Synthesis and Analgesic Activity of Cridanimod–Monoterpene Conjugates

Cridanimod (acridoneacetic acid), the active ingredient of the immunomodulatory drug Cycloferon®, has a well-characterized antiviral profile, but an unexplored analgesic potential. Bornane monoterpenes, notably (+)-camphor and (−)-fenchone, are established scaffolds for analgesic drug design. We therefore synthesized thirteen cridanimod–monoterpene conjugates—seven amides, three acylhydrazones, and three esters—by EDCI/DMAP-mediated coupling or CDI/DBU-promoted esterification, and evaluated them in mice at an oral dose of 10.0 mg/kg in the acetic acid writhing and hot plate tests, with diclofenac sodium (10.0 mg/kg) as the reference. Acridoneacetic acid and Cycloferon® were themselves inactive in both assays. By contrast, six of the thirteen conjugates were significantly active (p < 0.05) in at least one test, with distinct leads in each: amide 19 matched diclofenac in the writhing test (−63.7% vs. −64.4%), whereas acylhydrazone 23 exceeded diclofenac in the hot plate test (+43.9% vs. +25.7%). The divergent profiles suggest that different structural subclasses engage peripheral and central antinociceptive mechanisms. Conjugation thus converts an analgesically inactive immunomodulator into a substance yielding analgesics that match or exceed a standard NSAID at the same oral dose, providing a promising starting point for the development of new pain-relieving agents.