Clinical testing of new antimalarial compounds*

Bull. Org. mond. Sante 1974, 50, 203-212 Bull. Wld Hlth Org. Clinical testing of new antimalarial compounds* C. J. CANFIELD 1 & R. S. ROZMAN2 More than 200 000 chemical compounds have been screened for antimalarial activity over the past 10 years by the US Army Antimalarial Drug Development Program. By means of extensive animal testing, 26 of these compounds were selected for clinical study in human subjects volunteering for such trials. Of these, 7 have received complete clinical trials and are in various stages of field evaluation, 4 are currently undergoing clinical trial, and 2 are still awaiting testing in volunteer subjects. Thus far, 2 compounds ( WR 33 063 and WR 30 090) have demonstrated greater activity against drug-resistant Plasmodium falciparum than any other known drug. Several other compounds presently being tested in human subjects are even more potent. The US Army Antimalarial Drug Development Program was organized in 1963 with the aim of developing drugs for the prevention or treatment of malaria due to chloroquine-resistant strains of P. fal- ciparum (1). It was designed as a complete pro- gramme to include the screening of available chemi- cals from a variety of sources as well as the synthesis of new compounds following successful leads. Of the more than 200 000 compounds tested over the past 10-year period, about 3% were active in primary screening tests. Approximately 170 of the most active compounds were selected for advanced testing in simian systems. Preclinical pharmacological and toxicological studies were performed on 36 selected compounds, and Notices of Claimed Investigational Exemption for a New Drug, known as INDs, for clinical and field testing were submitted to the US Food and Drug Administration for 27 of these. The purpose of this paper is to describe and illustrate the process used in the selection of these 36 compounds and to summarize the recent clinical results obtained with 26 of the 27 investigational new drugs. METHODS Drugs considered likely candidates for preclinical testing are initially chosen by members of the Divi- * From the Walter Reed Army Institute of Research, Washington, D.C. 20012, USA. This report represents con- tribution number 1241 to the US Army Research Program on Malaria. Presented at the Symposium on Malaria Research, Rabat, Morocco, 1-5 April 1974. 1 Special Assistant for Malaria Research. 2 Assistant Chief, Department of Pharmacology. sion of Medicinal Chemistry, Walter Reed Army Institute of Research (WRAIR). These decisions are based on the results of antimalarial efficacy studies in a variety of primary and secondary animal test systems (2). Particular attention is paid to the compound's activity against resistant strains of human plasmodia in vitro and in Aotus trivirgatus, the owl monkey (3, 4). Whenever feasible, several structurally similar compounds are evaluated con- currently in these systems and the most active ana- logue is selected for advancement to preclinical pharmacological and toxicological studies. A 3-kg batch of the selected drug is prepared by larger scale laboratory production, and its composition, purity, and stability are confirmed by independent assay. Acute and subacute tolerance studies are per- formed in at least 2 animal species in accordance with the guidelines laid down by the US Food and Drug Administration (5). Particular emphasis is given to the establishment of a safe or tolerated dose and to the determination of sensitive methods for monitoring target organ toxicity produced by higher doses. Reversal of toxicity may be studied by drug withdrawal and/or administration of specific antago- nists, e.g., folinic acid in the case of drugs that inhibit dihydrofolate dehydrogenase.1 Preliminary studies of drug absorption, metabolism, excretion, and tissue localization are performed with radio-labelled com- pounds. In some cases, the results of these routine preclini- cal studies and experience with similar compounds indicate a need for special pharmacological studies, ' Previously known as dihydrofolate reductase. 3180 - 203- 5 204 C. J. CANFIELD & R. S. ROZMAN including testing for potential phototoxicity or car- diovascular toxicity. All preclinical data are then evaluated by the Department of Pharmacology, WRAIR, to assess the potential benefit/risk ratio of the drug. If human trials are recommended, a portion of the 3-kg batch is formulated. The composition, purity, weight varia- tion, and stability of the drug formulation are confirmed by independent assay. Recently, new for- mulations have been evaluated in addition for drug dispersion, dissolution, and bioavailability. A Notice of Claimed Investigational Exemption for a New Drug is then prepared with a protocol for initial phase I human tolerance studies. A double- blind two-by-two rising dose design is used. The particular dosage schedule, clinical studies, and laboratory examinations to be employed are deter- mined on the basis of the preclinical information and are thus individualized for each drug. All data are then reviewed by four additional groups: (a) an ad hoc study group of civilian scientists; (b) the Investigational Drug Review Board of the US