Limited options provided by the current standard of care for the patients suffering from myeloproliferative diseases (MPDs) prompted an extensive search for the underlying molecular mechanisms of these disorders. Recent discovery of a single activating mutation (V617F) in JAK2 kinase gene associated with the development of the polycythemia vera (PV), essential thrombocythemia (ET) and chronic idiopathic myelofibrosis (CIMF) opened up a possibility to develop highly targeted therapies against these debilitating ailments. To that end, we engineered cytokine-independent Ba/F3 cell line expressing the V617F mutant of JAK2 to screen a focused small molecule library for potential inhibitors of JAK2 V617F -dependent proliferation. We confirmed the ability of hit compounds to inhibit proliferation of JAK2-dependent tumor cell lines using UKE-1 and SET-2 cells carrying the V617F JAK2 mutation. A FACS-based phosphoSTAT5 assay was then used to demonstrate that the hits directly targeted mutant JAK2. JAK3 activity of each compound was evaluated in IL-2-dependent CTLL-2 cell line using phosphoSTAT5 FACS and proliferation assays. To avoid hits with nonspecific antiproliferative activity, the hits were tested in JAK2-independent MOLT4, A549 and H1299 cell lines. Compound hits with the desirable properties were further evaluated for their ability to inhibit JAK2, JAK3 and other kinases in the context of T cell, B cell, or mast cell activation using a variety of cell-based assays as well as in the in vitro biochemical assays. We identified a number of compounds that potently inhibit growth of the two V617F mutant cell lines with EC50s varying from 20 to 500 nM, but do not affect proliferation of control cell lines MOLT4, A549 and H1299 to the same degree. These compounds induce strong and highly specific suppression of STAT5 phosphorylation with IC50s of 10 to 200 nM in SET-2 and V617F JAK2 expressing Ba/F3 cells. One of the hits with the desirable biological and pharmacokinetic profiles was further evaluated in V617F JAK2 Ba/F3 engraftment mouse model where it demonstrated significant extension of survival at 150 and 200 mg/kg bid. Such potent JAK2 inhibitors could become the basis for the next generation of compounds targeting JAK2-dependent myeloproliferative diseases.

Author notes

Disclosure: Employment: All authors are employees of Rigel, Inc. Ownership Interests:; Employees of Rigel Inc., a publicly traded company, may have stock and stock options. Research Funding: This research is sponsored by Rigel, Inc.