`Third Edition
`
`Applied Pharmacokinetics
`Principles of Therapeutic Drug Monitoring
`
`Edited by
`
`William E. Evans, Pharm.D.
`First Tennessee Professor of
`_ Clinical Pharmacy and Pediatrics
`University of Tennessee,
`Memphis, TN
`and
`i
`
`Chair, Pharmaceutical Division
`St. Jude Children’s Research Hospital
`
`Jerome J. Schentag, Pharm.D.
`“ox... Professor of Pharmaceutics and Pharmacy
`State University of New York at Buffalo
`Buffalo, NY
`and
`
`Director, Clinical Pharmacokinetics Laboratory
`Millard Fillmore Hospital
`
`William J. Jusko, Ph.D.
`Professor of Pharmaceutics
`
`School of Pharmacy
`State University of New York at Buffalo
`Buffalo, NY
`
`Assistant Editor
`
`MaryV.Relling, Pharm.D.
`Assistant Member
`Pharmaceutical Division
`
`St. Jude Children’s Research Hospital
`and
`
`Assistant Professor of Clinical Pharmacy
`University of Tennessee
`Memphis, TN
`
`J. Wack, RDR, CRR, CCR
`
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`Other Publications by Applied Therapeutics, Inc.:
`Applied Therapeutics: The Clinical Use of Drugs, Sth edition
`Edited by Mary Anne Koda—Kimble and Lloyd Y, Young
`ISBN0-915486-14-8
`Basic Clinical Pharmacokinetics, 2nd edition
`byMichaelE. Winter
`ISBN 0~915486-08-3
`Bedside Clinical Pharmacokinetics, revised edition
`by Carl C. Peck, Dale P. Conner, and M. Gail Muzphy.
`ISBN 0-915486-10_5
`Clinical Clerkship Manual
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`by Philip D. Hansten and John R. Horn
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`Wandbook ofApplied Therapeutics, 2nd edition
`by Mary Anne Koda—Kimble, Lloyd Y. Young,
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`Copyright © 1992 by Applied Therapeutics, Inc.
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`Previous editions ofAppliedPharmacokinetics: Principles ofTherapeutic Drug
`Tyonitoring:
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`second edition, © 1980 first
`editi
`;
`Therapeutics, Inc.
`D
`inst edition by Applied
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`ona or transmitted, any form or by any means, electronic, mechanical,
`Ph
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`
`198
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`Ree
`1-1
`1-1
`
`

`

`
`
`Contents
`
`Acknowledgments ..........c:cccecccssssssessessescsussssscsesessesseacsusessesaavatatavarevsversavavavevesececcce iv
`
`Contributing Authors... ccccsecsssssescsesssssssssavsussvsssvasssasstavsesavavaressvastsavacaseesecescecesv
`
`Editorial Review Board ...........scccsscsesessetssessssesssevesssvessarsesanscevsueusacatssassvasavsvesesesecesxv
`
`Notice to the Reader 0... esccccseesensecsessesseesseseacsssesssusssusssssessacsescavavavavereeseecs xxli
`
`Preface to Third Edition oo... cccessesssssssessessseessesessssssevesesesesssarersacaesessesesassessaes xxiii
`
`Applied Pharmacokinetics—A Prospectus .......cssesssssssssecsesesseessecscevsesatsesessreeees P-1
`Gerhard Levy
`
`1. General Principles of Applied Pharmacokinetics ......c.c.ccsccssscssssssecssssessseescaas 1-1
`William E. Evans
`
`2. Guidelines for Collection and Analysis of Pharmacokinetic Data ..........0.....2-1
`William J. Jusko
`
`3. Analysis of Pharmacokinetic Data for Individualizing
`Drug Dosage Regimens... cesccsscssesenssetssccscsnssecseseeseeseseetteseeseneeeeseeeereetenss 3-1
`Carl C. Peck, David Z. D’Argenio, and John H. Rodman
`
`4, Pharmacodynamics... cccssssessccsesescsssesesssceseeesseesssssesessersssavecaeeeeseasesersaneees 4-1
`Richard L. Lalonde
`
`5. Influence of Protein Binding and Use of
`Unbound (Free) Drug Concentrations... sesiseseesersssereseeseceesteeeesseneenns 5-1
`Janis J, MacKichan
`
`6. Influence of Liver Function on Drug Disposition.......cccesessecseetseeeteersees 6-1
`Kim L.R. Brouwer, George E. Dukes, and J. Robert Powell
`
`7. Genetic Polymorphisms of Drug Metabolism .......escesecrseeereseerereeetseeeseesees 7-1
`Mary V. Relling and William E. Evans
`
`8. Influence of Renal Function and Dialysis on Drug Disposition..................... 8-1
`Gary R. Matzke and Stephen P. Millikin
`
`9. Special Pharmacokinetic Considerations in the Elderly .........scsssereseeen9-1
`Michael Mayersohn
`
`10. Special Pharmacokinetic Considerations in Children... ssessseserereesees 10-1
`Rebecca L. Milsap, Malcolm R. Hill, and Stanley J. Szefler
`11. Special Pharmacokinetic Considerations in the (0)b=) 11-1
`Robert A. Blouin and Mary H.H. Chandler
`
`xix
`
`

