Exploration of the DTrp-NMeLys motif in the search for potent somatostatin antagonists

Previous studies from this laboratory demonstrated that N-methylation at Lys(5) residue in somatostatin octapeptide antagonist analogues increased the GH release inhibition potency by as much as 300%. We have now further investigated N-methylation of this Lys(5) residue in conjunction with a number of N- and C-terminal modifications previously found to give highly potent somatostatin receptor antagonists. Synthetic analogues were tested in a functional assay for their ability to inhibit somatostatin-inhibited GH release from rat pituitary cells in culture and to displace 125I-labeled somatostatin from CHO cells transfected with the five known human somatostatin receptors. Several interesting observations resulted from the study. Replacement of liphophilic Nal(8) at the C-terminus with a hydrophilic His(8) resulted in the increased affinity and selectivity for type 2 receptor to give the most potent antagonist analogue yet discovered (K(i), 1.5 nM), although in the rat pituitary cells inhibitory activity on somatostatin inhibited GH release decreased somewhat. A His(3) substitution within the cyclic portion of the analogues retained pituitary cell potency and affinity for type 2 receptor as did substitution with Bip(8) and Fpa(1). Replacement of Cpa(1) with Iph(1) did not effect the affinity for type 2 receptor significantly, but did decrease the effects on rat cell GH release. Iph(3) within-ring substitution increased the selectivity for sst(2) appreciably although the affinity for that receptor was considerably decreased. Substitution of Npa(3) resulted in good selectivity for sst(2) receptor. Replacement of Nal(8) with D-Trp(8) also increased the selectivity for type 2 receptor. Use of a 'bivalent ligand' approach in which two peptides were joined by 4,4'-biphenyldicarbonyl as a spacer destroyed the affinity for all the subtypes, however, the bivalent ligand formed with the Ahp spacer displayed significant affinity and high selectivity for the type 2 receptor.