Leukotriene A4 Hydrolase (LTA4H) is a bifunctional zinc metalloenzyme that comprises both epoxide hydrolase and aminopeptidase activity, exerted by two overlapping catalytic sites. The epoxide hydrolase function of the enzyme catalyzes the biosynthesis of the pro-inflammatory lipid mediator leukotriene (LT) B4. Recent literature suggests that the aminopeptidase function of LTA4H is responsible for degradation of the tripeptide Pro-Gly-Pro (PGP) for which neutrophil chemotactic activity has been postulated. It has been speculated that the design of epoxide hydrolase selective LTA4H inhibitors that spare the aminopeptidase pocket may therefore lead to more efficacious anti-inflammatory drugs. In this study, we conducted a high throughput screen (HTS) for LTA4H inhibitors and attempted to rationally design compounds that would spare the PGP degrading function. While we were able to identify compounds with preference for the epoxide hydrolase function, absolute selectivity was not achievable for highly potent compounds. In order to assess the relevance of designing such aminopeptidase-sparing LTA4H inhibitors, we studied the role of PGP in inducing inflammation in different settings in wild type and LTA4H deficient (LTA4H KO) animals but could not confirm its chemotactic potential.  Attempting to design highly potent epoxide hydrolase selective LTA4H inhibitors, therefore seems to be neither feasible nor relevant.

The cytosolic metalloenzyme leukotriene A4 hydrolase (LTA4H) is the final and rate-limiting enzyme in the biosynthesis of pro-inflammatory leukotriene B4 (LTB4). Preclinical studies have validated this enzyme as an attractive drug target in chronic inflammatory diseases. Despite several attempts, no LTA4H inhibitor has reached the market, yet. Herein, we disclose the discovery and preclinical profile of LYS006, a highly potent and selective LTA4H inhibitor. A focused fragment screen identified hits that could be cocrystallized with LTA4H and inspired a fragment merging. Further optimization led to chiral amino acids and ultimately to LYS006, a picomolar LTA4H inhibitor with exquisite whole blood potency and long-lasting pharmacodynamic effects. Due to its high selectivity and its ability to fully suppress LTB4 generation at low exposures in vivo, LYS006 has the potential for a best-in-class LTA4H inhibitor and is currently investigated in phase II clinical trials in inflammatory acne, hidradenitis suppurativa, ulcerative colitis, and NASH.

Increased airway hyperresponsiveness and epithelial remodeling in asthmatic LTA 4 H-KO mice may be mediated by CysLTs rather than elevated tripeptide PGP.The recent article by Patel et al.(1) characterizes the biological response of an allergic pulmonary inflammation on the background of leukotriene A 4 hydrolase (LTA 4 H)-deficient mice. The authors demonstrate that, although many parameters of inflammation were reduced in the absence of LTA 4 H, the mice exhibited increased airway resistance, airway remodeling, and goblet cell activation.Work from this group previously suggested that accumulation of the tripeptide proline-glycine-proline (PGP) in the absence of LTA 4 H would lead to enhanced neutrophilic inflammation in models of pulmonary infection (2, 3); in this model of sterile allergic inflammation, however, no such enhanced neutrophilic inflammation could be observed. Our recent publication demonstrated that PGP is actually not chemotactic for neutrophils in LTA 4 H knockout (KO) mice nor in human cells and provides an explanation for the observations by Patel et al. on neutrophils in this sterile inflammatory setting (4). Therefore, our conclusion was that the enhanced neutrophil response previously observed in infection models is likely a compensatory immune response to the infection in the absence of LTA 4 H rather than driven by PGP elevation.Similarly, we believe that the observations on airway hyperresponsiveness and epithelial remodeling in this recent article by Patel et al. may not be due to the elevation of the PGP peptide but could actually be caused by the effects of the cysteinyl leukotrienes (CysLTs) in asthma, which are known to induce bronchoconstriction, airway remodeling, goblet cell activation, and mucus production (5). It is well known that inhibition of specific enzymes of the arachidonic acid pathway can lead to the elevation of lipid mediators due to pathway shunting. We have observed that absence of LTA 4 H can lead to the elevation of CysLTs after stimulation of immune cells in vitro or in inflammatory settings in vivo in LTA 4 H KO mice ( Fig.

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