Investigations on the structure-activity relationships of verapamil

An investigation was carried out towards a qualitative and quantitative structure-activity relationship of verapamil based on an analysis of the frequency-dependent negative inotropic action exerted in cat papillary muscles by various groups of verapamil derivatives. (1) Substituents of the benzene ring near the asymmetric carbon atom and the isopropyl group were found to be no essential substituents for the frequency-dependent negative inotropic action of verapamil but to have a strong influence on the potency of the drug. (2) Both the tertiary amino nitrogen and the two benzene rings are essential for the frequency-dependent negative inotropic action of verapamil (3) The molecular importance of the N-methyl group is probably based on steric effects. (4) Investigations of the verapamil derivative H 1 revealed that a quaternization of the drug is followed by a total loss of effectiveness. (5) No significant correlation of the biological activity of verapamil derivatives with the partition coefficient P has been obtained. (6) Hansch analysis with verapamil derivatives of group A (= different substitution of the benzene ring near C) shows that the variance of biological activity can be optimally correlated to a combination of the Hammett constant and the molar volume. Hansch analysis of group B (= exchange of isopropyl group) led to the conclusion that hydrophobic effects are responsible for the influence of the isopropyl substituent.