Previously, we found that intrathecal (i.t.) injection of PACAP induces long-lasting mechanical allodynia in mice and increases the expression of Nr4a1 (Nuclear receptor subfamily 4, group A, member 1) mRNA in the spinal cord. We also developed a novel small-molecule antagonist of Nr4a1, named NRA-8, using docking-based in silico screening followed by in vitro/vivo pharmacological assays. NRA-8 (i.t.) showed to be able to inhibit the induction of PACAP-induced long-lasting mechanical allodynia. In this study, based on the structure of NRA-8, we synthesized derivative compounds with potent antagonistic activity on Nr4a1 and examined their analgesic effects on pain model mice.
We synthesized 15 novel derivative compounds of NRA-8 (NRA-801 to 815) and identified that 4 derivatives (NRA-811, NRA-813 to 815) show more potent antagonist activity than NRA-8. Among 4 derivatives, the effects of NRA-811 and 815 on mechanical allodynia were investigated. I.t. injection of Nr4a1 antagonists (1 nmol) ameliorated the spinal nerve ligation-induced mechanical allodynia. In bone cancer pain model mice which are induced by transplantation of NCTC2472 cells into the femur, oral administration of Nr4a1 antagonists (10 and 30 mg/kg) dose-dependently ameliorated the mechanical allodynia. In both cases, NRA-815 showed the strongest effects than NRA-811 or NRA-8. Inhibiting Nr4a1 may be one of the strategies in the treatment of intractable pain.