CHZ868

Our study identifies JAK1 and JAK2 resistance variants against the type I JAK2 inhibitors ruxolitinib, fedratinib, and lestaurtinib. The sensitivity of these resistant variants towards the type II JAK2 inhibitor CHZ-868 indicates that this mode of type II JAK2 inhibition is a potential therapeutic approach against ruxolitinib refractory leukemia. This also proposes the development of potent and specific type II JAK2 inhibitors using ruxolitinib-resistance variants as a prototype.

We report on a novel mechanism of AXL-MAPK-driven escape from type II JAK2 inhibition, which is targetable at different nodes. This highlights AXL as mediator of acquired resistance warranting inhibition to enhance sustainability of JAK2 inhibition in MPN.

Our findings suggest that ruxolitinib and baricitinib bind the active phosphorylated form of JAK and lead to a paradoxical cellular accumulation of functionally defective pJAK. Upon inhibitor withdrawal, the primed pJAKs are de-repressed and initiate a pSTAT signaling cascade, ultimately resulting in high levels of uPA. In contrast, CHZ868, which binds the inactive JAK kinase conformation, does not lead to pJAK accumulation, a pSTAT cascade, or uPA production.

Finally, we identify a JAK2 G993A mutation that confers resistance to the type II JAK2 inhibitor CHZ868 but not to our analogs. These data provide a template for identifying novel type II kinase inhibitors and inform further development of agents targeting JAK2 that overcome resistance.

After 48h monotherapy treatment by CHZ868, we detected decreased cell viability (20-75% at IC50), which increased in the combination with dexamethasone...Importantly, combination of BIBF1120 and CHZ868 showed a synergistic effect (-45%, at IC50)...Indeed, active caspase 3 increased after ruxolitinib and chloroquine (autophagy inhibitor) combination (+20% vs ruxolitinib alone, p<0.01)...Instead, AT9283 (p<0.001 vs LCLs), Fedratinib (p<0.01 vs LCLs) and Gandotinib (p<0.05 vs LCLs) were found to be potent, specific, and non-toxic JAK2 inhibitors. Additionally, this extended screening led us to identify drugs, not belonging to JAK inhibitors, specific and non-toxic for rearranged JAK2 cohort, such as Birinapant (Smac mimetic), JQ1 (BRD4 inhibitor), Fludarabine (Chemotherapy) among the others. CHZ868 is a promising drug for the treatment of JAK2 fusions. Further studies will focus on effective and specific novel drugs found to be highly effective and specific on JAK2 rearrangements in BCP-ALL.

In combination with dexamethasone, a further decrease of viability was observed...Importantly, combination of BIBF1120 and CHZ868 showed a synergistic effect (-45%, at IC50)...Indeed, active caspase 3 increased after ruxolitinib and chloroquine (autophagy inhibitor) combination(+20% vs ruxolitinib alone, p<0.01)...Conclusion CHZ868 is a promising drugfor the treatment of JAK2 fusionsBCP-ALL. Further studies will include combination with standard chemotherapy drugs, by reducing the intensity and toxicity of chemotherapy.

In combination with dexamethasone, we assessed a further decrease of viability between 10 to 95%...Importantly, combination of BIBF1120 and CHZ868 showed a synergistic effect (-45%, at IC50)...Indeed, active caspase 3 increased when ruxolitinib was given in combination with chloroquine, an autophagy inhibitor (+20% vs ruxolitinib alone, p<0.01)...CHZ868 is a promising candidate for treatment of BCP-ALL carrying JAK2 fusions, showing high efficacy and specificity, both ex vivo and in vivo . Further studies will include combination with standard chemotherapy drugs with the aim to maintain its efficacy by reducing the intensity and toxicity of chemotherapy.

Ruxolitinib (rux) Phase II clinical trials are underway for the treatment of high-risk JAK2-rearranged (JAK2r) B-cell acute lymphoblastic leukemia (B-ALL)...Using in vitro models of JAK2r B-ALL, JAK2 p.G993A conferred resistance to six type-I JAK inhibitors and the type-II JAK inhibitor, CHZ-868. Using computational modeling, we postulate that JAK2 p.G993A enabled JAK2 activation in the presence of drug binding through a unique resistance mechanism that modulates the mobility of the conserved JAK2 activation loop. This study highlights the importance of monitoring mutation emergence and may inform future drug design and the development of therapeutic strategies for this high-risk patient cohort.