antimalarial w. peters h. 5qaantimalarial drugsmayaffect oneormorephases ofthis cycle. thecompounds...

Postgraduate Medical Journal (August 1973) 49, 573-583.

Antimalarial drugs and their actions

W. PETERSM.D., B.S., D.T.M. & H.

Department of Parasitology, Liverpool School of Tropical Medicine, Liverpool L3 5QA

SummaryNew antimalarial drugs are required, partly becauseof the emergence of drug resistant strains of malariaparasites and partly because better compounds areneeded to cure relapsing tertian malaria. In reviewingthe diverse modes of action of currently used anti-malarials, against a background of the patho-genesis of malaria, attention is drawn to deficienciesin our knowledge. Even less do we understand how themalaria parasite becomes resistant to certain drugs,in particular chloroquine. New approaches to theproblem include the application of combinations ofexisting antimalarials, and the search for new drugs onan unprecedentedly vast scale. Out of over a quartermillion compounds that have recently been screened, ahandful are now in clinical trial and are showing greatpromise for the treatment of multiple resistantfalciparum malaria. The paper concludes by sum-marizing current recommendations for the prophylaxisand therapy of malaria due to drug resistant parasites.

AT the turn of the century malaria was still endemicin swampy areas of England such as the Thamesestuary. With increasing urbanization, land utiliza-tion and swamp clearance, breeding of the anophe-line mosquito vectors came under control andautocthonous malaria rapidly faded away. Today,however, we are witnessing a resurgence of malaria,not acquired within these shores, but imported bytravellers returning from the many parts of thetropical world where the transmission of this diseasecontinues on a vast scale. In spite of some 20 yearsof international campaigns to control or eradicatethis threat, malaria still occurs in territory occupiedby some 1000 million people, nearly one third of theworld's population.The scale of international travel to and from the

world's more wealthy countries is increasing on anunprecedented scale so that malaria, like other'exotic' diseases, preventable as it may be, neverthe-less is on the increase. In the U.K. an average of120 cases of malaria a year have been reportedbetween 1960 and 1970, with a mortality of 4-7%(Bruce-Chwatt, 1971). Most fatalities were due to the

malignant tertian malaria parasite, Plasmodiumfalciparum. The mortality from falciparum malariaalone was 16%. This figure is appalling, especiallyso since (a) malaria is preventable and (b) treatable.Most fatalities occur because the diagnosis is missed,and the diagnosis is missed in most instances simplybecause the physician fails to ask the patient or hisrelatives the one question, 'Where have you been,and when?' This lesson is brought home by com-paring falciparum mortality rates between civilianand military cases in the United States. Between1966 and 1969, 10% of civilians with falciparummalaria died there (Neva et al., 1970). Amongmilitary personnel the fatality rate was only 0-3%(Canfield, 1972).A massive drive to eradicate malaria was spon-

sored by the World Health Organization on thebasis of guidelines laid down in 1957 (W.H.O.,1957). It was based primarily on the deployment ofDDT and other chlorinated hydrocarbon insecticidesto break the cycle of malaria transmission by killingvector anophelines as they rested in houses. Anti-malarial drugs were given as a supplementarymeasure, partly for the relief of acute malaria andpartly to speed the elimination of the parasite poolin the human population. Both prongs of this two-forked attack have been badly blunted by thedevelopment of drug resistance. By the end of 1968some fifteen vector anopheline species had becomeresistant to DDT, thirty-six to dieldrin and thirteenof the fifteen to both. Already one species (Anophelesalbimanus in Central America) is resistant to thenew generation of insecticides, the organophosphates.On the parasite side the situation is no better. It hastaken very little time fromthelarge-scaleintroductionof a new drug to the appearance of parasites with theability to survive it (Table 1). This has been aparticularly severe problem in the case of P. falci-parum.New antimalarial drugs are needed, not only

because of the resistance problem, but also becauseexisting drugs do not include any that are able toproduce a radical cure of relapsing tertian malariadue to Plasmodium vivax unless given in subtoxicdoses for at least a week.

