Topical methadone and meperidine analgesic synergy in the mouse

Topical analgesics have many potential advantages over systemic administration. Prior work has shown potent analgesic activity of a number of topical opioids in the radiant heat tail-flick assay. The current study confirms the analgesic activity of morphine and extends it to two other mu opioids, methadone and meperidine. Combinations of topical morphine and lidocaine are synergistic. Similarly, the combination of methadone and lidocaine is synergistic. While there appeared to be some potentiation with the combination of meperidine and lidocaine, it did not achieve significance. Systemically, prior studies have shown that co-administration of morphine and methadone was synergistic. The combination of morphine and methadone was also synergistic when given topically. In contrast, the combination of morphine and meperidine was not synergistic systemically and it was not synergistic topically. Thus, the pharmacology of topical opioids mimics that seen with systemic administration. Their activity in the topical model supports their potential utility while the local limitation of their actions offers the possibility of a reduced side-effect profile.