Studies on Synthetic Antimalarial Drugs

Drug-resistance in Plasmodium gallinaceum, and the persistence of paludrine-resistance after mosquito transmission

1. Attempts, made over a period of more than 6 months, to produce a strain of Plasmodium gallinaceum resistant to mepacrine failed.

2. A strain of P. gallinaceum resistant to daily doses of 0·08 mg./20 g. of pamaquin was obtained after approximately 8 months' treatment with the drug.

3. A strain of P. gallinaceum resistant to the maximum dose of paludrine which the chicks would tolerate (1·0 mg./20 g. twice daily) was obtained after 4½ months' treatment with gradually increasing doses of the drug.

4. The exo-erythrocytic parasites of P. gallinaceum formed in chicks infected with a paludrine-resistant strain are themselves resistant to the drug. It is not, however, considered probable that the development of resistance is due to the action of the drugs upon the exo-erythrocytic parasites, as the technique used was one which reduced their number to a minimum.

5. A strain of P. gallinaceum resistant to paludrine showed a normal sensitivity to mepacrine and pamaquin, but was resistant to 4430, the methyl homologue of paludrine.

6. Chicks carrying a latent infection of the normal strain of P. gallinaceum were immune to infection with the paludrine-resistant strain; and conversely chicks carrying a latent infection of the paludrine-resistant strain were immune to infection with the normal parasite.

7. A paludrine-resistant strain of P. gallinaceum retained its resistance to this drug after five cyclical passages through mosquitoes without intervening drug treatment.

Mechanisms of resistance of malaria parasites to antifolates

Antifolate antimalarial drugs interfere with folate metabolism, a pathway essential to malaria parasite survival. This class of drugs includes effective causal prophylactic and therapeutic agents, some of which act synergistically when used in combination. Unfortunately, the antifolates have proven susceptible to resistance in the malaria parasite. Resistance is caused by point mutations in dihydrofolate reductase and dihydropteroate synthase, the two key enzymes in the folate biosynthetic pathway that are targeted by the antifolates. Resistance to these drugs arises relatively rapidly in response to drug pressure and is now common worldwide. Nevertheless, antifolate drugs remain first-line agents in several sub-Saharan African countries where chloroquine resistance is widespread, at least partially because they remain the only affordable, effective alternative. New antifolate combinations that are more effective against resistant parasites are being developed and in one case, recently introduced into use. Combining these antifolates with drugs that act on different targets in the parasite should greatly enhance their effectiveness as well as deter the development of resistance. Molecular epidemiological techniques for monitoring parasite drug resistance may contribute to development of strategies for prolonging the useful therapeutic life of this important class of drugs.

Antimalarial drugs and the mosquito transmission of Plasmodium

It is well-known that whenever possible, the treatment of patients with malaria should include measures to prevent them transmitting the infection to others. This is particularly important for P. falciparum, where the gametocytes can survive for a much longer period than the asexual stages. Not all antimalarials are gametocytocidal or sporontocidal and those that are may have particular disadvantages or lose their effectiveness because of resistance. Even drugs that have no obvious gametocytocidal or sporontocidal activity may have other effects. These include the possibility of increasing transmission, either by affecting the parasite within an individual host or by selection for parasite strains with increased potential for infecting the mosquito vector. This review summarises the available information on the properties of antimalarials in relation to mosquito transmission and highlights the need for more attention to be paid to this aspect of drug action.

Single dose pharmacokinetics of proguanil and its metabolites in healthy subjects

1. Plasma and whole blood concentrations of proguanil and its two major metabolites cycloguanil (CG) and 4-chlorophenylbiguanide (CPB) were measured by a sensitive h.p.l.c. technique in nine healthy adult male volunteers after a single oral dose of proguanil 200 mg. 2. Proguanil was absorbed with a median time to peak plasma concentration of 3 h (range 2-4 h). 3. Peak plasma concentrations of proguanil ranged between 150 and 220 (median 170) ng ml-1 compared with 12 to 69 (median 41) ng ml-1 for the active antimalarial metabolite CG, and 3 to 16 (median 11) ng ml-1 for CPB. Peak (mean +/- s.d.) plasma CG concentrations occurred 5.3 +/- 0.9 h and peak CPB concentrations occurred 6.3 +/- 1.4 h after oral administration of proguanil. 4. Whole blood concentrations of proguanil were approximately five times higher, and whole blood CPB concentrations were four times higher than corresponding plasma values, whereas plasma and whole blood concentrations of CG were similar. 5. A triexponential function was fitted to these data; mean (+/- s.d.) values for the AUC were 3046 +/- 313 ng ml-1 h for proguanil, 679 +/- 372 ng ml-1 h for CG and 257 +/- 155 ng ml-1 h for CPB. 6. Plasma and whole blood concentrations of proguanil and its metabolites declined in parallel with terminal elimination half-lives estimated as 16.1 +/- 2.9 h and 15.7 +/- 2.4 h, respectively. Mean residence times in plasma and whole blood were estimated as 21.2 +/- 4.9 and 19.3 +/- 2.4 h.

Problems concerned with gametogenesis in Haemosporidiidea, with particular reference to the genus Plasmodium

Although many interesting observations have been made upon gametogenesis in Haemosporidiidea many fundamental problems await solution. Our knowledge of the nuclear changes occurring during the development of the gametes and the fertilization of the female gamete is still incomplete. Little is known of the factors which control gametocyte production, though there is evidence that in some species, i.e. Leucocytozoon simondi and Haemoproteus columbae in the duck, it is influenced by the physiological stage of the host. The type of host and the method of transmission have also been shown to affect the numbers of gametocytes formed in some species. No correlation has been found in Plasmodium, the genus in which this subject has been studied extensively, between acquired immunity and gametocyte formation. Although a broad relationship has been demonstrated in Plasmodium between the density of gametocytes in the blood and the oocyst rate and density assessed in susceptible mosquitoes, hosts heavily infected with apparently ripe gametocytes may prove non-infective to susceptible mosquitoes. In birds heavily infected with P. gallinaceum, serum taken at the peak of parasitaemia has been shown to affect adversely the infectivity of gametocytes to mosquitoes. Pamaquin and primaquine, in small doses, inhibit the development of gametocytes completely, whereas proguanil may not prevent the formation of oocysts though it inhibits their subsequent development. High densities of gametocytes have been observed in patients treated with proguanil or sulphamethazine. An actual in crease in the number of gametocytes produced, as compared with the untreated control strain, was observed in strains of P. gallinaceum in process of developing resistance to sulphadiazine, proguanil or 2:4-diamino-6:7-diisopropylpteridine as a result of treatment with those drugs.

The effect of sulphadiazine, proguanil and 2:4-diamino-6:7-diisopro-Pylpteridine upon gametocyte production in Plasmodium gallinaceum (Brumpt, 1935)

In strains ofPlasmodium gallinaceumpassaged by blood-inoculation, through young birds treated with small, but effective doses of proguanil, sulphadiazine or 2:4-diamino-6:7-diisopropylpteridine, the numbers of gametocytes produced were much greater than in the parent strain passaged by the same method.

The effect of the drugs upon gametocyte production appears to be a long term one, since no change in gametocyte numbers was observed in birds treated with proguanil or sulphadiazine for only a few days.

The relationship of these drugs to nuclear division and the synthesis of nucleic acids is discussed in the light of the observed increase in gametocyte-production in drug-treated strains ofP. gallinaceum.