Background: Opioid-induced constipation (OIC) is a distressing side effect during opioid analgesia and is mainly mediated by gastrointestinal μ opioid receptors. Methylnaltrexone, a peripheral μ opioid receptor antagonist with restricted ability to cross the blood-brain barrier, may alleviate OIC without reversing analgesia. We performed a meta-analysis to assess the efficacy and safety of methylnaltrexone for the treatment of OIC.
Methods: We searched PubMed, Embase, and Cochrane Library for randomized controlled trials that compared methylnaltrexone with placebo for the treatment of OIC. The primary efficacy outcome was rescue-free bowel movement (RFBM) within 4 hours after the first dose. Secondary efficacy outcomes included RFBM within 24 hours after the first dose, RFBM ≥3 times per week, and need take rescue laxatives. The primary safety outcome was any adverse events. Secondary safety outcomes included abdominal pain, diarrhea, nausea, vomiting, and flatulence. Relative risks (RR) and 95% confidence interval (CI) were pooled using random-effects model with the intention-to-treat principle. We used the GRADE approach to assess the certainty of the evidence.
Results: Eight trials with 2,034 participants were included. Compared with placebo, methylnaltrexone significantly increased RFBM within 4 hours after the first dose (8 trials; 1,833 participants; RR 3.74, 95% CI 3.02-4.62; I2 = 0%; high-certainty evidence), RFBM within 24 hours after the first dose (2 trials; 614 participants; RR 1.98, 95% CI 1.52-2.58; I2 = 9%; moderate-certainty evidence), and RFBM ≥3 times per week (3 trials; 1,396 participants; RR 1.33, 95% CI 1.17-1.52; I2 = 0%; moderate-certainty evidence) and decreased need to take rescue laxatives (3 trials; 807 participants; RR 0.73, 95% CI 0.63-0.85; I2 = 0%; moderate-certainty evidence). For safety outcomes, there was no difference in any adverse events between the two groups (8 trials; 2,034 participants; RR 1.11, 95% CI 0.99-1.23; I2 = 34%; moderate-certainty evidence), including diarrhea, nausea, vomiting, and flatulence; but for the most commonly reported adverse events, the abdominal pain was higher in methylnaltrexone group than that in placebo group (6 trials; 1,813 participants; RR 2.30, 95% CI 1.29-4.08; I2 = 62%; moderate-certainty evidence).
Conclusions: Methylnaltrexone is an effective and safe drug for treating OIC. But the safety of abdominal pain should be considered.
Trial registration: PROSPERO (CRD42020187290).