`

`12-1
`
`
`
`= =B
`
`aeau
`
`Edes
`
`
`eeoeaebsPo
`
`Dietary Influences on Drug Disposition sestearearnnernnennne
`.eeeeaeeeseasuaeesa sSeite = Seeceieeeene « 2~OR
`
`
`12.
`Mary H.H. Chandler and Robert A. Blouin
`e a
`aRa
`servececeestecnneHET =e GNA OTEO RSET SOS 13-1
`Rg
`Theophylline .......--ssssecrsseseeeneeseseseee cesernetneeecie
`13.
`DavidJ. Edwards, Barbara J. Zarowitz, and Richard L. Slaughter
`gf
`a
`;
`a
`aevnevecenenseneceasenasenaeeretesneateesserasenaesens 14-1
`Aminoglycosides.......sssssssserssrerssreenneneetsetss
`Darwin E. Zaske
`VANCOMYCINssssseresssssssneetnsnensneeeeesennennsnsgeesTeeTEtOETOeee 15-1
`Gary R. Matzke
`Chloramphenicol .....ssssscssccececeeeesessessssssnnnnssesesseecnsnnsssnnnanasenensssesssrenssceeesettee 16~1
`Milap C. Nahata
`Dual Individualization with Antibiotics: Integrated Antibiotic
`ManagementStrategies for Use in Hospitals .........esssssseesrsssesresteessceees 17-1
`Jerome J. Schentag, Charles H. Ballow, Joseph A. Paladino,
`and David E, Nix
`Commentary on DualIndividualization with Antibiotics.......csssserseen 18-1
`Michael N. Dudley
`
`14.
`
`15.
`
`16.
`
`17.
`
`18.
`
`AE
`
`S
`Br!
`
`
`
`19.
`
`20.
`
`ae
`
`22.
`
`23.
`
`24.
`
`25.
`
`206.
`
`27,
`
`28.
`
`29.
`
`TEVONTIGIINGY « vcancuoseteosn eeu 0+ (REDHEE: «<eeersens eenstac anew oeTenee oat reser se sasee= > 19-1
`Gene D. Morse
`
`DUSONAN creceowerescceeesessosscecsonoestorervovsconges opseaursnscwenteneuustenseuesereesecesrscanerses eee sss 20-1
`Richard H. Reuning, Douglas R. Geraets, Mario L. RocciJr.,
`and Peter H. Vlasses
`
`Lidocaine 0... eeeecsscssssssssssesssssscssseecsseneessesecasseecstensecseceeesseeesenscessteneeseeeesnaee®21-1
`John A. Pieper and Kenneth E. Johnson
`
`Procainamide .........cssessssssssssessessssessesssceesessoseseseeccsvevsssvevavsvescarsaseceesvavesercaters22-1
`James D. Coyle and John J. Lima
`
`QUITEGINE oo essescessseecsesseecssesssessecsecessssccsssarssvcaussesusnsaesasavavavassessaseuaaseavacenee23-1
`Clarence T. Ueda
`
`Beta Blockers.......csssssssscssssssesssessssssssesssecssssersasstssrecessssuessavarcasavavesevavevsneueass 24-1
`David J. Kazierad, Karen D, Schlanz, and Michael B. Bottorff
`PHeNytOin .esssesecessssssscsssecscsssssssssseresseecesstssssssssssasstesesstescesuccosscossesccessccersercesee25-1
`Thomas N. Tozer and Michael E. Winter
`Carbamazepine, Valproic Acid, Phenobarbital, and Ethosuximide .........--26-1
`René H. Levy, AlanJ. Wilensky, and Gail D. Anderson
`Corticosteroids... ccccsssssssessasesesecccsesec cc.
`William J. Jusko and Elisobork 4 dag
`Cyclosporine...cscs...
`Gary C. Yee and Daniel R Ci ee
`Methotrexat
`otrexate
`Wi1liam R ean and WilliamEbuonTTPOeeeeectcetteeeeeeeseteeeeeetereseesneent renesseee
`
`27-1
`
`28-1
`
`om
`
`eerie
`
`XX
`
`

