Effects of Simultaneous Inhibition of Fatty Acid Amide Hydrolase and Soluble Epoxide Hydrolase on Acute and Persistent Pain in Male Rats

Abstract

<b>Abstract ID 18607</b> <b>Poster Board 512</b> Chronic pain is the primary cause of disability worldwide. Opioids and nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used to treat persistent pain are commonly used to treat persistent pain. Opioids (e.g., morphine), produce significant side effects such as addiction, constipation, and sedation. Therefore, the treatment of pain requires identifying new drugs with novel mechanisms of action to produce pain relief without these harmful side effects. Fatty acid amide hydrolase (FAAH) and soluble epoxide hydrolase (sEH) are two enzymes responsible for regulating pain and inflammation. Previous studies have demonstrated that co-administration of a FAAH inhibitor and a separate sEH inhibitor provides greater pain relief than administration of just one enzyme inhibitor alone in rodents. Thus, we synthesized a novel dual inhibitor of FAAH and sEH, SP 4-5, that inhibits both enzymes simultaneously with just one drug. This approach has several advantages including limiting the potential for drug-drug interactions. We hypothesized that simultaneous inhibition of FAAH and sEH using SP 4-5 will alleviate pain in male Sprague Dawley rats with acute and persistent inflammatory pain. We used the hot plate and tail flick tests to first assess acute pain. We administered varying doses of SP 4-5 (1 and 3 mg/kg), ketoprofen (30 mg/kg), or vehicle via intraperitoneal injection and measured the latency for the rat to respond. Administration of SP 4-5 and ketoprofen did not change the latency for the rat to flick its tail from a noxious heat source. Additionally, administration of SP 4-5 or ketoprofen did not alter the latency for a rat to respond on the hot plate test. We injected dilute formalin into the hindpaw of the rat and observed licking and guarding behavior of the injected paw in two phases of the Formalin Test. Administration of SP 4-5 and ketoprofen alleviated pain in the second, but not first, phase of the Formalin Test. To assess the efficacy of SP 4-5 in persistent inflammatory pain, rats received an intraplantar injection of 0.1 mL Complete Freund9s Adjuvant (CFA) to induce hindpaw inflammation. Twenty-four hours later, the rats were administered different doses of SP 4-5 (1 and 3 mg/kg) or the nonsteroidal anti-inflammatory drug ketoprofen (30 mg/kg) and re-tested on both tests. Neither SP 4-5 nor ketoprofen alleviated pain 24 hours post-CFA. To test the effects of acute inflammatory pain, SP 4-5 and ketoprofen were injected 2.5 hours post-CFA. In this experiment, 3 mg/kg SP 4-5 and 30 mg/kg ketoprofen increased mechanical, but not thermal, withdrawal thresholds. Lastly, effective doses of SP4-5 do not depress voluntary wheel running in naive animals suggesting that the drug may not have adverse locomotor effects. Based on these data, simultaneous inhibition FAAH and sEH seems especially effective at alleviating pain against acute inflammatory pain as demonstrated using hindpaw injections of formalin and CFA. SP 4-5 is not effective against acute thermal pain, which is consistent with most NSAIDs. Further studies must establish the utility of dual FAAH/SEH inhibition on varying degrees of inflammatory pain and identify effective chemical strategies to increase the duration and magnitude of antinociception in&nbsp;vivo.