A third life for burimamide. Discovery and characterization of a novel class of non-opioid analgesics derived from histamine antagonists.

Burimamide, a histamine (HA) derivative with both H2- and H3-blocking properties, induces antinociception when injected into the rodent CNS. Several related compounds share this property, and structure-activity studies have shown that this new class of analgesics is distinct from known HA antagonists. The prototype, named improgan, shows a preclinical profile of a highly effective analgesic, with activity against thermal, mechanical and inflammatory nociception after doses that do not alter motor balance or locomotor activity. Improgen analgesia is not blocked by opioid antagonists and is observed in opioid receptor knock-out mice. Unlike morphine, improgan does not induce tolerance after daily dosing. Extensive in vitro pharmacology studies have excluded known histaminergic, opioid, serotonergic, GABAergic and adrenergic receptor mechanisms, as well as 50 other sites of action. The improgan-like analgesic activity of some HA congeners suggests an analgesic action on a novel HA receptor, but further studies are required to substantiate this. Studies in progress are characterizing the sites and mechanisms of action of improgan, and developing brain-penetrating derivatives that could be useful for clinical pain.