Soluble epoxide hydrolase inhibition, epoxygenated fatty acids and nociception.

The Arachidonic Acid CascadeA simplified depiction of the formation of major metabolite families generated from arachidonic acid. AA is metabolized by cytochrome P450 enzymes into anti-inflammatory lipid mediators while the COX and LOX enzymes produce largely pro-inflammatory metabolites. The epoxyeicosatrienoic acids (EETs) are rapidly metabolized by the soluble epoxide hydrolase enzyme (sEH) to their corresponding diols the dihydroxyeicosatrienoic acids (DHETs). Inhibitors of sEH block this degradation and stabilize EET levels in vivo while reducing DHETs. The EETs suppress the transcription of the COX-2 isozyme and thus affect the AA cascade in an unexpected manner. Key cyclooxygenase metabolites include: (PGE2) prostaglandin E2, (PGD2) prostaglandin D2, (PGI2) prostaglandin I2 (prostacyclin), (TXA) thromboxane. Key lipoxygenase metabolites include: (5-HETE) 5-Hydroxyeicosatetraenoic acid, (12-HETE) 12-Hydroxyeicosatetraenoic acid, (LTB4) leukotriene B4. Other PUFAs can enter this pathway yielding among other metabolites, a variety of EFAs.

A dual sEH/COX-2 inhibitor efficiently blocks painA potent dual sEH/COX-2 inhibitor significantly alleviates intraplantar LPS induced allodynia measured by a von Frey mechanical nociception test (n=6 per group). The sEHI and co-administration of celecoxib and sEHI also attenuate the allodynia but not as effectively. Doses of celecoxib (CLX), sEHI (t-AUCB), or the dual inhibitor (10 mg/kg each s.c.) were compared to a combination of CLX + t-AUCB and an LPS control. The sEHI and combination treatment were significantly anti-allodynic when compared to the LPS control (P=0.02 and P<0.001 respectively). However the sEH/COX-2 dual inhibitor significantly (P<0.001) attenuated allodynia at both time points and was more effective than celecoxib or sEHI alone and the combination. Results are presented as percent of baseline mechanical withdrawal threshold on the y-axis.

EFAs and epoxyeicosatrienoic acid ethanolamides (EET-EAs) share structural featuresArachidonic acid is enzymatically converted to four regioisomers of epoxyeicosatrienoic acids (EETs) by cytochrome P450s. Here one regioisomer 11,12-EET is depicted. Arachidonic acid is a precursor in the synthesis of endogenous cannabinoids anandamide and 2-arachidonyl glycerol (not shown). Once synthesized from N-arachidonoyl phosphatidylethanolamine (NAPE) anandamide can be epoxygenated by cytochrome P450s to form regioisomers of EET-EAs which have similar functional properties with anandamide. Anandamide or 2-arachidonyl glycerol upon degradation by FAAH or MAGL releases arachidonic acid as a product which can act as substrate for cytochrome P450s to be converted to EETs.

Cannabinoid receptors play a role in sEHI mediated anti-nociceptionThe soluble epoxide hydrolase inhibitor AEPU administered topically significantly attenuates thermal hyperalgesia when compared to control (CTL) LPS treated rats. Male Sprague Dawley rats (n=4 per group) were administered intraplantar 10ug/paw LPS in one hind paw followed by 10mg/rat AEPU and monitored using the modified Hargreaves’ thermal hyperalgesia test. This anti-hyperalgesic effect of the sEHI AEPU is blocked by a 2mg/rat topical administration of the selective CB2 receptor antagonist AM 630. However, the CB1 selective antagonist AM 251 displays no effect on sEHI mediated anti-hyperalgesia. Results are presented as percent of baseline thermal withdrawal latency on the y-axis.