Lipolytic enzymes as therapeutic targets
Lipids are essential components of the cell membrane that play a variety of dynamic roles in mediating and controlling a wide array of cellular activities including membrane structure and organization, metabolic and gene regulation, protein structure and function, energy storage and signaling pathways [1]. Lipolytic enzymes, i.e. the enzymes that catalyze the hydrolysis of lipids, play key roles in nearly all cells and organisms producing a variety of bioactive metabolites. Potent and selective inhibitors of lipolytic enzymes help to elucidate their physiological functions and associated metabolic pathways. Here, I would like to focus the attention on two lipolytic enzymes of high medicinal interest, namely phospholipase A 2 (PLA 2 ) and monoacylglycerol lipase (MAGL).
Phospholipase A 2 enzymes catalyze the hydrolysis of the ester bond at the sn-2 position of glycerophospholipids releasing free fatty acids, including arachidonic acid, and lysophospholipids. Arachidonic acid and its various eicosanoid metabolites are critical second messengers in inflammation, pain, and many pathophysiological processes, while lysophospholipids is reported to form PAF, another important second messenger in inflammation, as well as other pathophysiological processes. The PLA 2 superfamily currently consists of fifteen groups and many subgroups of which a number of enzymes differ in primary sequence, structure and catalytic mechanism [2]. The three predominant types of phospholipase A 2 (PLA 2 ) found in human tissues are the cytosolic (such as the GIVA cPLA 2 ), the secreted (such as the GIVA sPLA 2 ), and the calcium-independent (such as the GVIA iPLA 2 ) enzymes. Although these enzymes display common enzymatic activities, each PLA 2 plays distinct roles in the molecular signaling and metabolism of phospholipids. Therefore, the development of selective inhibitors for individual PLA 2 enzymes is necessary in order to target PLA 2 -specific signaling pathways; however this is challenging due to the observed promiscuity of known PLA 2 inhibitors.
The research on low molecular weight inhibitors of PLA 2 started more than fifteen years ago. At that time the interest was focused on secreted PLA 2 . Eli Lilly discovered a potent and selective inhibitor of sPLA 2 which entered clinical trials for the treatment of severe sepsis. However, the trials were terminated at Phase II stage, because the efficacy was poorer than expected. On September 2008, Anthera Pharmaceuticals announced that they had reached agreement with the Food and Drug Administration (FDA) in the USA on a Phase III protocol for the selective sPLA 2 inhibitor varespladib in acute coronary syndrome.
As a result of various studies, cytosolic GIVA cPLA 2 is considered the main provider of arachidonic acid, although it has been shown that in macrophages and other cells, GIVA cPLA 2 and secretory PLA 2 work together to release arachidonic acid. Various classes of synthetic compounds have been studied as inhibitors of human GIVA cPLA 2 and for their in vivo effects. Among them, the 2-oxoamide inhibitor AX048 is the first systemically bioavailable compound with a significant affinity for GIVA cPLA 2 , which produces a potent anti-hyperalgesia [3]. Using the synthetic inhibitor arachidonyl trifluoromethyl ketone (AACOCF 3 ), it was demonstrated that GIVA cPLA 2 plays an important role in the pathogenesis of experimental autoimmune encephalomyelitis (EAE), the animal model of multiple sclerosis [4]. However, the various in vivo activities of AACOCF 3 should be viewed with some caution, since this inhibitor is not selective for GIVA cPLA 2 and may inhibit other lipolytic enzymes, such as calcium-independent GVIA iPLA 2 , fatty acid amide hydrolase etc. This year, Wyeth has reported that clinical trials to evaluate the efficacy in humans of the indole-based potent and selective GIVA cPLA 2 inhibitor efipladib have been initiated [5].
Another major intracellular PLA 2 , the calcium-independent GVIA iPLA 2 , has not become a target for the development of therapeutic agents up to now. One of the reasons was the lack of selective GVIA iPLA 2 inhibitors. Most recently, a pentafluoroethyl ketone has been identified as a selective GVIA iPLA 2 inhibitor [6] providing the opportunity to explore the role and functions of GVIA iPLA 2 . Using this inhibitor as a tool, the role of GVIA iPLA 2 in neurological disorders is currently under investigation.