Cyclic penicillamine containing enkephalin analogs display profound delta receptor selectivities

The cyclic, penicillamine(beta, beta dimethylcysteine)-containing enkephalin analogs, [D-Cys2, L-Pen5]-and [D-Cys2, D-Pen5]enkephalin and the corresponding bis-penicillamine analogs, [D-Pen2, L-Pen5]-and [D-Pen2, D-Pen5]enkephalin were synthesized and evaluated for opioid activity in the guinea pig ileum (GPI) and mouse vas deferens (MVD) bioassays and in rat brain and neuroblastoma-glioma cell membrane binding assays. These analogs all displayed delta receptor selectivity as assessed by IC50(GPI)/IC50(MVD) ratios and by their relative potencies for displacing [3H]naloxone (NAL) vs. [3H] [D-Ala2, D-Leu5]enkephalin (DADLE) from rat brain membrane preparations. For [D-Pen2, L-Pen5]- and [D-Pen2, D-Pen5]enkephalin the observed IC50(GPI)/IC50 (MVD) ratios (1088 and 3164) and IC50NAL/IC50DADLE ratios (371 and 175) represent a vast improvement over previously reported delta receptor selective ligands.