The chemotherapy of Plasmodium berghei. II. Antagonism of the action of drugs

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.

In vitro activities of and mechanisms of resistance to antifol antimalarial drugs

Certain drugs that interfere with folate metabolism (sulfones, sulfonamides, and inhibitors of dihydrofolate reductase) play an important role in the chemotherapy and prophylaxis of malaria. The activities and mechanisms of action of these drugs are regarded as similar in most respects to their activities against procaryotic microorganisms. Believed incapable of utilizing intact exogenous folates, plasmodia have been regarded as dependent on de novo synthesis of required folate cofactors. The present investigation, conducted in pursuit of a method for testing the in vitro susceptibility of Plasmodium falciparum to antifol antimalarial drugs, produced evidence that earlier assumptions about the folate metabolism of this organism are not correct. Three of four isolates of P. falciparum were successfully maintained in a culture medium depleted of folic acid and p-aminobenzoic acid. The antimalarial activities of sulfonamides and dihydrofolate reductase inhibitors were, furthermore, variably antagonized by the presence of folic acid and p-aminobenzoic acid in the culture medium. Optimum conditions for assessment of antifol antimalarial activity in vitro therefore require precise control of these factors in the culture medium. Our results suggest that resistance to antifol antimalarial drugs involves a complex of factors related to both the de novo synthesis of active folate cofactors and the ability to utilize exogenous intact folates in various forms.

POTENTIATION OF SULPHADIMIDINE BY 2,4-DIAMINO-6,7-DI-ISOPROPYLPTERIDINE AND OTHER 6,7-DISUBSTITUTED 2,4-DIAMINOPTERIDINES AGAINST EIMERIA INFECTIONS OF CHICKS

Six 2,4-diaminopteridines substituted in the 6 and 7 positions were tested as potentiators of sulphadimidine against Eimeria tenella. 2,4-Diamino-6,7-di-isopropylpteridine and 2,4-diamino-1'-propylindolo(2',3':6,7)pteridine were selected for further study. Both potentiated sulphadimidine against E. tenella and the di-isopropylpteridine potentiated it against E. necatrix. A 10% mixture of the di-isopropylpteridine with sulphadimidine (or other sulphonamides) appears to offer advantages in the treatment of E. tenella and still more of E. necatrix.

The action of 2:4-diamino-6:7-diisopropylpteridine upon Plasmodium gallinaceum and its relation to other compounds which are pteroylglutamic acid antagonists

1. Two strains ofPlasmodium gallinaceumwere made resistant to 2:4-diamino-6:7-diisopropylpteridine (0/129) by treatment with that drug.

2. The 0/129-resistant strains were resistant to proguanil, pyrimethamine, 2:4-diamino-6:7-diphenylpteridine (0/63) and 2:4-diamino-5-(p–chlorophenoxy)-6-methylpyrimidine (48–210), but not to sulphadiazine.

3. In one strain treated with 0/129, the development of resistance to that drug itself preceded resistance to proguanil, and resistance to proguanil preceded resis tance to pyrimethamine.

4. A strain ofP. gallinaceummade resistant to 0/63 was resistant to proguanil, pyrimethamine and 0/129, but not to sulphadiazine.

5. The action of 0/129 and proguanil uponP. gallinaceumwas not antagonized byp–A.B., though in the minimum effective dose their action was antagonized by relatively large doses of P.G.A.

6. Whereas the action of sulphadiazine uponP. gallinaceumwas antagonized competitively byp–A.B., it was antagonized by P.G.A. only when the sulphadiazine was given in small doses.