`

`
`
`BO. Heparin ......scesssesseseeseessssssecssescessveesssicassasseanssecussecsesssvevessvssecssataeasavaavatenes 30-1
`Robert J. Cipolle and Keith A. Rodvold
`
`BL. Warfarinr....c.cecccssescscsecsessescasseenesestessesesesesesacsesesecsrscscscsvavasecueacarsvaravseetavacsass 31-1
`R. Stephen Porter and William T. Sawyer
`
`32. Salicylates .... ccc seessseecssscsessseescsescsesesesessssesescssvaceecssecsevareaeevscaeens 32-1
`Sydney H. Dromgoole and Daniel E. Furst
`
`33. Cyclic Antidepressants 0.0... cece seessesssesestsseretssssesssesescsesssccssscssscecavsesseavesers 33-1
`C. Lindsay DeVane and C. Rick Jarecke
`
`34, Lithium 20.0 eee eececeeeesceeecceceasessesesecsesassesnesessesseaeseesseeseussussesacseesateasesaes 34-1
`Stanley W. Carson
`
`Tinex woe eecesesecerseracsecseecsessesenevssscveveescsesssarsenseteaesseneegeaeatesseasacsesenesseatensssenseesenteses I-1
`
`Xxi
`
`

`

`
`
`(cid:19)(cid:14)(cid:18)
`
`
`
`Chapter 2
`
`
`
`Guidelines for Collection
`and Analysis of
`Pharmacokinetic Data
`
`William J. Jusko
`
`|Batons in both theoretical and applied pharmacokinetics over the past decades
`have emphasizedthe utilizationofthe principles of physiological pharmaco-
`kinetics and the use of noncompartmental approaches to analysis of drug
`disposition data. Physiological pharmacokinetics involves the deployment of
`pharmacokinetic models and equations based on anatomical constructions and
`functions, such as tissue masses, blood flow, organ metabolism and clearance,
`specific drug input rates and sites, and processes of partitioning, binding, and
`transport. While the complete applications of physiologic systems analysis may
`require extensive models,’ even the simplest ofpharmacokinetic treatments should
`have a physiologic basis for interpretation. Noncompartmental techniques in
`pharmacokinetics can serve in this regard. This term applies to curve analysis
`methodsof data treatment which do not require a specific model and whichyield
`the prime pharmacokinetic parameters, such as systemic clearance (CL) and
`steady-state volumeofdistribution (V,,), which summarize the major elimination
`and distribution properties.
`This chapter is intended to provide an overview of major components of
`experimentally applied pharmacokinetics. A summary is provided of the most
`relevant concepts, models, equations, and caveats which may be useful in the
`design, analysis, and interpretation of pharmacokinetic studies. References are
`provided for more completedetails of the assumptions, derivations, and applica-
`tions of these guidelines and relationships. This material may be helpful as a
`checklist in designing animaland/or human experiments in pharmacokinetics and
`in reviewing drug disposition reports; with greater elaboration,it has served as a
`basis for a graduate course in physiological pharmacokinetics.
`
`

