The novel small-molecule antagonist of PAC1 receptor attenuates formalin-induced inflammatory pain behaviors in mice.

We recently developed PA-8, a novel small-molecule antagonist of PACAP type 1 (PAC1) receptor. In the present study, we examined whether PA-8 was effective against formalin-induced inflammatory pain in mice. Both intrathecal and oral administration of PA-8 resulted in the dose-dependent attenuation of the second phase of formalin-induced nociceptive responses. PA-8 also inhibited c-fos upregulation in the ipsilateral dorsal horn of the spinal cord. The results suggested that PACAP-PAC1 receptor signaling system in the spinal cord were primarily involved in the transmission of inflammatory pain, and PA-8 could be useful for the development of novel analgesics for treating inflammatory pain.

Design and synthesis of pyrido[2,3-d]pyrimidine derivatives for a novel PAC1 receptor antagonist.

Since PA-8 (5-(4-(Allyloxy)-3-methoxyphenyl)-2-amino-5,8-dihydro-3H,6H-pyrido[2,3-d]pyrimidine-4,7-dione) was recently identified as a novel small-molecule antagonist of the pituitary adenylate cyclase-activating polypeptide (PACAP) type I (PAC1) receptor, a series of pyrido[2,3-d]pyrimidine derivatives have been designed, synthesized and subsequently evaluated for antagonistic activity on the PAC1 receptor. In this study, we synthesized 21 derivatives based on the PA-8 structure. Among them, the compound 2o (2-Amino-5-(3-trifluoromethoxy-phenyl)-5,8-dihydro-3H,6H-pyrido[2,3-d]pyrimidine-4,7-dione) showed more potent antagonistic activities than PA-8. Intrathecal (i.t.) injection of 2o blocked the induction of PACAP-induced aversive behaviors and mechanical allodynia in mice, and the effects were more potent than those of PA-8. A single i.t. injection of 2o also inhibited spinal nerve ligation (SNL)-induced mechanical allodynia. Repeated intraperitoneal administration of 2o gradually reduced the SNL-induced mechanical allodynia, and this effect appeared earlier than for PA-8. In addition, 2o exhibited a favorable ADME and pharmacokinetics profiles. These results suggest that 2o may become an analgesic for the treatment of neuropathic pain.

Synthesis of a novel and potent small-molecule antagonist of PAC1 receptor for the treatment of neuropathic pain.

We recently identified novel small-molecule antagonists of the PACAP type I (PAC1) receptor using docking-based in silico screening followed by in vitro/vivo pharmacological assays. In the present study, we synthesized 18 novel derivatives based on the structure of PA-9, a recently developed antagonist of the PAC1 receptor, with a view to obtain a panel of compounds with more potent antagonistic and analgesic activities. Among them, compound 3d showed improved antagonistic activities. Intrathecal injection of 3d inhibited both pituitary adenylate cyclase-activating polypeptide (PACAP) and spinal nerve ligation-induced mechanical allodynia. The effects were more potent than PA-9. Compound 3d also showed anti-allodynic effects following oral administration. Hence, our results suggest that 3d may become an orally available analgesic in the treatment of the neuropathic pain.