Army; (c) the US Food and Drug Administration; and (d) an institutional peer review committee affiliated with the facility at which the studies will be performed. The primary responsibility of this last group is to review the moral and ethical con- siderations relating to the proposed studies, particu- larly to ensure that the subjects are fully informed and that consent is granted voluntarily. If all concerned agree, human trials begin in one of the 4 clinical facilities.' The results of phase I trials are continuously monitored and reviewed by the principal investigator at the clinical facility and by members of the Depart- ment of Pharmacology, WRAIR. If no serious ad- verse reactions are encountered during phase I stu- dies, pilot phase II efficacy studies are subsequently performed either with the maximum tolerated dose determined in phase I studies or, if a lower dose is desired, with the anticipated effective dose projected from animal data (4). The specific design and Plasmodium spp. used in the pilot phase II studies vary with the type of antimalarial activity expected. A number of sensitive 1 The 4 principal investigators at these facilities and their affiliated institutions are: J. D. Arnold, Harry S. Truman Research Laboratory, Kansas City, Mo.; P. E. Carson, Laboratory for Tropical Diseases, Rush-Presbyterian- St. Luke's Medical Center, Chicago, Ill.; D. F. Clyde, University of Maryland School of Medicine, Baltimore, Md.; A. W. Czerwinski, Oklahoma Medical Research Unit, Uni- versity of Oklahoma Medical Center, Oklahoma City, Okla. and resistant strains of P. falciparum with varying patterns of drug responses are available, as well as P. vivax; infections can be either blood or mosquito induced. All volunteers are carefully monitored by frequent clinical and laboratory evaluations after exposure to the disease. Treatment is initiated when patency develops, and the therapeutic response is observed. In cases where the response is too slow or recrudescence occurs, radical cure is produced by the administration of an agent or agents known to be effective against the strain used. For phase II studies to be expanded with a new drug, the pilot studies must have shown that it has a greater therapeutic activity or a lower toxicity than that of existing drugs. Drugs that meet either crite- rion undergo further phase II studies to determine spectrum of activity, minimum effective dose, thera- peutic index, and optimum dosage regimen. These studies are aided by continuing animal pharmaco- logy and toxicology experiments, determinations of drug concentrations in biological fluids, and assess- ments of drug interactions. The exact details of the expanded phase I studies, as well as of any addi- tional phase I or animal studies needed, are dictated by continuing reassessment of all results by represen- tatives from all levels of the programme. RESULTS Thirty-two different chemical compounds and 4 combinations of compounds have been selected for clinical trial since the inception of the programme. Many hundreds more showed varying degrees of activity in one or more test systems, but only the most active within each class of drugs were selected. Although Notices of Claimed Investigational Exemption were prepared for all 36 of these drugs or drug combinations, only 27 were submitted for approval to be taken to clinical trial because the remaining 9 were considered too toxic on the basis of preclinical animal studies. All 27 INDs submitted were approved. However, 1 drug never came to clinical trial because of the observation of human intolerance to a related compound. At clini- cal trial, 13 of the remaining 26 drugs showed insufficient activity or were poorly tolerated; these studies were therefore discontinued. Of the remain- ing 13 drugs, 7 have undergone complete clinical trials and are in various stages of field evaluation, 4 are currently receiving clinical trial, and 2 are awaiting testing in volunteer subjects. The 26 compounds or combinations selected for NEW ANTIMALARIALS 205 Table 1. Chemical names of compounds referred to by code number in the text WR 3090 6- (2-methylphenyl) -2,4,7-pteridinetriamine WR 4 809 1 - [4- [(7-chloro-4-quinoxalinyl) amino] benzoyl] -4-methylpiperazine WR 5 677 2- (1 ,3-diphenyl-2-butenylidene) hydrazinecarboxamide monohydrochloride WR 6798 4,4'-sulfonylbisbenzamide WR 14 997 1 -aminocyclopentanecarboxylic acid WR 17 206 1,4- bis(trichloromethyl) benzene WR 25 979 N- (aminoiminomethyl) -N'- (4-nitrophenyl) urea monohydrochloride WR 30 090 a- [(dibutylamino) methyl] -6,8-dichloro-2- (3,4-dichlorophenyl) -4-quinolinemethanol hydrochloride WR 33 063 6-bromo-a- [(diheptylamino)methyl]-9-phenanthrenemethanol hydrochloride WR 38 839 1- [(3,4-dichlorophenyl)methoxy]-1,6-dihydro-6,6-dimethyl-1,3,5-triazine-2,4-diamine hydrochloride WR 40 070 5-(1,3-benzodioxol-5-ylmethyl)-2,4-pyrimidinediamine WR 81 844 N- (3,4-dichlorophenyl) -N"- [4- [(1 -ethyl-3-piperidinyl)amino] -6-methyl-2-pyrimidinyl] guanidine dihydrochloride WR 122 455 3,6-bis(trifluoromethyl) -a- (2-piperidinyl) -9-phenanthrenemethanol hydrochloride WR 142 490 2,8-bis(trifluoromethyl) -a- (2-piperidinyl)-4-quinolinemethanol hydrochloride WR 158 122 6- [(2-naphthalenyl)sulfonyl]-2,4-quinazolinediamine