r 0.99.The largest back-calculated prediction error for CL wasassociated with dogs (65% error) and swine (47% error).The back-calculated prediction error for Vss was 13%(dog) and 21% (swine). The relationship between termi-nal elimination half-life, fraction unbound in plasma (fu)

for all six species included in this study, as well as thebioavailability of an oral dose, is provided in Table 5.

Similarly, Cox et al. (2004) examined the allometricrelationship for CL and Vss following ciprofloxacinacross a variety of mammalian species (including cow,pig, sheep, dog, rat, monkey, goat, buffalo and humans),

and reports the following estimated relationships:CL 20.6 mL= min BW0.815r 0.95;

Vss 3.5 L BW0.947

r 0.93.

In that same study, Cox and colleagues examined theallometric relationship for enrofloxacin. The resultingplot of log weight versus logCL or log Vss showed sub-stantial scatter (resulting equations were CL = 15.9(mL/min) BW0.764r= 0.77; Vss= 6.0 (L)BW

0.724

r= 0.81). However, even when there are very high val-ues forr, allometric scaling may not accurately extrapo-late CL or Vss to an unknown animal species. In

particular, large extrapolation errors can occur whentrying to predict pharmacokinetic parameter values forlarge species such as horses and cattle (Mahmood, Mar-tinez, Hunter, personal communication). On the otherhand, there have been numerous cases, when equationswith low values ofr provided highly accurate extrapola-tions to the unknown animal species (Mahmood, 2001).

Table 4Summary of pathways involved in the elimination of fluoroquinolones in veterinary species

Drug Species References Elimination pathway

Danofloxacin Cattle Heitzman (1998) Equal amounts of drug-related material in urine and feces, with90% and 57%being parent compound in urine and feces, respectively.

Difloxacin Dog Federal Register (1998) 80% of dose eliminated in the feces. Approximately 20% of the dose is eliminated astwo major metabolites: the glucuronide and the desmethyl derivative.Seguin et al. (2004)

Enrofloxacin Dog Boothe et al. (2002) Enrofloxacin is extensively metabolized by the liver, being transformed intociprofloxacin, as well as several other minor metabolites. Somewhat lowerciprofloxacin levels are observed in cats (20% of total compound appears asciprofloxacin in blood) as compared to dogs and cattle (50% of total compoundappears as ciprofloxacin in blood).

Cat McKellar et al. (1998)Cattle Albarellos et al. (2004)

Marbofloxacin Dog EMEA (2004) Excreted primarily in urine with limited biotransformation. Small amounts ofdesmethyl and N-oxide metabolites present in dogs, pigs, rats and cattle.Cat

Orbifloxacin Dog Matsumoto et al. (1997) Ratio of label found after 72 h in urine versus feces as 28%/15% (dog) and 45%/18%(cat). In cat, 4% of the drug in urine appeared as the N-hydroxylated compound. Indog, 13% appeared as the glucuronidated compound.

Cat Matsumoto et al. (1998)

Table 5Interspecies differences in absorption, protein binding and terminalelimination half-life of moxifloxacin across animal species

Species F(%) T1/2 % fu

Mouse (male) 78 0.93 69Rate (male) 78 1.2 63Monkey (female) 52 6.9 62Dog (female) 91 8.6 71Minipig (female) 54 5.7 63Human (male) 82 13 55

FromSiefert et al. (1999).



10/19

Measured differences in the terminal elimination half-lives and bioavailability of the fluoroquinolones ap-proved for use in veterinary species within the US areprovided inTable 6.

5.5. Pharmacokinetic/pharmacodynamic relationships

The relationship between the pharmacokinetics andmicrobiological activity (pharmacodynamics) of the flu-oroquinolones may be used to help determine the doseneeded to achieve a desired clinical outcome. From apharmacokinetic/pharmacodynamic (PK/PD) perspec-tive, fluoroquinolones are associated with concentra-tion-dependent killing (Zhanel, 2001).

Parameters attributed to this include the area underthe 24-h serum (or plasma) concentration versus timecurve (AUC024h) divided by MIC of the clinical isolateor, if the MIC of the clinical isolate is unknown, then theMIC90 of bacteria of the same genus and species as the

clinical isolate, (AUC/MIC) and the peak serum or plas-ma concentrations (Cmax) divided by the MIC, orMIC90, (Cmax/MIC). These parameters are closely re-lated and both are important for ensuring efficacy.Examples of variables that can influence the PK/PDrelationship associated with a successful clinical out-come are provided in Table 7.

Recently, Mouton et al. (2005) published a manu-script to standardize the PK/PD terminology used inthe literature. Some of the important points noted intheir review are:

1. AUC should be expressed in terms of unbound drug.If multiple dosing regimens are applied, AUC shouldbe measured over a 24-h dosing interval at steadystate.

2. AUC/MIC is often expressed in terms of the dimen-sion of time. However, since it is measured over aset period of incubation (generally 1824 h), this ratiocan be expressed as a dimensionless value.

3. T > MIC represents the cumulative percentage of a24-h period that the drug concentration exceeds theMIC at steady-state pharmacokinetic conditions.

4. In vitro PAE is the period of suppression of bacterialgrowth after a brief exposure of an organism to anantimicrobial compound (unit = time). In this case,drug has been removed.

5. In vivo PAE represents the difference in time for thenumber of bacteria in a tissue of treated versus con-

trol animals to increase 1 log10 over values observedwhen drug concentrations in serum or at the infectionsite fall below the MIC (unit = time). The in vivoPAE includes any effects associated with sub-MICconcentrations.

6. Sub-MIC effect is any effect of an antimicrobial on amicro-organism at concentrations below the MIC(unit = time).

7. Post-antibiotic sub-MIC effect is the effect of sub-MIC drug concentrations on bacterial growth follow-ing serial exposure to drug concentrations exceedingthe MIC (unit = time).

