INVESTIGATION OF THE N-SUBSTITUENT CONFORMATION GOVERNING POTENCY ANDMU-RECEPTOR SUBTYPE-SELECTIVITY IN (-(3R,4R)-DIMETHYL-4-(3-HYDROXYPHENYL)PIPERIDINE OPIOID ANTAGONISTS())

A study of the binding site requirements associated with the N-substit uent of (+)-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (4) derivat ives was undertaken using a set of rigid vs flexible N-substituents. T he study showed that compounds 7-9 bearing the trans-cinnamyl N-substi tuent most closely reproduced the potency at the opioid receptor of th e flexible N-propylphenyl or N-propylcyclohexyl analogues previously r eported. Neither the N-substituted cis-cinnamyl nor the cis-phenylcycl opropylmethyl compounds 10 and 11, respectively, showed high affinity for the opioid receptor. However, the N-trans-phenylcyclopropylmethyl compound 12 closely approximated the affinity of compounds 7-9. Additi onally, we found that free rotation of the phenyl ring is necessary fo r high affinity binding and mu receptor subtype selectivity as the pla nar N-substituted thianaphthylmethyl and benzofuranylmethyl compounds 13 and 14 had significantly lower binding affinities. Altogether, thes e findings suggest that the high binding affinity, selectivity, and an tagonist potency of N-propylphenyl or N-propylcyclohexyl analogues of (+)-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) (4) are achieved via a confor mation wherein the connecting chain of the N-substituents is extended away from piperidine nitrogen with the appended ring system rotated ou t-of-plane relative to the connecting chain atoms. This conformation i s quite similar to that observed in the solid state for 5, as determin ed by single crystal X-ray analysis. Additionally, it was found that, unlike naltrexone, N-substituents bearing secondary carbons attached d irectly to the piperidine nitrogen of 4 suffer dramatic losses of pote ncy vs analogues not substituted in this manner. Using a functional as say which measured stimulation or inhibition of [S-35]GTP-gamma-S bind ing, we show that the trans-cinnamyl analogues of (+)-(3R,4R)-dimethyl -4-(3-hydroxyphenyl)piperidine (4) retain opioid pure antagonist activ ity and possess picomolar antagonist potency at the mu receptor.