WR 171 669 v-(dibutylamino)-1,3-dichloro-6-(trifluoromethyl)-9-phenanthrenepropanol hydrochlor- ide WR 171 952 N- (2-bromo-4,5-dimethoxyphenyl) -N- [2- (diethylamino)ethyl] -N',N'-diethyl- 1,2- ethanediamine [i.e., RC-12] 1,5-naphthalenedisulfonate clinical trials, representing 14 broad classes of com- pounds, are described in detail below. Their chemical names are given in Table 1. Sulfones Dapsone was a known antimalarial drug tested early in the programme. Doses of 300 mg daily given for 3 weeks produced haemolysis (6). However, smaller doses (25-50 mg/day) were tolerated without difficulty, and dapsone was shown to be an effective suppressive agent in 85% of volunteers challenged with drug-resistant strains of P. falciparum (7). In similar suppressive studies, 25 mg of dapsone admin- istered daily to volunteers, who also received chloro- quine (300 mg of base) and primaquine (45 mg of base) weekly, protected 920% of the subjects (8). However, methaemoglobinaemia occurred in some cases (9). After field studies had confirmed the therapeutic advantage of this combination regimen (10, 11), it was used during the Viet-Nam conflict by US military personnel during field operations in areas of high endemicity for P. falciparum. Dapsone also improved cure rates when used as an adjunct in the treatment of proven cases of falciparum malaria (12). Despite the use of dapsone, malaria continued to be a major problem in Viet-Nam (13). In addition, a few cases of agranulocytosis occurred in personnel receiving dapsone (14). An 8: 1 commercial fixed combination of dapsone and pyrimethamine I has been used by other investi- gators both for the treatment of drug-resistant falciparum malaria and as a suppressive agent. Although few studies have been performed with this combination in volunteer subjects, field studies in Thailand showed a reduction in malaria transmission rates from 9.2/100 per month with weekly chloro- quine to 2.3/100 per month with weekly dapsone and pyrimethamine (15). WR 6 798 was developed and tested as an alterna- tive to dapsone. Single doses as great as 4 800 mg did not show detectable toxicity. However, mild haemo- lysis was observed in volunteers who received 3 200 mg twice weekly for 4 weeks (16). The weekly administration of 400-800 mg protected 88 % of 1 Maloprim ®), The Wellcome Foundation Ltd, Euston Road, London, N.W.l, England. 206 C. J. CANFLELD & R. S. ROZMAN volunteers exposed to mosquitos infected with drug- resistant P. falciparum (17). During similar studies in which variable weekly doses of WR 6 798 were administered for 8 weeks concurrently with chloro- quine/primaquine, 90-97% of volunteers were protected (16). However, methaemoglobinaemia occurred in a significant number of the subjects. Clinical trials with a 16: 1 fixed combination of WR 6 798 and pyrimethamine (400 mg of WR 6 798 and 25 mg of pyrimethamine administered weekly) showed it to be well tolerated (18). In these studies, 93% of the volunteers challenged with mosquitos infected with drug-resistant P. falciparum were pro- tected. In addition, the few volunteers who were also challenged with P. vivax did not become infected. Field trials with this combination are presently under way in Thailand. Pteridines WR 3 090 was the only pteridine tested. The largest doses administered during tolerance studies were 480 mg daily for 3 days and 320 mg daily for 6 days. Six of the 9 volunteers receiving one or the other of these dosage regimens had immature neutro- phils in their peripheral blood, and some also devel- oped neutropenia. During pilot efficacy studies, doses as large as 320 mg daily for 3 days produced only temporary parasite suppression in volunteers infected with drug-sensitive P. falciparum or P. vivax and no parasite suppression in volunteers infected with drug- resistant P. falciparuim. In addition, 3 volunteers who received WR 3 090 in these latter studies also devel- oped neutropenia, anaemia, and/or thrombocyto- penia exceeding that usually seen during malaria infections. 4-Aminoquiniolines WR 4 809 was proposed for evaluation because animal studies revealed no evidence of cross-resis- tance with chloroquine. Tolerance studies in volun- teers progressed to doses of 450 mg daily for 3 days. At these dose levels, some volunteers appeared se- dated and some complained of nausea, vomiting, and diarrhoea. Efficacy studies with maximum tolerated doses ofWR 4 809 in volunteers infected with repre- sentative strains of P. falciparum showed only tem- porary suppression of parasitaemia in all cases, including those infected with drug-sensitive strains. Amopyroquine,1 a known antimalarial drug, was also proposed for trial because of animal and in vitro 1 Propoquin ®, Parke, Davis & Co., Plymouth Road, Ann Arbor, Mich., USA. evidence of activity against chloroquine-resistant Plasmodium spp. Normal volunteers received initial doses of 600 mg,2 followed by 300 mg 6 h later and an additional 300 mg on each of the 2 succeeding days. They complained of anorexia, nausea, and diarrhoea during the period of drug ingestion. Little effect on blood parasite levels was seen in 8 volun- teers infected with chloroquine-resistant P. falci- parum and given the same dosage