Synthesis and α1-antagonist activity of new prazosin- and benextramine-related tetraamine disulfides

Analogues of prazosin that bear a benextramine-related polyamine backbone exhibit different antagonism toward alpha1-adrenoreceptor subtypes

Hybrid tetraaamine disulfides 4-9 were synthesized by combining the structural features of prazosin (1), a competitive alpha1-adrenoreceptor antagonist, and benextramine (2), an irreversible alpha1/alpha2-adrenoreceptor antagonist, and their biological profiles at alpha1-adrenoreceptor subtypes were assessed by functional experiments in isolated rat vas deferens (alpha1A), spleen (alpha1B), and aorta (alpha1D). To verify the role of the disulfide moiety on the interaction with alpha1-adrenoreceptor subtypes, carbon analogues 10-15 were included in this study. All quinazolines lacking the disulfide bridge behaved, like 1, as competitive antagonists, whereas all polyamine disulfides displayed a nonhomogeneous mechanism of inhibition at the three subtypes since they were, like 2, noncompetitive antagonists at the alpha1A and alpha1B subtypes while being, unlike 2, competitive antagonists at the alpha1D. In particular, the blocking effects were characterized by a decrease of the maximal response to noradrenaline that was affected only slightly by washings. Probably the alpha1A and alpha1B subtypes bear in the binding pocket a suitable thiol function that would suffer an interchange reaction with the disulfide moiety of the antagonist and which is missing, or not accessible, in the alpha1D subtype. Polyamines 8, 9, and 14, among others, emerged as promising tools for the characterization of alpha1-adrenoreceptors, owing to their receptor subtype selectivity. Finally, the effect of nonbasic substituents on the phenyl ring of prazosin analogues 16-28 on potency and selectivity for the different subtypes can hardly be rationalized.

Search for selective antagonists at alpha 1-adrenoreceptors: neutral or negative antagonism?

In this article the use of competitive antagonists as tools in receptor characterization and classification is discussed. It is pointed out that caution is required in receptor characterization because negative antagonism (inverse agonism) rather than neutral antagonism could play a relevant role. This implies that antagonists should be evaluated not only with regard to their affinity, but also with regard to their ability to affect the equilibrium between the two receptor states, namely active and inactive states. Since affinity and efficacy of a negative antagonist are system dependent the use of negative antagonists as competitive antagonists in receptor characterization may give rise to false differences in receptor subtypes. Finally, this article summarizes recent developments in the design of new alpha 1-adrenoreceptor antagonists which are structurally related to prazosin or WB 4101.