Lipoxin A4 (LXA4) mimics shown reduce inflammation by inhibiting the p38-MAPK pathway-further proof of concept for LXA4 analogues in preclinical development

DailyUpdates 11^th August: Lipoxin A4 (LXA4) mimics shown reduce inflammation by inhibiting the p38-MAPK pathway-further proof of concept for LXA4 analogues in preclinical development

The p38-MAPK pathway has long been targeted by the pharmaceutical industry in an attempt to identify new anti-inflammatory agents.  The development of p38-MAPK inhibitors has however been impeded by adverse effects.  Boston University researchers have now demonstrated that the lipoxin A4 (LXA4) mimic, ATLa inhibits the p38-MAPK cascade explaining in part its anti-inflammatory efficacy and offering a new approach to this promising pharmaceutical target.  Although ATLa has a poor pharmacokinetic profile, Berlex is currently developing analogues more suitable for clinical use.

Treatments for inflammatory diseases comprise a $20 billion market that shows no signs of slowing in growth.  Some of the most prevalent inflammatory diseases include psoriasis, asthma, rheumatoid arthritis, COPD, multiple sclerosis and inflammatory bowel disease.  The variety of molecular targets for these inflammatory conditions is immense and includes receptors and enzymes involved in the synthesis, removal and bioactivity of the eicosanoids. 

Synthesis of the ecosanoids originates with arachidonic acid which is converted to the prostaglandins through the prostaglandin synthase pathways.  Arachidonic acid can also be oxygenated by lipoxygenases to make HPETEs. 5-lipoxygenase catalyzes the production of leukotriene (LT) A4 which is hydrolyzed to produce LTB4 or has glutathione added by a glutathione S-transferase to produce LTC4 and LTD4.

Both the prostaglandins and the leukotrienes are involved in the initiation and maintenance of inflammatory responses and are proinflammatory.  In LeadDiscovery’s recent DiscoveryDossier, “Rheumatoid arthritis: Current treatments and prospects for new drugs” the concept of developing agents that block the prostaglandin and/or the leukotrioene pathways is evaluated and from available evidence it is concluded that such an approach has little effect on the progression of disease.  Likewise, in a second report evaluating emerging treatments of asthma it was also concluded that targeting these pro-inflammatory mediators was of limited benefit.

In contrast to the leukotrienes (LTs) and the prostaglandins, the lipoxins (LXs) are involved in the regulation and resolution of inflammation and are anti-inflammatory.  Arachidonic acid is oxygenated by 15-lipoxygenase to form 15-HETE. Within leukocytes, 15-HETE is in turn converted to LXA4 by 5-lipoxygenase and epoxide hydrase.

The anti-inflammatory properties of LXA4 have been reported however few mimics have been developed and LXA4 therefore represents an under-exploited field. This lack of development is due to the metabolic and chemical instability of LXA4 and its analogues. The metabolic instability of the natural product, LXA4, is the result of a series of steps initiated by the rapid oxidation of the 15(S) alcohol by prostaglandin dehydrogenase (PGDH). A number of endogenous analogues of LXA4 have been identified and one of these, 15-epi-LXA4, or aspirin-triggered LXA4, has been shown to be equipotent in in vitro assays to LXA4 but was a poorer substrate for PGDH.  Further development has led to the synthesis of a synthetic analogue, 15-epi-16-(p-fluoro)phenoxy-LXA4 (ATLa, ATLa2).

The potent anti-inflammatory and immunomodulatory properties of ATLa were demonstrated in a series of in vivo models, including an allergic airway inflammation model, a T-cell dependent skin inflammation model, a dextran sulfate induced colitis model, and an adaptive immunity model with 5-LO knockout mice. Taken together, these studies provide evidence that ATLa can directly or indirectly modulate T cell effector function in the setting of Th1- and Th2-dependent inflammation

and adaptive immunity.

researchers have now investigated the mechanism of action of ATLa in greater detail.  In particular they have recently reported in the Journal of Immunology that ATLa blocked the phosphorylation of leukocyte-specific protein 1, a downstream component of the p38-MAPK cascade in fMLP-stimulated neutrophils. ATLa was found to reduce the phosphorylation/activation of several other components of the p38-MAPK pathway (MAPK kinase 3/MAPK kinase 6, p38-MAPK, MAPK-activated protein kinase-2). These results indicate that ATLa exerts its anti-inflammatory effects, at least in part, by blocking activation of the p38-MAPK cascade in neutrophils, which is known to promote chemotaxis and other proinflammatory responses by these cells.

Recent successful development of anti-cytokine therapeutics including those that block TNF-alpha and IL-1, and more recently IL-6 has spurred research into identifying cytokine production and signaling pathways. p38-MAPK plays a pivotal role in the production of TNF and IL-1, and furthermore it is also required for the cellular response to these cytokines. The concept of developing p38-MAPK inhibitors for the treatment of inflammatory diseases is strong and the development of small molecule inhibitors of cytokine signaling and production holds potential advantages over existing biological therapies that prevent binding or neutralize the cytokines per se. These advantages include improved half-life and reduced immunogenicity, host immune response and cost. MAPK inhibitors have shown activity in animal models of inflammation and importantly effects are observed in animals with established joint inflammation as well as in animals pretreated with inhibitors prior to the induction of disease. Many companies have been involved in the development of p38-MAPK inhibitors however side effects have to date limited development.  The demonstration that ATLa inhibits the p38-MAPK cascade therefore offers a new approach to the treatment of inflammatory disorders.

Although ATLa has enhanced metabolic stability over LXA4 in vivo, the pharmacokinetics of ATLa remain poor which, in addition low chemical stability has precluded further development.  Of note however Berlex researchers have recently reported a novel analogue of ATLa, ZK-994, with improved stability and efficacy in a number of animal models.  

The paper “A stable aspirin-triggered lipoxin A4 analog blocks phosphorylation of leukocyte-specific protein 1 in human neutrophils”( J Immunol. 2004 Aug 1;173(3):2091-8) is highlighted in today’s edition of DailyUpdates.

Further

In depth pharmaceutical reports

• Rheumatoid Arthritis - Novel Biologics Drive Innovation in the Global RA Market
• Biotech in Autoimmune/Inflammatory Disease 2004: Revolutionizing the Market
• Rheumatoid arthritis: Emerging drug discovery targets and therapeutic candidates
• Rheumatoid Arthritis - DMARDs Gain Ground as Biologics Competition Intensifies
• Asthma Therapeutics: New treatment options and emerging drug discovery targets
• Asthma and COPD - Combinations breathe life into the asthma and COPD markets

Papers:

• Novel 3-oxa lipoxin A4 analogues with enhanced chemical and metabolic stability have anti-inflammatory activity in vivo. J Med Chem. 2004 Apr 8;47(8):2157-65.
• Lipoxins and novel 15-epi-lipoxin analogs display potent anti-inflammatory actions after oral administration. Br J Pharmacol. 2004 Aug 9 [Epub ahead of print]
• Aspirin-triggered lipoxin A4 and B4 analogs block extracellular signal-regulated kinase-dependent TNF-alpha secretion from human T cells. J Immunol. 2003 Jun 15;170(12):6266-72.
• An aspirin-triggered lipoxin A4 stable analog displays a unique topical anti-inflammatory profile. J Immunol. 2002 Dec 15;169(12):7063-70.
• Local and systemic delivery of a stable aspirin-triggered lipoxin prevents neutrophil recruitment in vivo. Proc Natl Acad Sci U S A. 1999 Jul 6;96(14):8247-52.