Treatment Options Expand Beyond JAK Inhibition for Patients With Myelofibrosis

Aaron Gerds, MD, MS

The only curative approach to treating myelofibrosis (MF) at this time is hematopoietic stem cell transplant, but the treatment landscape for this patient population continues to expand, particularly since the identification of the JAK-STAT pathway as a potential target in 2005. Although the introduction of novel agents like JAK inhibitors have been promising for the treatment of patients with MF, there are more agents coming down the pipeline as well that will impact the way physicians treat this population.¹

During the National Comprehensive Cancer Network (NCCN) 2020 Virtual Congress: Hematologic Malignancies, Aaron Gerds, MD, MS, assistant professor of medicine (hematology and medical oncology), Cleveland Clinic Taussig Cancer Institute, reviewed the current treatment landscape for patients with MF and what’s to come for the treatment of this patient population as clinical trials continue to advance the field.

“Treatments of MF focus on the symptoms of the disease. This is a very symptom-forward disease,” Gerds explained during his presentation, pointing toward the 4 biggest challenges in treating MF.

Overall, 80% of patients have splenomegaly, 70% experience MF-associated symptoms, 60% to 85% have anemia or cytopenia, and the life expectancy is shortened, with the average time from diagnosis to death being 5 to 6 years in all comers.

Currently, the treatment landscape includes hydroxyurea to control counts, while the JAK inhibitors like ruxolitinib (Jakafi) and fedratinib (Inrebic) are known to control the symptoms and splenomegaly. Patients with lower grades of fibrosis can be treated with interferons as well.

For patients who are anemic, some of the available treatment options include lenalidomide (Revlimid), thalidomide (Thalomid), and danazol, and patients who are moving from MF into an acute leukemia can be treated currently with chemotherapy agents such as azacitidine and decitabine.

JAK Inhibition Remains Cornerstone of MF Treatment

A promising advancement in the treatment landscape of MF includes the identification of the JAK-STAT pathway, which is targeted with JAK inhibitors. The first JAK inhibitor to receive approval from the FDA was ruxolitinib, followed by fedratinib. Additional agents from this class of drugs are in development as well, which are showing different clinical benefits than what have been observed with these initial agents.

“Targeting the JAK-STAT has really been the centerpiece for the treatment of MF," said Gerds, "and you can see JAK inhibitors are all over the place, from low risk to high risk, from the top to the bottom, it's everywhere. It has really become a cornerstone treatment for MF.”

Ruxolitinib received its approval in 2011 on the basis of the COMFORT-1 and COMFORT-2 studies, in which ruxolitinib was compared with placebo or best available therapy (BAT), respectively, in intermediate-2 and high-risk patients with MF. Overall, the agent was able to induce spleen volume reduction of ≥35% in 41.9% of patients by week 24 compared with 0.7% with placebo (P <.0001), while spleen volume was reduced in 28.5% by week 48 in the COMFORT-2 study versus 0% with BAT (P <.0001).^2,3

Although ruxolitinib does not kill MF cells, a survival benefit has also been associated with ruxolitinib. According to a pooled analysis from both the COMFORT-1 and COMFORT-2 studies, the median overall survival (OS) in the ruxolitinib arms was 5.3 years (95% CI, 4.7-not evaluable [NE]) compared with 3.8 years (95% CI, 3.2-4.6) in the control arm (HR, 0.70; 95% CI, 0.54-0.91; P = .0065).

Fedratinib received approval in 2019 based on findings from the open-label phase 2 JAKARTA-2 and the randomized placebo-controlled phase 3 JAKARTA clinical trials. JAKARTA demonstrated that 47% of patients receiving 400 mg fedratinib and 49% receiving 500 mg had spleen volume reduction ≥35% at week 24, while in JAKARTA-2, this was achieved in 53% of patients with intermediate/high-risk MF who were resistant to prior ruxolitinib treatment and 63% of those who were intolerant to the therapy.^4,5

Among other JAK inhibitors coming done the pipeline now for the treatment of MF, the 2 agents that are furthest along include pacritinib and momelotinib, which have also demonstrated interesting activity in clinical trials. However, no 2 JAK inhibitors are alike, Gerds explained during his presentation.