Regarding the duration of a post-antibiotic effect(PAE), mathematical models have recently been devel-oped to help predict the duration of the PAE basedupon variables such as the terminal elimination half-life(T1/2), the rate of bacterial killing (e), the slope of thecurve relating the magnitude of bacterial kill versus drugconcentration (c= the Hill Coefficient), the estimatednumber of viable bacteria at the infection site, the

Table 6

Interspecies comparison of oral bioavailability and terminal elimination half-life for fluoroquinolones used in veterinary species

Enrofloxacin Danofloxacin Marbofloxacin

F(PO) Terminal T1/2(h) F(IM) Terminal T1/2(h) F(IM) Terminal T1/2(h)

Chicken 101 15.6Turkey 61 3.9Cattle 8 15.4 78 2.9 100 4.2Pig 76 6.8Sheep 95.7 3.35Dog 91 4.9 8.6Horse 60 5.6 88 4.7Goat 100 7.2

Based upon information fromAliAbadi and Lees (2002), Carretero et al. (2002), Greene and Budsberg (1993), Mann and Frame (1992), McKellaret al. (1998), Siefert et al. (1999) and Waxman et al. (2001) .

Table 7Factors influencing the accuracy of efficacy predictions based uponPK/PD relationships

1 Inoculum effect2 Pathogen growth rate (generation time)3 Growth phase of the invading organism (active versus dormant)4 Microbial biofilm5 Host response to the pathogen

a. Integrity of the immune systemb. Diffusivity of drug through host exudatesc. Change in pH at site of the infection

6 Site of the infectiona. Natural barriers (e.g., bloodbrain barrier)b. Tissue perfusionc. Nature of the exudate



11/19

maximum number of bacteria (or attainable bacterialdensity), the concentration associated with 50% of themaximal effect (EC50), and the MIC of the targetedpathogen (Mouton and Vinks, 2005a, b). Consider-ations associated with the development of a concentra-tion versus effect model, from which terms such as

Emax, EC50and ccan be derive, has been extensively re-viewed elsewhere (Toutain, 2002; Toutain et al., 2002).Within any given bacterial population, there is the

possibility of bacterial subpopulations that are less sus-ceptible to the antimicrobial agent. As demonstrated byBlaser et al. (1987)andDrusano (2004), unless these lesssusceptible pathogens are killed, succeeding microbialgenerations will re-populate the infection site withpathogens whose MIC values are higher than thosefound within the initial infection. Accordingly, ensuringadequate exposure following an initial dose of a fluor-oquinolone is as important as insuring that high drugconcentrations occur after repeated administration.

Drug concentrations need to be adequate to either de-stroy the existing bacterial population at the site of theinfection or (at least) to reduce its size to the point wherethe hosts defence mechanisms can successfully controland eliminate the remaining pathogens.

Cmax/MIC ratios may be particularly important inthe presence of bacteria with higher MIC values or inthe presences of rapidly proliferating bacteria (Craigand Dalhoff, 1998). With the latter, rapidly proliferatingbacteria have a greater likelihood of undergoing a muta-tional event that could lead to the genesis of a less sus-ceptible population. In infectious disease processes

where there is a high bacterial burden (inoculum effect),the risk of a mutational event is increased due simply tothe laws of probability (Craig and Dalhoff, 1998; Dru-sano et al., 1993). In these cases, to ensure maximumkilling, the targetedCmax/MIC ratios are approximately10 12 (Drusano et al., 1993; Preston et al., 1998). Suchratios ensure increased killing of susceptible organisms,an increased killing or inhibition of organisms withhigher MICs and thus, fewer surviving organisms forthe host defences to scavenge during the troughs of sys-temic drug concentrations. High drug concentrations,relative to the MIC, may also contribute to increasedPAE. For those bacteriadrug combinations that exhibita PAE, in vivo PAEs have been shown to be longer thanin vitro PAEs for most organisms. b-Haemolytic strep-tococci are notable exceptions. Thus, optimizing theCmaxto MIC ratio will delay the re-growth of the path-ogen, sometimes by several hours. The result of usingthis type of dosing regimen is that there are fewer organ-isms remaining that can reproduce into a resistantsubpopulation.

Despite the large body of information suggesting thatfluoroquinolones are highly effective in the presence ofhighCmax/MIC values, exceptions to this rule have beenobserved. For example, in the case ofBacillus anthracis,

hollow fibre studies suggest that AUC/MIC was morepredictive of success as compared to Cmax/MIC (Dezielet al., 2001). This result relates to the findings describedbyMacGowan et al. (2000, 2001), where time to kill 99%of the inoculum depends on Cmax/MIC, but the abilityto maintain this decrease in microbial counts (termed

area under the bacterial killing curve, or AUBKC) is re-lated to AUC/MIC (in vitro test conditions). If theduration of time between doses is extended beyond24 h (as was tested for gemifloxacin), effectiveness mayalso depend upon the duration of time that drug concen-trations exceed the MIC (T> MIC) (MacGowan et al.,2001).

When a Cmax/MIC ratio of 10 is not possible, thecontribution of time of exposure becomes increasinglyimportant and the target for efficacy defaults to theAUC/MIC ratio (Owens and Ambrose, 2002a). AUC/MIC also serves as the pivotal PK/PD index when theinfection is caused by relatively slow growing bacteria,

when there is little or no PAE that will contribute tobacterial killing, or when the MIC for the pathogen isrelatively low. A variety of animal model studies (basedon infections caused by Gram-negative organisms) haveshown that AUC/MIC values of 100 or greater havebeen needed to insure host survival (Craig, 1998; Craigand Dalhoff, 1998; Thomas et al., 1998). This value of100 translates to drug concentrations equalling approx-imately four times the MIC throughout the 24-h dosinginterval, corresponding to the findings by Walker, 2000(in vitro difloxacin concentrations needed to ensure abactericidal effect against the clinical isolates of dogs

presenting with recurrent urinary tract infections). Theimportance of AUC/MIC is consistent with clinical out-comes observed with moxifloxacin (Vesga et al., 1996),ciprofloxacin (Forrest et al., 1993) and grepafloxacin(Forrest et al., 1997).

