PA-8 is a small molecule receptor antagonist for PACAP Type I (PAC1) that is selective, effective, and orally active. PA-8 inhibits PacAP-induced CREB phosphorylation at PAC1- receptors, but does not inhibit VPAC1- or vpac2 receptors. PA-8 also inhibited PacAP-induced elevation of cAMP levels in vitro (IC50 = 2 nM) and PacAP-induced aversive response and mechanical atopic pain after intrathecal injection in vivo.

cAMP:2 nM

In PAC1/CHO cells, PA-8 (10 pM to 10 nM; 30 minutes) dose-dependently inhibits PACAP (1 nM)-induced CREB phosphorylation. In VPAC1/CHO and VPAC2/CHO cells, PACAP (1 nM) also induces CREB phosphorylation; however, PA-8 (10 pM to 10 nM) does not inhibit this effect [1].

Treatment with PA-8 (100 pmol/5 μL; via intrathecal injection; single dose; in male ddY mice) effectively suppresses PACAP-induced aversive reactions and mechanical allodynia in vivo [1]. Additionally, oral administration of PA-8 (3-30 mg/kg) leads to a dose-dependent reduction in the second phase of formalin-induced pain responses. PA-8 also significantly reduces c-fos expression in the spinal cord's ipsilateral dorsal horn [2]. This showcases PA-8's potential in mitigating nociceptive behaviors and neuronal activation in a PACAP-administered male ddY mouse model (6 weeks old; 100 pmol PACAP) [1], confirming its analgesic and anti-allodynic effects in vivo.