Receptor reserve reflects differential intrinsic efficacy associated with opioid diastereomers.
Structure-activity relationships built around receptor binding or cell-based assays are designed to reveal physiochemical differences between ligands. We hypothesized that agonist receptor reserve may provide a unique approach to distinguish structurally-related agonists exhibiting similar functional characteristics. An intracellular calcium activation assay in Chinese Hamster Ovary (CHO) cells expressing cloned human mu-opioid receptors was developed. We examined two isomers exhibiting indistinguishable receptor binding and in vitro potency profiles. Oxymorphone, a clinically-available congener of codeine has at least two active diastereomeric metabolites (6alpha- and 6beta-oxymorphols) found to be similar for mu-opioid receptor binding affinity (K(d) = 15 versus 14 nM) and calcium activation (EC(50) = 22 versus 14 nM). Calcium activation was then inhibited in CHO cells in a concentration-dependent manner using the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA). Under these conditions, approximately 10-fold greater receptor reserve was found for 6alpha-oxymorphol compared to 6beta-oxymorphol. This difference between the oxymorphols corresponded to a rank order of intrinsic efficacy (Emax): DAMGO > oxymorphone = 6alpha-oxymorphol = oxycodone > 6beta-oxymorphol. In addition, 6alpha-oxymorphol exhibited greater relative potency than the 6beta-oxymorphol in mouse tail-flick, hot-plate and phenylquinone writhing antinociceptive assays, regardless of route of administration. Thus the beta-FNA/calcium model provides a novel, cell-based approach to distinguish structurally related mu-opioid agonists, and in the specific case of the oxymorphols, receptor reserve differences provided a means to bridge functional in vitro and in vivo models.
Carliss
Richard D S
RD
University of South Alabama, Mobile, AL, USA. rcarliss@gmail.com
Keefer
James F
JF
Perschke
Scott
S
Welch
Sandra
S
Rich
Thomas C
TC
Weissman
Arthur D
AD
D000701:Analgesics, Opioid / Q000378:metabolism / Q000494:pharmacology*; D000818:Animals; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D006801:Humans; D051379:Mice; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors* / Q000378:metabolism; D012965:Sodium Chloride / Q000494:pharmacology; D013237:Stereoisomerism
D000701:Analgesics, Opioid; D017450:Receptors, Opioid, mu; D012965:Sodium Chloride; D009271:Naltrexone; C024524:beta-funaltrexamine