Studies report that the AUC/MIC values needed toensure a successful therapeutic outcome may be differentfor infections caused by Gram-negative and Gram-posi-tive bacteria. For Gram-negative organisms, the esti-mated AUC/MIC ratios needed to ensure bacterialcure and prevent the selection of resistant strains is esti-mated to be approximately 125 (Forrest et al., 1993). Incontrast, the AUC/MIC ratio for Gram-positive bacte-ria is considerably lower, approximately 3050 for anumber of drugmicrobe combinations (Ibrahim et al.,2002; Preston et al., 1998; Wright et al., 2000). Studiesinvolving the third and fourth generation fluoroquino-lones suggest that for Gram-positive organisms AUC/MIC values are substantially lower whenCmax/MIC val-ues are P10 (Nightingale et al., 2000). For example,Forrest et al. (1997) noted that in 73 patients withchronic acute bacterial exacerbations of chronic bron-chitis, ratios of P175 were needed when grepafloxacinwas used to treat infections associated with Gram-nega-tive organisms while ratios ofP92 were adequate to



12/19

treat Gram-positive infections. An excellent overview ofin vitro, animal model and human clinical pharmacody-namic trials conducted with various fluoroquinolonesand across a variety of bacterial species is provided byWright et al. (2000).

Pharmacokineticpharmacodynamic relationships

can serve as a valuable guide for obtaining initial esti-mates of doses that may be needed to achieve a desiredclinical response, to modify a dosing regimen in patientspresenting with altered clearance, or to scale a dosebased upon susceptibility information on the invadingpathogen. However, it is incorrect to use these relation-ships as a mechanism for either assessing product effec-tiveness (in lieu of clinical data) or for comparingproducts. There are numerous factors that pharmacoki-netics and pharmacodynamics alone cannot accuratelypredict. For example, in vitro MIC values do not pro-vide information on time to kill, time to maximum kill,log change within a fixed time or the maximum reduc-

tion in viable bacterial counts (MacGowan and Bowker,2002). Furthermore, serum drug concentrations do notnecessarily reflect a compounds ability to diffuse intothe site of infection and into the bacterial cell.

Along with concerns about microbial susceptibilitypatterns, a drugs bactericidal activities can vary withthe intracellular pH versus drug pKa, oxygen content,and intracellular enzymatic activity (Butts, 1994). More-over, drug potency is often considered in terms of MIC,which is a static effect on microbial growth. The MICmay not be the same as a compounds minimum bacte-ricidal concentration (MBC). Since both the MIC and

MBC values are in vitro estimates, they do not reflecta drugs rate of killing, the effect of serum on antimicro-bial activity, PAE, or post antimicrobial sub-MIC effects(Craig and Dalhoff, 1998). Moreover, while some fluo-roquinolones demonstrate in vivo and in vitro activityagainst stationary phase bacteria (e.g., ofloxacin andciprofloxacin), others (norfloxacin) do not (Lode et al.,1998). In addition, once a certain AUC/MIC orCmax/MIC ratio is achieved for a particular compound,further increases in these ratios may not improve clinicalefficacy. As indicated earlier, at high FQ concentrations,both RNA synthesis and protein synthesis can be inhib-ited, thereby causing a decrease in bactericidal activity(Lode et al., 1998). Ultimately, it is the integrity of thehost immune response that will determine the effective-ness of the targeted pharmacokineticpharmacody-namic relationship (Andes and Craig, 2002; Toutainet al., 2002).

The establishment of pharmacokineticpharmacody-namic relationships within the human pharmaceuticalcommunity is generally determined for a particulardrugbacterium combination, requiring that tens ofthousands of subjects be evaluated. It is only throughthe collection of huge amounts of data that specific cri-teria can be developed.

6. Toxicities

Fluoroquinolone toxicities are, for the most part,dose and animal species dependent (Bertino and Fish,2000). Most reactions are considered to be minor andreversible upon discontinuing treatment. In human

medicine, there have been reports of photosensitivity,drugdrug interactions, central nervous system effects(including seizures, ataxia, dizziness, insomnia, restless-ness, somnolence and tremors), and crystalluria (leadingto obstructive uropathy). Many of these toxicities havealso been reported in dogs and cats. In addition, in vet-erinary species, reported toxicities include gastrointesti-nal disturbances (such as nausea, vomiting anddiarrhoea), arthropathies in young animals, especiallydogs, and ocular toxicities (including retinal degenera-tion in cats and subcapsular cataracts for certainfluoroquinolones).

Some fluoroquinolones have been known to induce

QT prolongation in sensitive people (Cubeddu, 2003).In a high risk patient, this could lead to Torsades dePointe (Owens and Ambrose, 2002a,b). Similarly, QTcprolongation following high doses of some fluoroquino-lones, such as sparfloxacin, has been reported in dogs(Satoh et al., 2000). Other fluoroquinolones not ap-proved for veterinary use, but which have been associ-ated with prolongation of cardiac repolarisation(canine model), include gatifloxacin and moxifloxacin.Conversely, sitafloxacin, a different third generation flu-oroquinolone, has no effect on canine cardiac repolarisa-tion (Chiba et al., 2004).

Tendonitis and spontaneous tendon rupture havebeen reported in people during or following therapywith fluoroquinolones. Consequently, the effect of enro-floxacin on tendon cell cultures of mature and juvenilehorses has been investigated (Yoon et al., 2004). Enro-floxacin was found to inhibit cell proliferation, inducemorphological changes, decrease total monosaccharidecontent, and alter small proteoglycan synthesis at theglycosylation level in equine tendon cell cultures. Theseeffects are more pronounced in juvenile tendon cells thanin adult equine tendon cells.

