Structure-activity studies of the Phe(4) residue of nociceptin(1-13)-NH2: Identification of highly potent agonists of the nociceptin/orphanin FQ receptor

A total of 32 compounds was prepared to investigate the functional role of Phe(4) in NC(1-13)NH2, the minimal sequence maintaining the same activity a s the natural peptide nociceptin. These compounds could be divided into thr ee series in which Phe(4) was replaced with residues that would (i) alter a romaticity or side chain length, (ii) introduce steric constraint, and (iii ) modify the phenyl ring. Compounds were tested for biological activity as (a) inhibitors of the electrically stimulated contraction of the mouse vas deferens; (b) competitors of the binding of [H-3]-NC-NH2 to mouse brain mem branes; and (c) inhibitors of forskolin-stimulated cAMP accumulation in CHO cells expressing the recombinant human OP4 receptor. Results indicate that all compounds of the first and second series were inactive or very weak wi th the exception of [N(CH3)Phe(4)]NC(1-13)-NH2, which was only 3-fold less potent than NC(1-13)-NH2.Compounds of the third series showed higher, equal , or lower potencies than NC(1-13)-NH2. In particular, [(pF)Phe(4)]NC(1-13) -NH2 (pF) and [(pNO(2))Phe(4)]NC(1-13)-NH2 (pNO(2)) were more active than N C(1-13)-NH2 by a factor of 5. In the mVD, these compounds showed the follow ing order of potency: (pF) = (pNO(2)) greater than or equal to (pCN) > (pCl ) > (pBr) > (pI) = (pCF(3)) = (pOCH(3)) > (pCH(3)) > (PNH2) = (pOH). (oF) a nd especially (mF) maintained high potencies but were less active than (pF) , Similar orders of potency were observed in binding competition and cAMP a ccumulation studies. There was a strong (r(2) greater than or equal to 0.66 ) correlation between data observed in these assays. Biological activity da ta of compounds of the third series were plotted against some Hansch parame ters that are currently used to quantify physicochemical features of the su bstituents. In the three biological assays agonist potency/affinity positiv ely correlates with the electron withdrawal properties of the groups in the p-position of Phe(4) and inversely with their size.