regimen of amopy- roquine, only 2 of them showing temporary suppres- sion of parasitaemia. Parasitaemia in a single patient with mosquito-induced P. vivax malaria cleared but reappeared 17 days later. Diaminopyrimidines Trimethoprim 3 with sulfalene 4 is a combination that had some success early in the programme. In normal volunteers trimethoprim alone was tolerated in doses as great as 2 g daily without adverse effects (19). Larger doses produced gastrointestinal side effects, as did doses greater than 1.5 g daily in volunteers with malaria. Subjects infected with drug-sensitive P. falciparum were cured with 0.75 g daily for 5 days, but 1.5 g daily for 7 days cured only 3 out of 8 volunteers infected with a moderately drug-resistant strain. However, rapid clearance of parasites occurred, even in subjects who showed subsequent recrudescence. The curative po- tential of trimethoprim was markedly improved when 0.5 g was given with 0.75 g of sulfalene for a single day (20). This combination cured 10 out of 11 volunteers infected with the same resistant strain of P. falciparum. Continued clinical studies with other drug-resis- tant strains of P. falciparum showed lower curerates. Only 8 out of 12 volunteers were cured with the same or greater doses of the combination in one trial (21). The administration of 1.5 g of trimethoprim together with 1.0 g of sulfalene in a single dose cured only 20 out of 26 patients in a pilot field study in Viet-Nam (22). Extending the period of administration of this drug combination to several days failed to improve the cure rate in volunteers infected with highly drug- resistant strains of P. falciparum (23). Treatment failures appeared to be related to the bioavailability characteristics of sulfalene rather than to parasite resistance. 2All doses of amopyroquine are expressed as mg of base. 3Syraprim ®, Burroughs Wellcome Co., Research Triangle Park, N.C., USA. ' Kelfizina ®, Soc. Farmaceutici Italia (Farmitalia), Milan, Italy. NEW ANTIMALARIALS Studies designed to detect possible exoerythrocytic activity of the combination disclosed no activity against secondary tissue forms of P. vivax even when the combination was administered in maximally tolerated doses for 14 days (K. H. Rieckmann, unpublished observations, 1973). WR 40 070 is an analogue of trimethoprim that was tested because animal experiments showed it to possess greater activity than trimethoprim and revealed no evidence of cross-resistance in triazine- resistant strains. In tolerance studies in normal volunteers, gastrointestinal intolerance occurred with doses of 500 mg daily for 3 days. Two volunteers infected with drug-resistant P. falciparum received 400 mg daily for 2.5 or 4 days without any appre- ciable effect on their parasitaemia. Studies of this compound were therefore discontinued. Guanidines WR 5 677, extensively studied in animals, was of great interest because of its demonstrated activity against drug-resistant strains of rodent malaria para- sites. Studies in healthy volunteers revealed dose- related gastrointestinal intolerance at dose levels of 110 mg daily or greater for 3 days. Transient de- creases in the haematocrit were observed in subjects who received 390 mg daily for 3 days. Human efficacy studies indicated that 150 mg of the com- pound given daily for 3 days had no significant effect on either drug-sensitive or chloroquine-resistant strains of P. falciparum. These volunteers also devel- oped gastrointestinal side effects. WR 25 979 has been used as an anthelminthic in patients and has also exhibited some antimalarial activity in both animals and man. Graduated phase I studies in healthy volunteers showed that 0.8 g daily for 6 days caused hepatotoxic effects and delayed transitory orthostatic hypotension. Little or no efficacy was demonstrated by a dosage regimen of 0.6 g daily for 3 days in volunteers infected with P. vivax or with sensitive or resistant strains of P. falciparum. WR 81 844 was very active in both animal and in vitro test systems. Tolerance studies in healthy volun- teers were performed with doses of up to 530 mg daily for 3 days. This amount of drug produced nausea, abdominal pain, and diarrhoea. Transitory, possibly drug-related, leucopenia was observed in a few volunteers at intermediate doses only. Doses of 375 mg daily for 6 days were tolerated without adverse effect. Three volunteers infected with repre- sentative strains of P. falciparum were treated with a total of 720 mg of WR 81 844 during 2-4 days. No drug-related adverse effects were observed but the subjects' parasitaemia increased until additional drugs were administered. Two other volunteers, one with drug-sensitive P. falciparum and one with blood-induced P. vivax infection, received 375 mg daily for 6 days and were both cured. However, interest in the compound waned because of its lack of effect on drug-resistant strains. Amino acids WR 14 997, an antineoplastic drug, was found to have antimalarial activity in animal test systems. Tolerance studies in healthy volunteers revealed no adverse effects with doses as great as 2 g daily for 3 days. A single dose of 2 g or repeated doses of 1.6 g daily for 3 days administered to volunteers with induced falciparum