New Therapies in Development for Myelofibrosis

Srdan Verstovsek, MD, PhD

Building on the transformative impetus from the first Food and Drug Administration (FDA)-approved Janus kinase (JAK) 1/2 inhibitor, ruxolitinib (Jakafi), in the clinical landscape of myeloproliferative neoplasms (MPNs), we are entering a new era of multiple JAK inhibitors and other diverse classes of drugs in rapid clinical development. Advancements in elucidating the pathophysiology of MPNs have spurred significant progress in developing novel promising agents or combination regimens with ruxolitinib to treat patients who are unresponsive to standard treatments or have specific clinical needs.

Myelofibrosis

In myelofibrosis (MF), the most aggressive MPN, with an average survival of 5 to 7 years, abnormal clonal hematopoietic stem cell proliferation in the bone marrow (BM) leads to liberation of pro-inflammatory cytokines and extensive fibrosis, causing progressive pancytopenia, especially anemia and thrombocytopenia, along with splenomegaly and other symptoms, compromising quality of life.¹

JAK Inhibitors

For nearly a decade, ruxolitinib has been the centerpiece therapy for patients with MF, markedly improving splenomegaly and constitutional symptoms and providing survival benefit.2 The second FDA-approved JAK2 inhibitor, fedratinib (Inrebic), may actually be a good second-line option for patients who are ruxolitinib-resistant with intermediate-2 and high-risk MF (primarily thrombocytopenic and characterized by platelet counts 50−100 × 109/L).3 At present, 2 ongoing phase 3 clinical trials, the single-arm FREEDOM trial (NCT03755518) and the double-arm FREEDOM 2 trial (NCT03952039), are assessing the efficacy and safety of fedratinib in patients with MF who are resistant/refractory/intolerant to ruxolitinib. The FREEDOM trials are important because the previous JAKARTA studies (NCT01523171, NCT01437787) were placed on hold or terminated given concerns for the development of Wernicke encephalopathy. Pacritinib is a potent inhibitor of both JAK2 and fms-related receptor tyrosine kinase

3, or FLT3, but does not affect JAK1. Pacritinib is being evaluated in comparison with the physician’s choice in an ongoing phase 3 trial (PACIFICA; NCT03165734) in patients with MF and severe thrombocytopenia (baseline platelet count < 50 × 10⁹/L) at the optimal dose determined in the PAC203 study (200 mg twice daily; NCT03165734).³ Successful clinical development of pacritinib will provide a non-myelosuppressive JAK2 inhibitor for frontline treatment of patients with MF who have severe thrombocytopenia, a setting currently lacking approved drugs. Another JAK1/2 inhibitor that is in advanced clinical development and complements its predecessors is momelotinib, possessing the exclusive attribute to improve anemia, which becomes severe in patients with MF.³ At present, momelotinib is undergoing evaluation in patients who are symptomatic and anemic with advanced MF, previously treated with a JAK inhibitor, in a phase 3 trial (MOMENTUM; NCT04173494); the comparator drug is danazol.

Drug Combinations with Ruxolitinib

Targeting anemia and thrombocytopenia. Given that patients with MF experience disease-associated and JAK inhibitor-induced anemia, several clinical trials have been evaluating drugs counteracting anemia, as monotherapies or in combination with ruxolitinib, in patients with MF-associated anemia.⁴ Currently, a global, multicenter phase 2 trial is under way to evaluate the safety and efficacy of luspatercept-aamt (Reblozyl), an activin receptor ligand trap that enhances late-stage erythropoiesis in patients with anemia and MF, including ruxolitinib-treated, transfusion-dependent individuals; a phase 3 trial (INDEPENDENCE) is planned for 2020. Interim results of the phase 2 study demonstrated significant efficacy of luspatercept-aamt, achieving reduction in red blood cell transfusion burden in ruxolitinib-treated patients with MF. Thalidomide (Thalomid), an immunomodulatory agent, significantly improved anemia and thrombocytopenia (platelet counts increased in 60% of patients) in a phase 2 trial evaluating ruxolitinib-treated patients with MF and baseline thrombocytopenia (NCT03069326).⁵

