Structural Insights for Drugs Developed for Phospholipase D Enzymes

In recent years human phospholipase D enzymes (PLD1 and PLD2 isozymes) have emerged as drug targets for various diseases such as cardiovascular disease, cancer, infectious diseases and neurodegenerative conditions such as Alzheimer's and Parkinson's disease. The interest in PLD as a drug target is due to the fact that PLD enzymes belong to a superfamily of phospholipases that are essential to intracellular and extracellular signaling. Many bioactive lipid signaling molecules are generated by these enzymes including phosphatidic and lysophosphatidic acid, arachidonic acid, and diacylglycerol (DAG). More specifically PLDs are part of one pathway that generates phosphatidic acid which is a precursor to many lipids in the intracellular de novo pathway. The lipids produced from PA regulate many cellular events considered hallmarks of pathogenesis in cells; including proliferation, migration, invasion, angiogenesis, and vesicle transport. Hence, human PLD is a valid target for a variety of drug therapies.

The focus of this review is phospholipase D inhibitory molecules. A survey of structure-based drug design studies for PLD enzymes was done by searching several literature databases. Studies that focused on the structural aspects of phospholipase D were compiled and analyzed for content. Particular attention was given to studies involving inhibitory molecules as the focus of this work. In addition, the protein data bank (PDB) was surveyed for three dimensional structures of PLD. Structural investigation via in silico docking utilizing the available three dimensional coordinates of PLD and recent potent PLD isozyme specific inhibitors was performed to gain insights into the mode of binding by drugs designed to inhibit PLDs.

Beginning with halopemide and derivatives such as FIPI (5-fluoro-2- indoyly des-chlorohalopemide) leading to PLD isozyme selective inhibitors such as novel triazaspirone-based series of PLD inhibitors, structures and IC50 values presented were found to be in the nanomolar range for either human PLD1 or PLD2. Selective oestrogen receptor modulators (SERMS), compounds used in the treatment of oestrogen-receptor-positive breast cancer, inhibited mammalian PLD enzymes in the low micromolar range. The first universal PLD inhibitor developed was devoid of the 6-OH moiety necessary for oestrogen receptor binding and anti-proliferation action. The universal PLD inhibitor contains a N,N-dimethylamino moiety which is known to reduce SERM activity and was found to inhibit several PLDs in the low micromolar range. The literature analyzed revealed a systematic approach to the biochemical evaluation of modes of binding of these inhibitors to the PLD enzymes. Finally, docking studies of several of the more potent PLD inhibitors correlates with biochemical studies with two modes of inhibitor binding to PLD: active site binding and allosteric binding.

PLD inhibitors from diverse backgrounds continue to be developed as research progresses to the most potent and highly selective human PLD inhibitors with low or no off target activities. Docking studies strongly suggest both competitive (active site) and allosteric binding of these inhibitors to PLD. The three dimensional structure of PLD co-crystallized with potent inhibitors will be paramount to confirm the modes of binding for these molecules to PLD.