`
`AAC
`Journals ABMnrg
`
`MINIHEUIEW
`
`Pharmacokinetic and Pharmacodynamic Considerations in
`Antimalarial Dose Optimization
`
`Nicholas J. White
`
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`Antimalarial drugs have usually been first deployed in areas ofmalaria endemicity at doses which were too low, particularly for
`high—risk groups such as young children and pregnant women. This may accelerate the emergence and spread of resistance,
`thereby shortening the useful life of the drug, but it is an inevitable consequence o fthe current imprecise method ofdose fi nd—
`ing. An alternatiVe approach to dose finding is suggested in which phase 2 studies concentrate initially on pharmacoldnetic—
`pharmacodynamic (PKHPD) characterization and in viva calibration of in vitro susceptibility information. PD assessment. is
`facilitated in malaria because serial parasite densities are readily assessed by microscopy, and at lowr densities by quantitative
`PCR, so that initial therapeutic responses can be quantitated accurately. If the in viva MIC could be characterized early in phase
`2 studies, it would provide a sound basis for the choice of dose in all target populations in subsequent combination treatments.
`Population PK assessments in phase 213 and phase 3 studies which characterize PK differences between different age groups, clin-
`ical disease states, and human populations can then be combined with the PK—PD observations to provide a sound evidence base
`for dose recommendations in different target groups.
`
`he primary objective of treating severe malaria is to save life.
`Other considerations such as preventing recrudescence or mi—
`nor toxicity are secondary. 1n uncomplicated malaria. the main
`objective of antimalarial drug treatment is cure of the infection.
`Speed of response is also important. as this reflects the rate at
`which the disease is controlled and the corresponding reduction
`in the risk of progression to severe malaria. Lesa—Serious adVerSe
`effects therefore become a more important factor in determining
`dose. The therapeutic response in malaria is determined by the
`concentration profile (pharmacokinetics Phil) of active antima—
`larial drug or drugs in the blood (as the asexual parasites which
`cause malaria pathology are confined to the blood). tlieirintrinsic
`pharmacodynamic {PD} properties, the susceptibility of the in—
`fecting parasites to the drugls), the number of asexual malaria
`parasites in the blood, and the activity of host-defense mecha-
`nisms. Ideally, antimalarial treatment should be 100% effective in
`everyone, but this may not be possible without producing toxicity
`or recommending a long course of treatment with consequent
`poor adherence.
`[t is now recommended that all antimalarial
`treatments for uncomplicated malaria should aim at :1 >95% cure
`rate for the blood-stage infection it). In recent years. a general
`agreement has been reached on methods ofclinical and parasito—
`logical assessment to measure the cure rates in casas of uncompli~
`cared falciparum malaria (L—3). ln Plasmodinm virus: and P. [ii-vile
`infections, persistent
`liver—stage parasites (hypnozoitesj cause
`later relapses, despite cure of the blood~stage infection, which
`complicates therapeutic assessment. These infections require ad—
`ditional treatment with Seaminoquinolines [radical cure). Re-
`lapses are often genetically heterologous and cannot be distin—
`guished reliably from recrudescen ces or new infections. This
`necessitates a different approach for assessment of treatment Elfi—
`cacy in the relapsing malariasfiwhicli is yet to be agreed upon.
`Many ofthe antimalarial drugs in current use were introduced
`at suboptimal doses. For various reasons, quinine, sulfadoxine—
`pyrimethamine, primaquine {for radical cure oftropical frequent
`relapsing P. vivax infections). mefloquine. halofantrine. artemis-
`
`inin derivatives, artemether-lumefantrine, and dihydroartemis—
`inin—piperaquine (i.e., 7 of the 12 current antimalarials) were all
`deployed initially at doses which were too low in some or all age
`groups. Pyrimethamine and sulfadoxine (310535 for children were
`extrapolated from experience in Caucasian and Asian adults.
`Their pharmacohineticproperties Were not studied in younger age
`groups before widespread deployment in Africa, where children
`are the main target group [4). The dose was too low in young
`children. The primaquine dose regimen (15 mg baseiday adult
`dose} was developed largely on the basis of studies of the long—
`latency Korean vivax malaria, but this close was then recom—
`mended widely in areas with the more resistant tropical relapse P.
