Conformationally restricted analogs of somatostatin with high mu-opiate receptor specificity

A series of cyclic, conformationally restricted analogs of somatostatin have been prepared and tested for their ability to inhibit the binding of [3H]naloxone and [D-Ala2, D-Leu5] [3H]enkephalin to rat brain membranes. The most potent analog, D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 where Pen is penicillamine in [D-Phe5, Cys6, Tyr7, D-Trp8, Pen11]somatostatin-(5-12)-octapeptide amide, exhibited high affinity for mu-opiate receptors (IC50 value of [3H]naloxone = 3.5 nM), being 7800 times more potent than somatostatin. The cyclic octapeptide also displayed high mu-opiate receptor selectivity with an IC50 [( D-Ala2,D-Leu5]enkephalin)/IC50 (naloxone) ratio of 271. The high affinity and selectivity of the somatostatin analog for mu-opiate receptors may be of use in examining the physiological role(s) of the mu-opiate receptor.