Affiliation: Institute of Pharmaceuticaland Medicinal Chemistry, University of Munster, Hittorfstrasse 58-62, D-48149 Münster, Germany.
Arachidonic acid derivatives, like prostaglandins and leukotrienes, and the platelet-activating factor (PAF) are highly active substances with diverse biological actions. Elevated levels of these lipid mediators in response to a variety of stimuli have been implicated in the pathology of many inflammatory diseases. The rate limiting step in the generation of prostaglandins, leukotrienes and the PAF, respectively, is the cleavage of the sn-2- ester of membrane phospholipids by phospholipase A2. To date four main groups of phospholipases are known, which comprise the secretory, the calcium-independent, the cytosolic and the lipoprotein-associated phospholipases A2. From these the a-subtype of cytosolic phospholipases A2 (cPLA2α) appears to be the most likely candidate to catalyze this hydrolysis, since the enzyme is highly selective for arachidonoyl-containing phospholipids and is tightly regulated by receptor-stimulated mechanisms (calcium influx and phosphorylation). Moreover, experiments with cPLA2α knockout mice have provided further evidence for the central role of this enzyme in inflammation. Therefore, inhibition of cPLA2α activity is an attractive approach to the control of inflammatory disorders. In this article we describe the different assays applied for the evaluation of cPLA2α inhibitors in vitro and in vivo. Furthermore, we present the structures and inhibition data of known cPLA2α inhibitors and discuss the problems associated with the development of a clinical active drug candidate. Since it is difficult to compare the in vitro inhibition data of enzyme inhibitors as far as they are monitored with different assays, we also present such data for some interesting cPLA2α inhibitors determined with the same assay.