GI side-effects of a possible therapeutic GRF analogue in monkeys are likely due to VIP receptor agonist activity

Growth hormone (GH) is used or is being evaluated fbr efficacy in treatment of short stature, aspects of aging, cardiac disorders, Crohn's disease, an d short bowel syndrome. Therefore, we synthesized several stable growth hor mone-releasing factor (GRF) analogues that could be therapeutically useful. One potent analog, [D-Ala(2),Aib(8,18)Ala(9, 15, 16, 22, 24-26.)Gab(27)]hG RF(1-27)NH2 (GRF-6), with prolonged infusion caused severe diarrhea in monk eys; however, it had no side-effects in rats. Because GRF has similarity to VIP/PACAP and VIPomas cause diarrhea, this study investigated the ability of this and other CRF analogues to interact with the VIP/PACAP receptors. R at VPAC(1)-R (rVPAC(1)-R), human VPAC(1)-R (hVPAC(1)-R), rVPAC(2)-R and hVP AC(2)-R stably transfected CHO and PANC 1 cells were made and T47D breast c ancer cells containing native human VPAC(1)-R and AR4-2J cells containing P AC(1)-R were used. hGRF(1-29)NH2 had low affinity for both rVPAC(1)-R and r VPAC(2)-R while VIP had a high affinity for both receptors. GRF-6 had a low affinity for both rVPAC(1)-R and rVPAC(2)-R and very low affinity for the rPAC(1)-R. VIP had a high affinity, whereas hGRF(1-29)NH, had a low affinit y for both hVPAC(1)-R and hVPAC(2)-R. In contrast GRF-6, while having a low affinity for hVPAC(2)-R, had relatively higher affinity for the hVPAC(1)-R . In guinea pig pancreatic acini, all GRF analogues were full agonists at t he VPAC1-R causing enzyme secretion. These results demonstrate that in cont rast to native hGRF(1-29)NH2 GRF-6 has a relatively high affinity for the h uman VPAC(1)-R but not for the human VPAC(2)-R, rat VPAC(1)-R, rat VPAC(2)- R or rat PAC(1)-R. These results suggest that the substituted GRF analog, G RF-6, likely causes the diarrheal side-effects in monkeys by interacting wi th the VPAC(1)-R. Furthermore, they demonstrate significant species differe nces can exist for possible therapeutic peptide agonists of the VIP/PACAP/G RF receptor family and that it is essential that receptor affinity assessme nts be performed in human cells or from a closely related species. (C) 2001 Published by Elsevier Science, Inc.