Do Resting Plasma β-Endorphin Levels Predict Responses to Opioid Analgesics?

Abstract

Objectives Clinically-feasible predictors of opioid analgesic responses for use in precision pain medicine protocols are needed. This study evaluated whether resting plasma beta-endorphin (BE) levels predicted responses to an opioid analgesic, and whether chronic pain status or gender moderated these effects. Methods Participants included 73 individuals with chronic low back pain (CLBP) and 88 healthy controls, all using no daily opioid analgesics. Participants attended two identical laboratory sessions during which they received either intravenous morphine (0.08 mg/kg) or saline placebo, with blood samples obtained before drug administration to assay resting plasma BE levels. Once peak drug activity was achieved in each session, participants engaged in an ischemic forearm pain task (ISC) and a heat pain task. Morphine analgesic effects were derived reflecting the difference in pain outcomes between placebo and morphine conditions. Results In hierarchical regressions, significant Type (CLBP vs. Control) X BE interactions (p’s<.05) were noted for morphine effects on ISC tolerance, ISC intra-task pain ratings, and thermal VAS unpleasantness ratings. These interactions derived primarily from associations between higher BE levels and smaller morphine effects restricted to the CLBP subgroup. All other BE-related effects, including gender interactions, for predicting morphine analgesia failed to reach statistical significance. Discussion BE was a predictor of morphine analgesia for only 3 out of 9 outcomes examined, with these effects moderated by chronic pain status but not gender. On the whole, results do not suggest that resting plasma BE levels are likely to be a clinically useful predictor of opioid analgesic responses.

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