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574 W. Peters

TABLE 1. Appearance of drug resistance

Year resistance firstYear recognized in man,

Drug introduced and where

'Antifols'Proguanil 1948 1949 (experimental)

1950 Java (in field use)Pyrimethamine 1952 1952 GambiaCycloguanilembonate 1963 1964 S. Rhodesia

Sulphadoxine 1964 1968 CambodiaMepacrine 1935 1946 New GuineaChloroquine 1945 1961 Colombia (but probably

seen and notrecognized inPanama, 1956)

Pathogenesis of malaria and the place of antimalarialdrugs in prevention and treatmentThe malaria parasite undergoes two phases of

asexual reproduction in man, the first in paren-chymal cells of the liver and the second in red bloodcells. From parasites of the latter phase are derivedgametocytes which undergo further developmentculminating in sexual reproduction only after theyare ingested by a suitable mosquito. The mosquitophase terminates in the production of infectiveforms, the sporozoites that enter the circulation of anew mammalian host together with secretions of themosquito's salivary glands. The cycle is representedin Fig. 1.

* cc: PrOPb I cs aC i

-in

1 +:$ (mu C*}'.C').:cy*6.SA _

(Noc-b

FIG.I. Diagram oflife-cycle ofmalaria parasites showingsites of action of antimalarials.

Antimalarial drugs may affect one or more phasesof this cycle. The compounds are given names thatdescribe the type of action for which they arenormally employed, e.g. chloroquine is known as a'blood schizontocide', whereas primaquine is usuallydescribed either as an 'anti-relapse drug', a 'tissueschizontocide' or a 'gametocytocide'; the first two ofthese both describe the action of primaquine on the

secondary exoerythrocytic liver schizonts that areresponsible for relapses in benign tertian malariadue to P. vivax and ovale. (The existence of thisphase in Plasmodium malariae is still in dispute.)The pathogenic effects of malaria are all related to

the asexual erythrocytic stages of the parasites (i.e.the direct damage they do to their host cells), themore distant effects due to the products of theirmetabolism, and the immunological response of thehost. The direct damage consists of acute haemo-lysis, both of infected and uninfected red cells, withits attendant anaemia. The indirect damage is causedby various factors, such as the release of toxic mat-erials of unknown nature (see review by Maegraith& Fletcher, 1972), which trigger a chain of eventsthat include changes in capillary permeability, theproduction of pharmacologically active peptides,anoxic anoxia, and possibly some degree of dissemi-nated intravascular coagulation. These events mayoccur to a lesser or greater degree in malaria causedby any of the species of Plasmodium that infect manbut are usually more severe in falciparum infection.This type of disease, justifiably known as 'malignanttertian malaria', is aggravated in addition by thetendency of the infected erythrocytes to adhere tocapillaries of the deep circulation, and particularly tothose of the brain. In this situation interferencewith cerebral microcirculation can lead to the con-dition known as cerebral malaria.

Disturbances of renal vascular function may giverise to decreasing glomerular filtration and anuriawith all its consequences. Sudden haemolytic crisesassociated with falciparum malaria can produce thecondition known as 'blackwater fever'. The patho-genesis of this syndrome is still incompletelyunderstood. It is known that it used to be associatedwith the administration of quinine and that incidenceof this complication has diminished greatly inrecent years, but it is also possible that host geneticfactors such as G-6-PD deficiency may play a role.

Treatment of malaria consists of the administra-tion of specific drugs to destroy the intraerythrocyticparasites and of non-specific supportive measuresaimed at correcting the general disturbances of hostfunction. Certainhighlyeffectiveblood schizontocidesin fact possess general pharmacological propertiesthat are of value also in a general manner. The anti-inflammatory action of chloroquine, for example,undoubtedly plays a role in the remarkably rapidresponse of a child with cerebral malaria to systemicadministration of this compound.A few drugs are of value, not only for their action

on the asexual blood stages, but also as a means ofblocking the transmission of the parasites throughthe anopheline vectors. Such drugs are pyri-methamine which has a potent sporontocidal action,and primaquine which appears to act upon the



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Antimalarial drugs 575

gametocytes, rendering them non-infective. Prima-quine, while having some action too upon theasexual blood stages, is too toxic to be employedgenerally for treatment, but is at present the drugof choice for the elimination of the secondary tissuestages of relapsing malarias (i.e. 'radical cure').

Malaria may be prevented by drugs such as pro-guanil that inhibit the development of the pre-erythrocytic liver phase (i.e. 'causal prophylactics')or by any blood schizontocide that is able to sup-press the erythrocytic asexual parasites. In time,with continuing exposure to malaria infection, andespecially with the repeated stimulus of the erythro-cytic parasitaemia, even at sub-threshold levels, adegree of immunity may be built up that permits theindividual to tolerate further infections without theaid of continuous chemoprophylaxis and withoutclinical symptoms. However, undue stress, such assecondary infection, trauma or pregnancy, mayreduce the protection afforded by the immunemechanisms, and acute malaria may ensue. In thecase of infection due to P. malariae, pathogenicchanges occasionally occur in the kidneys or otherorgans due to the deposition of antigen-antibodycomplexes. Current concepts on immunity in malariawere reviewed recently by McGregor (1972).