`

`2-2
`
`Chapter 2: Guidelines for Collection and Analysis
`
`CONTEXT OF PHARMACOKINETICS
`okinetic analysis must be made in context of, be consistent with, and
`A pharmac
`Is
`:
`d disposition character-
`explainthe array of basic data regarding the properties an
`istics of the drug.
`.
`.
`.
`The tasks of model and equationselection andinterpretation of data require a
`fundamental appreciation and integration of principles of physiology, pharmacol-
`ogy, biochemistry, physicochemistry, analytical methodology, mathematics, and
`statistics. Pharmacokinetics has derived from these disciplines, and the relevant
`aspects of many of these areas mustbe considered in reaching any conclusions
`regarding a particularset of data. The physicochemical properties of a drug such
`as chemical form (salt, ester, complex), stability, partition coefficient, pKa, and
`molecular weight can affect drug absorption,distribution, and clearance. A drug
`disposition profile mustbe correlated withstudiesof structure-activity, disposition
`in alternative species, perfused organ experiments, tissue or microsomal metabo-
`lism,tissue drug residues,disease-state effects, and pharmacology and toxicology.
`For example, a much larger LD,for oral doses of a drug comparedwithparenteral
`administration may be indicative of either poor gastrointestinal absorption (low
`aqueous solubility?) or a substantial first-pass effect. Drug metabolism pathways
`may differ between species, but the biotransformation rate (V,,, and K,,) of
`microsomes, homogenates, or perfused organs can often be applied directly to
`whole-body disposition rates and often correlate between species.'?
`In general, the pharmacokinetic model and analysis should either conform to,
`or accountfor, the knownproperties and accumulated datarelated to the drug. One
`set of disposition data may misrepresent the characteristics of the drug because of
`any one or combination of reasons. Experienced judgmentis usually required in
`the final interpretation of any experimental findings and analysis.
`
`
`
`ARRAYOF BASIC DATA
`
`Pharmacokinetic studies often serve to answer specific questions about the
`properties of a drug. For example, a limited experimental protocol can easily
`resolve the question of howrenal impairmentaffects the systemic clearanceof an
`antibiotic. In the total design and implementation of pharmacokinetic studies, an
`ideal and complete array of experimental data should include a number of
`considerations:
`A. The dosage form should be pre-analyzed. All calculations stem from
`knowledgeof the exact dosegiven [e.g., CL = dose / AUC (area underthe plasma
`concentration-time curve)}]. Most commercial dosage forms are inexact, and
`content uniformity should be examined.Vials or ampules of injectables typically
`contain some overage and require analysis or aliquoting for administration of a
`precise dose. Solid dosage forms are required to yield an average of the stated
`quantity of drug with limited variability, but both injectable and solid forms may
`be inaccurate for pharmacokinetic purposes. Manninen and Koriionen‘ provide an
`excellent example of both the variability and lack of stated quantity of digoxin in
`many commercial tablets. One product contained a range of 39% to 189% of the
`
`
`
`

`

`
`
`Chapter 2: Guidelines for Collection and Analysis
`
`2-3
`
`stated 0.25 mg dose of digoxin, while the most uniform product, Lanoxin,
`exhibited a range of about 95% to 106% for one batch of drug. To evaluate the
`potential uncertainty of the dose of drug used in disposition studies, it may be
`necessary to collect and analyze replicate doses of the product used. Poorly soluble
`and highly potent drugs are of most concern regarding erratic formulation.
`B. Accuracy in administration of the dose should be confirmed. All doses
`should be timed exactly for starting time and duration of administration. For ease
`in subsequentcalculations, pharmacokinetic equations can be usedto correct data
`from short-term infusion studies to the intercepts expected after bolus injection.
`The particular materials used in drug administration may cause loss of drug. In
`one of the most dramatic examples, MacKichanetal.° found immediate loss of
`about 50% of a dose of intravenous diazepam by adsorption during passage
`throughthe plastic tubing of an infusionset. Inline filtration can also significantly
`reduce the potency of drugs administered intravenously.®
`C. Attention to methods and sites of blood collection is needed. Ideally, blood
`samples should be collected by direct venipuncture in clean glass tubes without
`anticoagulant. Otherwise, the presence of possible artifacts should be tested. In
`the absence of any in vitro artifacts, serum and plasma concentrations are usually
`identical, and these terms are commonly used interchangeably. However, there
`are several reasons why they may notbe identical. For example, the presence of
`heparin can result in increased free fatty acid concentrations, causing altered
`plasma-protein binding.’ Also, the type of blood collection tube or anticoagulant
`maybeafactor. If protein binding is temperature dependent,it may be necessary
`to centrifuge the blood sample at 37 °C to avoid changes in red cell-plasma
`distribution of some compounds.’ These problems primarily pertain to weak
`bases, such as propranolol and imipramine, for which binding to a, acid glyco-
`protein is appreciable and displacement alters plasma-red cell drug distribution.
`Plasma or serum protein binding andred cell partitioning should be measured
`at 37 °C over the expected range of plasma drug concentrations. Both rate and
`degree of binding and uptake are theoretically important. This information may
`be especially needed for interpretation or normalization of nonlinear disposition
`patterns.
`Sometimes thesite of blood collection and the presence of a tourniquet canalter
`the composition of the blood sample: serum proteins, calcium, and magnesium
`concentrationsrise by 5% to 13% during venous stasis.'°
`One of the major assumptions employed in most pharmacokinetic studies is
`that venous blood collected from one site adequately reflects circulating arterial
`blood concentrations. For practical purposes, venous blood samples are usually
`collected. The pharmacokinetic analysis may need to be somewhat qualified,
`because arterial and capillary blood concentrations may differ markedly from
`venous blood concentrations of many drugs.'! The AUCofarterial versus venous
`blood is expected to be identical for a non-clearing organ, and thus the principal
`difference expected is in distribution volumes. Physiologically, organ uptake of
`drugs occurs from thearterial blood, and clearance organ models are based on
`arterial-venous extraction principles.
`
`