Ruxolitinib is a JAK1/2 inhibitor, while fedratinib targets JAK2 and also hits other targets, including FLT3, which may be the cause of some of the particular off-target effects observed with this agent, like diarrhea and nausea. Pacritinib is a JAK2 inhibitor that has very little sensitivity for JAK1, but it is known to have some off-target effects due to also hitting FLT3 and IRAK1, which may be important in this agent. Momelotinib, on the other hand, is a JAK1/2 inhibitor, but it also has off-target effects in ACVR1, which is suspected to help with anemia in some patients.

“We can say that there's room for all these JAK inhibitors in the treatment of MF because they all are a little bit different, and they can be applied to different populations of patients with MF,” Gerds explained. “For example, momelotinib has a positive effect on patients with anemia.”

Momelotinib has been evaluated in 2 large randomized phase 3 studies, the SIMPLIFY 1 and SIMPLIFY 2 clinical trials, in which momelotinib was evaluated in patients who were naïve to JAK inhibition (n = 432) and those who were previously treated with ruxolitinib (n = 156), respectively.

Spleen volume reductions of ≥35% at week 24 were observed in 26.5% receiving momelotinib versus 29% with ruxolitinib (P = .011) in the SIMPLIFY 1 study and in 7% who received momelotinib versus 6% with BAT (P= .90) in the SIMPLIFY 2 study. The total symptom score reduction at week 24 with momelotinib was 28.4% versus 42.2% with ruxolitinib (P = .98) in the SIMPLIFY 1 study and 26% with momelotinib versus 6% with BAT (P = .0006) in the SIMPLIFY 2 study.^6,7

The JAK1/2 inhibitor momelotinib appears to have the potential to improve anemia via suppression of hepcidin, Gerds said. Momelotinib has been shown to decrease production of hepcidin and to increase serum iron and erythropoiesis, which leads to transfusion independence and an increase in hemoglobin. This served as the rationale for a phase 2 study of 41 transfusion-dependent patients with MF, in which 41% of patients converted to transfusion independence and 78% of non–transfusion-independent patients achieved ≥50% decrease in transfusions with momelotinib.⁸

The phase 3 MOMENTUM (NCT04173494) study has been initiated to evaluate momelotinib at 200 mg daily plus placebo against danazol, which is a therapeutic approach for treating anemia, at 600 mg daily plus placebo. Patients enrolled in the study are randomized 2:1 to either the momelotinib or danazol arm. After spleen progression in the control arm, patients are able to cross over to receive momelotinib. This is a global study being conducted in North America, the European Union, and Asia Pacific. Patients must have received prior JAK inhibitor therapy and have symptomatic disease to be included in the study, as well as have anemia. This trial will be able to validate the value of momelotinib in treating anemic patients with MF.

Pacritinib has been evaluated in 3 key studies, including the phase 2 PAC203 study, and the phase 3 PERSIST-1 and PERSIST-2 studies. PAC203 was a dose-finding study in higher-risk patients with MF who previously received ruxolitinib, while PERSIST-1 included higher-risk JAK inhibitor–naïve patients with any degree of anemia or thrombocytopenia and PERSIST-2 included patients with platelet counts ≤100,000/mcL, allowing for prior JAK inhibitor treatment as well.

Spleen volume reduction ≥35% at week 24 occurred in 19% of patients in the pacritinib arm versus 5% with BAT (P =.0003) in PERSIST-1, 18% in the pacritinib arm versus 3% with BAT (P =.001) in PERSIST-2. The spleen volume reduction ≥35% at week 24 in 18% of patients who received pacritinib in PAC203, and the total symptom scores reduced ≥50% in 7.4% of patients, which was also observed in PERSIST-2 in 25% of those receiving pacritinib and 14% BAT.^9,10

Pacritinib was temporarily placed on a clinical hold due to an increased signal for potential cardiac and bleeding complications, but upon a second look at the data from the PERSIST studies, investigators determined that this was a very high-risk population that are very thrombocytopenic and prone to bleeding events.¹¹

Without the JAK1 inhibition in pacritinib, thrombocytopenia is not as concerning as with other JAK inhibitors, Gerds said. The aim of the ongoing PACIFICA (NCT03165734) study is to potentially fulfill the unmet need for patients with MF who have platelet counts less than 50,000 and who are at risk for thrombocytopenic events.

PACIFICA, a randomized phase 3 study, is now ongoing to determine the efficacy of pacritinib compared with the physician’s choice of therapy. The primary end point for the trial is spleen volume reduction at 24 weeks, and secondary end points include total symptom score at 24 weeks, OS, and patient global impression change at 24 weeks. Crossover is not allowed in this study.