The evolution of antimalarialsEver since Pelletier & Caventou isolated the active

principle of quinine in 1820, research workers havebeen endeavouring to produce a synthetic productwith the properties of the original antimalarial butwithout its toxicity. One of the first compounds tobe developed which proved to have antimalarial

properties of a modest nature was methylene blue,and further work on this type of chemical structureled in the 1920's to the evolution of a number of8-aminoquinolines. In the early 1930's pamaquinewas developed by Schulemann and his collaboratorsof the I.G. Farbenindustrie. It proved to be a verypoor blood schizontocide, although it was found tohave considerable activity against exoerythrocyticstages. Later the 9-aminoacridine mepacrine wasdeveloped by other German workers and mepacrineproved, unlike its predecessor, to be a highly effectivecompound for the treatment of acute malaria. Themepacrine type of structure was later simplified anda series of 4-aminoquinolines resulted. The mostactive of these eventually proved to be chloroquine,which received its first clinical challenge duringthe Second World War. Other derivatives of the sameseries were introduced within a few years and theseincluded such compounds as amodiaquine, whichhas come into very wide use, and the related deriva-tives amopyroquine and cycloquine. Furtherdevelopment of the 8-aminoquinolines led to a muchless toxic analogue of pamaquine, namely prima-quine. The structures of these compounds areillustrated below (Figs. 2 and 3).The 8-aminoquinolines have an entirely different

mode of action from quinine, mepacrine or the4-aminoquinolines. It is now known that the8-aminoquinolines suppress the mitochondrial func-tions of malaria parasites, at least in the exoerythro-cytic stages, and possibly also the activity of struc-tures equivalent to mitochondria in the erythrocyticforms. Moreoever, it seems very likely that it is notthe 8-aminoquinolines themselves that are active

Me H H Me H

N ®H N H> N H

OMe Me OMe

Primaquine Pentaquine Quinocide

Me Me

N® N®~~~

0Me0

Pamaquine 5 6 quinoline -quinonederivative

FIG. 2.



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576 W. Peters

H -/; MeMe 0 Me

H N® HN~~( N®( , E

0 Me

cI ~~~~~CI ci N

Chloroquine Amodiaquine Mepacrine

FIG. 3.

but 5,6-quinolinequinone derivatives formed bymetabolization of the 8-aminoquinolines in themammalian organism. This, however, remains tobe proven.During the search for new antimalarials in the

Second World War it was discovered that manysulphonamides and sulphones have good anti-malarial activity against avian malaria, but thepromise of these compounds was not borne out byclinical studies against human malaria parasites.It is now known that this was partly because manyof the compounds tested were rapidly excreted inman, and some modern, more long-acting sulphon-amides have been shown now to have a far moresatisfactory blood schizontostatic activity (Fig. 4).

Also during the Second World War research pro-gramme a new type of chemical structure was evolvedby chemists of I.C.I. from which the compound weknow as proguanil emerged. Proguanil proved tobe by far the most active compound known, and inaddition it was the only truly causal prophylacticantimalarial ever discovered up until that time.Shortly afterwards pyrimethamine was produced byother British workers in the Wellcome researchorganization. It was only when the mode of actionof proguanil was revealed that it was realized inretrospect how similar pyrimethamine was to theactive metabolite of the earlier drug, which we nowcall cycloguanil (Fig. 5).

Chloroquine, together with amodiaquine, prima-quine, proguanil and pyrimethamine provided anarmamentarium of synthetic antimalarials that for anumber of years following the Seond World Warappeared to provide all that could be desired in thefield of malarial chemoprophylaxis and chemo-therapy. It was, however, only a matter of a very fewyears before resistance began to appear in the fieldto proguanil and pyrimethamine. Nevertheless, thisproved no cause for alarm, since parasites whichappeared to be resistant to those compounds stillresponded to treatment with chloroquine or mepa-crine, and in certain cases (when it was still used),quinine. It was indeed only in about 1959 when thefirst cases of P. falciparum infection resistant tochloroquine were found, that serious concern beganto be shown about a possible breakdown in ourdrug weapons against malaria. Since this happenedto coincide with a period in history when consider-able numbers of non-immune troops were beingdeployed in a highly malarious part of SoutheastAsia, namely the Republic of South Vietnam, thecoincidence of the emergence of chloroquine resis-tance and of a major war set in train a series ofresearch programmes which, up to the present time,have culminated in the study of well over 200,000new compounds in the search for better antimalarials(Peters, 1970; Kinnamon, 1972). Furthermore,considerable effort was devoted to studying the mode

N ~~~~~~~N-NN H2 _ S2 NH _ NH2 SO2 NH / OCH3

N

Sulphadiazine OH3C Sulphamethoxypyridazine

NH SO2 NH /< N NH NS2 NH N

OH3C CH30 OH3CSulphadimethoxine Sulphormethoxine

H2N SO2 / \ NH2 CH2CONH / S02 / NOCCH2

Dapsone DADDS

FIG. 4.



