Preclinical characterization of atiprimod, a novel JAK2 AND JAK3 inhibitor

We herein report on the activity of the JAK2/JAK3 small molecule inhibitor atiprimod on mouse FDCP-EpoR cells carrying either wild-type (JAK2 ^WT) or mutant (JAK2 ^V617F) JAK2, human acute megakaryoblastic leukemia cells carrying JAK2 ^V617F (SET-2 cell line), and human acute megakaryocytic leukemia carrying mutated JAK3 (CMK cells). Atiprimod inhibited more efficaciously the proliferation of FDCP-EpoR JAK2 ^V617F (IC₅₀ 0.42 μM) and SET-2 cells (IC₅₀ 0.53 μM) than that of CMK (IC₅₀ 0.79 μM) or FDCP-EpoR JAK2 ^WT cells (IC₅₀ 0.69 μM). This activity was accompanied by inhibition of the phosphorylation of JAK2 and downstream signaling proteins STAT3, STAT5, and AKT in a dose- and time-dependent manner. Atiprimod-induced cell growth inhibition of JAK2 ^V617F–positive cells was coupled with induction of apoptosis, as evidenced by heightened mitochondrial membrane potential and caspase-3 activity, as well as PARP cleavage, increased turnover of the anti-apoptotic X-linked mammalian inhibitor of apoptosis (XIAP) protein, and inhibition of the pro-apoptotic protein BCL-2 in a time- and dose-dependent manner. Furthermore, atiprimod was more effective at inhibiting the proliferation of peripheral blood hematopoietic progenitors obtained from patients with JAK2 ^V617F-positive polycythemia vera than at inhibiting hematopoietic progenitors from normal individuals (p = 0.001). The effect on primary expanded erythroid progenitors was paralleled by a decrease in JAK2^V617F mutant allele burden in single microaspirated BFU-E and CFU-GM colonies. Taken together, our data supports the clinical testing of atiprimod in patients with hematologic malignancies driven by constitutive activation of JAK2 or JAK3 kinases.