Dextromethorphan/Morphine

While many pre-clinical and clinical studies have suggested that the addition of N-methyl-D-aspartate (NMDA) receptor antagonists, such as dextromethorphan (DM), to opioid analgesics, such as morphine (MS), may enhance the analgesic effects and prevent the tolerance that may result from chronic opioid administration, others have not. The potential for reduced doses, enhanced opioid analgesia, and decreased analgesic tolerance associated with the MS/DM combination were evaluated in a series of three large, randomized, double-blind, parallel group, phase 3, multicenter trials each of 3 months duration in patients with chronic, non-malignant, non-neuropathic pain. To evaluate these unique endpoints, novel study designs were employed. In Study A, patients received fixed doses of MS or MS/DM, based on the stable dose of MS/DM attained during a Run-in period; changes from baseline in average daily pain intensity were compared. In Studies B and C, patients self-titrated doses of MS or MS/DM, based on stable doses of MS or other opioids attained during Run-in periods, to maintain pain relief; percentage changes from baseline in MS (or MS-equivalent) doses were compared. No statistically significant differences between treatment groups in any primary or secondary efficacy variables were demonstrated in any trial. These results suggest that adding the NMDA antagonist, dextromethorphan, to opioids does not add any clinical benefit.

Endo (which acquired Algos in July 2000) has developed EN-3231, a combination of morphine and dextromethorphan, for the treatment of moderate-to-severe chronic pain. In October 1998, the FDA accepted for filing the company's NDA submission on EN-3231, which was submitted in August 1998. However, in August 1999, Algos received a 'non-approvable' letter from the FDA for EN-3231. In September 2000, the company met with the FDA to discuss what information the FDA required in order to approve EN-3231. The FDA requested a second pivotal trial, which Endo initiated promptly. Endo hoped to file its supplement to the NDA during 2002.