`vivax phenotypes (5). 111 Southeast Asia and Oceania, this dose is
`too low. Five—day primaquine regimens were deployed very widely
`for radical cure ofvivax malaria for ovsr 30 years—yet these reg—
`imens were largely ineffective. Fourteen—day courses are now rec—
`ommended. Mefloquine was first introduced at a single dose of] 5
`mg baselkg of body weight (-6, 7}. which may have hastened the
`emergence ofresistance (-8}. The total dose now recommended is
`25 mgikg divided over 2 or 3 days. The closes ofartemisinin deriv—
`atives used initially as monotherapy, and then subsequently in
`combination treatments {arteniether at 1.6 mgikgidose in arte—
`mether—lumefantrine and dihydroartemisinin at 2.5 mgikgidose
`togetherwith piperaquine}, may not provide maximal effectsin all
`patients. The initial treatment regimen of artemether—lumefan—
`trine deployed was a four—dose regimen which provided insuffi—
`cient lumefantrine and gave high failure rates (six doses are now
`recommended} {'9'}. The dose of dihydroartemisinin in the first
`
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`Antimicrobial Agents and Chemotherapy
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`p. 51392—580?
`
`December 2013 l.l'olume- 5? Number i2
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`Minireview
`
`formulations of the diltydroartemisinin—piperaquine combina—
`tion was <2 mgr‘kg (it is now 2.5 mg/kg, which may still be too
`low) {10-}. The pharniacokinetic properties ofpiperaquine are dif-
`ferent in children from in adults, and there is evidence that current
`
`dosing schedules in children may be suboptimal (l 1). After 3 cen-
`turies of reasonable dosing based originally upon the Schedule
`Romana. treatment recommendations for quinine in severe ma—
`laria were suddenly reduced in the l970s to a doSe as low as 5
`mgr‘kgiltl h. which is eight times lower than that now recom—
`mended. In contrast. the quinine loading dose in severe malaria
`was not introduced until the early 19805. and it is still not recom—
`mended universally (.12). The initial recommendation for artesu—
`nate treatment in severe cases ofrnalaria was a daily maintenance
`dose ofhalfthe initial dose (1.2 mgikg}. As oral bioavailability is
`approximately 60%. this corresponds to an oral dose of 2 mglkg
`{1-3, 14}. The currently recommended parenteral close is twice this
`and is the same as the recommended initial dose, 2.4 mgl'kgr‘day
`(1'. 15. 1th. Recent evidence suggests that this dose should be in—
`creased in young children (1'7).
`Optimizing drug dosing requires characterization ofthe phar—
`niacokinetic and pharmacodynamic properties of the drug in the
`target populations. There are four main determinants of the ther—
`apeutic response: antimalarial pharniacokinetics {affected byvari-
`ables such as coadministration with food. age, pregnancy, disease
`severity, vital organ dysfunction, partner drug, and coinfectionsi
`other drugs), parasite susceptibility {incorporating effects on dif—
`ferent stages of asexual parasite development. dormancy, propen-
`sity for resistance to develop, and level of resistan ce first selected],
`host defense (influenced by age, pregnancy, and transmission in-
`tensityiexposure history}. and parasite burden. In addition. mixed
`infections can be a factor. For antimalarials. ex vivo systems are
`useful for predicting resistance (18] and they provide valuable
`pharmacodynamic information (19}, but they are simply not
`good enough yet to replace in vivo evaluations for dose finding. In
`uncomplicated falciparum malaria,
`it
`is generally agreed that
`combinations, preferably,
`fixed—dose
`combinations
`{FDCL
`should be used. The same should apply to vivax malaria. although
`chloroquine and primaquine can be considered a combination.
