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Persistence of myelofibrosis treated with ruxolitinib
effective than ruxolitinib monotherapy in a mouse model of myelofibrosis.80 A clinical trial using the Hsp90 inhibitor AUY922 was terminated prematurely because of safety concerns, despite evidence of splenic responses.81 Other studies using the same drug documented a high frequency of visual alterations, including night blindness.82 An alterna- tive approach to target the accumulation of p-JAK2 is to use a proteolysis-targeting chimera (PROTAC). These small- molecule drugs contain two functional domains: one binds the target protein and the second engages an E3 ubiquitin ligase, thereby triggering proteasomal degradation of the target protein. JAK family PROTAC have been developed that recruit inhibitor of apoptosis protein leading to protea- somal degradation.83 An inhibitor of the de-ubiquitinase, USP9X, has been shown to accelerate the degradation of JAK2 in vitro.84 Whether any of these approaches could selectively target pathological accumulation of p-JAK2 in patients with myelofibrosis remains to be tested.
One of the pathways activated downstream of JAK2 in MPN is RAS/RAF/MEK/ERK. Ruxolitinib treatment of a cell line expressing JAK2 V617F inhibited downstream phosphorylated ERK1 and ERK2 (p-ERK1/2), but ruxoli- tinib treatment of a V617F mouse MPN model did not, despite effective suppression of other proteins down- stream of JAK2.46 Persistence of MEK/ERK signaling in vivo was found to be due to activation of PDGFRA by PDGF- BB signaling as a bypass pathway that enables persistence of MPN cells despite effective inhibition of JAK2 kinase activity. Combined inhibition of JAK2 and MEK with rux- olitinib and binimetinib (an approved treatment for RAF- mutant melanoma) was more efficient than either TKI alone in achieving regression of splenomegaly, fibrosis and JAK2 V617F allele burden.46
Conclusion
Despite the symptomatic benefits of currently available JAK2 inhibitors, there is a need for agents that have a greater effect on the disease clone and its natural history. JAK2 remains a crucial target in myelofibrosis, and dele- tion of JAK2 in a mouse model of myelofibrosis substan-
tially abrogates disease manifestations. Data from several groups have shown that a conformational change in JAK2 induced by the binding of ruxolitinib and other type 1 inhibitors may contribute to disease persistence by pre- venting JAK2 de-phosphorylation and proteasomal degra- dation, allowing heterodimerization of JAK2 with JAK1 or TYK2. Type 2 inhibitors of JAK2 do not cause this phe- nomenon and have greater impact on disease biology in a mouse model. The V617F mutation leads to a highly inflexible substitution, which may be indicative of a neo- morphic change in function, rather than non-specific over- activation. Several lines of evidence suggest that it may be possible to exploit the unique properties of the mutant pseudokinase to develop mutation-specific JAK2 inhibitors that spare normal hematopoiesis. Whereas cur- rent clinical trials are mostly targeting cooperating path- ways or pathways downstream of JAK2, these observa- tions suggest that novel approaches to the targeting of JAK2 could lead to substantial benefits for patients with myelofibrosis.
Disclosures
DMR has received research funding and honoraria from Novartis and BMS/Celgene unrelated to the current work. JJB, DTv and DTh declare that they have no conflicts of interest.
Contributions
DMR and DTh reviewed the literature and wrote the paper. JJB and DTv critically reviewed the content and wrote the paper.
Acknowledgments
We thank Suraiya Onnesha for assistance with the figures.
Funding
Research support for DMR was provided through the Medical Research Future Fund and The Hospital Research Foundation. Research support for JJB was provided by the National Health and Medical Research Council (APP1113577; APP1121755; APP1122999). Research support for DTh was provided through the National Health and Medical Research Council (APP1182564), Medical Research Future Fund, The Hospital Research Foundation and a CSL Centenary Fellowship.
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