Zafirlukast is a broad-spectrum thiol isomerase inhibitor that inhibits thrombosis without altering bleeding times

Holbrook, L.-M., Keeton, S. J., Sasikumar, P., Nock, S., Gelzinis, J., Brunt, E., Ryan, S., Pantos, M. M., Verbetsky, C. A., Gibbins, J. M. ORCID: https://orcid.org/0000-0002-0372-5352 and Kennedy, D. R. (2021) Zafirlukast is a broad-spectrum thiol isomerase inhibitor that inhibits thrombosis without altering bleeding times. British Journal of Pharmacology, 178 (3). pp. 550-563. ISSN 0007-1188 doi: 10.1111/bph.15291

Abstract/Summary

Background and purpose Multiple members of the thiol isomerase (TI) family of enzymes are present in, and released by platelets. Inhibition of these enzymes results in diminished platelet responses including aggregation, adhesion and thrombus formation. In recent years, the therapeutic potential of TI inhibition has been recognised and drug-development technologies used to identify selective small molecule inhibitors. To date, few pan-TI inhibitors have been characterised and the most studied, bacitracin is known to be nephrotoxic which prohibits its systemic therapeutic usage. Experimental approach We therefore sought to identify novel broad-spectrum inhibitors of these enzymes and test their effects in vivo. 3641 compounds were screened for inhibitory effects on the redox activity of ERp5, PDI, ERp57, ERp72 and thioredoxin (TRX) in an insulin turbidity assay. Of the lead compounds identified, zafirlukast (ZFL) was selected for further investigation. Key results When applied to platelets, ZFL diminished platelet responses in vitro. ZFL was antithrombotic in murine models of thrombosis but did not impair responses in a model of haemostasis. Since thiol isomerases are known to modulate adhesion receptor function, we explored the effects of ZFL on cell migration. This was inhibited independently of cysteinyl leukotriene receptor expression and was associated with modulation of cell-surface free thiol levels consistent with alterations in redox activity on the cell surface. Conclusion and implications We identify zafirlukast to be a novel, potent, broad-spectrum TI inhibitor, with wide ranging effects on platelet function, thrombosis and integrin-mediated cell migration. ZFL is antithrombotic but does not cause bleeding.