`When drugs are first developed, there is a limited window of op—
`portunity to define the dose—response (or concentrationveffect)
`relationship for the single new compound. but this opportunity
`must be taken {20). Once the drug is available only as an FDC, the
`dose ratio is. by definition. fixed and it is too late for optimization
`ofthe individual component doses. Characterizing the individual
`drug dose— response relationships is essential for rational dose op—
`timization, and so a good drug development approach involves
`documenting the blood concentrations that are associated with
`submaitimal antimalarial effects. Studies in animal models. partic—
`ularly with P. jitlciparnm, may be informative. but studies in hu-
`mans will also be needed. It is important to accept that this may
`result in temporary therapeutic failures in some vol unteers. There
`is a natural reluctance to accept this. but sensitive detection meth—
`ods to measure low parasite densities now provide us with safe
`methods that should avoid any risk or discomfort to the patient
`(2]). Suggestions are provided here for an alternative PK-PD ap-
`proach for dose finding which, ifvalidated, may improve and ac—
`celerate dose finding and so avoid systematic underprescribing
`and thus underdosing. It might also prove more rapid and less
`expensive. The primary objective is determination of the in viva
`MIC as the basis for rational dosing (the MIC is the concentration
`
`Total
`
`Detection limit
`
`parasites
`
`
`
`tsenfianArea“9|,MenueruorfiJo'queoeeyzduuwonpepemumog
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`Weeks
`
`FIG I Population PK PD responses lollowinga 3 -day Ircatmcnl will: a hypothet-
`ic al slowly eliminated :ml'irn alarial drug. The total numbers ofmalaria parasites in
`the body over time are depicted in blue in a range of patients presenting with
`parasile densities between approximately 50 and ZULDUOHLL The ranges ofdrilg
`concenlration profiles are shown in red. with the corresponding ranges of parasi-
`tological responses in blue. Parasilclnia levels cannot be counted reliably by Ini
`croscopy below 50nd [Corresponding to ' 100,000,000 parasites in the body ol'an
`adnll}. The M'PC is the lowest lslrmd,plasma1 or free plasma col Icel'ltrnlion which
`produces Ille Inzuilnulrl parasllicitlal eilecl {Len Ihe maximum parasite reduction
`n1hr)l.‘lhis corresponds lo the concentration associated wilh first slowing of the
`first order (log linear) decline in panniternia.
`
`at which the parasite multiplication factor per asexual cycle is 1}. It
`is necessary first to consider the factors which affect the pharma—
`cokinetic properties of antim alarials in malaria and then to con—
`sider antimalarial pharmacodynamics and how PK—PD relation—
`ships should be assessed.
`
`PHARMACOKINETICS
`
`The pharrnacokinetic (PK) properties of antimalarial drugs are
`often altered in patients with malaria compared with healthy sub—
`jects. The PK properties therefore change as the patient recovers.
`PK properties are also often significantly different in important
`patient subgroups such as young children and pregnant women
`{22). Several of the antimalarial drugs, notably those which are
`hydrophobic and lipophih'r.1 are poorly absorbed after oral or in—
`tram uscular administration and Show wide interindividual differ—
`
`ences in concentration profiles. In general. this variation in blood
`concentrations is inversely proportional to bioavailabiljty, which
`emphasizes the importance of improving bioavailability in drug
`development. Increasing bioavailability provides the twin benefits
`of reducing the required dose and thus the cost ofthe drugs and
`reducing the individual probabilities of underdosing or overdos—
`ing. In considering antimalarial dosing in the past, we tended to
`concentrate on mean or median values ofPK variables, but itis the
`
`patients with the lowest blood concentrations who are most likely
`to fail treatment and facilitate the emergence of resistance and
`those with the highest concentrations who are most likely to ex—
`perience drug toxicity (23). These extremes need to be defined.
`which means that characterizing the distributions of PK variables
`in important target groups is as important as assessing their mea—
`sures of central tendency (Fig. 1). Characterizing these distribu—
`tions well eventually requires sampling of relatively large numbers
`ofpatients, which in turn usually necessitates sparse sampling and
`population PK modeling. Optimal design approaches can be used
`to ensure that the information is gathered most efficiently (24). It
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`December 2013 Volume 5? Number ‘IZ
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`Minireview
`
`is essential that key patient groups such as young children and
`pregnant women are studied specifically, and there should be a
`postregistration commitment to this if such investigations have
`not been conducted during preregistration studies. There may
`also be clinically relevant pharmacogenetic differences in drug
`metabolism between different ethnicgroups. Thus. characterizing
`the distributions ofpharmacokinetic variables is a gradual process
`accrued during phase 2 and phase 3 of drug development, but it
`must continue into phase 4 to cover all relevant populations.
