The antimalarial triazine WR99210 and the prodrug PS-15: folate reversal of in vitro activity against Plasmodium falciparum and a non-antifolate mode of action of the prodrug.

Abstract

Abstract. We have studied the reversal of activity against Plasmodium falciparum of WR99210, a triazine antimalarial drug, and of the pro-drug PS-15 by folic acid (FA) and folinic acid (FNA). Folic acid and FNA inhibit the growth of P. falciparum in vitro at concentrations Ͼ 10 Ϫ4.5 and 10 Ϫ3.5 mol/L, respectively. The activity of pyrimethamine against Kenyan strains M24 and K39 is reduced 10-12-fold by 10 Ϫ5 mol/L of FA, and virtually eliminated by 10 Ϫ5 mol/L of FNA. Folates do not antagonise the action of WR99210 against Kenyan strains, and only partially antagonize the action of WR99210 action against the Southeast Asian strains V1/S and W282. Similarly, FA and FNA exerted weak or no antagonism of the action of PS-15. The inability of folates to antagonize the action of WR99210 can be explained in terms of high drug-enzyme affinity, but this does not account for the inability of FA and FNA to antagonize PS-15. These results suggest that action of PS-15 against P. falciparum is primarily due to a non-folate mechanism.Chemotherapy remains the mainstay in the control and treatment of malaria. However, parasite resistance to the available antimalarial drugs is spreading rapidly. Chloroquine is now almost totally ineffective in most parts of Africa and reports indicate that resistance to pyrimethaminesulfadoxine, the current first-line treatment for chloroquineresistant parasites in Africa, will soon become widespread, rendering the drug useless. 1 Therefore, there is an urgent need for new antimalarial drugs. WR99210 and its prodrug PS-15 represent a new class of highly active antifolate drugs that could be used in combination therapy against multidrugresistant parasites.WR99210 (4,6-diamino-1,2-dihydro-2,2-dimethyl-1-[(2,4, 5-trichlorophenoxy)propyloxy]-1,3,5-triazine), is a close structural analog of the antimalarial antifolate drug cycloguanil, which has pronounced activity in vitro against all Plasmodium falciparum strains studied to date. 2-5 PS-15 (N-(3-(2,4,5-trichlorophenoxy)propyloxy)-NЈ-(1-methylethyl)-imidocarbonimidodiamide hydrochloride), a close analog of proguanil, 6,7 is a prodrug of WR99210 developed to overcome the low bioavailability and gastrointestinal intolerance that occurs with oral dosages of WR99210 in humans. 8 Several factors suggest that both WR99210 and PS-15 have antifolate activity directed against parasite dihydrofolate reductase: the close structural similarity between PS-15 and proguanil, and between WR99210 and cycloguanil; 4 the ability of folinic acid (FNA) to reverse the gastrointestinal intolerance caused by WR99210; 6 and the fact that both compounds interact synergistically with sulfonamide. 6,7 Conversely, the high level of activity of WR99210 against all isolates tested, 4-6 the lack of cross-resistance with other dihydrofolate reductase (DHFR) inhibitors, 1,4 and the synergistic interaction between PS-15 and atovoquone, a drug that inhibits parasite mitochondrial electron transport, 6,9 collectively imply that both WR99210 and PS-15 may inhibit P. falciparum by additi...

Chemosensitization of Plasmodium falciparum by Probenecid In Vitro

“…The second receptor, the folate binding protein or folate receptor, is specific for oxidized folate derivatives such as folic acid (1,24). Studies have demonstrated that both the oxidized and reduced folate (folic and folinic acid, respectively) can cross the membranes of the red blood cells parasitized with P. falciparum, because they can both negate the effects of antifolate drugs (12,42). Methotrexate, an analog of folic acid, uses the RFC receptor to gain intracellular access and has been shown to inhibit P. falciparum at IC 50 s similar to those that inhibit human cells (nanomolar range) (10,39).…”

Preclinical Evaluation of the Antifolate QN254, 5-Chloro-N′6′-(2,5-Dimethoxy-Benzyl)-Quinazoline-2,4,6-Triamine, as an Antimalarial Drug Candidate

“…QN254 was compared to the antimalarials PM and cycloguanil (inactive on QM PfDHFR), as well as the triazine WR99210, a potent inhibitor of all mutant PfDHFRs, including QM PfDHFR (20). The results of these experiments are summarized in Table 1.…”

“…The antifolate triazine WR99210 (3,22) is potent against P. falciparum bearing quadruple mutations of DHFR at S108N, N51I, C59R, and I164L (QM PfDHFR) (20). However, WR99210 has shown limited efficacy in vivo due to poor oral bioavailability and displayed some gastrointestinal toxicity.…”

Selective killing of the human malaria parasite Plasmodium falciparum by a benzylthiazolium dye

“…Importantly, the methods employed in our study cannot distinguish between folic acid and PUR-1 competition for uptake through the PSAC channel on the outer membrane of the infected cell vs. competition for uptake across a carrier protein located on the parasite plasma membrane. However, taken together with the findings of Nzila et al (Nzila, et al, 2003) and Wang et al (Wang, et al, 1999) it seems reasonable to believe that a selective inhibitor of the NPP will block uptake of exogenous folic acid from the bloodstream and function synergistically with selective inhibitors of parasite dihydrofolate reductase, such as WR99210 (Canfield, et al, 1993,Kinyanjui, et al, 1999 and "biguanide" prodrugs of the Jacobus series (Jensen, et al, 2001). …”