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NH2 CH30 NH2

N N

/ / \> NH2 CH3O \ CH2 / \ NH2

N ~~~~~~~~~~~~~~N

C2H5 CH3O

Pyrimethamine Trimethoprim

CH3

C / NH-C-NH C-NH-CH

~~11 11 \NH NH CH3

Proguani

NH2 NH2 OH

Ci I N>< N CH2

CH2 CH2

OH

COOH

Cycloguanil embonate

FIG. 5.

of action of even the older compounds, since thistopic had been largely ignored up to this time.Fortunately a new and very useful laboratory modelhad been developed during the interim period. Thiswas a malaria parasite of rodents which was readilyadaptable to ordinary laboratory mice, namelyPlasmodium berghei. This has proved extremelyuseful for large-scale screening of drugs for anti-malarial activity and has proven to be a far bettermodel for human malaria than the avian parasitePlasmodium gallinaceum that was the basis of mostdrug screening during the Second World Warprogramme. Still more recently an even better modelhas been discovered in a combination of humanP. falciparum in the little South American monkeyknown as Aotus trivirgatus. This is the only simianspecies in which human malaria parasites willfairly readily develop.

Mode of action of chloroquine and quinine-typecompounds

P. berghei has been used extensively for the study

of the mode of action of chloroquine. It has longbeen known that this drug is highly concentrated inparasitized erythrocytes, and it is now clear that theconcentration takes place very rapidly in the para-sites themselves. The immediate effect of exposure tochloroquine is clumping of the malarial pigment,which is maximal after a period of about 60 min.The malaria pigment that forms within vesicles inwhich the parasites digest host cell haemoglobin isprobably the end product of the breakdown of thehaemoglobin protein chains. Interference with thisprocess by chloroquine must quite clearly alsointerfere with the production of the basic amino acidbuilding blocks required by the parasites. At a laterstage of its action chloroquine induces a breakdownof the larger RNA particles of the malaria parasites.Electron micrographs show that chloroquine leadsto the development of large cytolysosomes whichcontain all the haemozoin together with a consider-able number of malarial ribosomes. A number ofcompounds have been shown by Warhurst et al.(1971) to interfere with this process of cytolysosome



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formation (Type B). On the other hand, several com-pounds can be seen to have the same type of actionas chloroquine, namely the induction of haemozoinclumping and cytolysosome formation (Type A)(Table 2). Drugs that produce such an effect are

TABLE 2. Mode of action of chloroquine and quinine-related compounds

Type A Type B

Cause clumping self: No clumping, but inhibitChloroquine chloroquine-inducedMepacrine clumping:Amodiaquine QuinineAmopyroquine Phenanthrene-methanols

(Can be doubly protonated (e.g. WR 122,455)at physiological pH) Quinoline-methanols

(e.g. WR 142,490)(Single protonatable N)

other 4-aminoquinolines and mepacrine. Quininedoes not induce clumping of haemozoin but inhibitsthe clumping which itself is produced by exposingthe parasites to chloroquine. It is now known thatthe inhibition of chloroquine-induced clumpingfollows exposure of the parasites (at a suitable intervalbefore their exposure to chloroquine), to inhibitorsof protein synthesis, ofRNA synthesis and inhibitorsof parasite respiration. Quinine itself is a com-petitive antagonist of chloroquine in this clumpingsystem. Other chemical structures, such as quinolinemethanols and phenanthrene methanols, that havecertain structural similarities to quinine, exert asimilar inhibition of chloroquine-induced clumpingof malarial pigment to quinine itself (Warhurstet al., 1972). Thus the study of the action of otheragents on chloroquine-induced haemozoin clumpingin malaria parasites has proved to be an extremelyuseful tool, both for the study of the mode of actionof new drugs and indeed for the study of the physio-logical processes of the malaria parasite itself (Home-wood et al., 1972). This technique has the advantagethat it enables us to examine what is happeninginside the intact host-parasite complex, whereasmost other biochemical techniques necessitate theseparation of the parasite from the host, which inturn produces an artificial situation, if not death ofthe parasites, in the process. As a final stage in itsmode of action chloroquine probably intercalateswith the nucleic acids of the parasite, and in sodoing prevents the normal replication of plasmodialnucleic acids.