`Malaria is often worst in remote rural areas. The recent devel—
`
`opment ofsimple methodologies such as drug measurement from
`capillary blood filter paper samples (25. 26) will facilitate commu-
`nity—based assessments in remote settings and make sampling of
`infants and children feasible. Thus, population PK information
`will eventually be needed in all important target groups (i.e.. in—
`fants, children, pregnant women. lactating women, malnourished
`patients, patients receiving antituberculosis lanti—TB] and antiret—
`roviral drugs. etc.) {22) to provide optimal dose recommenda—
`tions. There is currently limited bioanalytical capacity to support
`such studies. but there are international schemes to assist antima—
`
`larial drug measurement and ensure the accuracy of the results,
`which should facilitate future laboratory bioanalytical capacity
`development in tropical countries (27. 28}.
`In drug development. where a new compound has not been
`used previously, there is little information on distributions of PK
`variables and so the important but difficult issue is to determine
`how much PK—PD information is enough to decide upon a dosage
`recomm end ation. For safety reasons, the PK information is usu-
`ally gathered in the following standard sequence: experimental
`animals, healthy normal volunteers, adult patients with uncom—
`plicated malaria. children, and, much later, infants and pregnant
`women.
`
`PHARMACODYNAMICS
`
`(i) Action ofthedrugs. The antimalarial drugs differ in their stage
`specificities of action against malaria parasites. The S—amino—
`quinolines are unusual in killing pre—erythrocy‘tic—stage parasites,
`hypnozoites. and mature ganietocytes ofP.fi.ilciparnm but having
`weak activity against its asexual stages (2-9). They are more active
`against asexual stages of P. vivax and P. lcuuwlesi. All other antiv
`malarial drugs in current use kill the asexual and sexual stages of
`sensitive P. Vii-'(LY, P. mnlnrinc, P. ovals, and P. knowlesi and the
`asexual stages and early gametocytes (stages 1 to [1]) ofsensitive P.
`julcrparnm. but they do not kill the mature P. jitltiipumm gameto-
`cytes (stage V). The artemisinins have a broader range ofeffect on
`developing P. jiilciparmn sexual stages, as they also kill stage IV
`and younger stage Vgametocytes. Atovaquone and the antifols kill
`preerythrocytic stages and have sporontocidal activity in the mos—
`quito {interfering with oocyst formation and therefore blocking
`transmission). Apart from the 8—aminoquinolines, none of these
`drugs have significant effects on P. vivax or P. made hypnozoites.
`Even within the asexual cycle there are differences in antimalarial
`activity in relation to parasite development. None ofthe currently
`used drugs have significant effects on very young ring stages or
`mature schizonts. and all have their greatest effects on mature
`trophozoites in the middle of the asexual cycle (36). In addition,
`the artemisinins (and other antimalarial peroxides) have substan-
`tial ring—stage activity which underlies their life—saving benefit in
`treatment ofsevere falciparum malaria (15, 16. 31}. Several anti-
`malarials. notably. some antibiotics with antimalarial activity.
`
`have greater effects in the second than in the first drug—exposed
`asexual cycle (23]. The pharmacokinetic—pharmacodynamic rela—
`tionships (PK—PD) have not been very well characterized for any
`ofthese activities.
`
`(ii) In vivo pharmacodynamic measures. In severe malaria.
`the primary therapeutic concern is the speed of parasite killing
`and, in particular. the killing of circulating ring—stage parasites
`before they mature and sequester (39, 3] ). Rapid killing ofyoung
`P. jiilcipurrrrn parasites by artemisinin and its derivatives explains
`much of the superiority of artesunate over quinine in the treat~
`ment ofsevere falciparum malaria [15, 16). in uncomplicated ma—
`laria, rapid ring—form killing is also important, as it contributes to
`the speed ofpatient recovery, but the main therapeutic objective is
`to reduce para site multiplication. Once iurtinlala rial treatment is
`started. then. after a variable lag phase, parasite killing in vivo
`approximates to a first-order process {32-34) as represented by
`the following equation:
`
`P, — Pue' tr
`
`(1 i
`
`where P[ is the parasitemia level at any time I after starting treat—
`ment, PD is the parasitemia level immediately before starting treat—
`ment. and kg, is the first—order parasite elimination rate constant.
`The parasite clearance half- life is therefore 0.693hl‘r. In equation I.
`parasite killing equates with parasite removalfrom the circulation.
`but in falciparum malaria (but not the other malarias} there is an
`additional major factor removing parasites from the circulation.