`

`i ”
`2-4
`Chapter 2: Guidelines for Collection and Analysis
`
`D. Serum (or blood) concentration datafollowing intravenousinjection (bolus
`or infusion) provides partial characterization of drug disposition properties.
`Accurate assessmentof volumesofdistribution, distribution clearance (CL), and
`systemic clearance (CL) can best be attained with intravenous washoutdata.
`E. Serum (or blood) concentration data following oral doses of the drug in
`solution and common dosageforms provides additionalpharmacokinetic param-
`eters related to absorption andintrinsic clearance. The doses(or resultant serum
`or blood concentrations of drug) should be comparable to those from the intrave-
`nous dose. These data permit assessment of either oral clearance (CL,,.;) or
`bioavailability (F), and of the mean absorption time (MAT).If relevant, other
`routes of administration should be studied. For these, the FDA guidelines for
`bioavailability studies should be consulted."
`F. Three dosage levels (both oral and intravenous) should be administered to
`span the usual therapeutic range of the drug to permit assessment of possible
`dose-dependence(nonlinearity) in absorption,distribution, and elimination.
`G. Urinary excretion rates of drug (as a function of time, dose and route of
`administration) should be measured to accompany the above studies. Urinary
`excretion is often a major route of drug elimination, and analyses permit quanti-
`tation of renal clearance (CL). Collection of other excreta or body fluids (feces,
`bile, milk, saliva) may permit determination of other relevant elimination or
`distributional pathways.
`
`
`
`
`
`
`
`
`
`Dose
`
`Central
`
`U
`L
`Compartment
`
`rl
`
`
`
`Tissue
`
`Compartment
`
`
`
`Uptake_
`———
`Release
`
` [fens Clearance SerumGentamicinConcentration,ug/ml!
`
`
`16
`12
`10
`14
`
`Time, Days
`
`
`ipigure2-1: Plasma concentration-time profile for gentamicin disposition during multiple
`dosing In @patientshowing theprolonged terminalphase caused by strongtissue binding. These
`lata were characterized with a two-compartment model (inset) which included predictio
`if
`drug remaining in the body at the time ofdeath ofthe patients. Datafrom reference 15 -
`
`
`
`

