Cycloguanil is the active metabolite of the antimalarial prodrug proguanil. Cycloguanil is formed from proguanil by the cytochrome P450 (CYP) isoforms CYP2C19 and CYP3A in human liver microsomes. It is an inhibitor of dihydrofolate reductase (DHFR; Kis = 1.5 and 0.79 nM for the P. falciparum and P. berghei enzymes, respectively). It is active against ten P. falciparum field isolates (IC50s = 0.12-1,400 μg/ml). Cycloguanil reduces parasitemia in a mouse model of P. berghei infection (ED50 = 2 mg/kg). It also reduces parasitemia in a rhesus monkey model of P. cynomolgi infection when administered at a dose of 0.3 mg/kg.
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