Structure−Activity Relationships in 4-Aminoquinoline Antiplasmodials. The Role of the Group at the 7-Position

Antiplasmodial activities versus the chloroquine sensitive D10 strain of Plasmodium falciparum of a series of N ,N 1-diethyl-N -(4-quinolinyl)-1,2-ethanediamines with 11 different substituents at the 7-position on the quinoline ring have been investigated in vitro. Electron-withdrawing groups at the 7-position have been shown to lower the pK a of both the quinoline ring nitrogen atom and the tertiary amino nitrogen in the alkyl side chain. The quinoline nitrogen pK a ranges from 6.28 in the nitro derivative to 8.36 in the amino derivative, while the tertiary amino nitrogen has a pK a ranging between 7.65 in the trifluoromethyl derivative and 10.02 in the amino derivative. Calculation suggests that the resulting pH trapping of these compounds in the parasite food vacuole ranges between about 7% of that observed in chloroquine for the NO2 derivative and 97% in the amino derivative. A direct proportionality between antiplasmodial activity normalized for pH trapping and β-hematin inhibitory activity was observed. Activity could not be correlated with any other observed physical parameter. The β-hematin inhibitory activity of these derivatives appears to correlate with both the hematin−quinoline association constant and the electron-withdrawing capacity of the group at the 7-position (Hammett constant). For the compounds under investigation, the hematin association constant is in turn influenced by the lipophilicity of the group at the 7-position.