WHI-P258, a quinazoline compound, binds to the active site of JAK3 with an estimated Ki of 72 µM. WHI-P258 does not inhibit JAK3 and does not affect the thrombin-induced aggregation of platelets even at 100 μM[1][2].
[1]. Sudbeck EA, et al. Structure-based design of specific inhibitors of Janus kinase 3 as apoptosis-inducing antileukemic agents. Clin Cancer Res. 1999 Jun;5(6):1569-82. [2]. Tibbles HE, et al. Role of a JAK3-dependent biochemical signaling pathway in platelet activation and aggregation. J Biol Chem. 2001 May 25;276(21):17815-22.