• Edward Michna, Arnold J Weil, Marc Duerden, Seth Schulman, Wenjin Wang, Evan Tzanis, Haiying Zhang, Dahong Yu, Amy Manley, and Bruce Randazzo.
• Pain Management Center, Brigham and Women's Hospital, Boston, Massachusetts 02467, USA. emichna@partners.org
• Pain Med. 2011 Aug 1;12(8):1223-30.

ObjectiveMethylnaltrexone, a selective peripherally acting mu-opioid receptor antagonist, effectively treats opioid-induced constipation (OIC) in patients with advanced illness and shows efficacy in patients with chronic nonmalignant pain. The objective was to identify patients who achieved maximal treatment effect based on response to initial four methylnaltrexone doses.DesignA post hoc analysis of a randomized, double-blind, placebo-controlled study evaluating patients with OIC and chronic nonmalignant pain who received 12 mg subcutaneous methylnaltrexone daily for 4 weeks was performed to determine if response to the first four methylnaltrexone doses predicted overall response during the study. Patients receiving ≥8 doses were included.Outcome MeasuresPatients having ≥3 rescue-free bowel movements (RFBMs)/week; change from baseline in RFBMs/week; percentage of doses with RFBMs within 4 hours after dosing.ResultsOf 137 patients, 58 patients (42.3%) had RFBMs after ≥2 of four doses. Among those with response to ≥2 of four doses, 81% had ≥3 RFBMs/week vs. 43% for those with response to <2 of four (P < 0.0001). Those with RFBMs after ≥2 of first four doses averaged 4.8 RFBMs/week vs. 2.0 RFBMs/week for those with <2 of four (P < 0.0001). Percentage of subsequent injections resulting in RFBMs within 4 hours was 45.9 ± 27.6 for those with response to ≥2 of four doses vs. 17.1 ± 19.1 for those with response to <2 of four (P < 0.0001). Abdominal pain was the most frequently reported adverse event.ConclusionEarly response to ≥2 of first four doses of methylnaltrexone identified patients who demonstrated a particularly robust effect of treatment over the duration of use.Wiley Periodicals, Inc.

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