Preclinical Analysis of Linsitinib for the Treatment of Polycythemia Vera

Even after the discovery of JAK2^V617F in majority of Polycythemia Vera (PV) patients and the generation of JAK2 inhibitors, the treatment of PV remains to be improved. An increased understanding of aberrant signaling in PV cells may yield novel drug targets for this patient population. The formation of erythroid colonies in the absence of exogenous erythropoietin (Epo), so called Epo-independent erythroid colonies (EECs), is a pathognomonic hallmark of PV. Over 20 years ago, Axelrad and colleagues proposed that EEC formation is not due to Epo-independence. Rather, they postulated that PV cells are hypersensitive to Insulin Growth Factor-1 (IGF-1). Subsequently, increased IGF-1 receptor (IGF-1R) phosphorylation was demonstrated in PV MNCs. In addition, serum concentrations of IGF-1 binding protein (IGF-1BP) are significantly elevated in PV patients. It has been shown that IGF-1 cooperates with JAK2^V617F to increase JAK/STAT signaling, elevating constitutive phosphorylation of STAT factors and thereby augmenting aberrant proliferation. We therefore hypothesized that inhibition of IGF-1 signaling would ameliorate the PV phenotype, even in the presence of JAK2^V617F.

The dual IGF-1R and insulin receptor (IR) kinase inhibitor linsitinib (formerly called OSI-906) was used to investigate the contribution of the IGF-1 pathway to PV pathophysiology. In methylcellulose colony assays, linsitinib significantly reduced the EEC formation from PV MNCs. In contrast, in the presence of EPO, colony formation from either PV or healthy control MNCs was not affected. These data suggest that linsitinib specifically inhibits growth of the PV clone. Linsitinib likewise significantly reduced the cytokine independent proliferation of Ba/F3 cells expressing JAK2^V617F but did not affect proliferation of Ba/F3 cells expressing either JAK^WT or JAK2^V617F in the presence of cytokine (mIL3). These data again argue that IGF-1 signalling is required for aberrant, cytokine independent growth. In addition to reducing proliferation, Linsitinib also induced apoptosis in IL3-independent JAK2^V617F transduced Ba/F3 cells. Using the JAK2^V617F bone marrow transplant (BMT) model, we investigated the effect of linsitinib on the PV phenotype in vivo. Linsitinib treatment significantly reduced the elevated RBC count, hematocrit and hemoglobin in this PV mouse model. In addition, the white blood cell count was decreased.

The data presented here provide a pre-clinical rationale for expanding the investigation of linsitinib as well as similar IGF-1 pathway inhibitors for the treatment of PV. The effect of combining JAK2 and IGF-1 inhibition may prove especially promising in this patient population and is currently being investigated.