Atiprimod Inhibits the Growth of JAK2 Tyrosine Kinase Over-Expressing Cells “In Vitro” and “Ex Vivo”.

Background:

A novel somatic single point non-synonymous mutation (1849 G to T) in the tyrosine kinase JAK2, rendering the enzyme constitutively active, has been reported in classical myeloproliferative disorders (MPD), specifically, in greater than 90% of patients with polycythemia vera (PV), and 50% of patients with essential thrombocythemia (ET) and myelofibrosis with myeloid metaplasia (MMM). Consequently, JAK2 tyrosine kinase has become a significant target for the development of specific inhibitors of its activity with possible therapeutic benefits for patients with classical MPD. We report here the results of a study using atiprimod, an anti-inflammatory and anti-cancer compound belonging to the azaspirane class of cationic amphiphilic drugs in JAK2 over-expressing cell lines and patient samples. In contrast to TKI inhibitors, this drug is causing downregulation of JAK2 protein by facilitating its degradation, possibly through the ubiquitin-proteosome pathway (Faderl, S. et al, Leukemia Research 2006 in press).

Methods and Results:

We have examined the growth inhibitory effect of atiprimod against two mouse FDCP cell lines transfected with erythropoietin receptor (Epo-R) and, either wild-type JAK2 (JAK2^WT) or mutant JAK2 (JAK2^V617F). Atiprimod growth inhibition, measured using 72 hours MTS assay, was significant against both mutant JAK2^V617F cells (IC₅₀ 0.42μM) and wild-type JAK2^WT cells (IC₅₀ 0.65μM). Atiprimod was found to induce apoptosis in cell lines as evidenced by an increase in the mitochondrial membrane potential and augmented annexin V positivity by flow cytometry. Mononuclear cells, isolated from peripheral blood of PV patients (JAK2^V617F positive), were cultured in cytokine supplemented serum free media for expansion of erythroid progenitors. Expansion spanned a three week, three step sequential treatment of the erythroid progenitor cells with specific combinations of cytokines. Finally, treatment of expanded erythroid progenitor cells with atiprimod for 24 and 48 hours resulted in increased annexin V positive cells, in both, a dose and time dependent manner.

Conclusion:

Our preliminary pre-clinical results suggest that atiprimod might be a beneficial medication for MPD patients carrying the JAK2^V617F mutation. Atiprimod is currently being tested in a phase I clinical study.