In addition to their specific antimalarial effect,chloroquine, mepacrine and quinine have markedanti-inflammatory properties, which are perhapsattributable to their ability to stabilize lysosomalmembranes, although it may prove that they alsohave a further action, such as the inhibition ofprostaglandin production by certain target tissues in

HH

0

\N

Br

WR 33063

F3C

WR 122455

H CF,

H

H \ 8~ \-H1

00

CH,0 <Qi

Quinine

H

II

00

N

C c

CI

WR 30090 WR 142490

FIG. 6.

the host organism. Current work will be directed toinvestigating this possible mode of action of anti-malarials. The anti-inflammatory effects of suchdrugs as chloroquine are invaluable as a non-specificmeasure in the treatment of acute malaria. For thisreason the threatened loss of chloroquine from ourarmamentarium was a particularly serious blow.

Mode of action of antimetabolitesSulphonamides and sulphones exert their effect

on malaria parasites in the same way as they do onbacteria, namely by inhibiting the utilization of para-aminobenzoic acid. It appears that their effectivenesslies in the pharmacokinetic properties of theindividual compounds in the host (Tables 3 and 4).

TABLE 3. Half-life of sulphonamides and sulphones

Approximate 'half-life' in man (hrs)

'Medium-acting' 'Long-acting'

Sulphaphenazole 10 Sulphadimethoxine 35Sulphamethoxazole 11 Sulphalene 60Sulphadiazine 17 Sulphadoxine* 120Dapsone 25 Diformyldapsone 84

* Sulphormethoxine.

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Antimalarial drugs

TABLE 4. Half-life of repository drugs

Approximate 'half-life' (weeks)

Mouse Man

Cycloguanil embonate 6-8 24Acedapsone* 6-13 7

* Dadds.

For example, sulphonamides with a prolongedhalf-life in man are proving to be remarkablyeffective antimalarial agents, as indeed are suchnewer sulphone derivatives as the diformyl analogueof dapsone.

Proguanil and its active metabolite cycloguanil,pyrimethamine and trimethoprim inhibit the con-version of dihydrofolate to tetrahydrofolate in themalaria parasite. Their selectively toxic action againstmalaria parasites is due to the fact that they are morestrongly bound to the enzyme of the parasites than tothe enzyme of the host (Ferone, Burchall& Hitchings,1969). If sulphonamides or sulphones are given at thesame time as one of these antifols, a marked potentia-tion is observed. This is found not only in systemsemploying the blood forms of malarial parasites butalso in the mosquito stages, and indeed even inthe liver stages in the few cases where this hasbeen examined. The same situation applies not onlyin experimental laboratory models (Peters, 1968) butalso in the treatment of human malaria and parti-cularly in P. falciparum.One of the problems, apart from any question of

drug resistance, has been the desirability of havingantimalarial compounds which can be given atirregular intervals under field conditions whereregular chemoprophylaxis is an impracticable pro-position. In their search for such long-acting com-pounds, the late Paul Thompson and his colleaguesof the Parke-Davis organization developed poorlysoluble salts of cycloguanil and of dapsone. Thesewere found to have extremely long activity, both inmice and in man, when given in an injectablerepository formulation (for formulae of cycloguanilembonate and acedapsone, see Figs. 5 and 4 respec-tively). Moreover, it was hoped that by giving acombination of cycloguanil pamoate and acedap-sone one could avoid stimulating the developmentof parasite strains resistant to one or other of theindividual components. Unfortunately this did notprove to be the case when the mixture of these twocompounds was applied in extensive field trials.Nevertheless, even cycloguanil pamoate aloneproved in human volunteers to protect againstsporozoite challenge with both P. falciparum andP. vivax, in some cases for more than 1 year. Underfield conditions the duration of protection was farless, usually only about 3 months, but even thiscould have offered a considerable advantage for the

control of malaria if drug resistance had notappeared within a very short time.

Resistance to the antifols appears to be due to thedevelopment by the parasites of a mutant enzymewith a decreased affinity for the drug (Ferone, 1970).Moreover, the mutant enzyme is produced in alarger quantity than the normal enzyme, thus com-pensating for the presence of the inhibitory drug.Nevertheless, if such a resistant strain, or indeed astrain which is resistant both to an antifol such aspyrimethamine and to a sulphonamide, is exposed toa combination of these two compounds, the com-bination proves to be effective. This has been shownnot only by ourselves and others in experimentalsituations (Peters, 1971), but also by our Americancolleagues in human volunteers infected withmultiple-resistant strains of P. falciparum.

