Metkephamid, a Systemically Active Analog of Methionine Enkephalin with Potent Opioid δ-Receptor Activity

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Abstract
Metkephamid is an analog of methionine enkephalin that retains high affinity for the δ receptor and is a systemically active analgesic. Since it is at least 100 times more potent than morphine as an analgesic when placed directly into the lateral ventricles, and is 30 to 100 times more potent on the δ receptor and yet is roughly equipotent on the μ receptor in vitro, it is concluded that it probably produces analgesia by action on δ receptors as well as, or rather than, on μ receptors. It has less tendency to produce respiratory depression, tolerance, and physical dependence than standard analgesics, and it is presently undergoing clinical trial.