SB1518 is an innovative pyrimidine-based macrocycle that shows a unique kinase profile with selective inhibition of Janus Kinase-2 (JAK2; IC 50 ¼ 23 and 19 nM for JAK2 WT and JAK2 V617F , respectively) within the JAK family (IC 50 ¼ 1280, 520 and 50 nM for JAK1, JK3 and TYK2, respectively) and fms-like tyrosine kinase-3 (FLT3; IC 50 ¼ 22 nM). SB1518 shows potent effects on cellular JAK/STAT pathways, inhibiting tyrosine phosphorylation on JAK2 (Y221) and downstream STATs. As a consequence SB1518 has potent anti-proliferative effects on myeloid and lymphoid cell lines driven by mutant or wild-type JAK2 or FLT3, resulting from cell cycle arrest and induction of apoptosis. SB1518 has favorable pharmacokinetic properties after oral dosing in mice, is well tolerated and significantly reduces splenomegaly and hepatomegaly in a JAK2 V617F -driven disease model. SB1518 dose-dependently inhibits intra-tumor JAK2/ STAT5 signaling, leading to tumor growth inhibition in a subcutaneous model generated with SET-2 cells derived from a JAK2 V617F patient with megakaryoblastic leukemia. Moreover, SB1518 is active against primary erythroid progenitor cells sampled from patients with myeloproliferative disease. In summary, SB1518 has a unique profile and is efficacious and well tolerated in JAK2-dependent models. These favorable properties are now being confirmed in clinical studies in patients with myelofibrosis and lymphoma.

The phosphotyrosine residues of receptor tyrosine kinases serve as unique binding sites for proteins involved in intracellular signaling, which contain SRC homology 2 (SH2) domains. Since overexpression or activation of the pp60C-src kinase has been reported in a number of human tumors, including primary human breast carcinomas, we examined the interactions of the SH2 and SH3 domains of human SRC with target proteins in human carcinoma cell lines. Glutathione S-transferase fusion proteins containing either the SH2, SH3, or the entire SH3/SH2 region of human SRC were used to affinity purify tyrosine-phosphorylated proteins from human breast carcinoma cell lines. We show here that in human breast carcinoma cell lines, the SRC SH2 domain binds to activated epidermal growth factor receptor (EGFR) and pl85HER2/neu. SRC SH2 binding to EGFR was also observed in a nontumorigenic cell line after hormone stimulation. Endogenous pp60c-src was found to tightly associate with tyrosinephosphorylated EGFR. Association of the SRC SH2 with the EGFR was blocked by tyrosyl phosphopeptides containing the sequences surrounding tyrosine-530, the regulatory site in the SRC C terminus, or sequences surrounding the maEjor sites of autophosphorylation in the EGFR. These results raise the possibility that association of pp6(C-src with these receptor tyrosine kinases is an integral part of the signaling events mediated by these receptors and may contribute to malignant transformation.Signaling mediated by receptor tyrosine kinases, such as the epidermal growth factor receptor (EGFR), requires receptor autophosphorylation on tyrosine (1). These phosphotyrosine residues serve as unique binding sites for proteins that contain SRC homology 2 (SH2) domains. This protein motif recognizes phosphotyrosine in a sequence-specific manner, and the in vivo specificity of proteins for their cognate phosphotyrosine residue in the receptor is maintained in in vitro binding assays using the isolated SH2 domains of these proteins. Such domains are found in a number of proteins involved in intracellular signaling including p85, the noncatalytic subunit of phosphatidylinositol 3-kinase; GAP, the GTPase-activating protein of p2lras; and phospholipase Cy (2). Constitutive activation of these signaling pathways is apparent in many malignancies. Human breast cancers often overexpress two closely related receptor tyrosine kinases, EGFR or p185HER2/neu, and amplification of these genes is correlated with poor clinical prognosis (3-5). It has recently been reported that many primary human breast tumors also show elevated activity of pp6c-src (6), suggesting that this protein may play a role in carcinoma of the breast. These results indicate that activation of downstream events mediated by both receptor and nonreceptor tyrosine kinases may be critical in some types of human neoplasia. Evidence for similar interactions between receptor and nonreceptor tyro-The publication costs of this article were defrayed in part by page charge payment. This article must the...