Other reported adverse events have been associatedwith drugdrug interactions. The following interactionshave been reported to occur in humans via mechanismsthat may be relevant to veterinary species. Therefore, itmay be prudent to approach, with caution, the con-comitant use of fluoroquinolones and the followingentities:

1. Ciprofloxacin and theophylline (Radandt et al.,1992): These interactions result from fluoroquino-lone metabolism by the P450 CYP 1A2 isoenzyme.Fluoroquinolone-mediated inhibition of this enzymeprevents the metabolism/inactivation of methylxan-thines such as caffeine and theophylline causing



13/19

excess CNS and cardiac stimulation. This potentialinteraction may have veterinary relevance in thatciprofloxacin is a metabolite of enrofloxacin.

2. Probenecid and renal elimination: probenecid maydecrease the renal clearance of fluoroquinolones(Aminimanizani et al., 2001). Multiple drug effects on

cardiac repolarisation: to minimise risk of Q-Tprolongation and subsequent Torsades de Pointe, flu-oroquinolones should not be co-administered withother Q-T prolonging drugs such as erythromycin,disopyramide, or antidepressants such as amitriptyline(Curtis et al., 2003; Owens and Ambrose, 2002a,b).

7. Fluoroquinolones and resistance

The development of fluoroquinolone resistance viamutations in topoisomerases has been studied exten-

sively. For detailed discussion, a number of excellent re-views are in the literature (Drlica and Malik, 2003;Hawkey, 2003; Hooper, 2002). Resistance is mediatedprimarily by target mutations in DNA gyrase (topoiso-merase II) (Nakamura et al., 1989; Yoshida et al.,1990), with secondary mutations in topoisomerase IVcontributing to higher levels of resistance (Vila et al.,1996). Amino acid substitutions that can result inbacterial resistance have been localized to a specifictopoisomerase subdomain termed the quinolone resis-tance-determining region (QRDR) within gyrA (Yos-hida et al., 1988, 1990) and parC (Khodursky et al.,

1995). InE. coli, most mutations associated with quino-lone resistance occur in the QRDR at serine 83 (Ser83)and aspartate 87 ofgyrA, and at serine 79 and aspartate83 ofparCand at analogous sites in other species (Be-bear et al., 2003; Takiff et al., 1994; Taylor and Chau,1997). DNA sequence analysis ofStaphylococcus aureusand Streptococcusgenes shows that the situation can bereversed in Gram-positive bacteria, where topoisomer-ase IV (encoded by grlAand grlB) is the primary fluor-oquinolone target (Munoz and De La Campa, 1996; Nget al., 1996). In both cases, mutations decrease thequinolone affinity for the enzyme/DNA complex (Max-well and Critchlow, 1998), and allow DNA replicationto continue in the presence of fluoroquinolone concen-trations that are inhibitory to wild-type cell growth.

Among Gram-negative organisms, quinolone resis-tance typically develops in a stepwise manner. A singleQRDR mutation, usually at Ser83, confers resistanceto nalidixic acid and decreases susceptibility to fluoro-quinolones (ciprofloxacin MICs 0.1251lg/mL from awild-type baseline of 0.0150.03lg/mL). Secondarymutations in thegyrAQRDR lead to overt fluoroquino-lone resistance (ciprofloxacin MICs P4lg/mL). How-ever, this does not hold true for all Gram-negativebacteria. In Campylobacter, which lacks topoisomerase

IV, a single mutation in gyrA is sufficient to imparthigh-level ciprofloxacin MICs (32lg/mL) (Wang et al.,1993). This feature helps explain the higher prevalenceof resistance in Campylobacter, compared with E. coli,from food animals exposed to fluoroquinolones (VanBoven et al., 2003).

In addition to structural mutations in the topoiso-merase genes, fluoroquinolone resistance is mediatedby decreased permeability of the bacterial cell wall andby the activity of energy-dependent efflux pumps. Fluor-oquinolone resistance due to target mutations typicallyresults in decreased susceptibility or resistance to othermembers of the class for both veterinary and humandrugs (Everett et al., 1996; Piddock et al., 1998). Resis-tance due to decreased permeability and active efflux of-ten are less specific, generating cross-resistance tomultiple classes of antimicrobials (Poole, 2000).

It appears that most fluoroquinolones cross theGram-negative outer membrane through protein chan-

nels called porins (Nikaido and Vaara, 1985), althoughsome may diffuse directly across the lipid bilayer. Thus,the number and size of the porins can contribute to theintrinsic susceptibility of bacteria to quinolone agents.Resistance due to decreased quinolone influx is generallyreflected in low-level changes in susceptibility and mayhelp to contribute to differences in potency among differ-ent fluoroquinolone derivatives. Porin deficiency hasbeen associated with quinolone resistance in E. coliand Pseudomonas. For example, mutations of theE. coli porin OmpF produced about a 2-fold increasein quinolone MICs (Alekshun and Levy, 1999).

It is difficult to experimentally assess the role of por-ins without also accounting for effects due to efflux. Per-meability changes mediated by altered porins are oftenpart of a coordinated cellular response to the presenceof numerous toxic agents, which includes simultaneousupregulation of efflux. In E. coli, de-repression in regu-latory loci such as marA or soxS leads to decreasedfluoroquinolone susceptibility via simultaneous upregu-lation of the AcrAB-TolC efflux pump (Okusu et al.,1996) and downregulation of the OmpF porin (Cohenet al., 1988). This mechanism confers decreased suscep-tibility to a large number of other antimicrobial agentsin addition to fluoroquinolones. Analogous regulatoryloci exist among other species of bacteria (Cohenet al., 1993).

