Page 30 - 2021_05-Haematologica-web

D.M. Ross et al.
occurs relatively abruptly and leads to near-complete loss of therapeutic benefit, typically over a number of months. In contrast, loss of response to ruxolitinib in myelofibrosis is characterized by a gradual waning of the initial thera- peutic benefit, sometimes associated with late-emerging cytopenia that is likely to represent progression of the underlying disease rather than direct toxicity of the TKI.
In chronic myeloid leukemia, around half of cases of secondary resistance are associated with the emergence of single amino acid changes in the BCR-ABL1 kinase domain which impair binding of the TKI or stabilize the active conformation of the protein.75 Ruxolitinib-resistant kinase domain mutations have been isolated using in vitro mutagenesis screening, and such mutations have also been observed clinically in patients with acute lymphoblastic leukemia treated with ruxolitinib.76 However, fewer than 50% of resistant clones after saturation mutagenesis in the presence of ruxolitinib harbor additional mutations in JAK2.49 Fedratinib, another ATP-competitive JAK2 inhibitor in clinical use, appears to be less susceptible to this pattern of resistance because it inhibits not only ATP binding, but also the binding of peptide substrates to JAK2.77 However, JAK2 kinase domain mutations have not yet been observed in clinical samples from myelofibrosis patients treated with JAK inhibitors, which could indicate either incomplete inhibition of JAK2 or that myelofibrosis is not critically dependent on the kinase activity of JAK2 in the way that chronic myeloid leukemia is addicted to a fusion oncogene.
Strategies to improve inhibition of JAK-STAT signaling
Here we discuss various approaches that might be employed to achieve more complete inhibition of JAK- STAT signaling (summarized in Table 1) with the aim of improving treatment response, and ultimately finding
treatments with greater disease-modifying potential. Type 2 kinase inhibitors, such as the BCR-ABL1 inhibitor, ima- tinib, are ATP-competitive inhibitors that bind the inactive conformation of the kinase. All of the JAK2 inhibitors that have been tested clinically to date for myelofibrosis are type 1 inhibitors. Since JAK2 V617F is a weak activating mutation (relative to constitutive activation of fusion kinases, such as BCR-ABL1 and PCM1-JAK2) it does not strongly favor the active conformation. In contrast, the KIT D816V mutation that is commonly found in systemic mastocytosis leads to a bias in favor of the active confor- mation of the stem cell factor receptor (which KIT encodes). Midostaurin and avapritinib (both type 1 inhibitors) have significant clinical activity against KIT D816V: they show greater potency against KIT D816V than against wild-type KIT, whereas the converse is true for type 2 inhibitors, such as imatinib and sunitinib.78 An experimental type 2 JAK inhibitor, CHZ868, did not result in accumulation of p-JAK2 and led to more potent sup- pression of myelofibrosis cells.49,51 This compound is not being developed for clinical use, but these pre-clinical data suggest that the development of type 2 JAK inhibitors for clinical use might offer advantages over the available TKI for myelofibrosis. A potential risk of this approach is greater suppression of normal hematopoiesis.
The JH2 pseudokinase domain of JAK2 binds ATP, and mutation of certain residues abrogates ATP binding, as well as leading to loss of the activated JH1 tyrosine kinase activity in V617F.79 In a mouse model of JAK2 V617F MPN, co-mutation with K581A prevented the development of polycythemia. Notably, mutation of the same residue had little or no effect on cytokine signaling through wild-type JAK2, potentially opening the opportunity for the devel- opment of mutation-specific allosteric inhibitors that tar- get this site.
JAK2 is a client protein of Hsp90, and inhibition of Hsp90 leads to degradation of JAK2 (including p-JAK2). Combining ruxolitinib with an Hsp90 inhibitor was more
Table 1. Possible strategies to inhibit JAK2 and downstream signaling in myeloproliferative neoplasms.
Current standard of care
Type 1 inhibitors
Alternative JAK inhibitors
Type 2 inhibitors Allosteric inhibitors
Drugs to degrade JAK2
Hsp90 inhibitors De-ubiquitinase inhibitors
Proteolysis-activating chimeras (PROTAC)
Inhibitors of downstream signaling
MEK/ERK inhibitors
Mechanism of action
ATP-competitive inhibitors that bind the active conformation of JAK2
ATP-competitive inhibitors that bind
the active conformation of JAK2 Non-ATP-competitive inhibitors of
the pseudokinase domain or substrate binding
Inhibit chaperone function to expose JAK2 to degradation
Small molecules that promote ubiquitination without specificity for JAK2 Designed to target specific proteins
to degrade
Inhibitors of bypass signaling in the presence of JAK inhibitor
Examples
Ruxolitinib Fedratinib Momelotinib Pacritinib
CHZ868 BBT594 LS104
AUY922
WP1130 -
Trametinib Binimetinib Ulixertinib
Notes
Lead to accumulation of p-JAK2 Limited disease-modifying potential
Do not lead to accumulation of p-JAK2 Could confer greater specificity for JAK2
V617F over wild-type JAK2
Clinical development discontinued because of ocular toxicity
Could reduce accumulation of p-JAK2
in combination with a type 1 JAK2 inhibitor Could reduce accumulation of p-JAK2
in combination with a type 1 JAK2 inhibitor
Synergistic with type 1 JAK2 inhibitor in experimental models
Not tested clinically in MPN
Approaches that have been used clinically (in any disease) are shown in bold.
1250
haematologica | 2021; 106(5)