Elucidation of the vasoactive intestinal peptide pharmacophore for VPAC₂ receptors in human and rat and comparison to the pharmacophore for VPAC₁ receptors

Vasoactive intestinal peptide (VIP) functions as a neurotransmitter involved in a number of physiological and pathological conditions. The actions of VIP are mediated through VPAC₁ and VPAC₂. In contrast to VPAC₁, which has been extensively studied, little is known about the pharmacology of VPAC₂. In this study we investigated the VIP pharmacophore for VPAC₂ by using alanine and D-amino acid scanning. We found significant species differences, and the human VPAC₂ (hVPAC₂) expressed in Chinese hamster ovary (CHO) cells, which have been used in previous studies, differed significantly from the native hVPAC₂ in Sup T₁ cells and hVPAC₂ expressed in PANC1 cells. There was a close agreement between binding affinities and potencies for VPAC₂ activation. The amino acids whose backbone or side chain orientations were most important for high affinity potency are Asp³, Phe⁶, Thr⁷, Thr¹⁰, Arg¹², Tyr²², and Leu²³, whereas the side chains of Ser², Asp⁸, Asn⁹, Gln¹⁶, Val¹⁹, Lys²⁰, Asn²⁴, and Ser²⁵ are not essential. Comparison of the VIP pharmacophore between hVPAC₁ hVPAC₂ demonstrated that the side chains of Thr⁷, Tyr¹⁰, Thr¹¹, and Tyr²² were much more critical for high affinity for the hVPAC₂ than the hVPAC₁. In contrast, the orientation of the side chain of Asn²⁴ was more important for high affinity for the hVPAC₁. This study shows that in assessing the pharmacophore of VIP analogs for the VPAC₂, important species differences need to be considered as well as the expression system used. These results of our study should be useful for designing VPAC subtype-selective analogs, simplified analogs, and possibly metabolically stable analogs.