Autotaxin (ATX) is a key enzyme involved in generation of the profibrotic lipid, lysophosphatidic acid (LPA). Autotaxin, through its enzymatic lysophospholipase D activity, cleaves a choline head group from lysophosphatidylcholine (LPC) to generate LPA, a bioactive lipid involved in a number of key cellular processes including inflammation, proliferation and apoptosis. LPA has been implicated in a variety of fibrotic diseases from liver and lung to kidney and skin (Stoddard & Chun, 2015, Biomol Ther). LPA binds to multiple G protein-coupled receptors to mediate the downstream biology; activity of one of those receptors, LPA1, is currently under clinical investigation in IPF and scleroderma.
PharmAkea has prepared potent autotaxin inhibitors in a number of different chemical series. In a collaboration with Cayman Chemical, we generated X-ray crystal structures of several of our key inhibitors bound to human autotaxin. As a result, we have a strong understanding of how our compounds bind and block autotaxin activity. Interestingly, each series binds in a unique fashion occupying either the active site or the channel leading to the enzyme surface. By combining our biochemical analyses with the crystal structure, we have shown that our lead compound PAT-409 is an allosteric, non-competitive inhibitor that binds to the proposed exit channel for newly synthesized LPA. The information gained from the crystallography work has been useful in understanding the modes of enzyme inhibition as well as the design of new inhibitors. Some of these results have been described in a recent publication: Stein et al, 2015, Mol. Pharmacol.
PharmAkea’s lead autotaxin inhibitor, PAT-409, demonstrated robust reduction of fibrosis in a model of nonalcoholic steatohepatitis (NASH). Mice were fed a diet deficient in choline, which is known to cause a NASH-like phenotype. Supplementing this diet with 0.1% methionine and 60% fat content allows the mice to maintain weight neutrality and have less peripheral consequences (CDAA-HFD; Matsumoto et al., 2013 Int. J. ExpPathol.). PAT-409 reduces fibrosis as assessed histologically (PSR score) and biochemically (hydroxyproline) and, to a lesser degree, reduces the concomitant steatosis.
By inhibiting the generation of LPA, autotaxin inhibitors have potential to show broader effects than compounds selectively inhibiting the downstream receptors. Inhibition of LPA1R has shown great promise preclinically and is currently under clinical evaluation in IPF. When compared with the efficacy of PAT-409, the LPA1R antagonist, AM152 (aka BMS986020), was ineffective in the CDAA-HFD mouse model of NASH, suggesting that additional LPA receptors are relevant in NASH and inhibition of LPA generation is a better therapeutic approach.
By middle age, approximately 12% of the general population have NASH with up to 35% prevalence in the morbidly obese population (Williams et al., 2011 Gastroenterology; Reha et al. 2014 Am Surg). By directly targeting the fibrotic pathways with an autotaxin inhibitor in conjunction with a metabolic modulator, multi-factorial efficacy in NASH is possible.
Autotaxin expression is elevated in the high endothelial venules (HEV) of the gut suggesting that it plays a role in inflammatory cell infiltration and damage in the context of inflammatory bowel disease (IBD) (Hozumi 2013 Lab Invest). PAT-409 was evaluated in a rat model of IBD where dinitrobenzoic acid (DNBS) was administered intracolonically to cause tissue damage and development of fibrotic lesions. PAT-409 dramatically protected against the lesions and further reduced inflammatory cytokine expression and fibrosis at the site of injury. Fibrotic lesions and strictures are a known complication in Crohn’s disease suggesting that an autotaxin inhibitor represents an important new modality in the treatment of IBD.