malaria produced only a minimal effect on their parasitaemia. Studies in which these same doses of WR 14 997 were administered during the prepatent period after challenge with mosquitos infected with P. falciparum or P. vivax failed to disclose any evidence of exoerythrocytic activity, and the studies were discontinued. Xylenes WR 17 206 has been used as an anthelminthic drug in patients with Clonorchis sinensis infections and was found to have antimalarial activity in animal test systems. Phase I testing in healthy volunteers indicated that the drug was tolerated at a dose level of 10 g daily for 6 days, but volunteers who received 17.5 g for 3 days had gastrointestinal side effects. WR 17 206 produced temporary suppression of chlo- roquine-resistant P. falciparum parasitaemia in only 1 of the 2 subjects who received 4 g daily for 3 days. A single 4-g dose caused temporary suppression of parasitaemia in a subject infected with drug-sensitive P. falciparum. Daily doses of 4.5 g for 4 days cleared, but did not cure, a blood-induced P. vivax infection. Further studies were not performed because the drug was slow-acting and large doses were required. 4-Quinolinemethanols WR 30 090 was the first drug of this class to be evaluated in man as a part of the present drug development programme. It was well tolerated when administered to healthy volunteers in doses as great as 1.4 g daily for 10 days (J. D. Arnold, unpublished observations, 1973). A few subjects developed ephemeral phototoxic effects unrelated to dose 207 C. J. CANFIELD & R. S. ROZMAN (24). All volunteers infected with drug-sensitive P. falciparum were cured after treatment with 0.7 g of WR 30 090 daily for 6 days. In addition, the drug cured about 90% of volunteers infected with multi- drug-resistant strains of P. falciparum. A pilot field study confirmed the efficacy of this compound against naturally acquired P. falciparum from Viet- Nam (25). Seven patients with multiple recrudes- cences following treatment with standard antima- larial drugs were treated and cured with WR 30 090, as were 88% of patients with acute infections in Viet- Nam. Continued field efficacy studies have shown a cure rate of 86% in patients with acute falciparum malaria in Thailand from an area of known multi- drug parasite resistance (A. P. Hall et al., unpub- lished observations, 1974). WR 142 490 is a long-acting chemical analogue of WR 30 090 with greater potency against malaria in animal test systems. Tolerance studies in normal volunteers currently in progress have thus far shown the drug to be well tolerated in single doses as large as 1.75 g (G. M. Trenholme et al., unpublished observations, 1974). No evidence of phototoxicity has been detected. Single doses of 0.4 g administered to 13 volunteers infected with multi-drug-resistant P. falciparum suppressed parasitaemia in all subjects and cured 2 of them. Single doses of 1.0 g cured all 6 subjects with similar infections who were tested. A single case of blood-induced P. vivax infection was treated and cured with 0.4 g. Continued clinical studies are under way. 9-Phenanthrenemethanols WR 33 063 was the first drug of this class to be utilized against human malaria infections in the present US Army Antimalarial Drug Development Program. During tolerance studies, it was adminis- tered to healthy volunteers in doses as great as 4.6 g daily for 10 days without showing toxicity (26). A regimen of 1.6 g daily for 6 days was uniformly curative in volunteers infected with drug-sensitive strains of P. falciparum and was also curative in about 80% of volunteers infected with a variety of drug-resistant strains. The drug has been confirmed to be active against naturally acquired falciparum malaria (25). Thirteen patients who had suffered multiple recrudescences following treatment with all standard antimalarial drugs and 92% of 25 patients with acute infections in Viet-Nam were treated and cured with WR 33 063. Continued studies in Thai- land also yielded cure rates of 92% in patients with acute falciparum malaria (H. E. Segal et al., unpub- lished observations, 1974; A. P. Hall et al., unpub- lished observations, 1974). WR 122 455 is an analogue of WR 33 063 that exhibited significantly greater activity in animal test systems. Phase I studies in healthy volunteers re- vealed gastrointestinal intolerance to doses of 720 mg/day or greater if administered for more than 1 day. Single doses as great as 880 mg suppressed parasitaemia temporarily in volunteers with induced infections by sensitive or multi-drug-resistant strains of P. falciparum. A single patient infected with drug- sensitive P. falciparum was cured with 480 mg of WR 122 455 administered daily for 6 days. WR 171 669 was the third drug of this class to progress to human studies during the present pro- gramme. In animals it was shown to be more active than WR 33 063 and better tolerated than WR 122 455. Tolerance studies in healthy volunteers have been initiated only recently but thus far no intolerance has been observed to doses as large as 360 mg administered during 1 day. Triazines WR 38 839 was administered to healthy volunteers in doses as large as 1.4 g daily for 3 days without adverse effects. The administration of 1.3 