Polycythemia vera (PV) is an acquired myeloproliferative, clonal stem cell disorder characterized by cytokine hypersensitivity. Several groups reported a clonal mutation in the pseudo kinase domain of the Janus kinase 2 (Jak2) protein, substituting phenylalanine at position 617 with valine (V617F) that causes constitutive activation of the JAK/STAT pathway in PV patients. Chronic myelogenous leukemia is another myeloproliferative disorder associated with aberrant tyrosine kinase (bcr-abl) that arises from t(9:22) translocation. Constitutive bcr-abl tyrosine kinase activity suppresses apoptosis leading to survival and proliferation of cells at low cytokine concentrations. Imitanib has been developed to inhibit the bcr-abl kinase activity and has had impressive therapeutic efficacy. However, imitanib also inhibits other tyrosine kinases. Clinical benefits of imitanib in PV have been reported. Mouse FDCP cells over-expressing either the wild-type Jak2 (JAK/W) or two cell lines with the V617F mutation (V617F), one with cytokine hypersensitivity and the other cytokine independent, were created. We examined the effect of imitanib by MTT proliferation assay and propidium iodide staining analysis. No appreciable changes in the proliferation and DNA content were observed in all three cell-lines after imitanib treatment at ~1μM (the concentration effective for the bcr-abl expressing cells and achievable in vivo in imitanib treated patients). However, after 72h of exposure, the cells expressing JAK2 V617F mutant showed 50% inhibition of growth at ~6μM imitanib with no significant effect on the growth of cells expressing JAK/W. To further understand the mechanism of growth inhibition of V617F cells by imitanib, we examined the metabolism of these cells since the constitutively active tyrosine kinases has been demonstrated to change the glucose metabolism. Imitanib treatment (5μM) for 72h caused about 30% decrease in the glucose uptake in V617F cells with only a marginal (~5%) decrease in glucose uptake was observed in the JAK/W cells. We then examined the effect of imitanib on in vitro expanded native human erythroid progenitors (CD71 and CD235a positive cells) from three normal and four PV individuals who expressed variable proportion of mutant JAK2 V617F alleles. In this small study, between 18–30% decrease in the proliferation of the cells from PV patients was seen with 1–2μM of imitanib compared to 8% seen with normal erythroid progenitors; however, the in vitro expansion was accompanied by a decrease of proportion of cells with JAK2 mutation (see abstract Prchal et. Al. at this meeting). Further, in ongoing clinical studies, one of 5 tested PV patients who showed an excellent clinical response to imitanib therapy had decrease of V617F mutation from 58 to 19% in circulating granulocytes. We conclude that high concentration of imitanib is required to achieve the cytotoxic effects in reporter cells transfected with JAK2 mutation that are not readily achievable in vivo; however, lower doses (but still significantly higher than those needed for bcr-abl expressing cells) are effective in native PV progenitors. These data do not exclude possible useful therapeutic effect of imitanib in PV either alone or in combination with other drugs. Amos Gaikwad, Ph.D. and Srdan Verstovsek, M.D. contributed equally to this study.