Berlex researchers develop lipoxin A4 (LXA4) mimics for the treatment of inflammation

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, multiple sclerosis and inflammatory bowel disease (to view LeadDiscovery's feature on autoimmune/inflammatory disorders click here). The number 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 form HPETEs. 5-lipoxygenase catalyzes the production of leukotriene (LT) A4 which is in turn hydrolyzed to produce LTB4; alternatively the addition of a glutathione moiety in the presence of glutathione S-transferase produces LTC4 and LTD4.

Both the prostaglandins and the leukotrienes are involved in the initiation and maintenance of inflammatory responses and are proinflammatory. Agents that block the prostaglandin or the leukotrioene pathways are however of limited efficacy in various inflammatory disease settings (for a full discussion see our recent reports evaluating molecular targets for rheumatoid arthritis or asthma therapeutics).

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

A stable analog of LXA4 has recently been reported to block both airway hyper-responsiveness and pulmonary inflammation in a murine model of asthma. Likewise the transgenic expression of human LXA4 receptors in murine leukocytes significantly inhibits airway inflammation. These data along with findings from earlier studies showing that LXA4 modulates LTC4-induced airway obstruction in human asthmatics support the development of LXA4 mimics for the treatment of asthma.

Since LXA4 was reported in 1984, its trihydroxytetraene structure has attracted the attention of synthetic chemists; however development of LXA4 analogues has been limited by their metabolic and chemical instability. 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) to form inactive catabolites.

The initial design of metabolically stable LXA4 analogues focused on identifying poor substrates for PGDH, which maintained potency in in vitro assays. A number of endogenous analogues of LXA4 have been identified and one of these, 15-epi-LXA4 (aspirin-triggered LXA4), has been shown to be equipotent in in vitro assays to LXA4 but was a poorer substrate for PGDH. This observation spurred researchers to synthesize a stable 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.

Although ATLa has enhanced metabolic stability over LXA4 in vivo, the pharmacokinetics of ATLa remain poor which, in addition to low chemical stability precluded further development. In a recent study by Guilford et al, Berlex researchers found that ATLa is metabolized by beta-oxidation; in contrast ZK-994, a trienyne analogue of the tetraene, ATLa, was not disposed of in this fashion. Consequently, ZK-994 displayed considerably improved pharmacokinetic properties in the rat and was chemically stable. Furthermore the efficacy of this analogue was comparable if not improved compared to ATLa in a cutaneous mouse model of inflammation.

Although ATLa has enhanced metabolic stability over LXA4 in vivo, the pharmacokinetics of ATLa remain poor which, in addition to low chemical stability, creates challenges for development. In a recent study by Guilford et al, Berlex researchers found that ATLa is metabolized by beta-oxidation; in contrast the 3-oxa analog, ZK-142, and the corresponding trienyne, ZK-994, were not disposed of in this fashion. Consequently, both ZK-142 and ZK-994 displayed improved pharmacokinetic properties in the rat, while ZK-994 is more chemically stable. The efficacy of the 3-oxa analogue s, ZK-142 and ZK-994, was comparable if not improved compared to ATLa in a cutaneous mouse model of inflammation.

In a more recent British Journal of Pharmacology paper Berlex researchers in collaboration with Brigham and Women's Hospital and have published data comparing the anti-inflammatory efficacy of endogenous LXA4 and LXB4, ATLa2, ZK-142, and ZK-994 after intravenous, oral, and topical administration in mice. LXA4, LXB4, ATLa2, ZK-142, and ZK-994 were orally active, exhibiting potent systemic inhibition of zymosan A-induced peritonitis at very low doses. Intravenous ZK-994 and ZK-142 also potently attenuated hind limb ischemia/reperfusion-induced lung injury. At similar doses ATLa had no significant protective action.

These data therefore support the further development of LXA4 analogues such as ZK-142 and ZK-994. The improved pharmacokinetic properties and stability of such compounds may finally allow the anti-inflammatory properties of this ecosanoid to be effectively harnessed.

Featured on TherapeuticAdvances for LeadDiscovery, Sept. 2004

Adapted from Guilford et al, J Med Chem. 2004 Apr 8;47(8):2157-65. and Bannenberg et al, Br J Pharmacol. 2004 Aug 9 [Epub ahead of print]