g daily for 3 days to volunteers infected with drug-resistant P. falciparum caused nausea and diarrhoea, and was not curative (K. H. Rieckmann, unpublished obser- vations, 1973). Half this dose was well tolerated and was curative in volunteers infected with drug-sensi- tive P. falciparum. Limited studies were made of the concurrent administration of WR 38 839 and sulfa- diazine, but gastrointestinal side effects were seen at relatively low dose levels. The maximum tolerated daily dose was 0.3 g ofWR 38 839 together with 2.0 g of sulfadiazine given for 3 days. The association of these 2 drugs apparently had pre-erythrocytic tissue schizontocidal activity in volunteers infected with P. falciparum that was not observed with the indivi- dual drugs (27). WR 99 210, a proprietary drug, is a chemical analogue of WR 38 839 that has not shown signifi- cant evidence of cross-resistance to pyrimethamine in animal test systems. It has been approved for study, and human trials will be performed during 1974. Naphthoquinones Menoctone was proposed for evaluation on the basis of good evidence that it had causal prophylac- tic activity in mice. Healthy volunteers who received 650-800 mg daily for 3 days developed gastro- 208 NEW ANTIMALARIALS intestinal symptoms (R. D. Powell, unpublished observations, 1968). In doses of 500 mg daily for 3 days the compound was ineffective as either a blood schizontocide or a gametocytocidal agent in volunteers infected with chloroquine-resistant P. fal- ciparum. No causal prophylactic effect was seen in volunteers who received 500 mg daily for 3 days after challenge with either drug-sensitive or drug-resistant P. falciparum. Pyridines Clopidol was a coccidiostat with known activity against animal plasmodia. It was administered to healthy volunteers in a variety of dosage regimens. Total doses of about 7.8 g or greater given during 3-10 day periods almost invariably produced transi- tory and spontaneously reversible paresthesias. Some volunteers also developed ephemeral ptosis, diplopia, changes in visual accommodation, ataxia, deep ten- don reflex hyperactivity, or clouding of conscious- ness. Total doses of 6 g or less administered during 3 days to volunteers infected with a variety of strains of P. falciparum did not produce these neurological abnormalities, but effected only temporary suppres- sion of parasitaemia. No further studies were done with clopidol. Quinazolines WR 158 122 was the first drug of this class evalu- ated in man for antimalarial activity. Animal studies had shown it to be the most potent drug (in mg/kg) tested. Tolerance studies in healthy volunteers de- monstrated that the drug was without adverse effects when given in doses as great as 1.3 g daily for 3 days. In efficacy studies in a few patients infected with drug-sensitive P. falciparum, WR 158 122 produced temporary suppression of parasitaemia in doses of 250 mg daily for 3 days and cured a single patient treated with 1.0 g daily for 3 days. WR 158 122 with sulfadiazine is a new drug com- bination that exploits the proven potentiation of dihydrofolate dehydrogenase inhibitors by 4-amino- benzoic acid (PABA) competitors. This will permit the treatment of drug-resistant strains of malaria parasites with WR 158 122 while minimizing concern about the possible induction of resistance. Clinical trials have been approved and will be initiated shortly. Pyrocatechols WR 171 952, or RC-12 NDS,was an old drug that had previously shown exoerythrocytic activity in animal studies. The drug was administered to healthy volunteers in doses as great as 1.25 g daily for 7 days without evidence of intolerance (D. F. Clyde, unpublished observations, 1974). This dosage regi- men did not delay or prevent the occurrence of parasitaemia when the drug was administered during the prepatent period to volunteers who were exposed to P. vivax-infected mosquitos. In addition, when volunteers with acute vivax malaria were treated with standard amounts of chloroquine followed by 1.25 g of WR 171 952 daily for 7 days, relapses occurred in all subjects. No further studies were undertaken since exoerythrocytic activity was not detected at the dose levels used. DISCUSSION A comprehensive discussion of the specific com- pounds tested is beyond the scope of this review. The clinical results summarized are generally self-explan- atory; for many of them, details can be found elsewhere; for others, clinical trials are still in pro- gress. However, some general comments about the US Army Antimalarial Drug Development Program may be in order. Of the compounds screened during the 10-year span of this programme, the proportion that has come to clinical trial is small compared with the proportion that came to clinical trial in the Co- operative Wartime Program on a Survey of Antimalarial Drugs, undertaken during the Second World War (28). The major reason for this difference is the dramatic change that has since occurred in the amount and sophistication of preclinical information required for human trials, as reflected by the present US Food and Drug Administration guidelines. These restrictions would have severely handicapped antimalarial drug development if it had not been for the fact that significant improvements have also been made in our ability to evaluate potential antimalarial activity. Large numbers of compounds can be screened with relative ease (29) and the most active can then be evaluated in a simian host infected with the target parasite (4). The predictive ability of this P. falciparum-Aotus system has been good. Although specific dose correlations have been at variance in some instances, this has been attributed to differences between the simian and human hosts in drug metabolism and/or absorption. Other drug testing systems have also improved predictive ability, such as the in vitro evaluation of compounds against clinical strains of P. falciparum (3) and the testing of the efficacy of orally administered drugs against P. berghei (30). 