Discovery of Bioavailable Inhibitors of Secretory Phospholipase A2

Abstract

Substrate-mimetic inhibitors of sPLA2 with submicromolar in vitro potency were discovered by use of a novel dual substrate screening strategy. In vivo evaluation of selected inhibitors in the rat carrageenan paw edema model of inflammation, however, indicated that in vitro potency was not a good predictor of in vivo activity. Studies of the metabolic stability of early examples of these inhibitors suggested that the metabolic lability of these compounds was a major contributing factor to the observed weak in vivo activity. In an attempt to achieve improved in vivo activity, we prepared and tested compounds designed to overcome the observed metabolic instability. The design of the new compounds involved two types of changes in the inhibitor molecules. First, the C-2 ester moiety was replaced with an amide function so that direct cleavage by stomach acid and blood esterases at this site was minimized. Second, omega-oxidation of the decanamide moiety was eliminated by substitution of hydrogen with fluorine in this position. Compounds containing fluorine in the terminal positions of the alkyl chain retained sPLA2 inhibitory activity and also possessed improved in vitro metabolic stability and pharmacokinetic parameters relative to nonfluorinated inhibitors in this series. As exemplified by GW 4776, improvements in metabolic stability alone, however, were not sufficient to ensure oral activity. Thus, GW 4776 did not show oral activity in the carrageenan edema model and had only modest activity after i.v. dosing in the same model. In fact, the results for GW 9624 and GW 8219 suggested that factors in addition to potency of sPLA2 inhibition and metabolism affect the observed in vivo activity. Despite the fact that these two compounds varied only by a single oxygen-to-sulfur substitution, one was active whereas the other was not. One possible explanation for the observed variability is a compound-dependent difference in the rate of equilibration into tissue. This possibility is relevant as both the carrageenan paw edema model and the phorbol ester edema model involve a localized inflammation. No measurements were made to assess differences in the distribution of the different inhibitors between the blood and the localized site of inflammation. In summary, a series of bioavailable inhibitors of sPLA2 was prepared using an iterative approach that combined medicinal chemistry, in vitro and in vivo evaluation of biological activity, and metabolic and pharmacokinetic studies. Although some compounds in the series showed in vivo activity, the anti-inflammatory effect observed in animal models was modest and a decision was made to abandon sPLA2 as a molecular target for the development of anti-inflammatory agents.