AT9283, a Potent Inhibitor of JAK2, Is Active in JAK2 V617F Myeloproliferative Disease Models.

AT9283 is a potent inhibitor of JAK2, JAK3, mutant Abl kinase (T315IAbl) and Aurora kinases A and B all of which have an IC₅₀ <5nM. The compound is currently in early phase clinical development for hematological malignancies. Multitargeted kinase inhibitors may be of particular value as anti-proliferative agents as a consequence of their ability to inhibit several signalling pathways simultaneously. Many of the kinases targeted by AT9283 lie in signalling pathways activated by oncogenes and may contribute in a positive way to the anti-tumour action of the compound. Here we describe the characterisation of the anti-tumour effects of AT9283 in models of JAK2-dependent disease The JAK2 V617F mutation has been identified in more than 95% of patients with Polycythemia vera and in 50 to 60% of patients with Essential Thrombocythemia and myelofibrosis. JAK2 is a key modulator of cytokine signalling, transducing signals from cell surface receptors via the JAK/STAT pathway. The point mutation renders the kinase constitutively active and induces cytokine-independent proliferation of cell lines harboring mutated JAK2. AT9283 was profiled against two cell lines harboring JAK2 V617F (HEL, SET-2) as well as two lines with a dependence on cytokine signalling through wild type endogenous JAK2 for survival (BA/F3 wt, TF-1, stimulated with IL3). These lines were compared with two additional leukaemia cell lines not dependent upon JAK signalling for survival, K562 (Ph+CML) and HL60 (N-Ras PML). In each case the dominant effect of the compound is to inhibit cell growth through JAK inhibition where the survival of the cell line depends on signalling through the JAK2 pathway. In contrast the cell lines less dependent on JAK2 for survival show a polyploid phenotype indicative of Aurora kinase inhibition. In HEL cells AT9283 inhibits phosphorylation of JAK2 pathway markers such as phospho-STAT5 ^(Tyr694) at 100nM, a dose consistent with concentrations required to inhibit the proliferation of these cells. Consequently, the cell cycle profiles generated when either K562 or HL60 cells are treated with AT9283 suggest that endoreduplication derived from Aurora inhibition is the dominant phenotype. In a JAK driven system the dominant phenotype observed is induction of apoptosis via inhibition of JAK. In an in vivo model, administration of a single dose of AT9283 to the JAK-dependent HEL xenograft, inhibited phosphorylation of STAT5 indicating JAK2 inhibition in the tumour tissue and an efficacy study demonstrated growth inhibitory effects of 9283 in this model. Finally the activity of AT9283 was tested in primary assays using cells taken either from healthy volunteers or patients diagnosed with MPD. These data together support further clinical investigation of the compound in patients with a myeloproliferative disorder.