Discovery, structure-activity relationship studies, and anti-nociceptive effects of 1-phenyl-3,6,6-trimethyl-1,5,6,7-tetrahydro-4H-indazol-4-one as novel opioid receptor agonists

The μ-opioid receptor (MOR) is the major opioid receptor targeted by most analgesics in clinical use. However, the use of all known MOR agonists is associated with severe adverse effects. We reported that the 1-phenyl-3,6,6-trimethyl-1,5,6,7-tetrahydro-4H-indazol-4-ones are novel opioid receptor agonists. Subsequent structural modification resulted in the potent MOR/KOR (κ-opioid receptor) agonists 19, 20, and 21. Testing the analgesic effect of these in WT B6 mice (tail-flick test) gave ED₅₀ values of 8.4, 10.9, and 26.6 mg/kg, respectively. The 1-phenyl-3,6,6-trimethyl-1,5,6,7- tetrahydro-4H-indazol-4-one core could be addressed in 1 or 2 synthetic steps with moderate to high percent of yield. In the adenylyl cyclase assay, compound 19 displayed a MOR/KOR agonist profile, with IC₅₀ values of 0.73 and 0.41 μM, respectively. Current results suggest that compound 19 is a promising lead to go further development and in vitro/in vivo adverse effects studies.