Vol. 303, Issue 2, 445-460, November 2002

Elucidation of the Vasoactive Intestinal Peptide Pharmacophore for VPAC₂ Receptors in Human and Rat and Comparison to the Pharmacophore for VPAC₁ Receptors

Hisato Igarashi, Tetsuhide Ito, Tapas K. Pradhan, Samuel A. Mantey, Wei Hou, David H. Coy and Robert T. Jensen

Digestive Diseases Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland (H.I., T.I., T.K.P., S.A.M., W.H., R.T.J.); and Peptide Research Laboratories, Department of Medicine, Tulane University Health Sciences Center, New Orleans, Louisiana (D.H.C.)

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, Tyr¹⁰, Arg¹², Tyr²², and Leu²³, whereas the side chains of Ser², Asp⁸, Asn⁹, Gln¹⁶, Val¹⁹, Lys²⁰, Lys²¹, Asn²⁴, and Ser²⁵ are not essential. Comparison of the VIP pharmacophore between hVPAC₁ and 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.