Pharmacotherapeutic group: Laxatives, Peripheral opioid receptor antagonists, ATC code: A06AH01
Mechanism of action
Methylnaltrexone bromide is a selective antagonist of opioid binding at the mu-receptor. In vitro studies have shown methylnaltrexone bromide to be a mu-opioid receptor antagonist (inhibition constant [Ki] = 28 nM), with 8-fold less potency for kappa opioid receptors (Ki = 230 nM) and much reduced affinity for delta opioid receptors.
As a quaternary amine, the ability of methylnaltrexone bromide to cross the blood-brain barrier is restricted. This allows methylnaltrexone bromide to function as a peripherally acting mu-opioid antagonist in tissues such as the gastrointestinal tract, without impacting opioid-mediated analgesic effects on the central nervous system.
Clinical efficacy and safety
Opioid-induced constipation in adult patients with chronic non-cancer pain (12 mg dose)
The efficacy and safety of methylnaltrexone bromide in the treatment of opioid-induced constipation in patients with chronic non-cancer pain were demonstrated in a randomized, double‑blind, placebo‑controlled study (Study 3356). In this study, the median patient age was 49 years (range 23‑83); 60% were females. The majority of patients had a primary diagnosis of back pain.
Study 3356 compared 4-week treatment regimens of methylnaltrexone bromide 12 mg once daily and methylnaltrexone bromide 12 mg every other day with placebo. The 4-week, double-blind period was followed by an 8‑week, open-label period during which methylnaltrexone bromide was to be used as needed, but no more frequently than once daily. A total of 460 patients (methylnaltrexone bromide 12 mg once daily, n = 150, methylnaltrexone bromide 12 mg every other day, n = 148, placebo, n = 162) were treated in the double-blind period. Patients had a history of chronic non-cancer pain and were taking opioids with stable doses of at least 50 mg of oral morphine equivalents per day. Patients had opioid‑induced constipation (< 3 rescue medication-free bowel movements per week during the screening period). Patients were required to discontinue all previous laxative therapy.
The first co-primary endpoint was the proportion of patients having a rescue free bowel movements (RFBMs) within 4 hours of the first dose administration and the second the percentage of active injections resulting in any RFBM within 4 hours during the double-blind phase. A RFBM was defined as a bowel movement that occurred without laxative use during the previous 24 hours.
The proportion of patients having an RFBM within 4 hours of the first dose was 34.2% in the combined methylnaltrexone bromide group versus 9.9% in the placebo group (p<0.001). The mean percentage of methylnaltrexone bromide resulting in any RFBM within 4 hours were 28.9% and 30.2% respectively for the once daily and every other day dose groups compared with 9.4% and 9.3% respectively for the corresponding placebo regimen for daily and every other day dosing (p <0.001) during the double blind phase.
The key secondary endpoint of adjusted mean change from baseline in weekly RFBMs was 3.1 in the methylnaltrexone bromide 12 mg once daily treatment group, 2.1 in the methylnaltrexone bromide 12 mg every other day treatment group, and 1.5 in the placebo treatment group during the 4-week double-blind period. The difference between methylnaltrexone bromide 12 mg once daily and placebo of 1.6 RFBMs per week is statistically significant (p < 0.001) and clinically meaningful.
Another secondary endpoint evaluated the proportion of patients with ≥3 RFBMs per week during the 4‑week double-blind phase. This was achieved in 59% of the patients in the group receiving daily methylnaltrexone 12 mg (p<0.001 vs. placebo), in 61% of those receiving it every other day (p<0.001 vs. placebo), and in 38% of the placebo treated patients. A supplementary analysis evaluated a hard endpoint of the percentage of patients achieving ≥3 complete RFBMs per week and an increase of ≥1 complete RFBMs per week in at least 3 of the 4 treatment weeks. This was achieved in 28.7% of the patients in the group receiving daily methylnaltrexone 12 mg (p<0.001 vs. placebo), in 14.9% of those receiving it every other day (p=0.012 vs. placebo), and in 6.2% of the placebo treated patients.
There was no evidence of a differential effect of gender on safety or efficacy. The effect on race could not be analysed because the study population was predominantly Caucasian (90 %). Median daily opioid dose did not vary meaningfully from baseline in either methylnaltrexone bromide-treated patients or in placebo-treated patients.
There were no clinically relevant changes from baseline in pain scores in either the methylnaltrexone bromide or placebo-treated patients.
The use of methylnaltrexone bromide for treating opioid-induced constipation beyond 48 weeks has not been evaluated in clinical trials.
