AT THE 2015 ASCO ANNUAL MEETING
CHICAGO (FRONTLINE MEDICAL NEWS) – Pacritinib, an investigational oral inhibitor of Janus kinase 2 (JAK2), reduced splenomegaly and alleviated other symptoms in patients with myelofibrosis, in a randomized phase III trial reported at the annual meeting of the American Society of Clinical Oncology.
After 24 weeks of treatment, patients in the pacritinib arm were about four times more likely to have a sizable reduction in spleen volume than peers in the best available therapy arm, reported lead study author Dr. Ruben A. Mesa, deputy director of the Mayo Clinic Cancer Center in Scottsdale, Arizona.
Of special note, the drug was not associated with increased anemia or thrombocytopenia. In fact, it was safe in the subset of patients who had thrombocytopenia at baseline, a group currently having an unmet need for treatments because they cannot receive ruxolitinib (Jakafi), a dual JAK1 and JAK2 inhibitor that is associated with thrombocytopenia.
“Based on these preliminary results, pacritinib may represent a very important agent for individuals with advanced disease and may have impact on the disease course,” Dr. Mesa commented. Additionally, the findings warrant studies of combination therapy with other potentially disease-modifying agents in myeloproliferative neoplasms.
“I think pacritinib for myelofibrosis represents an advance in our field,” commented invited discussant Dr. Lloyd E. Damon, a professor of medicine and director of hematologic malignancies and bone marrow transplant at the University of California, San Francisco.
The trial’s findings have a number of implications going forward, he said. “There are several avenues yet to explore with these types of agents; for instance, what is the role of JAK inhibitors in those who are actually JAK2 mutated, and for that matter, those who are actually calreticulin mutated, and for that matter, those for whom there is no known mutation? The JAK inhibitors so far are directed against wild type,” he noted. “And should we be seeking to develop agents which are very specific against JAK gene products that are mutated or calreticulin gene products that are mutated vis-a-vis what we are seeing in the FLT3 inhibitors?”
Session attendee Dr. Harry Erba, a professor of medicine and director of the hematologic malignancy program, University of Alabama at Birmingham, commented, “With ruxolitinib, I’d always assumed that the improvement in quality of life was due to the JAK1 inhibition and decreasing of inflammatory cytokine signaling. In this study, the benefit in terms of the total symptom score was maybe a little bit less robust, comparing two very different studies – only 20% or 25%.”
“I think when it comes to the mechanism of symptom improvement, JAK2 probably still remains a key piece. As we look at the entire portfolio of JAK inhibitors that have been tested, we see improvement in symptoms whether they hit JAK1 or not,” Dr. Mesa replied.
“Was there a difference in the responses between the spleen-related and the more inflammatory-related symptoms?” Dr. Erba further asked. “Also, the other thing I was struck by, with ruxolitinib, it seems to be such a quick response in terms of quality of life and symptoms, and here there just seemed to be a more gradual improvement in time.”
“We did not see a strong difference between spleen- and non–spleen-related improvements,” Dr. Mesa replied, and the majority of responses seen with pacritinib were still “fairly rapid,” occurring within 4-8 weeks.
The CTI-funded trial – known as PERSIST-1 (A Randomized Controlled Phase 3 Study of Oral Pacritinib Versus Best Available Therapy in Patients with Primary Myelofibrosis, Post-Polycythemia Vera Myelofibrosis, or Post–Essential Thrombocythemia Myelofibrosis) – was unique in allowing patients to enroll regardless of platelet count, Dr. Mesa noted.
The patients were randomized in 2:1 ratio to receive pacritinib or best available therapy. The latter typically consisted of off-label agents such as erythropoietin-stimulating agents, immunomodulatory drugs, and hydroxyurea; ruxolitinib was not permitted. Crossover was allowed, and 79% of patients in the best available therapy arm eventually did go on to receive pacritinib.
Median follow-up was 8.4 months. In intention-to-treat analyses, at 24 weeks, 19.1% of patients in the pacritinib arm had a reduction of at least 35% in spleen volume, compared with only 4.7% in the best available therapy arm (P = .0003). The findings were similar in the subsets with a platelet count of less than 50,000 per microliter (22.9% vs. 0%) and less than 100,000 per microliter (16.7% vs. 0%).
Patients in the pacritinib arm were less likely to die if they had a spleen volume reduction of at least 20%, but longer follow-up is needed to determine if the drug improves survival, according to Dr. Mesa, who disclosed that he receives honoraria from and has a consulting or advisory role with Novartis, and receives research funding from Celgene, CTI, Incyte, and Gilead Sciences.
The proportion of patients having at least a one-half reduction in Total Symptom Score was 24.5% with pacritinib and 6.5% with best available therapy (P < .0001).
“We did not see any significant drug-emergent thrombocytopenia,” Dr. Mesa reported. In fact, among patients who entered the trial with a platelet count of less than 50,000 per microliter, those in the pacritinib arm had a significant, steady improvement in platelet count. “This could be multifactorial, from reduced splenic sequestration amongst other beneficial features,” he proposed
Among patients who were red cell transfusion dependent at baseline, 25.7% in the pacritinib arm achieved transfusion independence, compared with none in the control arm (P = .04).
The most common nonhematologic grade 3 or 4 adverse event with pacritinib was diarrhea (5% vs. 0%), while the most common hematologic grade 3 or 4 adverse event was anemia (16.8% vs. 15.1%).
Dr. Mesa noted that an ongoing sister trial, PERSIST-2, is still open to accrual. “This is a trial exclusively for patients with thrombocytopenia and allows individuals who have previously received JAK inhibitor therapy, with patients being randomized to the dose tested in the PERSIST-1 study or a b.i.d. dosing with similar goals and endpoints,” he elaborated.
