Structural modification of 5-alkyl-4-phenyl-thiazole-based fatty acid amide hydrolase and soluble epoxide hydrolase inhibitors to alleviate orofacial inflammatory pain in the rat

• Eleven 5-alkyl-4-phenylthiazole FAAH/sEH inhibitors were synthesized • 4j showed potent dual inhibition of both enzymes (FAAH 18.7 nM; sEH 25.1 nM) • 4j reversed acute orofacial inflammatory pain in female rats with no motor effects • This effect in vivo was comparable to the antinociception provided by sumatriptan • 5-alkyl-4-phenylthiazole FAAH/sEH inhibitors may be useful to treat orofacial pain Orofacial pain is difficult to manage. Treatments like nonsteroidal anti-inflammatory drugs (NSAIDs), and serotonin agonists provide limited relief or produce side effects. These limitations highlight the need for alternative strategies that reduce orofacial pain without adverse effects. One approach is polypharmacology, where a single compound modulates multiple targets involved in pain. Fatty acid amide hydrolase (FAAH) and soluble epoxide hydrolase (sEH) are regulators of pain that can be targeted simultaneously to enhance antinociceptive efficacy. We previously reported that the 4-phenylthiazole-based dual FAAH/sEH inhibitor SW-17 does not alleviate acute orofacial pain in rats, and it showed low stability in rat liver microsomes. We introduced electronic and steric modifications to the thiazole and phenyl rings of the 4-phenylthiazole scaffold to improve the stability and better understand structure-activity relationship of this set of analogs. An 11-compound library was synthesized by varying alkyl groups at position 5 of the thiazole ring and substituents on the phenyl ring. Several analogs exhibited nanomolar potency at both enzymes. The most potent compound, 4j , exhibited IC 50 values of 18.7 nM and 25.1 nM for human FAAH and sEH, respectively. 4j was evaluated in rat liver microsomes where it exhibited relatively low metabolic stability but better than SW-17 . ADMET studies indicated that 4j possesses promising safety features. At 3 mg/kg, 4j reversed acute orofacial inflammatory pain, whereas SW-17 was ineffective. This effect was comparable to sumatriptan and did not reduce voluntary wheel running. These findings imply that optimized dual FAAH/sEH inhibitors can alleviate orofacial pain without affecting normal behavior.