`and that is cytoadherence. Only parasites in the first third of the
`asexual cycle circulate, and the more mature parasites are seques—
`tered. This complicates interpretation of the parasite clearance
`curve following treatment with drugs which do not kill ring-form
`parasites, as initial declines in parasitemia result mainly from se—
`questration and not drug effects (33). Parasite killing can be ex—
`pressed as the parasite reduction ratio (PER). which is the frac—
`tional reduction in parasite numbers per asexual cycle. or the
`reciprocal of ring-form k1,. per cycle {32). This cancels out the
`effects ofcytoadherence, as the parasite populations are assessed at
`the same stages ofdeveloprnent separated by one cycle. The shape
`ofthe concentration~effect relationship in viva is assumed always
`to be sigmoid. as it is in vitro (Fig. 2), per the following equation:
`
`k _ kin-1.x" [Cn/ECSHH l {:nl
`
`where k is the parasite killing rate and ileum is the maximum par—
`asite killing rate (i.e., the maximum effect, orljnm) for that drug in
`that infection, C is the concentration of drug in blood or plasma.
`ECE,J is the blood or plasma concentration resulting in 50% of the
`maximum effect, and n is a parameter defining the steepness ofthe
`dose—response relationship. For most drugs, maximum effects are
`probably achieved initially. The evidence for this is the lack of a
`relationship between peak concentrations and parasite clearance
`(the exception is quinine treatment of severe malaria without a
`loading dose, which provides submaximal effects in some pa~
`tients} (1'2). So while concentrations exceed the minimum para—
`siticidal concentration {MPC).lc in equation I is equal to kum. It
`should be noted that each end ofthe sigmoid curve approaches 0%
`and 100% effects asymptotically—“so the MPC is an approxima—
`tion, whereas the ECSU is a more robust and precise estimate. Once
`antimalarial concentrations in blood decline to a level below the
`
`MPC. the parasite killing rate declines (see the “Antimalarial phar—
`macokinetic—pharmacodynamic relationships" section below).
`For drugs in current use, maximum PRRs range from approxi—
`
`5794 aacasrnorg
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`Antimicrobial Agents and Chemotherapy
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`
`Log concentration
`
`FIG 2 The concentration cll'ect relationship; for antimalarial drugs, the effect
`is parasite killing, which can be measured in different ways. The Ema); is the
`maximum parasite killing Iliat a drug can produce, which translates in viva into
`the maximum parasite reduction ratio. The litjffl, is the blood or plasma con
`cent ration providing 50% of matimum killing. The median and range values
`for a hypothetical population ofmalaria parasites are shown in blue. and lllc
`dislribulinu of average drug levels in palieuts is shown as :1 Yeti bell-shaped
`curve ( i.e., concentrations are log normally distributed}. Clearly, some of the
`patients have average drug levels below the MPG and would not have maxi
`mum responscswilh this dose regimen.
`
`mately 10—fold to approximately 10,000—fold reductions in para—
`sitemia per asexual cycle. The mean values and their variance in
`vivo have not been established for several important antimalarial
`drugs in current use (notably lumefantrine and piperaquine}. and
`for others, where monotherapies have been evaluated. the esti—
`mates are often imprecise. There is no evidence for saturation of
`parasite clearance, but, obviously, the higher the initial bio—
`mass, the longer it takes to eliminate all the parasites from the
`body (3-3). Consequently, patients with high—biomass infec—
`tions need more antimalarial drug exposure than those with
`low—biomass infections.
`
`(ii) In vitro susceptibility. For antimalarial effects, the shape
`and position ofthe concentration-effect curve studied ex viva de-
`pends on the susceptibility of the infecting parasites and the PD
`readout (typically, for blood stages. inhibition ofgrowth or mat—
`uration, inhibition of hypoxanthine uptake. inhibition ofprotein
`or nucleic acid synthesis, etc). Furthermore, each in vitro method
`assesses a slightly different section ofthe asexual life cycle, which
`may result in important differences between methods in the re-
`suits for drugs with ring—stage activity. It is not clear exactly how
`the effects ofthese static drug concentrations in a small volume of
`dilute blood in the laboratory correspond with in vivo effects (1-8,
`19, 35). Neither is the relationship between inhibition of parasite
`growth and subsequent inhibition of multiplication well estab~
`Iished. inhibition of growth is measured in most in Vin-o tests,
`whereas in in viva patient studies, inhibition of multiplication
`( parasite clearance} is recorded. In the absence of in viva i nforma—
`tion on the concentration—effect relationship. for predictive mod~
`cling purposes the slopes ofthe linear segments of the in vitro and
`in vim sigmoid concentration-effect relationships have been as—
`sumed to be similar (8, 35), but whether or not such an assump—
`tion is justified remains to be established. Most agree that the
`antimalarial drug concentration that is biologically relevant in as—
`sessilig blood~stage effects is the (unbound) fraction in plasma.
