Minimal-Size, Constrained Corticotropin-Releasing Factor Agonists with i−(i+3) Glu−Lys and Lys−Glu Bridges

In three earlier publications (Miranda et al. J. Med. Chem. 1994, 37, 1450−1459; 1997, 40, 3651−3658; Gulyas et al. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 10575−10579) we have hypothesized that covalent constraints such as side-chain-to-side-chain lactam rings would stabilize an α-helical conformation shown to be important for the recognition and binding of the CRF C-terminus 30 residues, to CRF receptors. These studies led to the discovery of useful CRF antagonists such as α-helical CRF (α-hel-CRF) and Astressin both in vitro and in vivo. To test the hypothesis that such lactam rings may also be modulating activation of the receptor when introduced at the N-terminus of CRF, we studied the influence of the successive introduction from residues 4 to 14 of a cyclo(i,i+3)[Lys i −Glu( i +3)] and a cyclo(i,i+3)[Glu i −Lys( i +3)] bridge on the in vitro potency of the agonist [Ac-Pro4,dPhe12,Nle21,38]hCRF(4 - 41) and related compounds. We have also introduced the favored cyclo(Glu30−Lys33) substitution found to be remarkable in several families of antagonists (such as Astressin) and in a number of CRF agonists and investigated the role of residues 4−8 on receptor activation using successive deletions. Earlier studies had shown that in both oCRF and α-helical CRF, deletion of residues 1−6, 1−7, and 1−8 led to gradual loss of intrinsic activity (IA) (from 50% IA to