Opioid-induced constipation in adult patients with advanced illness
The efficacy and safety of methylnaltrexone bromide in the treatment of opioid-induced constipation in patients receiving palliative care was demonstrated in two randomised, double-blind, placebo-controlled studies. In these studies, the median age was 68 years (range 21-100); 51 % were females. In both studies, patients had advanced terminal illness and limited life expectancy, with the majority having a primary diagnosis of incurable cancer; other primary diagnoses included end-stage COPD/emphysema, cardiovascular disease/heart failure, Alzheimer's disease/dementia, HIV/AIDS, or other advanced illnesses. Prior to screening, patients had opioid-induced constipation defined as either <3 bowel movements in the preceding week or no bowel movement for >2 days.
Study 301 compared methylnaltrexone bromide given as a single, double-blind, subcutaneous dose of 0.15 mg/kg, or 0.3 mg/kg versus placebo. The double-blind dose was followed by an open-label, 4‑week dosing period, where methylnaltrexone bromide could be used as needed, no more frequently than 1 dose in a 24-hour period. Throughout both study periods, patients maintained their usual laxative regimen. A total of 154 patients (methylnaltrexone bromide 0.15 mg/kg, n = 47; methylnaltrexone bromide 0.3 mg/kg, n = 55, placebo, n = 52) were treated in the double-blind period. The primary endpoint was the proportion of patients with a rescue-free laxation within 4 hours of the double-blind dose of study medicinal product. Methylnaltrexone bromide-treated patients had a significantly higher rate of laxation within 4 hours of the double-blind dose (62 % for 0.15 mg/kg and 58 % for 0.3 mg/kg) than placebo-treated patients (14 %); p<0.0001 for each dose versus placebo.
Study 302 compared double-blind, subcutaneous doses of methylnaltrexone bromide given every other day for 2 weeks versus placebo. During the first week (days 1, 3, 5, 7), patients received either methylnaltrexone bromide 0.15 mg/kg or placebo. In the second week, a patient's assigned dose could be increased to 0.30 mg/kg if the patient had 2 or fewer rescue-free laxations up to day 8. At any time, the patient's assigned dose could be reduced based on tolerability. Data from 133 (62 methylnaltrexone bromide, 71 placebo) patients were analysed. There were 2 primary endpoints: proportion of patients with a rescue-free laxation within 4 hours of the first dose of study medicinal product and proportion of patients with a rescue-free laxation within 4 hours after at least 2 of the first 4 doses of medicinal product. Methylnaltrexone bromide-treated patients had a higher rate of laxation within 4 hours of the first dose (48 %) than placebo-treated patients (16 %); p<0.0001. Methylnaltrexone bromide-treated patients also had significantly higher rates of laxation within 4 hours after at least 2 of the first 4 doses (52 %) than did placebo-treated patients (9 %); p<0.0001. Stool consistency was not meaningfully improved in patients who had soft stool at baseline.
In both studies, there was no evidence to suggest differential effects of age or gender on safety or efficacy. The effect on race could not be analysed because the study population was predominantly Caucasian (88 %).
Durability of response was demonstrated in Study 302, in which the laxation response rate was consistent from dose 1 through dose 7 over the course of the 2-week, double-blind period.
The efficacy and safety of methylnaltrexone bromide were also demonstrated in open-label treatment administered from Day 2 through Week 4 in Study 301, and in two open-label extension studies (301EXT and 302EXT) in which methylnaltrexone bromide was given as needed for up to 4 months (only 8 patients up to this point). A total of 136, 21, and 82 patients received at least one open-label dose in studies 301, 301EXT, and 302EXT, respectively. Relistor was administered every 3.2 days (median dosing interval, with a range of 1-39 days).
The rate of laxation response was maintained throughout the extension studies for those patients who continued treatment.
There was no significant relationship between baseline opioid dose and laxation response in methylnaltrexone bromide-treated patients in these studies. In addition, median daily opioid dose did not vary meaningfully from baseline in either methylnaltrexone bromide-treated patients or in placebo-treated patients. There were no clinically relevant changes in pain scores from baseline in either the methylnaltrexone bromide or placebo-treated patients.
Effect on cardiac repolarisation
In a double-blind, randomised, parallel-group ECG study of single, subcutaneous doses of methylnaltrexone bromide (0.15, 0.30 and 0.50 mg/kg), in 207 healthy volunteers, no signal of QT/QTc prolongation or any evidence of an effect on secondary ECG parameters or waveform morphology was detected as compared to placebo and a positive control (orally administered 400 mg moxifloxacin).