Primaquine is the least toxic and most effective of the 8-aminoquinoline antimalarial compounds. The mechanism by which 8-aminoquinolines exert their antimalar-ial effects is thought to be through a quinoline-quinone metabolite that inhibits the coenzyme Q-mediated respiratory chain of the exoerythrocytic parasite.

Primaquine is an important antimalarial because it is essentially the only drug effective against the liver (exo-erythrocytic) forms of the malarial parasite. The drug also kills the gametocytes in all four species of human malaria. Primaquine is relatively ineffective against the asexual erythrocyte forms. Primaquine finds its greatest use in providing a radical cure for P. vivax and P. ovale malaria.

Primaquine is readily absorbed from the gastrointestinal tract, and in contrast to chloroquine, it is not bound extensively by tissues. It is rapidly metabolized, and the metabolites are reported to be as active as the parent drug itself. Peak plasma levels are reached in 4 to 6 hours after an oral dose, with almost total drug elimination occurring by 24 hours. The half-life is short, and daily administration is usually required for radical cure and prevention of relapses.

Although primaquine has a good therapeutic index, a number of important side effects are associated with its administration. In individuals with a genetically determined glucose 6-phosphate dehydrogenase deficiency, primaquine can cause lethal hemolysis of red cells. This genetic deficiency occurs in 5 to 10% of black males, in Asians, and in some Mediterranean peoples. With higher dosages or prolonged drug use, gastrointestinal distress, nausea, headache, pruritus, and leukopenia can occur. Occasionally, agranulocytosis also has been observed.

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