Fatty acid amide hydrolase - A target for anti-inflammatory therapies?

Abstract: Anti-inflammatory drugs are a widely used class of therapeutic agents, but the use of non-steroidal anti-inflammatory drugs (NSAID) is hampered by their gastrointestinal side-effects. Recent reports that cyclooxygenase-2 inhibitors may cause cardiovascular events underline the importance of identifying new therapeutic strategies for the treatment of inflammation. One such target could be agents modifying the endogenous cannabinoid (endocannabinoid) system, since there is evidence that this system plays a role in our natural defence against inflammation. The levels of the endocannabinoid anandamide (arachidonoyl ethanolamide, AEA) are low under normal conditions, and stand under strict regulatory control of synthesising and degrading enzymes. Fatty acid amide hydrolase (FAAH) is the main enzyme degrading AEA, hydrolysing it to ethanolamine and arachidonic acid. The focus of this thesis lies in exploring the pharmacology of FAAH to evaluate its possibilities as a target for new anti-inflammatory drugs.In Papers I and II, the effects of the ambient pH on the properties of FAAH were investigated, since tissue pH is known to decrease under inflammatory conditions. In homogenates, it was found that the activity of FAAH decreased as the assay pH was decreased, consistent with the known pH profile of the enzyme. More importantly, the sensitivity of the enzyme to inhibition by FAAH inhibitors changed. In particular, the sensitivity of the enzyme to inhibition by the NSAID ibuprofen increased seventeen-fold as the assay pH decreased from 8.37 to 5.28. A similar pattern was found using intact C6 glioma cells when the extracellular, but not the intracellular pH was reduced. Thus, at an extracellular pH value of 6.2, (R)-ibuprofen, (S)-flurbiprofen and (R,S)-flurbiprofen inhibited the metabolism of AEA with IC50 values of 26, 14 and 15 µM, respectively. These values are in theory reachable upon normal dosing of the compounds. In Paper III, the effect of the selective FAAH inhibitor URB597 and the NSAID indomethacin were investigated in vivo upon the oedema response to carrageenan administration in the paw of anaesthetised mice. Both compounds reduced the oedema in a manner completely blocked by the CB2 receptor antagonist SR144528. In Paper IV, the effect of inflammation upon endocannabinoid synthesis was investigated in mice. Lipopolysaccharide-induced pulmonary inflammation was found not to affect the release of AEA to any obvious extent, and did not change the activities of the AEA synthesising enzymes N-acyl transferase or N-acyl phosphatidylethanolamine phospholipase D, or of FAAH in lung tissue.The results of this thesis would suggest that FAAH inhibitors can produce anti-inflammatory effects, and that the endocannabinoid system contributes to the actions of the NSAID indomethacin in the carrageenan model of inflammation, but that an increased endocannabinoid synthesis (a prerequisite for FAAH inhibition as a therapeutic strategy) is not an obligatory response to an inflammatory stimulus.