N-(3-acyloxy-2-benzylpropyl)-N′-(4-hydroxy-3-methoxybenzyl)thiourea derivatives as potent vanilloid receptor agonists and analgesics

A series of N-(3-acyloxy-2-benzylpropyl)-N′-(4-hydroxy-3-methoxybenzyl)thiourea derivatives were investigated as vanilloid receptor ligands in an effort to discover a novel class of analgesics. The proposed pharmacophore model of resiniferatoxin, which includes the C₂₀-homovanillic moiety, the C₃-carbonyl and the orthoester phenyl ring as key pharmacophoric groups, was utilized as a guide for drug design. The compounds were synthesized after several steps from diethylmalonate and evaluated in vitro in a receptor binding assay and in a capsaicin-activated channel assay. Additional evaluation of analgesic activity, anti-inflammatory activity and pungency was conducted in animal models by the writhing test, the ear edema assay, and the eye-wiping test, respectively. Among the new compounds, 23 and 28 were found to be the most potent receptor agonists of the series with K_i values of 19 nM and 11 nM, respectively. Their strong in vitro potencies were also reflected by an excellent analgesic profile in animal tests with ED₅₀ values of 0.5 μg/kg for 23 and 1.0 μg/kg for 28. Relative to capsaicin these compounds appear to be ca. 600 and 300 times more potent. Both 23 and 28 were found to be less pungent than capsaicin based on the eye-wiping test. However, the compounds did not show significant anti-inflammatory activity. A molecular modeling study comparing the energy-minimized structures of resiniferatoxin and 35 demonstrated a good correlation in the spatial disposition of the corresponding key pharmacophores. The thioureas described in this investigation, which were designed as simplified resiniferatoxin surrogates, represent a novel class of potent vanilloid receptor agonists endowed with potent analgesic activity and reduced pungency.