Accumulation Of Jak Activation-Loop Phosphorylation Promotes Type I Jak Inhibitor Withdrawal Syndrome In Myelofibrosis

Patients with myelofibrosis who are treated with the Type I ATP competitive JAK inhibitor ruxolitinib derive symptomatic benefit from improvement in cytokine-mediated symptoms, and show cytoreductive responses in disease manifestations, such as splenomegaly or leukocytosis. However, ruxolitinib treatment does not consistently reduce JAK2 mutant allele burden nor eradicate the disease. Secondly, when ruxolitinib is withdrawn there is rapid recrudescence of cytokine-mediated symptoms within days of treatment stopping and in rare cases this has led to a life-threatening “ruxolitinib discontinuation syndrome” characterized by acute relapse of disease symptoms, splenomegaly, worsening cytopenia, and a cytokine storm akin to septic shock. Paradoxically, Type I inhibitors can induce the accumulation of JAK activation loop phosphorylation for unknown reasons, despite blockade of kinase function and inhibition of STAT phosphorylation. Here, we developed a time-dependent assay in primary myelofibrosis samples to mimic ruxolitinib withdrawal syndrome and investigate a possible mechanism of action involving JAK2 activation loop phosphorylation.