`

`
`
`Chapter 2: Guidelines for Collection and Analysis
`
`2-5
`
`H. Many drug metabolites are either pharmacologically active or otherwise
`of pharmacokinetic interest. Phase I products such as hydroxylated or
`demethylated metabolites are most commonly either active or toxic.'’? Their
`measurement will allow evaluation of AUC and meanresidence time (MRT)and
`perhaps permit quantitation of metabolite formation and disposition clearances.
`I. Multiple-dose and steady-state experiments are necessary iftherapeutic use
`of the drug relies on steady-state concentrations. The duration of multiple-dosing
`in relation to the terminal half-life is crucial for ascertaining applicability to
`steady-state conditions. Comparative single- and multiple-dose studies permit
`further assessmentof linearity and/or allow determination of chronic or time-de-
`pendent drug effects, such as enzyme induction,’* unusual accumulation,’* or
`drug-inducedalterations in disposition. For example, aminoglycoside uptakeinto
`tissues is extremely slow and difficult to assess from single-dose studies. Multi-
`ple-dose washout measures (see Figure 2-1) led to observation of a slow disposi-
`tion phase for gentamicin whichwastheresult oftissue accumulation and release."
`J. Tissue analyses add reality and specificity to drug distribution characteris-
`tics. Comprehensivestudies in animals permit detection of unusualtissueaffinities
`while generating partition coefficients (K,;) for individual tissues (V,). This can
`lead to complete physiologic models for the drug in each species studied.'?
`Autopsy or biopsy studies in man may extend or complement pharmacokinetic
`
`8
`
`200
`
`WK
`
`b QQL
`
`uaa
`
`200
`
`300
`
`MEASURED AMOUNT IN THE BODY, mg
`
`Figure 2-2. Correlation of gentamicin accumulation in the body determined by pharmacokt-
`netic analysis ofserum concentration data (see Figure 2-1) and by direct analysis ofbody tissues
`obtained at autopsy from the same patients who were evaluated pharmacokinetically before
`death. Dotted line indicates correlation. Data from references 15 and 16.
`
`
`
`ou
`
`E 8a u
`
`wLE S
`
`e
`Re
`
`Z3x ai
`
`