Experimental investigation of resistance to chloroquineThe main laboratory model used up to 1948 for

the investigation of antimalarial drugs was theavian parasite P. gallinaceum in the chick. In thismodel it proved very easy to produce resistance tothe sulphonamides and to the antifols. It was, how-ever, by no means the same case with the newlydeveloped compounds of the chloroquine type.Numerous attempts to develop strains of P. gallin-aceum resistant to mepacrine or to chloroquinefailed. This induced a false sense of security in thepotential value of these modern synthetic anti-malarials. This sense of security was abruptlyshattered, however, when chloroquine-resistantstrains of P. falciparum were demonstrated first inSouth America, and then, within a year to two, inparts of Southeast Asia. Investigations were startedrapidly to try to discover how malaria parasites couldbecome resistant to chloroquine. One of the firstexperiments revealed that chloroquine-resistantP. berghei concentrated chloroquine to a muchlower degree than did normally chloroquine-sensitive parasites (Macomber, O'Brien & Hahn,1966). Peters, Fletcher & Staubli (1965) showedthat chloroquine-resistant P. berghei failed to pro-duce normal malarial pigment and that the parasiteslived in immature rather than in mature red cells.While investigating the respiration of normal andresistant parasites, Howells et al. (1970) found thatthere is a cyclical change in the mode of respirationof P. berghei between the mammalian and mosquito-stages.While the forms in the red blood cells utilized

anaerobic respiration of the Embden-Meyerhof type,the mosquito stages more fully utilized glucose byopening up pathways of the aerobic Krebs cycle. Itwas shown that blood stages of the highly chloro-quine-resistant parasites also utilized Krebs cyclepathways. The hypothesis was put forward that the

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W. Peters

parasites were chloroquine-resistant by virtue of theirability to 'switch on' the Krebs cycle enzymes pre-maturely, that is, in the red cells rather than waitingto go into the mosquito. It was suggested that thiswould enable the parasites to make up for any loss ofamino acids, which may be caused by the interferencein their digestive processes by chloroquine, by pro-ducing Krebs cycle intermediates which they couldthen transaminate to produce their own amino acids.Unfortunately, however, we were unable to demon-strate the presence of Krebs cycle enzymes in themalaria parasites themselves in electron microscopecytochemical studies.

Howells et al. (1972) have been able to show thatthere is indeed an increase in Krebs cycle enzymesin red cells inhabited by malaria parasites, but thatthe increase is an increase in the host cell enzymesand not in the parasite enzymes. It is relevant, how-ever, that this increase is much more marked in redcells inhabited by chloroquine-resistant P. bergheithan in red cells inhabited by drug-sensitiveorganisms. We believe now that, while this may bethe way in which parasites are able to survive oncethey are resistant to chloroquine, the chloroquineresistance itself is due to a decrease in high affinitybinding sites for chloroquine, as suggested originallyby Fitch (1969, 1970). This is probably associatedwith a simple mutation which is inherited in thenormal manner and which is stable through mos-quito passage. It is interesting to note that we foundin our experimental model that exposure of parasitesof chloroquine-resistant strains of P. berghei in themouse to chloroquine led to the production of higherinfection rates in mosquitoes subsequently feeding onthose mice than would appear if the mice had notreceived chloroquine prior to the mosquito feed(Ramkaran & Peters, 1969). This situation, if itapplies also to chloroquine-resistant P. falciparumin the field, may facilitate the spread of such resistantmutants. This perhaps is one way in which chloro-quine resistance has become so rapidly extendedfrom what were probably relatively limited foci inSouth America and Southeast Asia.What is perhaps a little more difficult to under-

stand is why chloroquine resistance in P. falciparumhas never yet been proven to exist anywhere on theAfrican continent. We believe that the different hostresponse to P. falciparum by African people mayplay a significant role in this phenomenon. Fortu-nately, the potentiating combinations of sulphon-amides or sulphones with antifols are effective evenagainst the vast majority of strains of P. falciparumwhich are resistant to chloroquine, to antifols alone,or indeed to sulphonamides and sulphones alone.There is, however, already some indication thatcertain strains are showing up which do not respondto these potentiating combinations as one would

hope. What is not clear at the moment is whetherthese failures are due to resistance of the parasitesor to some inherent difference in the host, such as amore rapid excretion or a failure of absorption orutilization of the compounds themselves. Thisclearly requires further investigation. It may reveal,for example, a genetic difference, such as a rapidacetylation and excretion of sulphonamides or afailure to metabolize certain compounds to theiractive derivatives.