209 C. J. CANFIELD & R. S. ROZMAN Extensive preclinical toxicological and pharmaco- logical studies have also presumably limited the number of drugs that would have been rejected because of human toxicity. Owing to the greater stringency of preclinical evaluation, the dose limiting factor in human trials, with rare exceptions, has been the occurrence of nausea and vomiting. These gastro- intestinal side effects are considered advantageous because the manifestations are obvious and revers- ible, and more serious dose-related toxicity can be avoided. Ability to predict antimalarial activity and toler- ance in man from multiple tests of drug activity in animals and extensive preclinical toxicological evalu- ations has improved steadily. In addition, standards for effectiveness have gradually risen. The combina- tion of trimethoprim and sulfalene is the only investigational drug evaluated for treatment of drug-resistant falciparum malaria during the first 5 years of the programme that is still considered to have even borderline effectiveness. Most drugs tested in volunteers during those years were existing compounds that were subsequently shown to have insufficient activity or to be poorly tolerated. How- ever, the synthesis of many new chemicals was achieved during this period by following successful leads, and the next 5 years saw the development of several promising new compounds. In the case of WR 33 063 and WR 30 090, success resulted from the resynthesis of compounds with known anti- malarial activity in animals; this illustrates the transition from clinical trials of existing compounds to clinical trials with newly synthesized compounds. These 2 drugs have undergone extensive trials in the volunteer clinical facilities collaborating with the programme and in field studies in south-eastern Asia. They are extremely well tolerated and are more active against drug-resistant P. falciparum than any other known drug. Even more promising are the initial results achieved with newly synthesized chemicals, including analogues of WR 33 063 and WR 30 090. Com- pounds that are very potent in animal test systems and that show little or no cross-resistance with existing drugs are now being evaluated in clinical trials. Foremost among these is WR 142 490. Initial clinical results in a few subjects indicate that a single dose of this drug has a rapid action against multi- drug-resistant P. falciparum. Several other new drugs are currently being evaluated in clinical trials, includ- ing WR 158 122, WR 122 455, and WR 171 669. To judge from preclinical data, these drugs should also have a broad spectrum of activity and produce no serious toxic effects. However, drug activity and tolerance in human trials cannot be predicted perfectly from the results of animal studies. The converse is also true, which is one obvious disadvantage of the present system of antimalarial drug development and screening. Drugs that might be active in man may be discarded because of their inactivity in animal tests. This drawback must be accepted for the moment since no reasonable alternative is available. However, as test- ing procedures are refined and as experience is gained in interpreting the large amounts of preclinical data, predictive ability should improve. Extensive antimalarial screening and preclinical evaluations are costly and time-consuming. The average interval between receipt of a new chemical and human trials is 3 years. It has become apparent that crash programmes, such as the one initiated during the Second World War, or even the present programme, cannot be expected to provide im- mediate solutions to new problems of antimalarial chemotherapy. The present programme has therefore not been terminated, and new compounds are con- tinuing to be synthesized and tested for antimalarial activity. The major emphasis of the US Army Antimalarial Drug Development Program has thus been to de- velop drugs for the treatment of drug-resistant P. fal- ciparum infections. Anticipated successes in this area have recently permitted a shift of emphasis to the development of drugs with exoerythrocytic activity. The only drug of this type that has come to clinical trial has been WR 171 952. New drugs with exo- erythrocytic activity in animal models are being dis- covered but it will be several years before any can be advanced to the clinical trial stage. In the mean- time, drug development and testing for agents useful against drug-resistant P. falciparum will continue. ACKNOWLEDGEMENTS Grateful appreciation is expressed to the many persons and institutions under contract to the US Army whose studies contributed to the development and testing of the antimalarial compounds reviewed. Many of the data were derived from unpublished reports, which were too numerous to be acknowledged individually. 