Corticotropin Releasing Factor (CRF) Agonists with Reduced Amide Bonds and Ser7Substitutions

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Abstract
Strategies to generate competitive antagonists of bioactive peptides include several possible structural modifications such as the introduction of d-residues and of reduced amide bonds, the substitution of amino acid side chains, dimerization of fragments, and deletion of part of the sequence, among others. Whereas we have identified the two most likely residues responsible for receptor activation in corticotropin releasing factor (CRF) (Ser7 and Leu8)1 and generated potent antagonists by deleting residues 1−8,2,3 the question remained as to whether we could generate CRF antagonists with enhanced affinity after reduction of amide bonds at the N-terminus of CRF or through subtle alteration of those residues' side chains. Reduced amide bond replacements (ψ[CH2NH]) between residues 6−9 in oCRF(5-41) (11, 12, 15) analogues consistently yielded potencies of 10ψ11 and 12ψ13 analogues 19 and 20, reduced amide bond replacements were generally well-tolerated in the longer hCRF(4-41) analogues, with the 7ψ8-, 8ψ9-, and 9ψ10-modified peptides (13, 14, 18) yielding potencies that were 2−4 times that of hCRF. Although somewhat promising as agonists, they were, however, 3−7 times less potent than the parent [d-Pro4Nle21,38]-hCRF(4-41) (2). Since O-alkylation of Tyr3 in vasopressin yields an antagonist, and since Ser7 is one of the eight fully conserved residues in the CRF family (inclusive of sauvagine, urocortins, and urotensins) and likely to be critical for receptor binding, we synthesized cyclo(30−33)[Ser(OMe)7,d-Phe12,Nle21,Glu30,Lys33,Nle38]Ac-hCRF(7-41) (22), which was found to exhibit full efficacy and 40% of the potency of cyclo(30−33)[d-Phe12,Nle21,Glu30,Lys33,Nle38]Ac-hCRF(7-41) (5). Other substitutions at position 7 included aminoglycine (23, 24) and alkylated and/or acylated [α or α‘-methyl (25−28), α‘-formyl (29, 30), α‘-formyl, α‘-methyl (31), α‘-acetyl (32), α‘-acetyl, α‘-methyl (33)], d- or l-aminoglycines. All analogues were active although less potent than the parent compound 2, and all elicited maximal ACTH response as compared to hCRF. The most potent analogue in this series (33) had the bulkiest side chain, Agl(Me, Ac), and was 60% and 80% as potent as the Ser7 analogue 5 and the Ala7 analogue 6, respectively. In conclusion, we found that neither reduction of the individual amide linkages between residues 6−11 and 12−13 nor introduction of a carbamide moiety in lieu of the side chain of Ser7 led to CRF antagonists.