The effect of parasite populations on the curative action of pyrimethamine

A non-resistant strain of P. falciparum was used to demonstrate a mechanism of acquired drug resistance. The dose of pyrimethamine required to cure a non-resistant P. falciparum infection depends directly on the population size of the parasitaemia. The surviving parasites demonstrate maximum resistance. This may explain widely different findings about the ease of resistance of P. falciparum to various drugs. This must also be considered as a mechanism for potentiation between two drugs effective on asexual forms of the parasite.