A novel selective prostaglandin E2 synthesis inhibitor relieves pyrexia and arthritis in Guinea pigs inflammatory models

[The Analgesic Effect of Aspirin on Inflammatory Pain in Rats Is Affected by Pain Intensity and Administration Timing]

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely known as painkillers. The analgesic action of NSAIDs is attributable to the inhibition of prostaglandin synthesis that occurs in response to blocking cyclooxygenase activity. The effective dose of NSAIDs can vary depending on pain intensity and administration timing; however, there are few studies on this. This study aimed to elucidate whether the analgesic effect of NSAIDs changes depending on the situation in which they are taken and we focused on the NSAID, aspirin (ASP). In a rat model of brewer's yeast-induced inflammation, pain caused by 20% (w/v) brewer's yeast-treatment was defined as "strong pain" and that caused by 2.5% (w/v) was defined as "weak pain". The analgesic effect of ASP (low-dose; 44 mg/kg or high-dose; 66 mg/kg) against strong pain was dose-dependent, but that against weak pain was the same. Furthermore, we defined drug administration after 3 h of brewer's yeast-treatment as "late administration" and that after 20 min as "early administration". In the case of strong pain, the analgesic effect of "late ASP administration" was dose-dependent, but that of "early ASP administration" was the same. These results suggest that low-dose NSAIDs have an analgesic effect against weak pain or when taken early.

Simultaneous Inhibition of PGE2 and PGI2 Signals Is Necessary to Suppress Hyperalgesia in Rat Inflammatory Pain Models

Prostaglandin E2 (PGE2) is well known as a mediator of inflammatory symptoms such as fever, arthritis, and inflammatory pain. In the present study, we evaluated the analgesic effect of our selective PGE2 synthesis inhibitor, compound I, 2-methyl-2-[cis-4-([1-(6-methyl-3-phenylquinolin-2-yl)piperidin-4-yl]carbonyl amino)cyclohexyl] propanoic acid, in rat yeast-induced acute and adjuvant-induced chronic inflammatory pain models. Although this compound suppressed the synthesis of PGE2 selectively, no analgesic effect was shown in both inflammatory pain models. Prostacyclin (PGI2) also plays crucial roles in inflammatory pain, so we evaluated the involvement of PGI2 signaling in rat inflammatory pain models using prostacyclin receptor (IP) antagonist, RO3244019. RO3244019 showed no analgesic effect in inflammatory pain models, but concomitant administration of compound I and RO3244019 showed analgesic effects comparable to celecoxib, a specific cyclooxygenase- (COX-) 2 inhibitor. Furthermore, coadministration of PGE2 receptor 4 (EP4) antagonist, CJ-023423, and RO3244019 also showed an analgesic effect. These findings suggest that both PGE2 signaling, especially through the EP4 receptor, and PGI2 signaling play critical roles in inflammatory pain and concurrent inhibition of both signals is important for suppression of inflammatory hyperalgesia.