Enhancing MPN Therapy: Type II JAK2 Inhibition Overcomes Resistance and Amplifies Efficacy in MPN Models

The discovery of JAK2 mutations in individuals with myeloproliferative neoplasms (MPN) has prompted the medical advancement of JAK2 inhibitors, with ruxolitinib being authorized for myelofibrosis (MF) treatment. However, existing JAK inhibitors, while alleviating symptoms, do not notably shrink the MPN clone in most patients. Our research has shown that MPN cells can become resistant to ruxolitinib and other type I JAK inhibitors, which is linked to the reactivation of JAK-STAT signaling and the formation of heterodimers between JAK2 and other JAK proteins, such as JAK1/TYK2. This suggests that JAK2 is activated by other JAK kinases in a trans manner. Our studies have been expanded to other type I JAK inhibitors under clinical development, revealing the same resistance mechanism and a lack of sensitivity to different JAK inhibitors in persistent MPN cells.

All JAK inhibitors in development are type I, targeting the active state of JAK2 kinase. We theorized that type II JAK inhibitors, which target the inactive state of JAK2, could be effective against resistant cells and enhance efficacy in MPN models. We explored the effectiveness of NVP-CHZ868, a novel type II JAK inhibitor, in MPN cells and in mouse MPN models. CHZ868 effectively inhibited the growth of cells with JAK2V617F mutation or TEL-JAK2 fusion and showed high sensitivity in JAK2/MPL-mutant cell lines. It induced greater apoptosis in JAK2-mutant SET2 cells compared to ruxolitinib and more significantly reduced JAK-STAT signaling.

Furthermore, CHZ868 was found to completely suppress JAK-STAT signaling in cells that had become resistant to type I JAK inhibitors, preventing the heterodimeric activation of JAK2. Importantly, cells that were insensitive to type I JAK inhibitors were still highly responsive to CHZ868, indicating that resistance to type I inhibitors does not confer resistance to type II.

In mouse models of JAK2/MPL-mutant MPN, CHZ868 demonstrated significant activity, normalizing various hematological parameters and reducing mutant allele burden by an average of 49%, suggesting a disease-modifying effect.

Our findings indicate that type I JAK inhibitors lead to resistance through JAK2 transactivation, while type II JAK inhibition with CHZ868 remains effective against resistant cells and shows increased activity in mouse MPN models. These results suggest that new JAK inhibitors have the potential to enhance target inhibition and therapeutic efficacy, offering a promising strategy for improving outcomes in MPN patients.