Elucidation of vasoactive intestinal peptide pharmacophore for VPAC₁ receptors in human, rat, and guinea pig

Vasoactive intestinal peptide (VIP) is a neurotransmitter involved in a number of pathological and physiological processes. VIP is rapidly degraded and simplified stable analogs are needed. VIP's action was extensively studied in rat and guinea pig. However, it is largely unknown whether its pharmacophore in these species resembles human. To address this issue we investigated the VIP pharmacophore for VPAC₁ (the predominant receptor subtype in cancers and widely distributed in normal tissues) by using alanine and D-amino acid scanning. Interaction with rat, guinea pig, and human VPAC₁ was assessed using transfected Chinese hamster ovary (CHO) and PANC1 cells and cells possessing native VPAC₁. Important species differences existed in the VIP pharmacophore. The human VPAC₁ expressed in CHO cells, which were used almost exclusively in previous studies, differed markedly from the native VPAC₁ in T47D cells. The most important amino acids for determining affinity are His¹, Asp³, Phe⁶, Arg¹², Arg¹⁴, and Leu²³. Ser², Asp⁸, Asn⁹, Thr¹¹, Val¹⁹, Asn²⁴, Ser²⁵, Leu²⁷, and Asn²⁸ are not essential for high-affinity interaction/activation. [Ala^{2,8,9,11,19,24,25,27,28}]VIP, which contained 11 alanines, was synthesized and it was equipotent to VIP at VPAC₁ receptors in all species and was metabolically stable. Our results show in any design of simplified VIP analogs for VPAC₁ it will be important to consider species differences and it is essential to use transfected systems that reflect the native receptor's pharmacophore. Last, with our results a simplified, metabolically stable VIP analog was identified that should be useful as a prototype for design of selective agonists/antagonists that could be useful therapeutically.