In vitro activities of novel antifolate drug combinations against Plasmodium falciparum and human granulocyte CFUs

Abstract

The potency of antimalarial dihydrofolate reductase inhibitors, alone and in synergistic combination with dihydropteroate synthetase inhibitors, against the Kenyan K39 strain of Plasmodium falciparum (pyrimethamine resistant) and against normal replicating human bone marrow cells in in vitro culture has been studied. Therapeutic indices and rank order of synergistic potency were derived. Trimethoprim, pyrimethamine, and the quinazolines WR159412 and WR158122 had the smallest therapeutic indices (1.39, 4.38, 2.56, and 90.0, respectively), while the three triazines clociguanil, WR99210, and chlorcycloguanil had the largest (3,562, 3,000, and 2,000, respectively). In rank order of decreasing activity against P. falciparum, the six most potent drug combinations were WR99210-dapsone, chlorcycloguanil-dapsone, WR158122-dapsone, WR159412-dapsone, WR159412-sulfamethoxazole, and chlorcycloguanil-sulfamethoxazole; pyrimethaminesulfadoxine was the least potent combination. These experiments form a basis for the selection of rapidly eliminated antifolate combinations for further clinical testing.

Identification and analysis of dihydrofolate reductase alleles from Plasmodium falciparum present at low frequency in polyclonal patient samples.

“…Clinically, all of these DHFR inhibitors are used in combination with sulfonamides or sulfones. 6,30 A number of studies have shown that resistance of P. falciparum to PSD is correlated with point mutations in both DHFR and dihydropteroate synthase (DHPS). 14-17 Analysis of both genes will likely be required to correlate completely the molecular analysis with the response of parasites to the combination drug.…”

“…[25][26][27][28][29] The DHFR inhibitors are always used clinically in combination with sulfonamides or sulfones. 2,6,30 Currently, pyrimethamine plus sulfadoxine (PSD) is replacing chloroquine as the first-line antimalaria drug in many countries in Africa. 30 Unfortunately, alleles with mutations at S108N, N51I, and C59R have already been observed in this region; this combination is strongly correlated with decreased susceptiblity to PSD.…”

“…Pyrimethamine, clociguanil, chlorcycloguanil, cycloguanil, and the experimental WR-99210 are antimalarial drugs that competitively inhibit DHFR in Plasmodium falciparum (Pf), the parasite that causes the most virulent form of human malaria. [2][3][4][5][6] Effective antimalarial chemotherapy is becoming increasingly difficult due to the spread of multiple drug-resistant strains of P. falciparum. [7][8][9] In contrast to most antimalarial drugs, the mechanism of resistance to antifolates has been well characterized.…”

Triazine Inhibits Toxoplasma gondii Tachyzoites In Vitro and In Vivo

“…It is also active against P. vivax sistance to the triazines has been demonstrated for plasmodia. In addition to the potency of triazines, the very high avidity of this compound for the P. falciparum DHFR differs from its avidity to mammalian DHFR (13)(14)(15), increasing the therapeutic/toxic ratio.…”

Novel Saccharomyces cerevisiae Screen Identifies WR99210 Analogues That Inhibit Mycobacterium tuberculosis Dihydrofolate Reductase

“…In the early 1980s, WR99210 was shown to selectively inhibit parasite growth in vitro, with a 50% inhibitory concentration (IC 50 ) in the nanomolar range (24). Despite better selectivity for the malaria parasite enzyme than for the enzyme in human cells in culture (42), trials of WR99210 with humans showed serious problems of bioavailability, and further development was abandoned. However, Canfield and colleagues (4) demonstrated that in vivo, the biguanide precursor of WR99210, PS-15, is converted to the active triazine form and that this formulation improved the bioavailability substantially.…”