In antimicrobial efflux systems, membrane-localizedproteins actively pump drug from the cell before it candiffuse to its primary target within the active site ofDNA gyrase. Because they are driven by the proton mo-tive force, energy uncouplers can be used to study theirrole in resistance. TheE. coligenome carries as many as30 potential efflux pumps, many of which mediate anti-microbial efflux. Some are effective for specific agents,whereas others protect against a variety of structurallydiverse compounds. In addition, a single bacterium



14/19

may contain multiple efflux pumps (e.g., AcrAB andcmlA) that are capable of extruding the same antimicro-bial agent. Constitutive and inducible efflux is a knownmechanism of fluoroquinolone resistance in both Gram-negative and Gram-positive bacteria, and may be moreimportant than secondary mutations in topoisomerase

IV genes. For example, it has been shown that deletionof the gene encoding the inducible AcrAB efflux pumpreduces ciprofloxacin MICs to near wild-type levels incells carrying topoisomerase mutations (Oethingeret al., 2000). In Campylobacter, where efflux mediatedby CmeAB is constitutive, fluoroquinolone MICs inwild-type cells are 34-fold higher than those typical ofE. coli. Insertional inactivation of CmeAB in C. jejunireduces ciprofloxacin MICs to levels near that of wild-typeE. coli(0.003lg/mL) (Luo et al., 2003). These find-ings have led some drug developers to examine bacterialefflux systems as potential targets for compounded anti-microbial therapeutics.

It has long been thought that bacterial fluoroquino-lone resistance disseminates exclusively via clonalexpansion under selective pressure. Recently, a plas-mid-mediated quinolone resistance gene (qnr) was de-scribed, first in clinical isolates of Klebsiellapneumoniae (Martinez-Martinez et al., 1998) and laterin E. coli (Jacoby et al., 2003; Wang et al., 2003). Thegene is located near sequences (qacEA1, sulI) typicallyassociated with class I integrons: the qnr gene encodesa 218 amino acid protein belonging to the pentapeptiderepeat family (Tran and Jacoby, 2002). In a concentra-tion-dependent manner, qnr functions by protecting

E. coli DNA gyrase, but not topoisomerase IV, frominhibition by ciprofloxacin (Tran and Jacoby, 2002).The qnr gene confers a small decrease in quinolone sus-ceptibility such that qnr+ strains are still consideredclinically susceptible. It has been suggested that the im-pact on clinical resistance results from the ability ofqnrto permit selection of topoisomerase mutants at concen-trations that normally would be toxic to the bacterium(Martinez-Martinez et al., 1998).

Each of the fluoroquinolone resistance mechanisms(target modification, decreased permeability, efflux andtarget protection) can occur simultaneously within thesame cell, thereby leading to very high resistance levels.To date, no mechanisms based on enzymatic inactiva-tion/modification of quinolones have been discovered.Because quinolones are synthetic antimicrobials withno known natural analogues, it is less likely that thistype of mechanism will emerge.

8. Dose optimization

Dose optimization reflects the results of a multidi-mensional evaluation that includes such factors as (Polk,1999):

1. Efficacy2. Risk of adverse events3. Contraindications4. Cost of therapy5. Patient compliance

Theoretically, this objective appears straightforward.However, there are numerous challenges to face whenstriving to optimize a dose under clinical conditions.Firstly, we must define our therapeutic objective. Is ita cessation of clinical signs of disease or microbialcure? If microbial cure is not obtained, how does thatimpact the potential for relapse or for the spread ofresistant microbial strains to other hosts? In this re-gard, it is of interest to note that based upon isolatesfrom the canine urinary tract at the University of Mis-souri-Columbia Veterinary Medical Diagnostic Lab,there has been an increase in the number of resistantorganisms isolated between 1992 and 2001. The clinical

relevance of these finding has not been determined(Cohn et al., 2003). Secondly, when selecting a drug,dose and dosing regimen, it is relatively uncommonto have identified, via culture, the pathogenic organismand its susceptibility pattern. Consequently, practiceexperience plus publicly available information on thevarious choices of antimicrobial agent is often the pri-mary consideration in drug selection and dose determi-nation. Finally, when treating food animal species,where numerous animals may be involved, there isthe concern of cost, ease of use, stress (animal and han-dler), residues and microbial safety (Food & Drug

Administration, 2004).The goal of any prescribing clinician is to identify a

therapeutic regimen that will have a reasonable chancefor success. At least in part, the availability of interpre-tive criteria and susceptibility breakpoints can providean invaluable guide to the selection of an appropriatetherapeutic intervention (Mouton, 2003). However, evenif breakpoints are available, there remains an enormousnumber of variables for the attending clinician to con-sider. These include:

1. What are the pharmacokinetic characteristics of thatdrug in my patient and what blood levels am I likelyto achieve with any particular dosing regimen? In thisregard, the use of Monte Carlo simulation and theconcept of selecting a dose on the basis of its proba-bility of therapeutic success is extremely attractive(Ambrose and Grasela, 2000; Dudley and Ambrose,2000). However, these methods are only as good asthe users estimates of the pharmacokinetic parame-ters, the population distribution of these parameters,and the MIC distribution of the pathogen (assumingthat the pathogen has been identified). Moreover,there are very few clinical situations in which theattending veterinarian will have access to (or the time



15/19

to run) these kinds of simulations. Therefore, thesetechnologies are currently limited to use as researchtools.

2. If the drug is intended for use in food producing ani-mals, how can the drug intake be optimized? This is aparticularly challenging question when dosing poul-

try, aquatic species or large pens of animals.3. What level of drug exposure is needed to insure thatpathogen load is reduced to where the host immunesystem can effectively eliminate the remainingpathogens?

4. How will the pathogen respond to this drug in vivo?This question needs to be addressed in relation to thepotential presence of biological barriers (host or bac-terial) as well as potential differences in bacterialin vivo versus in vitro growth rate.

5. Finally, for food producing animals, there is alwaysthe need to consider drug, dose, duration of dosingand the withdrawal time needed to minimize the risk

of violative residues. This can be particularly chal-lenging in aquatic species where residue depletionmay vary markedly as a function of species of fishor water temperature (Lucchetti et al., 2004). In addi-tion, environmental consequences of fluoroquinoloneuse for treating aquatic species needs also to be con-sidered (Robinson et al., 2005).

Ultimately, as aptly stated by Ron Polk in a 1999article regarding optimal use of antibiotics,

Far more is known about the basic chemistry of antibiot-

ics, their pharmacokinetics, and their mechanisms of

action and of resistance than is known about the practical

problem of what dose to give.

