Structure-activity relationships for ferriprotoporphyrin IX association and β-hematin inhibition by 4-aminoquinolines using experimental and ab initio methods

In order to probe structure-activity relationships of association with ferriprotoporphyrin IX (log K) and inhibition of β-hematin formation, a series of 4-aminoquinolines with varying substituents at the 7-position (X) have been synthesized. These have been further elaborated by introduction of two different R groups on the 4-amino nitrogen atom in the form of methyl (R = Me) and ethylamine (R = EtNH2) side chains. Data for a previously investigated series containing an N,N-diethyl-ethylamine side chain were also compared with the findings of this study. Experimentally, log K values for the simple 4-aminoquinoline series (R = H) were found to correlate with the hydrophobicity constant (π) of the group X. The log K values for the series with R = Me and EtNH2 were found to correlate with those of the series with R = H. The log of the 50% β-hematin inhibitory activity (log BHIA50) was found to correlate with log K and either meta (σm) or para (σp) Hammett constants for the series with R = Me and EtNH2, but not the simple series with R = H. To further improve predictability, correlations with ab initio electrostatic parameters, namely Mulliken and CHelpG charges were investigated. The best correlations were found with CHelpG charges which indicated that log K values can be predicted from the charges on atom H-8 and the group X in the quinolinium species computed in vacuum, while log BHIA50 values can be predicted from the CHelpG charges on C-7, C-8 and N-1 for the neutral species in vacuum. These correlations indicate that association and inhibition of β-hematin formation are separately determined. They also suggest that electron withdrawing groups at the 7-position, but not necessarily hydrophobic groups are required for hemozoin inhibition. The upshot is that the correlations imply that considerably more hydrophilic hemozoin inhibitors are feasible.