ANALOGS OF CAPSAICIN WITH AGONIST ACTIVITY AS NOVEL ANALGESIC AGENTS - STRUCTURE-ACTIVITY STUDIES .2. THE AMIDE BOND B-REGION

A series of compounds incorporating replacements for the amide bond ''B-region'' moiety of capsaicin have been synthesized, including vanillylamides and esters, homovanillic acid amides and esters, ureas, and thioureas. These have been tested in an in vitro assay for agonism (Ca-45(2+) influx into dorsal root ganglia neurones), which is predictive of analgesic activity, to investigate the requirements in this region of capsaicin for activity. N-(4-Hydroxy-3-methoxybenzyl)-N'-octylthiourea (14a) emerged as the most potent analogue (EC50 = 0.06 muM). An operational model based on multiple hydrogen-bonding interactions is proposed to explain the structure-activity profile observed. In combination with studies on the other regions of the capsaicin molecule these results describe a picture of the molecular interactions of capsaicin with its putative receptor.