Hopefully, the magnitude of this uncertainty willdiminish over time as more sponsors of veterinary anti-microbial agents work with organizations such as theCLSI to establish susceptible, intermediate and resistantbreakpoints (interpretive criteria) and as tools such asthe Veterinary Antimicrobial Decision System (http://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asp) become available to assist veterinarians in judgingtherapeutic options.

References

Agarwal, R., Mittal, R., Singh, A., 2000. Studies of ion-exchange resincomplex of chloroquine phosphate. Drug Development andIndustrial Pharmacy 26, 773776.

Albarellos, G.A., Kreil, V.E., Landoni, M.F., 2004. Pharmacokineticsof ciprofloxacin after single intravenous and repeated oral admin-istration to cats. Journal of Veterinary Pharmacology and Ther-apeutics 27, 155162.

Al Omran, M.F., Al Suwayeh, S.A., El Helw, A.M., Saleh, S.I., 2002.Taste masking of diclofenac sodium using microencapsulation.Journal of Microencapsulation 19, 4552.

Alekshun, M.N., Levy, S.B., 1999. The mar regulon: multipleresistance to antibiotics and other toxic chemicals. Trends inMicrobiology 7, 410413.

AliAbadi, F.S., Lees, P., 2002. Pharmacokinetics and pharmacoki-netic/pharmacodynamic integration of marbofloxacin in calfserum, exudate and transudate. Journal of Veterinary Pharmaco-logical Therapy 25, 161174.

Allen, A., Bygate, E., Clark, D., Lewis, A., Pay, V., 2000. The effect offood on the bioavailability of oral gemifloxacin in healthyvolunteers. International Journal of Antimicrobial Agents 16, 4550.

Ambrose, P.G., Grasela, D.M., 2000. The use of Monte Carlosimulation to examine pharmacodynamic variance of drugs:fluoroquinolone pharmacodynamics against Streptococcus pneu-moniae. Diagnostic Microbiology and Infectious Disease 38,151.

Aminimanizani, A., Beringer, P., Jelliffe, R., 2001. Comparativepharmacokinetics and pharmacodynamics of the newer fluoroqu-inolone antibacterials. Clinical Pharmacokinetics 40, 169187.

Andes, D., Craig, W.A., 2002. Pharmacodynamics of the newfluoroquinolone gatifloxacin in murine thigh and lung infectionmodels. Antimicrobial Agents and Chemotherapy 46, 16651670.

Appelbaum, P.C., Hunter, P.A., 2000. The fluoroquinolone antibac-terials: past, present and future perspectives. International Journalof Antimicrobial Agents 16, 515.

Araki, H., Ogake, N., Minami, S., Watanabe, Y., Narita, H., Tamai,I., Tsuji, A., 1997. Application of muscle microdialysis to evaluatethe concentrations of the fluoroquinolones pazufloxacin andofloxacin in the tissue interstitial fluids of rats. Journal ofPharmacy and Pharmacology 49, 11411144.

Ball, P., 2000. Quinolone generations: natural history or naturalselection. Journal of Antimicrobial Chemotherapy 46 (Suppl T1),1724.

Barbosa, J., Barron, D., Cano, J., Jimenez-Lozano, E., Sanz-Nebot,V., Toro, I., 2001. Evaluation of electrophoretic method versuschromatographic, potentiometric and absorptiometric methodolo-gies for determining pKa values of quinolones in hydroorganicmixtures. Journal of Pharmaceutical and Biomedical Analysis 24,10871098.

Barbosa, J., Barron, D., Jimenez-Lozano, E., 1999. Electrophoreticbehaviour of quinolones in capillary electrophoresis. Effect of pHand evaluation of ionization constants. Journal of Chromatogra-phy A 839, 183192.

Barron, D., Jimenez-Lozano, E., Barbosa, J., 2001. Prediction ofelectrophoretic behaviour of a series of quinolones in aqueousmethanol. Journal of Chromatography A 919, 395406.

Bebear, C.M., Renaudin, H., Charron, A., Clerc, M., Pereyre, S.,Bebear, C., 2003. DNA gyrase and topoisomerase IV mutations inclinical isolates of Ureaplasma spp. and Mycoplasma hominisresistant to fluoroquinolones. Antimicrobial Agents and Chemo-therapy 47, 33233325.

Bergogne-Berezin, E., 2002. Clinical role of protein binding of

quinolones. Clinical Pharmacokinetics 41, 741750.Bertino Jr., J., Fish, D., 2000. The safety profile of the fluoroquino-lones. Clinical Therapeutics 22, 798817.

Blaser, J., Stone, B.B., Groner, M.C., Zinner, S.H., 1987. Comparativestudy with enoxacin and netilmicin in a pharmacodynamic modelto determine importance of ratio of antibiotic peak concentrationto MIC for bactericidal activity and emergence of resistance.Antimicrobial Agents and Chemotherapy 31, 10541060.

Blondeau, J.M., 1999. Expanded activity and utility of the newfluoroquinolones: a review. Clinical Therapeutics 21, 340.

Boothe, D.M., Boeckh, A., Boothe, H.W., Wilkie, S., Jones, S., 2002.Plasma concentrations of enrofloxacin and its active metaboliteciprofloxacin in dogs following single oral administration ofenrofloxacin at 7.5, 10, or 20 mg/kg. Veterinary Therapeutics 3,409419.

http://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asphttp://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asphttp://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asphttp://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asphttp://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asphttp://www.vetmed.iastate.edu/departments/vdpam/pam/vads.asp


16/19

Butts, J.D., 1994. Intracellular concentrations of antibacterial agentsand related clinical implications. Clinical Pharmacokinetics 27, 6384.

Carretero, M., Rodriguez, C., San Andres, M.I., Fores, P., de Lucas,J.J., Nieto, J., San Andres, M.D., Gonzalez, F., 2002. Pharmaco-kinetics of marbofloxacin in mature horses after single intravenousand intramuscular administration. Equine Veterinary Journal 34,360365.