Summary of recommendations for prophylaxis andtherapyWhere there is no question of drug resistance

being present, prevention is simple. The regular con-sumption of chloroquine or of proguanil or pyri-methamine will provide adequate protection againstall the human malarias. In the case of an acuteattack of malaria developing because of the failureto take adequate prophylaxis, malaria is simpleenough to treat, provided that it is diagnosed rapidlyand accurately. Details of the dosage of the variousdrugs will be found in any medical textbook as wellas in the specialized monographs listed below (e.g.Ross Institute, 1972; Covell et al., 1955; Peters,1970).The problem arises where there is suspicion that

drug resistance may be present. It is essential thatthe physician who is concerned with giving advice onthis matter should make himself familiar with thesituation regarding antimalarial drug resistance inthe areas to which his patients are going.

If it is known that resistance to proguanil and/orpyrimethamine is present, then the drug of choicefor malaria prophylaxis is one of the 4-amino-quinolines, such as chloroquine or amodiaquine. Inparts of South America and Southeast Asia wherechloroquine resistance is known to occur, the regularconsumption of 200 mg of proguanil per day, oreven 25-50 mg of pyrimethamine once per week mayprovide adequate protection in spite of the fact thatsome measure of resistance may also be present tothese antifols. It is, however, not easy to prescribemalarial prophylaxis with confidence in areas suchas these where multiple drug resistance is known tooccur. Some take a sulphonamide-antifol combina-tion, but it is probably wiser not to do this, sincesuch potentiating combinations are still extremelyuseful drugs on which to fall back if one needs to forthe treatment of an acute attack. It is perhaps wiserto recommend simply a double dose of one of theantifols and to reserve the combination for treatmentin the event of a breakthrough.Most multiple-resistant strains of P. falciparum

will still respond, at least in the first instance, tointensive treatment with quinine. Fortunately,several new antimalarial drugs of the phenanthrene-

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Scal. 1*420,000

G.

CO=LOMiA.:7 . D. FGoU:?~~~~~~~~~~~~~o

ECUADOR G

PERUU.~~~~ ~ ~~~~~~~~~~~~~~~~~~~~~ .........

.8 R A Z I

BOLIVIA U

_~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

CHILE ~~PARAGUAY

ARGENTINA

-URGvAvFIG. 7. Distribution of chloroquine-resistant strains of P. falciparum in South and Central America. Stipple,chloroquine resistance proven; vertical hatching, chloroquine resistance suspected.

and quinoline-methanol series are currently beingtested which also are very effective against multiple-resistant strains of P. falciparum. So far it is only inthis species of parasite that resistance to chloro-quine has been clearly shown to occur. Chloroquineis still a very effective drug for the prevention ortreatment of malaria due to the other three parasitesof man.An exception, however, is in the radical cure of

infection due to relapsing P. vivax or ovale malaria.Here it is essential, in addition to chloroquine, to

provide a 14-day course of a tissue schizontocide,such as primaquine, in order to exclude the possibilityof relapses due to secondary exoerythrocytic schi-zonts. Primaquine is also extremely valuable in thetreatment of multiple drug-resistant malaria, not somuch from the point of view of the patient as of thecommunity. Primaquine has been shown clearly toretain a marked gametocytocidal action even againststrains of this parasite which are highly resistant tochloroquine, and to most other drugs as far as theasexual erythrocytic infection is concerned. An



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582 W. Peters

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FIG. 8. Distribution of chloroquine-resistant strains of P. falciparum in Southeast Asia. Stipple, chloroquineresistance proven; vertical hatching, chloroquine resistance suspected.

excellent statement of current recommendations formalaria chemoprophylaxis and chemotherapy willbe found in the publication of the Ross Institutelisted below.

AcknowledgmentsI should like to thank Mr S. N. McDermott for preparing

the figure and Mrs D. Steedman for the careful preparationof the manuscript.

ReferencesBRUCE-CHWATT, L.J. (1971) Malaria. Epidemiology. British

Medical Journal, 2, 91.CANFIELD, C.J. (1972) Malaria in U.S. military personnel

1965-1971. Proceedings of the Helminthological Societyof Washington, 39 (Suppl.), 15.

COVELL, G., COATNEY, G.R., FIELD, J.W. & SINGH, J. (1955)Chemotherapy of malaria. W.H.O. Monograph SeriesNo. 27. W.H.O., Geneva.