210 NEW ANTIMALARIALS 211 RISUMJ ESSAI CLINIQUE DE NOUVEAUX COMPOSES ANTIPALUDIQUES Le Programme de mise au point de medicaments antipaludiques de I'Armee des Etats-Unis d'Amerique a t lance en 1963 avec comme objectif la recherche et l'evaluation de medicaments efficaces contre les infections a Plasmodium falciparum resistant a la chloroquine. Au cours de cette periode de 10 ans, plus de 200 000 produits chimiques ont ete testes dont 6000 environ (3 %) ont fait preuve d'une activite antipaludique. Parmi ces der- niers, pres de 200 ont ete choisis et ont fait l'objet d'essais dans le paludisme simien. Enfin, apr&s une evaluation complete de l'efficacit6 et des proprietes toxicologiques, le nombre des composes retenus en vue d'essais cliniques sur des volontaires s'est trouve reduit a 26. L'article expose les methodes de selection utilisees aux divers stades et les resultats cliniques obtenus. Dans les premieres annees du programme, on a etudie l'action suppressive de la dapsone dans le palu- disme a P. falciparum resistant a la chloroquine avec des resultats qui ont justifie son emploi intensif pendant la guerre du Viet-Nam; d'autres composes sulfones sont actuellement a l'essai sur le terrain. Deux produits (WR 33 063 et WR 30 090) ont temoigne a l'egard de P. falciparum resistant aux medicaments d'une efficacite superieure a celle de tout autre antipaludique connu. 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ROZMAN DISCUSSION LEPES: Over 200 000 compounds have been screened by the United States Army for possible use as antimalarials, but the yield from this tremendous effort has been small. SCHMIDT: There are very good reasons for this seemingly small progress. First, the difficulty of the mission assumed by the Army Program-namely, to develop a drug or drugs not only fully effective against infections with chloroquine-resistant strains of P. falciparum but also as effective as chloroquine against infections with drug-sensitive strains, and as well tolerated; secondly, the expressed intent of the US Army to comply with every requirement of the United States Food and Drug Administration (FDA) and other regulatory groups for ensuring the safety of human volunteers. BRUCE-CHWATT: I admire the way in which the reported drug trials have been carried out by the US Army. These trials may be compared to the screening of more than 2 000 insecticides by WHO, which has resulted in 2 or 3 promising ones. It must be stressed that the insecticide trials involved anophelines and not human beings; in this respect the US Army trials were more difficult to carry out. CANFIELD: In answer to a prior question, cotri- moxazole has not been evaluated by the US Army Malaria Testing Program. However, it does not appear to be superior to a combination of tri- methoprim and sulfalene, which showed a cure rate of only 70%. The combination of tetracycline and quinine has also been tested and can be recom- mended for treatment of chloroquine-resistant falciparum malaria. We have attempted to induce resistance to some of the individual investigational drugs. We found, for example, that resistance to drugs of the dihydro- folate dehydrogenase type may be induced easily when such drugs are used alone. For this reason, the US Army testing programme is emphasizing the combination approach. For obvious reasons, at the time of treatment in volunteers parasitaemia is low. In field trials, condi- tions are different. Even so, patients with low para- sitaemia and with no obvious complications of malaria are selected initially. Following experience, patients having more severe infections are included. With regard to testing compounds for chemosup- pression, it must be emphasized that the function of any drug will depend on the duration of action. WR 142 490, for instance, has a half-life of 7-10 days when given as a single dose and may well turn out to be a highly effective suppressive prophylactic drug. SCHMIDT: Because resistance to it will eventually develop, every drug is dated in terms of effectiveness. Chloroquine has remained effective for a very long time but it is doubtful whether we will be so fortunate with other agents. When a drug is first introduced perhaps we should consider combining it routinely with another agent so as to delay resistance, rather than waiting for parasite resistance to develop and then trying serial treatment with another drug. MICHEL: Drug testing using animal systems and human volunteers is indispensable but the impor- tance of field testing should be recognized. Field conditions present many variables, including racial differences, the presence of other diseases and mal- nutrition, the degree of acceptance of new drugs, and even differences in strains of parasites encountered. We must therefore entrust field trials, including the selection of subjects, to national personnel working at the national and local levels. 212