Chiba, K., Sugiyama, A., Hagiwara, T., Takahashi, S., Takasuna, K.,Hashimoto, K., 2004. In vivo experimental approach for the riskassessment of fluoroquinolone antibacterial agents-induced longQT syndrome. European Journal of Pharmacology 486, 189200.

Chopra, R., Alderborn, G., Newton, J.M., Podczeck, F., 2002. Theinfluence of film coating on pellet properties. PharmaceuticalDevelopment and Technology 7, 5968.

Cohn, L.A., Gary, A.T., Fales, W.H., Madsen, R.W., 2003. Trends influoroquinolone resistance of bacteria isolated from canine urinarytracts. Journal of Veterinary Diagnostic Investigation 15, 338343.

Cohen, S.P., McMurray, L.M., Levy, S.B., 1988. marA locus causesdecreased expression of OmpF porin in multiple-antibiotic-resis-tant (Mar) mutants of Escherichia coli. Journal of Bacteriology170, 54165422.

Cohen, S.P., Yan, W., Levy, S.B., 1993. A multidrug resistanceregulatory chromosomal locus is widespread among enteric bacte-ria. Journal of Infectious Diseases 168, 484488.

Costerton, J.W., Stewart, P.S., Greenberg, E.P., 1999. Bacterialbiofilms: a common cause of persistent infections. Science 284,13181322.

Coulet, M., Morello, C., Cox, P., Lohuis, J., 2005. Pharmacokineticsof ibafloxacin in healthy cats. Journal of Veterinary Pharmacologyand Therapeutics 28, 3744.

Cox, S.K., Cottrell, M.B., Smith, L., Papich, M.G., Frazier, D.L.,Bartges, J., 2004. Allometric analysis of ciprofloxacin and enro-floxacin pharmacokinetics across species. Journal of VeterinaryPharmacological and Therapeutics 27, 139146.

Craig, W.A., 1998. Pharmacokinetic/pharmacodynamic parameters:rationale for antibacterial dosing of mice and men. ClinicalInfectious Diseases 26, 110.

Craig, W.A., Dalhoff, A., 1998. Pharmacodynamics of fluoroquino-lones in experimental animals. In: Born, G.V.R., Cuatrecas, P.,Ganten, D., Herken, H., Melmon, K.L., Starke, K. (Eds.),Handbook of Experimental Pharmacology. Springer, Berlin, pp.208232.

Craig, W.A., Ebert, S.C., 1989. Protein binding and its significance inantibacterial therapy. Infectious Disease Clinics of North America3, 407414.

Cubeddu, L.X., 2003. QT prolongation and fatal arrhythmias: a reviewof clinical implications and effects of drugs. American JournalTherapeutics 10, 452457.

Curtis, L.H., Ostbye, T., Sendersky, V., Hutchison, S., Allen LaPointe,N.M., Al Khatib, S.M., Usdin, Y.S., Dans, P.E., Wright, A., Califf,R.M., Woosley, R.L., Schulman, K.A., 2003. Prescription of QT-

prolonging drugs in a cohort of about 5 million outpatients.American Journal of Medicine 114, 135141.Dalhoff, A., 1989. A review of quinolones. In: Proceedings of an

International Telesymposium. J. R. Prous Science, Barcelona,Spain, pp. 277312.

Dautrey, S., Felice, K., Petiet, A., Lacour, B., Carbon, C., Farinotti,R., 1999. Active intestinal elimination of ciprofloxacin in rats:modulation by different substrates. British Journal of Pharmacol-ogy 127, 17281734.

Derendorf, H. 2003. Microdialysis-based PK/PD approaches in anti-microbial drug development. Available from: ,2003 (9-17-2004. Ref Type: Electronic Citation).

Deziel, M.R., Louis, A., Heine, H., Bush, K., Kao, M., Kelley, M.,Drusano, G.L., 2001. Evaluation of levofloxacin in a hollow fiber

infection model againstBacillus anthracisutilizing both human andRhesus Monkey pharmacokinetic profiles. Abstracts of the 41stAnnual Meeting of the Interscience Conference on AntimicrobialAgents and Chemotherapy. Abstract # A-1157, Chicago, IL.

Drlica, K., Malik, M., 2003. Fluoroquinolones: action and resistance:Current Topics in Medical Chemistry 3, 249282.

Drusano, G.L. 2002. Pharmacodynamics of fluoroquinolones. In:Proceedings of the 10th ISAP Symposium: Pharmacokinetics andPharmacodynamics (PK/PD): Towards Definitive Criteria. Milan,Italy, April 2728.

Drusano, G.L., 2004. Antimicrobial pharmacodynamics: criticalinteractions of bug and drug. Nature Reviews Microbiology 2,289300.

Drusano, G.L., Johnson, D.E., Rosen, M., Standiford, H.C., 1993.Pharmacodynamics of a fluoroquinolone antimicrobial agent in aneutropenic rat model of Pseudomonas sepsis. AntimicrobialAgents and Chemotherapy 37, 483490.

Dudley, M.N., Ambrose, P.G., 2000. Pharmacodynamics in the studyof drug resistance and establishing in vitro susceptibility break-points: ready for prime time. Current Opinions in Microbiology 3,515521.

Efthymiopoulos, C., Bramer, S.L., Maroli, A., 1997. Effect of food andgastric pH on the bioavailability of grepafloxacin. Clinical Phar-macokinetics 33 (Suppl. 1), 1824.

EMEA. European Agency for the Evaluation of Medicinal Products,Veterinary Medicines and Information Technology Unit. Marbo-floxacin summary report, 3/96. Available from: (Accessed June 4, 2004).

Escribano, E., Calpena, A.C., Garrigues, T.M., Freixas, J., Domenech,J., Moreno, J., 1997. Structureabsorption relationships of a seriesof 6-fluoroquinolones. Antimicrobial Agents and Chemotherapy41, 19962000.