`Total red cell concentrations are less informative as the parasitized
`
`Minirev'iew
`
`red cells behave very differently from their unparasitized counter-
`parts. In the patient, the blood concentrations of the antimalarial
`drug are changing constantly, and the parasite age distributions
`may differ considerably between patients. Ex vivo systems with
`changing antimalarial concentrations that are more biologically
`relevant than the simple static drug susceptibility assays have
`therefore been developed, and measurement ofmultiplication in—
`hibition can yield valuable information (19}. Rodent models
`capable of sustaining human malaria infections have also been
`developed recently {56). Human malaria infections in irnmuno—
`deficient mice allow PK—PD characterization and thus provide
`useful information in predicting therape otic responses in patients.
`These laboratory studies have the great advantage that parasites
`from many different locations or with known resistance profiles
`can be studied and compared. it is argued below that if the rela—
`tionship between the standard in Vin-n susceptibility measures
`{50% inhibitory concentrations [[CSOI,
`lCaU, etc.) and in vivo
`PK—PD responses in patients with malaria could be characterized.
`then this would facilitate dose finding.
`
`ANTIMALARIAL PHARMACOKINETIC-PHARMACODYNAMIC
`RELATIONSHIPS
`
`Some of the best research on antimalarial PK-PD relationships
`came from the period ofintense antimalarial drug investigation in
`the United States during and shortly after the Second World War
`(Fig. 3]. Studies were conducted to determine the optimum dos—
`ing strategies for mepacrine (atebrine, quinacrine), the Cinchona
`alkaloids, and both the 4— and S—arninoquinolines (37—39). Phar—
`macokinetic analysis had yet to he invented, and methods for
`quantitation ot‘clrugs in serum or plasma were in their infancy. but
`the spectrophotometric assays that were conducted still provided
`valuable information. Relatively large numbers of nonimmune
`adult male volunteers artificially infected with single "strains" of
`P. faldpnrum or P. vivnx received different dose regimens. serum
`levels were measured, and therapeutic responses were assessed.
`This research provided dose~response or concentration-effect re~
`lationships and led to the mepacrine loading—dose regimen. char—
`acterization of the comparative antimalarial effects of the four
`main Cinchona alkaloids (quinine, quinidine, cinchonine, and.
`cinchonidine), and development of the standard dosing regimen
`for chloroquine [one ofthe few antimalarial dose recommenda—
`tions which has stood the test of time). This was still the era of
`malaria therapy, and the war had focused military attention on
`malaria. Such volunteer studies are no longer possible today. Since
`that time, PK— PD relationships have been inferred mainly from
`clinical studies ofantimalarial treatment (8, 9, 4043).
`(i) PIC-PD correlates. Studies ofPK—PD relationships for anti—
`bacterial effects have shown that for some antibiotics (those with
`steep concentration-effect relationships and without postantibi~
`otic effects), bacterial killing is dependent on the duration for
`which the antibiotic exceeds the MIC for the bacterial population
`(“time above MIC”). For other antibiotics (where concentrations
`achieved with current regimens remain on the steep part ot'the
`concentration—effect relationship), it is the maximum concentra—
`tion achieved (Cum) or the related area under the plasma concen—
`tration—‘cime curve (AUC) that is the best correlate of bacterial
`killing (Fig. 4). These PK variables are all interrelated lie, the
`higher the Cm“, the larger the AU C and the longer the time above
`the MIC]. With some adjustments, these PK measures can be ap—
`plied to antimalarial effects {32). although correlates with parasite
`
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`December 2013 Volume 5? Number ‘IZ
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`aacasmmg 5795
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`InnoPharma Exhibit 1030.0004
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`Minireview
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`Plasma
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`Daily dose {Grams}
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`Effect
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`FIG 3 Dose—response relationships oblainecl between [he years [915 and 1946 for quinine in blood-induced Vivax malaria {McCoy strain} in volunteers (33).