Looking Ahead Beyond JAK Inhibition

In the COMFORT studies, the median time on ruxolitinib was around 3 years, but a real-world analysis demonstrated that the average may be much shorter, Gerds explained. Patients who discontinue treatment with ruxolitinib tend to do poorly, and the median OS is short. New treatments are needed to improve outcomes in this patient population.

Unlike in a disease like chronic myeloid leukemia, in which a single mutation could be targeted with a type of agent that would give significant long-term disease control, there are many other pathways outside of the JAK-STAT pathway that could also be targeted in MF, which is where research is now looking to. Momelotinib and pacritinib remain under evaluation in large randomized trials now, and these agents, as well as luspatercept for anemia, appear most promising in terms of becoming available for the treatment of patients with MF in the near future. However, other agents are coming down the pipeline as well that Gerds noted during his presentation.

Novel agent PRM-151 works well in reversing fibrosis in the bone marrow, and bromodomain and extraterminal (BET) inhibitors are also under evaluation in some ongoing studies that are heading into phase 3, such as CPI-0610 for the upfront and post-JAK inhibitor setting. BET inhibitors reduce inflammatory cytokine production in MF, and LSD1 inhibitors have been associated with epigenetic reprograming.

Another promising class of drugs coming down the pipeline for the treatment of MF include JAK2—type 2 inhibitors, which hit a different target than the known JAK inhibitors. PI3K inhibitors appear to suppress neoplastic clonal hematopoiesis via cell arrest and apoptosis, while SMAC activation, MDM2, and Aurora kinase A can potentially increase apoptosis.

There is some rationale for targeting the mutant CALR trap, which remains on the horizon for the treatment of patients with MF, as well as chimeric antigen receptor (CAR) T-cell therapies and other novel therapeutic approaches.

“Beyond that, we are only limited by our creativity and work that is being done by our colleagues in the lab, both basic science and translational labs,” Gerds concluded. More therapeutic treatments will be needed in order to delay progression in early disease, and lead to cure without transplant.

_References

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_{4. Pardanani A, Harrison C, Cortes JE, et al. Safety anf efficacy of fedratinib in patients with primary or secondary myelofibrosis: a randomized clinical trial. JAMA Oncol. 2015;1(5):643-51. doi:10.1001/jamaoncol.2015.1590}

_{5. Harrison CN, Schaap N, Vannucchi AM, et al. Janus kinase-2 inhibitor fedratinib in patients with myelofibrosis previously treated with ruxolitinib (JAKARTA-2): a single-arm, open-label, non-randomised, phase 2, multicentre study. Lancet Haematol. 2017;4(7):e317-e324. doi:10.1016/S2352-3026(17)30088-1}

_{6. Mesa RA, Kiladjian JJ, Catalano JV, et al. Mesa R, et al. SIMPLIFY-1: a phase iii randomized trial of momelotinib versus ruxolitinib in janus kinase inhibitor-naïve patients with myelofibrosis. J Clin Oncol. 2017;35(34):3844-3850. doi:10.1200/JCO.2017.73.4418}

_{7. Harrison C, Vannucchi AM, Platzbecker U, et al. Momelotinib versus best available therapy in patients with myelofibrosis previously treated with ruxolitinib (SIMPLIFY 2): a randomised, open-label, phase 3 trial. Lancet Haematol. 2018;5:e73-e81. doi:10.1016/S2352-3026(17)30237-5}

_{8. Oh, ST Talpaz M, Gerds AT, et al. ACVR1/JAK1/JAK2 inhibitor momelotinib reverses transfusion dependency and suppresses hepcidin in myelofibrosis phase 2 trial. Blood Adv. 2020 Sep 22;4(18):4282-4291. doi: 10.1182/bloodadvances.2020002662}

_{9. Mesa RA et al. Pacritinib versus best available therapy for the treatment of myelofibrosis irrespective of baseline cytopenias (PERSIST-1): an international, randomised, phase 3 trial. Lancet Haematol. 2017;4:e225-e236. doi: 10.1016/S2352-3026(17)30027-3}

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_{11. CTI biopharma announces removal of full clinical hold on pacritinib. News Release. CTI BioPharma Corp. January 5, 2017. Accessed October 11, 2020. https://prn.to/2GT8PuD}