In vitro and in vivo characterization of SGI-1252, a small molecule inhibitor of JAK2

Objective: Constitutive activation of the Janus kinase 2 (JAK2) due to a somatic mutation (JAK2^V617F) arising in hematopoietic stem cells plays a central role in the pathophysiology of myeloproliferative neoplasms (MPNs). To investigate the hypothesis that drugs that inhibit JAK2 have therapeutic potential, we developed a small molecule inhibitor, SGI-1252, that targets the adenosine triphosphate-binding and solvent pocket of the protein. Materials and Methods: Established cells lines each expressing different JAK2^V617F copy numbers, a cell line transfected with wild-type and mutant JAK2, ex vivo expanded erythroid progenitor cells from patients with MPNs, and a murine xenograft model were used to characterize the activity of SGI-1252. Results: In vitro studies showed that SGI-1252 potently inhibits the kinase activity of wild-type JAK2, JAK2^V617F and JAK1, but not JAK3. SGI-1252 blocked phosphorylation of signal transducers and activators of transcription 5, a downstream target of JAK2 and inhibited expression of the JAK2-dependent antiapoptotic gene BCL-X_L. Additional studies confirmed induction of apoptosis in JAK2^V617F-positive cell lines by SGI-1252. Moreover, cell lines transfected with either wild-type JAK2 or JAK2^V617F were equally susceptible to the antiproliferative effects of SGI-1252 and the antiproliferative activity of SGI-1252 toward ex vivo-expanded erythroid progenitors from patients with polycythemia vera and primary myelofibrosis appeared independent of the JAK2^V617F allele burden. Pharmacodynamic studies in a murine xenograft model demonstrated both anti-tumor activity and inhibition of signal transducers and activators of transcription 5 phosphorylation by SGI-1252, and the drug was active and well-tolerated whether delivered intraperitoneally or orally. Conclusions: Together, these studies support further development of SGI-1252 for clinical use.