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FERONE, R. (1970) Dihydrofolate reductase from pyrimetha-mine-resistant Plasmodium berghei. Journal of BiologicalChemistry, 245, 850.

FERONE, R., BURCHALL, J.J. & HITCHINGS, G.H. (1969)Plasmodium berghei dihydrofolate reductase. Isolation,properties, and inhibition by antifolates. MolecularPharmacology, 5, 49.

FITCH, C.D. (1969) Chloroquine resistance in malaria: adeficiency of chloroquine binding. Proceedings of theNational Academy of Sciences of the United States ofAmerica, 64, 1181.

FITCH, C.D. (1970) Plasmodium falciparum in owl monkeys:drug resistance and chloroquine binding capacity. Science,169, 289.

HOMEWOOD, C.A., WARHURST, D.C., PETERS, W. & BAGGA-LEY, V.C. (1972) Electron transport in intraerythrocyticPlasmodium berghei. Proceedings of the HelminthologicalSociety of Washington, 39 (Suppl.), 382.

HOWELLS, R.E., PETERS, W. & WARHURST, D.C. (1972)Physiological adaptability of malaria parasites. In: Com-parative Biochemistry of Parasites (Ed. by H. Van denBossche), p. 235. Academic Press, London.

HOWELLS, R.E., PETERS, W., HOMEWOOD, C.A. & WAR-HURST, D.C. (1970) Theory for the mechanism of chloro-quine resistance in rodent malaria. Nature, 228, 625.

KINNAMON, K.E. (1972) Personal communication.MACOMBER, P.B., O'BRIEN, R.L. & HAHN, F.E. (1966)

Chloroquine: physiological basis of drug resistance inPlasmodium berghei. Science, 152, 1374.

MAEGRAITH, B.G. & FLETCHER, K.A. (1972) The pathogenesisof mammalian malaria. In: Recent Advances in Parasito-logy (Ed. by B. Dawes), vol. 10, p. 49. Academic Press,London.

MCGREGOR, I.A. (1972) Immunology of malarial infectionand its possible consequences. British Medical Bulletin,28, 22.

NEVA, F.A., SHEAGREN, J.N., SHULMAN, N.R. & CANFIELD,C.J. (1970) Malaria: host-defence mechanisms and com-plications. Annals of Internal Medicine, 73, 295.

PETERS, W. (1968) The chemotherapy of rodent malaria.VII. The action of some sulphonamides alone or withfolic reductase inhibitors against malaria vectors andparasites. Part 2. Schizontocidal action in the albino mouse.Annals of Tropical Medicine and Parasitology, 62, 488.

PETERS, W. (1970) Chemotherapy and Drug Resistance inMalaria. Academic Press, London.

PETERS, W. (1971) The chemotherapy of rodent malaria.XIV. The action of some sulphonamides alone or withfolic reductase inhibitors against malaria vectors andparasites. Part 4. The response of normal and drug-resistant strains of Plasmodium berghei. Annals of TropicalMedicine and Parasitology, 65, 123.

PETERS, W., FLETCHER, K.A. & STAUBLI, W. (1965) Phago-trophy and pigment formation in a chloroquine-resistantstrain of Plasmodium berghei Vincke and Lips 1948.Annals of Tropical Medicine and Parasitology, 59, 126.

RAMKARAN, A.E. & PETERS, W. (1969) Infectivity of chloro-quine resistant Plasmodium berghei to Anopheles stephensienhanced by chloroquine. Nature, 223, 635.

Ross Institute (1972) Antimalarial Drugs, Bulletin No. 2(re-written). London School of Hygiene and TropicalMedicine, London.

WARHURST, D.C., HOMEWOOD, C.A., PETERS, W. & BAGGA-LEY, V.C. (1972) Pigment changes in Plasmodium bergheias indicators of activity and mode of action of antimalarialdrugs. Proceedings of the Helminthological Society ofWashington, 39 (Suppl.), 271.

WARHURST, D.C., ROBINSON, B.L., HOWELLS, R.E. & PETERS,W. (1971) The effect of cytotoxic agents on autophagicvacuole formation in chloroquine-treated malaria parasites(Plasmodium berghei). Life Sciences, 10, 761.

World Health Organization (1957) Expert Committee onMalaria, Sixth Report. W.H.O. Technical Report Series,No. 123. W.H.O., Geneva.



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antimalarial w. peters h. 5qaantimalarial drugsmayaffect oneormorephases ofthis cycle. thecompounds...

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