`Plasma concentrations after protein precipiialion were measured spectropliotomelricadly, which overestimales parenl compound concentralions. The left box
`shows the variable relationships between dose and mean plasma concentral'ituis. and the right graph shows the concentration effect relatioi'lsliip divided into
`three effect measures: class 1. no certain effect; class i], temporary suppression of parasitemia audior fever: class III, “permanent” effect. i.e.. absence ol
`parasitenlia for M days.
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`killing have not been established for most antimalarial drugs.
`Whereas rnost bacteria replicate every 20 to 40 min, asexual ma~
`lari'a parasites infecting humans replicate every I to 3 days. Symp—
`tomatic infections usually comprise one predominant brood of
`malaria parasites, but multiple genotypes are often present—par-
`ticularly in higher—transmission settings—and so within one host
`there may be subpopulations with different drug susceptibilities
`(and also different stages of asexual development). The lowest
`blood. plasma, or free plasma concentration which produces the
`maximum PRRis the MPC (Fig. 5}. These PK—PD variables reflect
`the antiparasitic effects of the antimalarial drug and host immu-
`nity and so are specific for an individual and that individual’s
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`AUGMIC
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`Concentration
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`Weeks
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`FIG 4 Plasma or blood concentration profile of a slowly eliminated aritirna
`larial drug showing an arbitrary MIC. The AUC is the area under theciirVe,ancl
`Ctnax is Ihe maximum concenlmtion in blood or plasma. AUC from 7 days to
`infinity is shown in darker pink. Blood concentrations are increasingly mea-
`sured on day Y in therapeutic assessments of slowly eliminated antimalarials
`{49).
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`infection. Innate host—defense mechanisms and acquired imm Line
`responses contribute significantly to therapeutic responses—ef—
`fectively shifting dose—response curves to the left. The contribu—
`tion of the host immune response, which may be significant even
`in previously no uimm une patients 44). has not been well char—
`acterized.
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`With current dosing for all antimalarial drugs except the cute
`misinin derivatives. drug elimination is sufficiently slow that the
`antimalarial eliects ofa treatment persist for longer than one asex—
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`Totalparasites é.
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`Detection Ilrnll'.
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`Weeks
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`FIG 5 Different therapeutic re'stISes to a slowly eliminated anlill‘lalari‘al
`ilmg in a malaria infection of mm parasites {parasite density,
`-2,000l|.|.l). The
`blood concentration profile in gray is shown in the background. Parasitologi
`cal responses range from fully sensitive {green} to highly resistant. {blue}. Each
`response is associated wil h :1 dil-l‘erenl level of snscep Iibili ty and lhns a dillerenl
`MIC and MPC [arrows polnlillg to collceniralion profile}. The inset repre-
`sents the concentration effect relationship {or Lllc lowest level of resistance
`(resulting in 11 Ian: failure} showing correapoi‘lding points for the MIC and
`MPC (orange curve}.
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`5796 aacasmorg
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`Antimicrobial Agents and Chemotherapy
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`lnnoPharma Exhibit 1030.0005
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`Minireview
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`ual cycle (8, 22. 32. 45). Indeed, many antimalarials have terminal
`elimination half—lives (t, mid) of several days or weeks. in order to
`cure the blood-stage infection in a nonimmune patient, antima—
`larial concentrations in blood {free plasma concentrations] must
`exceed the MIC for the infecting parasites until the last parasite is
`killed (8, 22, 23. .32. 45). The higher the initial parasite burden. the
`longer this takes. With host immunity. cure of malaria may be
`achieved even it'drug levels fall below the MIC before complete
`elimination ofall parasites {44). Thus, the time above the MIC is
`an important PK determinant oftherapeutic outcome, although
`the AUC above the MIC is also relevant. as the rate of parasite
`killing is determined by the concentratiomeffect relationship
`above the MIC for the infecting parasites and by the antimalarial
`concentration profile in the treated patient.
`Assuming that the parasites are exposed to the antimalarial
`drug at a sensitive stage, what duration of exposure in a single
`asexual life cycle is necessary for maximum effect? For sensitive
`parasites. it appears that up to 4 h of exposure is required [31),
`although for some drugs less time is needed. For artemisinin de