Uppal H1., Danilenko D1., Harstadt E1., Tarrant J1., Clarke E2., Dhawan P1., Kauss A1., McCray B1., Misner D1., Singh J1., Society of Toxicology 52nd Annual Meeting & ToxExpo 2013, Poster Abstract No. 1819. 1 Safety Assessment, Genentech, San Ramon, CA; 2 ReachBio, Seattle, WA.
Tyrosine Kinase 2 (Tyk 2) is a member of the Janus Activated Kinase (Jak) family. Tyk 2 is associated with signaling of pro-inflammatory cytokines, IL-12 and IL-23. Therefore, inhibition of Tyk 2 may potentially treat inflammatory diseases such as inflammatory bowel disease (IBD), rheumatoid arthritis, and psoriasis. Tyk 2 is also activated during thrombopoietin signal transduction, a pathway necessary for megakaryocyte development and platelet production. Studies were conducted in vitro and in vivo with potent Tyk 2 inhibitors with varying levels of JAK-family and general kinase selectivity to understand potential effects of these Tyk 2 inhibitors on platelet development.
Tyk 2 inhibitors were evaluated in vitro in human, mouse and cynomolgus monkey megakaryocyte colony forming cell assays. General cytotoxicity was evaluated by measuring cellular ATP levels. Direct effects of Tyk 2 inhibitors on platelet function (aggregation), energetics (oxygen consumption), and viability (ATP levels) were also evaluated in vitro. The effect of Tyk 2 inhibitors on platelet number was evaluated in short term (5-10 day) mouse studies. Tyk 2 and Jak mRNA expression was evaluated in human megakaryocytes by quantitative PCR to understand target expression.
Tyk 2 inhibitors caused a comparable reduction in the number of colony forming cells and the viability of megakaryocytes from human, mouse and nonhuman
primates. There were no direct effects of Tyk 2 inhibitors on the aggregation capacity, oxygen consumption, or ATP levels of human platelets in plasma. Tyk 2 inhibitors caused platelet reduction in mice. Tyk2 and Jak 1, 2, and 3 were comparably expressed in human megakaryocytes.
The in vitro and in vivo findings demonstrate that platelet reductions caused by multiple Tyk 2 inhibitors (with varying selectivity against other Jak family kinases) are likely due to effects on megakaryocyte development.