Everett, M.J., Jin, Y.F., Ricci, V., Piddock, L.J., 1996. Contributionsof individual mechanisms to fluoroquinolone resistance in 36

Escherichia coli strains isolated from humans and animals. Anti-microbial Agents and Chemotherapy 40, 23802386.

Federal Register. 63 Fed. Reg. 8122 (Feb 18 1998). 1998.Federal Register. 65 Fed. Reg. 64954 (Oct 31, 2000). 2000.Federal Register. 66 Fed. Reg. 21400 (April 30, 2001). [Docket No.

00N-1571]. 2001.Federal Register. 62 Fed. Reg. 50387 (Sept 25, 1997). 2004.Food & Drug Administration 2004. Center for Veterinary Medicine

Guidance for Industry #152: Assessment of the Effects of antimi-crobial Drug Residues from Food of Animal Origin on the HumanIntestinal Flora. Available from: (Accessed Sept 17, 2004).

Forrest, A., Chodosh, S., Amantea, M.A., Collins, D.A., Schentag,J.J., 1997. Pharmacokinetics and pharmacodynamics of oralgrepafloxacin in patients with acute bacterial exacerbations ofchronic bronchitis. Journal of Antimicrobial Chemotherapy 40(Suppl A), 4557.

Forrest, A., Nix, D.E., Ballow, C.H., Goss, T.F., Birmingham, M.C.,

Schentag, J.J., 1993. Pharmacodynamics of intravenous ciproflox-acin in seriously ill patients. Antimicrobial Agents and Chemo-therapy 37, 10731081.

Gajjar, D.A., Sukoneck, S.C., Bello, A., Ge, Z., Christopher, L.,Grasela, D.M., 2002. Effect of a high-fat meal on the pharmaco-kinetics of the des-F(6)-quinolone BMS-284756. Pharmacotherapy22, 160165.

Gellert, M., Mizuuchi, K., ODea, M.H., Itoh, T., Tomizawa, J.I.,1977. Nalidixic acid resistance: a second genetic characterinvolved in DNA gyrase activity. Proceedings of the NationalAcademy of Sciences United States of the America 74, 47724776.

Glaxo Wellcome. 2004. Glaxo Wellcome voluntarily withdraws Raxar(Grepafloxacin). Available from: (Accessed Sept 21, 2004).

http://www.aapspharmaceutica.com/inside/focus_groups/microdial/derendorf.pdfhttp://www.aapspharmaceutica.com/inside/focus_groups/microdial/derendorf.pdfhttp://www.emea.eu.int/pdfs/vet/mrls/007996en.pdfhttp://www.emea.eu.int/pdfs/vet/mrls/007996en.pdfhttp://www.fda.gov/cvm/guidance/guideline159.dochttp://www.fda.gov/cvm/guidance/guideline159.dochttp://www.fda.gov/medwatch/safety/1999/raxar.htmlhttp://www.fda.gov/medwatch/safety/1999/raxar.htmlhttp://www.fda.gov/medwatch/safety/1999/raxar.htmlhttp://www.fda.gov/medwatch/safety/1999/raxar.htmlhttp://www.fda.gov/cvm/guidance/guideline159.dochttp://www.fda.gov/cvm/guidance/guideline159.dochttp://www.emea.eu.int/pdfs/vet/mrls/007996en.pdfhttp://www.emea.eu.int/pdfs/vet/mrls/007996en.pdfhttp://www.aapspharmaceutica.com/inside/focus_groups/microdial/derendorf.pdfhttp://www.aapspharmaceutica.com/inside/focus_groups/microdial/derendorf.pdf


17/19

Greene, C.E., Budsberg, S.C., 1993. Veterinary use of quinolones. In:Hooper, D.C., Wolfson, J.S. (Eds.), Quinolone AntimicrobialAgents. American Society for Microbiology, Washington, DC, pp.473488.

Griffiths, N.M., Hirst, B.H., Simmons, N.L., 1994. Active intestinalsecretion of the fluoroquinolone antibacterials ciprofloxacin, nor-floxacin and pefloxacin; a common secretory pathway? Journal ofPharmacology and Experimental Therapeutics 269, 496502.

Guthrie, R.M., Jacobs, M., Low, D.E., Mandell, L., Slama, T., 2004.Treating resistant respiratory infections in the primary care setting:the role of the new quinolones. University of Cincinnati College ofMedicine Continuing Medical Education.

Hawkey, P.M., 2003. Mechanisms of quinolone action and microbialresponse. Journal of Antimicrobial Chemotherapy 51 (Suppl 1),2935.

Heitzman, R.J., 1998. Residues of some veterinary drugs in animalsand foods. In: 48th Meeting of the Joint FAO/WHO ExpertCommittee on Food Additives. FAO Food and Nutrition Paper41/10. Geneva, Switzerland, 1827 February 1997. Available from:http://www.fao.org/docrep/W8338E/w8338e07.htm> (Accessed 9/20/2004).

Hooper, D.C., 2002. Fluoroquinolone resistance among Gram-positive cocci. Lancet Infectious Diseases 2, 530538.

Ibrahim, K.H., Hovde, L.B., Ross, G., Gunderson, B., Wright, D.H.,Rotschafer, J.C., 2002. Microbiologic effectiveness of time- orconcentration-based dosing strategies in Streptococcus pneumoniae.Diagnostic Microbiology Infectious Diseases 44, 265271.

Inui, T., Taira, T., Matsushita, T., Endo, T., 1998. Pharmacokineticproperties and oral bioavailabilities of difloxacin in pig andchicken. Xenobiotica 28, 887893.

Jacoby, G.A., Chow, N., Waites, K.B., 2003. Prevalence of plasmid-mediated quinolone resistance. Antimicrobial Agents and Chemo-therapy 47, 559562.

Johnson, R.D., Door, M.B., Hunt, T.L., Jensen, B.K., Talbot, G.H.,1999.

pharmacology of the fluoroquinolonas

Documents