Synergistic combinations with ruxolitinib targeting epigenetics and JAK2 (TABLE). CPI-0610 is a selective bromodomain and extraterminal protein inhibitor that improved spleen volume, anemia, BM fibrosis, total symptom score, and transfusion dependence (alone or with ruxolitinib) in patients with MF who are enrolled in the global phase 2 MANIFEST study (NCT02158858).³ Furthermore, a phase 1 clinical trial combining an inhibitor of heat shock protein 90 (JAK2 is its chaperone protein), PU-H71, with ruxolitinib in patients with primary/secondary MF is under way (NCT03935555).³ The previous 2 trials are supported by preclinical data showing drug synergism. In a phase 2 trial of ruxolitinib/azacitidine (hypomethylating agent) in patients with MF, synergism was demonstrated in spleen length reduction and BM fibrosis improvement compared with ruxolitinib monotherapy (NCT01787487).⁵

Synergistic combinations with ruxolitinib targeting antiapoptotic proteins and JAK2. Navitoclax is an orally bioavailable inhibitor of the antiapoptotic B-cell lymphoma 2 (BCL2) family of proteins (primarily BCL extra-large [XL]). In preclinical studies, the nonclinical analogue of navitoclax, ABT-737, in combination with ruxolitinib showed synergism in inducing apoptosis of JAK2 V617F-driven MPN cell lines. Interim data from an ongoing phase 2 clinical trial evaluating navitoclax in combination with ruxolitinib in ruxolitinib-treated patients with MF (with baseline platelet count ≥ 100 × 10⁹/L) showed reduction in spleen volume and BM fibrosis (≥1 grade) and improvement in total symptom score in a proportion of the patients (NCT03222609).³

Telomerase Inhibitor

Imetelstat is a short oligonucleotide telomerase inhibitor that possibly prolonged median overall survival in patients with MF in the higher-dose (9.4-mg/kg) arm of the phase 2 IMbark study (NCT02426086).³ A phase 3 trial comparing imetelstat to best available therapy in patients with refractory MF is planned for early 2021.

Antifibrotic Agent

PRM-151, a plasma-derived analogue of the human antifibrotic protein pentraxin 2, improved BM fibrosis in mice models and patients with MF in preclinical and phase 1/2 clinical studies, respectively.³ The promising results merit a phase 3 trial, especially given the scarcity of antifibrotic agents.

Polycythemia Vera and Essential Thrombocythemia

The two relatively indolent MPN subtypes, polycythemia vera (PV) and essential thrombocythemia (ET), are characterized by abnorabnormal proliferation of myeloid cells, resulting in elevated blood counts (erythrocytosis and thrombocytosis in PV and ET, respectively), considerable risk of thrombosis and hemorrhage, and progression to secondary MF and acute myeloid leukemia (more common in PV than ET).⁶ In PV and ET, therapies are aimed at reducing risk of thrombosis, which is higher in patients over 60 years old or with a history of thrombosis, and in ET, when the calreticulin gene, CALR, is absent. A particularly promising agent for the two indolent MPNs is the long-acting ropeginterferon α-2b, which was approved in Europe for frontline treatment of high-risk patients with PV and without symptomatic splenomegaly on the basis of the PROUD/CONTINUATION-PV studies [EudraCT, 2012-005259-18 (PROUD-PV) and 2014-001357- 17 (CONTINUATION-PV)].⁷ The previous investigations demonstrated superiority of ropeginterferon α-2b versus hydroxyurea after 3 years of therapy. Besides awaiting possible approval of ropeginterferon α-2b to treat patients with PV in the United States, a phase 3 trial of ropeginterferon α-2b versus anagrelide in hydroxyurea-resistant/intolerant patients with ET has been planned to start in 2020. Givinostat, an inhibitor of histone deacetylases, demonstrated promising clinical responses (reduction in pruritus and thrombosis, and normalization of hematological parameters) in phase 1/2 studies in patients with JAK2 V617F– positive PV and is entering a phase 3 trial in 2021.⁷ Currently, hydroxyurea and ruxolitinib are the first- and second-line treatments for high-risk patients with PV, respectively, and hydroxyurea is the first-line treatment for ET.

Herein we highlighted an array of drugs— ranging from new JAK inhibitors to an antifibrotic agent, epigenetic modifiers, and telomerase and BCL-XL/BCL2 inhibitors—that are in early or advanced clinical development in MPN. We are looking forward to enrichment of the MPN arsenal with new disease-modifying agents complementing the clinical benefits of ruxolitinib and fulfilling unmet needs in this population.

_References:

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_{5. Gisslinger H, Klade C, Georgiev P, et al. Ropeginterferon alfa-2b versus standard therapy for polycythaemia vera (PROUD-PV and CONTINUATION-PV): a randomised, non-inferiority, phase 3 trial and its extension study. Lancet Haematol. 2020;7(3):e196-e208. doi:10.1016/S2352- 3026(19)30236-4}

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