`

`
`
`Chapter 2: Guidelinesfor Collection and Analysis
`2-6
`
`found to be extremely helpful (see Figure 2-2)
`tions. This approach was
`er
`ure
`aon
`om
`fgentamicin in man whichwasanticipated
`inconfirmingthe strongtissue binding o
`u
`on the basis of serum concentration profiles (see Figure 2-1).
`K. Suitable drug disposition studies inpatients with various diseases anda
`or given secondary drugsform the basis of clinical pharmacokinetics. Perturt a-
`tions in organ function, blood flow, or response will often alter drug disposition
`in a way that may warrant quantitative characterization. Generalprinciples rr
`not always apply, and each drug needs individualized study. For example, while
`hepatic dysfunction may diminish the rate of oxidation of many drugs, some
`compounds, such as oxazepam andlorazepam,are predominantly metabolized by
`glucuronide conjugation, a processlargely unaffected by liver diseases such as
`cirrhosis.:’ Each disease state may require evaluation of direct effects on pharma-
`cokinetic processes such as changesin renal clearance caused by kidney disease.
`However, indirect changes also require attention, such as the effects on both
`distribution and clearance caused by altered plasmaprotein binding.'* Finally,
`commonly encounteredpatient factors such as smoking habit"? and obesity may
`cause unusual changesin drug disposition and require specific study and notation
`in patient surveys.
`L. Many questions ofdrug disposition can be resolvedfrom selected, carefully
`designed studies, and alternative types of information may be sufficient to validate
`various assumptions and reduce experimental procedures. The investigator’s
`obligation is to adequately assessthe literature, to avoid unwarranted assumptions,
`and to seek experimental strategies that would resolve a proposed hypothesis.
`A comprehensive overview of pharmacokinetic needs in drug developmenthas
`been constructed by Balantetal.”°
`
`DRUG ASSAYS
`
`Certainty of specificity, sensitivity, and accuracy in measurementof drugs and
`their metabolites is a sine qua non in pharmacokinetics and deserves considerable
`attention. Guidelines for quality assurance in laboratory analyses have been
`concisely summarized by the American Chemical Society?! It is now common-
`place to report the linearity, the coefficient of variation of the assay at low and
`high drug concentrations, the minimum levelofdetection, and the procedures used
`to assure specificity and stability, especially in the presence of metabolites,
`secondary drugs, and in specimens from diseased patients. Microbiological assays
`are notoriously unreliable with problems due to other antibiotics and active
`metabolites. An extreme case of metabolite inclusionis in the use of radioisotopic
`tracers; total radioisotope counts generally yield total drug and metaboliteactivity
`and possibly the products ofradiolysis. Separation of parent drug andindividual
`metabolites is required for specificity. Microbiologic, enzymatic, and radioimmu-
`noassays are often of uncertain specificity, and matrix effects may require prepa-
`ration of standards in each patient’s pretreatment plasma. Most drug companies
`provide analytical-grade samplesof their drugs (and sometimes metabolites) to
`qualified investigators upon written request,
`
`
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`Chapter 2: Guidelines for Collection and Analysis
`
`2-7
`
`Sample Handling
`
`Coupled with assay reliability is concern forthe stability of drug in biological
`specimens, even in the frozen state. Ampicillin is unusual in thatit is less stable
`frozen than whenrefrigerated.*” Some drug esters, such as hetacillin (a prodrug of
`ampicillin), continue hydrolyzing in blood and during the bioassay. Penicillamine
`is unstable in the presence of plasma proteins, and immediate deproteination after
`blood collection avoids loss of reduced penicillaminebefore analysis.” Cyclospor-
`ine 1s best assayed in EDTArather than heparinized bloodasthe latter yields red
`cell aggregates that increase assay vartability.°* Measurementofdrug stability in
`blood will reveal whether hydrolysis can occur in blood or whether exposure to
`other body organs is required. Additional concerns in handling samples from a
`pharmacokinetic study include labeling and record-keeping procedures and doc-
`umentation of storage conditions.
`
`Sample Timing
`
`Appropriate pharmacokinetic evaluation requires properly timed specimens.
`The simplest and least ambiguous experimentis the determination of systemic
`plasma clearance during continuous infusionat steady state:
`
`k
`CL = —*
`Cy;
`
`(Eq. 2-1)
`
`wherek, is the infusion rate and C,, is the steady-state plasma or serum concen-
`tration. For this equation to apply, the infusion period must be sufficiently long
`(about five terminal disposition half-lives) to allow steady state to be attained.
`Alternatively, a loading dose or short-term infusion may be administered to more
`rapidly achieve equilibrium.”
`Practical and cost-effective methods are available for designing optimal sam-
`pling strategies for kinetic experiments where the number of specimensis lim-
`ited,such as in the clinic. Optimal designs largely depend on the likely “true”
`model parametervalues, the structure of the model, and the measurementerror. A
`sequential approachhas been advocatedwith pilot studies and a sampling schedule
`whichdistributes time points over the majorphases ofdrug disposition asthefirst
`step. Subsequent experiments can thenresolve a specific hypothesis.
`A commonandsevere problem in applied pharmacokineticsis the inadequate
`or incomplete measurement of drug washout from the system, either because of
`premature termination of sample collection or because of analytical limitations.
`The “true” terminal disposition phase must be examinedin order for most aspects
`of data treatment andinterpretation to be accurate. For example, the early distrib-
`utive phase of aminoglycoside disposition measured by bioassay had long been
`accepted as the only phase, yet more sensitive radioimmunoassays, lengthier
`sample collection, and evaluation of multiple-dose washout revealed the slower
`phase of prolongeddrug release from tissues (see Figure 2-1).
`The two summary physiologic parameters in pharmacokinetics, namely sys-
`temic clearance and steady-state volume of distribution, can be most easily
`
`

`

`2-8
`
`Chapter 2: Guidelines for Collection and Analysis
`
`calculated by use of the area underthe plasma concentration-time curve (AUC)
`and the area under the moment curve (AUMC). Both area values require extrap-
`olation of plasma concentrationsto timeinfinity, and the AUMCis, in particular,
`prone to exaggerated error from an inaccurate terminal slope.”If analytical or
`ethical constraints limit blood sample availability, extendedsaliva or urine collec-
`tion may aid in defining the terminal disposition slope while adding one or two
`other pharmacokinetic parametersto the analysis. Urine maybeparticularly useful
`in this regard (if renal clearanceis linear), as the sample volumeis large andurine
`concentrations often exceed plasma values by one or more orders of magnitude,
`The “midpoint” (C,,) is generally the most desirable time to collect blood
`samples to match an excretion interval in order to assess a time-dependent clearance
`process:
`
`
`Clearance = Excretion Rate _ Amount Excreted
`Cyy
`AUC
`
`(Eq. 2-2)
`
`The arithmetic mean time is acceptable for slow processes, but errors will be
`incurred if the kinetic process produces rapid changesin plasma concentrations.’
`It is commonto miss an early exponential phase of drug disposition because of
`infrequent blood sampling. For a polyexponential curve with intercepts C, and
`slopes A, the total AUCis:
`
`AUC = =i
`
`(Eq. 2-3)
`
`iv
`
`Separate from other compartments for drugs with h
`characterization ofthefirst-pass input.
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`

