Relationships between chemical structures of 8-aminoquinolines and their capacities for radical cure of infections with Plasmodium cynomolgi in rhesus monkeys

Evaluation of 200 8-aminoquinolines for the capacities to effect radical cure of infections with sporozoites of Plasmodium cynomolgi in rhesus monkeys led to identification of 34 derivatives with activity equal or superior to that of primaquine and to characterization of substituents on the quinoline nucleus and side chain that favored or prejudiced curative activity. Of the 34 derivatives, 19 were as active as primaquine, 9 were twice as active, and 6 were four times as active. With respect to nuclear substituents, all were methoxy substituted at position 6; 24 had one and 10 had two additional substituents. The additions with most favorable impact on activity included methyl substituents at positions 4 and 2 and alkoxy, fluoro, and a group of 3- or 4-substituted phenoxy substituents at position 5. With respect to 8-amino substituents, 14 of the 15 derivatives more active than primaquine, and 13 of the 19 as active as primaquine, carried a branched alkyl chain, four to five carbons in length, between the 8- and terminal amino groups. Proximity of branching to the 8-amino group could be an important determinant of curative activity; however, the effect of such branching was not predictable. All 15 derivatives more active than primaquine and a substantial fraction of those comparable to primaquine in activity have sufficient structural novelty to merit evaluation for tolerability and radical curative activity in humans, with reasonable prospects that one or more would be better tolerated than primaquine and superior to this drug for cure of Plasmodium vivax infections.