Inhibition of Plasmodium falciparum dihydropteroate synthetase and growth in vitro by sulfa drugs

Abstract

The Michaelis-Menten inhibitory constants (Kgs) and the concentrations required for 50% inhibition of the Plasmodiumfakciparum dihydropteroate synthetase were determined for six sulfa drugs. These drugs inhibited the in vitro growth of P. falciparum (50% lethal concentration) at concentrations of 30 to 500 nM; these concentrations were 100 to 1,000 times lower than the concentrations required for 50% inhibition and Kts (6 to 500 pM). The uptake of p-aminobenzoic acid was not inhibited by the sulfa drugs. However, infected erythrocytes took up more labeled sulfamethoxazole than did uninfected erythrocytes. Thus, the concentration of sulfa drugs by malaria parasites may explain how sulfa drugs inhibit in vitro growth of parasites through the inhibition of dihydropteroate synthetase.Antifolates are key antimalarial agents, particularly in the therapy and prophylaxis of chloroquine-resistant Plasmodium falciparum. Important antifolate antimalarial agents include pyrimethamine and chlorguanide, which inhibit dihydrofolate reductase, and sulfadoxine and dapsone, which inhibit dihydropteroate synthetase (DHPS). Activities of combinations of inhibitors of both enzymes, such as sulfadoxine and pyrimethamine (Fansidar), are synergistic (17).Currently, sulfadoxine-pyrimethamine is the drug of choice for the treatment of chloroquine-resistant malaria. Unfortunately, sulfadoxine-pyrimethamine cannot be used prophylactically because of its association with toxic epidermal necrolysis (Stevens-Johnson syndrome) (14). Furthermore, strains resistant to sulfadoxine-pyrimethamine have been isolated in Southeast Asia and Africa (5). Thus, there are serious limitations to the use of this agent in malaria control.Sulfonamides and sulfones are selectively toxic to malarial parasites because their target enzyme, DHPS, is not present in the mammalian host. DHPS catalyzes the following step in the de novo biosynthesis of folic acid: p-aminobenzoic acid (PAB) + 6-hydroxymethyldihydropterin pyrophosphate --dihydropteroic acid. Sulfa drugs have been shown to inhibit DHPSs from the murine malarial parasites Plasmodium berghei (7, 9) and Plasmodium chabaudi (16). In vitro, sulfadoxine and dapsone have been shown to inhibit P. falciparum growth and to act synergistically with pyrimethamine (4, 11). However, little is known about the relative potencies of various sulfa drugs against P. falciparum, either in terms of DHPS inhibition or their effectiveness in preventing growth of the parasite. The inhibition of P. falciparum growth in vitro by sulfa drugs is antagonized by PAB and folates (3, 18). The mechanism of antagonism, however, is not completely understood.In this study we compared the effects of sulfa drugs on the in vitro growth of P. falciparum and P. falciparum DHPS enzyme activity. Evidence for the uptake of sulfa drugs by the parasites is presented. Inhibition of PAB uptake by sulfa * Corresponding author. drugs as a possible alternative mechanism was also examined. MATERIALS AND METHODSParasite cultures of P. falciparum FCR3 were mai...

Malaria Chemotherapeutics Part I: History of Antimalarial Drug Development, Currently Used Therapeutics, and Drugs in Clinical Development

“…Homology modeling predicts dapsone (55) to be less affected by dhps mutations [242] which makes it one of the most active antimalarial agents in its class of sulfonamides and sulfones (approximately 10-fold more active than sulfadoxine (54) in vitro). [251] However, there is some degree of cross-resistance in the group of sulfonamides. The combination of dapsone and chlorproguanil (56) was evaluated in different clinical studies, [252][253][254] and was recently introduced as LapDap to the market.…”

Trimethoprim and sulfonamide resistance

“…The enzyme dihydropteroate synthase (DHPS; EC 2.5.1.15) catalyzes the formation of dihydropteroic acid in bacterial and some eucaryotic cells, like Pneumocystis carinii (179), Toxoplasma gondii (123), and Plasmodium falciparum (192). This enzyme activity is not present in human cells.…”

Sequence Variation of the Hydroxymethyldihydropterin Pyrophosphokinase: Dihydropteroate Synthase Gene in Lines of the Human Malaria Parasite, Plasmodium falciparum, with Differing Resistance to Sulfadoxine

“…Unfortunately, this cheap and successful combination (Fansidar) is itself increasingly compromised by the appearance of parasite strains re-sistant to both components . Alternative sulfa drugs [8], in combinations such as dapsone-chlorproguanil [9] and sulfisoxazole-proguanil [lo], are also being evaluated. However, their efficacy relies on the same antifolate synergism as sulfadoxine-pyrimethamine, and they are thus also likely to be compromised by resistance in due course.…”