`Chapter 2: Guidelines for Collection and Analysis
`
`2-9
`
`If the initial distributive phase is missing (area = C, /A,), then the error incurred
`in calculation of a clearance parameter (CL = dose / AUC) is
`
`% of CL error =
`
`100 |=!
`Ay
`
`
`AUC
`
`(Eq. 2-4)
`
`BASIC PHYSIOLOGIC PARAMETERS
`
`The evolution of complete physiologic models' and clearance concepts applied
`to perfused organ systems,”*” with therestrictions incurred bythe limited in vivo
`visibility offered by most blood or plasma drugdisposition profiles, has led to the
`employment of partial physiologic models for description of pharmacokinetic
`data. One such modelis shownin Figure 2-3. Its construction and use should be
`viewed with some conceptual flexibility, and this material will apply to linear
`processesunless stated otherwise.
`
`Volumes
`
`The drug in blood or plasma (C,) is considered to be part of the central
`compartment (V.). The minimum value of V, is plasma volume (V,), but, either
`because drug diffuses rapidly out of plasma or the numberof early time data are
`limited, the V, value often exceedsV..
`Drug whichislocated outside of V, or V.is, of course, present in tissues. The
`apparent volume of the tissue compartment (V;) has two basic determinants:
`physiologic weight or volume of each tissue (V,) and partition or distribution
`factors (K,,). In analysis of plasma concentration-time profiles, tissues must
`commonly be clustered together (including the clearing organs) thus:
`
`This equation leads to definition of one of the primary pharmacokinetic
`parameters with a physiologic basis, volumeof distribution atsteadystate (V.,):
`
`Vo= Ye + Vr
`
`(Eq. 2-6)
`
`Table 2-1. Physiological Determinants of Drug Partition or
`Distribution Ratios between Tissues and Plasma
`
`
`Plasmaprotein binding
`Active transport
`Tissue binding
`Donnonion effect
`Lipid partitioning
`pH differences
`
`
`

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`a
`
`Se
`
`
`
`2-10
`
`Chapter 2: Guidelines for Collection and Analysis
`
`If plasma andtissue binding are the sole determinants of nonhomogeneous
`distribution of drug in the body,then onedefinition of V,, 1S
`
`(Eq. 2-7)
`
`where f,, and f,, are the fractions of drug unbound in plasma and tissue.*? Other
`factors sa also contribute to the apparentpartition coefficient of drugs between
`tissues and plasma(see Table 2-1). Since, by definition, V, and LV, comprisetotal
`body weight (TBW),
`
`TBW = V,+ 2 Vi
`
`(Eq. 2-8)
`
`then the quotient of
`
`
`Vis
`Kb = TBW
`
`(Eq.2-9)
`
`defines the distribution coefficient (Kp), a physicochemical and physiological
`measure of the average tissue:plasma ratio of the drug throughout the body.
`Approximate values of K, and the primary rationalization of the size of Kp are
`
`Table 2-2. Distribution Coefficients (Kp) for Various Drugs and
`
`Inulin
`
`Ampicillin
`
`Probable Physiologic (Physicochemical) Cause
`
`
`V5
`Explanation/indication
`Drug
`Kp = TBW
`
`Indocyanine Green
`0.06
`Strong binding to plasma proteins and
`limited extravascular permeability.
`Distribution limited to plasma and
`interstitial fluid owingto large molecular
`weight (5500) andlipid insolubility.
`Limited intracellular distribution owingto
`poorlipid solubility (common to
`penicillins).
`Moderate plasma bindingand distribution
`primarily into total body water.
`Slight plasmabinding andfairly uniform
`distribution into total body water.
`Strong tissue binding (commonto
`aminoglycosides).
`Strong tissue binding to calcium in bone.
`Appreciablelipid partitioning.
`Strong binding to Na/K transport ATPasein
`cell membranes.