ORLANDO – Ruxolitinib is currently the only drug approved for the treatment of myelofibrosis, but a number of other therapies are in clinical trials and showing promise, according to Ruben A. Mesa, MD.
“Our field ... is rapidly in evolution,” he said at the annual conference of the National Comprehensive Cancer Network, adding that efforts are underway to determine where these drugs might fit in.
Sharon Worcester/Frontline Medical News
Dr. Ruben A. Mesa
Among those being considered specifically for myelofibrosis are pacritinib, momelotinib, PRM-151, and imetelstat, he said.
Pacritinib
This JAK2/FT3 inhibitor reduces splenomegaly and its related symptoms, but may also help patients who have low platelet counts and a worse prognosis. Pacritinib has demonstrated safety in that population, said Dr. Mesa of Mayo Clinic Cancer Center, Phoenix, Ariz.
Concerns about increased mortality related to risk of intracranial hemorrhage and cardiovascular events led the Food and Drug Administration to place a full clinical hold on pacritinib in February 2016. That hold was lifted in January 2017 when data from the randomized, controlled, phase III PERSIST-2 study, as presented at the American Society of Hematology annual meeting in December 2016, showed the risks did not hold up among study patients.
PERSIST-2 compared pacritinib doses of 400 mg once daily and 200 mg twice daily with best alternative therapy, which was ruxolitinib in most patients, Dr. Mesa said. He noted that the study included patients who had marked thrombocytopenia and were allowed prior JAK2 inhibitor exposure.
The 200-mg twice-daily dosing was superior in achieving spleen volume reductions greater than 35%: 22% of patients in the 200-mg dosing group vs. 15% in the 400-mg once-daily dosing group, compared with 3% of those receiving best available therapy. The twice-daily dosing group also experienced greater symptom improvement: Thirty-two percent in the 200-mg twice-daily group vs. 17% in the 400-mg once-daily group achieved at least a 50% reduction in total symptom scale scores, compared with 14% of those receiving best available therapy.
Additional studies of pacritinib will begin enrolling soon, Dr. Mesa said, noting that these studies will look at lower doses in an effort to identify the minimally effective dose with the optimal balance of safety and efficacy.
Momelotinib
Momelotinib, a JAK1/JAK2 inhibitor, was evaluated in two large recently concluded phase III trials (SIMPLIFY-1 and SIMPLIFY-2). SIMPIFY-1 compared momelotinib to ruxolitinib in the front-line setting, and showed momelotinib to be noninferior for reducing splenomegaly.
“However, it was inferior for improvement in the symptom burden,” Dr. Mesa said, noting that while there seemed to be a favorable difference in terms of anemia, the study was structured in such a way that the agent needed to be noninferior for both spleen and symptoms for the anemia response to be evaluable.
SIMPLIFY-2 evaluated momelotinib in patients who had not responded to ruxolitinib. In this second-line setting, momelotinib was not superior to the best alternative therapy, but since the vast majority of the ruxolitinib failure patients remained on ruxolitinib, it is “a bit of a confounded study to assess,” he said.
The top-line data from these studies were issued in a press release from the manufacturer (Gilead) in November 2016, and the complete results are expected to be made public in the near future, at which time more will be known about the next steps for momelotinib, he said.
If approved, pacritinib and momelotinib could ultimately be positioned as a front-line and/or second-line treatment for myelofibrosis, Dr. Mesa predicted.
There has been a goal, in terms of trial design, to see if there is a niche for these drugs in the front-line setting based on blood counts.
“Those recommendations would clearly be very much dependent on the risk, the safety, and the efficacy,” he said.
PRM-151
This antifibrosing agent was shown to be active in early-phase trials – including in stage 1 of an adaptive phase II trial. PRM-151 is currently being evaluated in the fully-accrued ongoing phase II PROMOTE study to determine whether it improves splenomegaly, symptoms, and cytopenia. The primary endpoint of the study is the bone marrow response rate. Study subjects are patients with primary myelofibrosis, post–polycythemia vera myelofibrosis, or post–essential thrombocythemia myelofibrosis, and grade 2-3 fibrosis, said Dr. Mesa, who is the principal investigator for the study.
Imetelstat
This telomerase inhibitor is being evaluated in the randomized, multicenter, phase II IMbark study, designed to assess spleen volume and total symptom score as primary end points. Earlier studies have shown deep responses in patients with myelofibrosis who were treated with imetelstat, Dr. Mesa said.
The IMbark study (NCT02426086) was originally designed to evaluate two dosing regimens administered as a single agent to participants with intermediate-2 or high-risk myelofibrosis who were refractory to or relapsed after JAK inhibitor treatment. Participants received either 9.4 mg/kg or 4.7 mg/kg intravenously every 3 weeks until disease progression, unacceptable toxicity, or study end.
According to information from Geron, which is developing the agent, enrollment of new participants is currently suspended following a planned internal data review, but enrollment “may be resumed after a second internal data review that is planned by the end of the second quarter of 2017.” If resumed, enrollment would be only to the higher-dose treatment arm; patients initially randomized to that arm may continue treatment, and those randomized to the lower-dose arm may see their dose increased at the investigator’s discretion.
If approved, PRM-151 and imetelstat would likely be positioned as second-line treatments for myelofibrosis, Dr. Mesa said, noting that determining which patients would be most likely to benefit from treatment with these agents would require a close look at the evidence from second-line studies.
Combination therapies
In addition to these investigational treatments, nearly 20 different combination treatments involving ruxolitinib plus another agent have been looked at to try to further improve activity. Some improvements in splenomegaly have been seen with combinations including ruxolitinib and either panobinostat (a histone deacytelase inhibitor), LDE225 (a hedgehog signaling pathway inhibitor), and BKM120 (a PI3-kinase inhibitor), he noted.
“For the area of greatest interest – which was to see incremental improvements in thrombocytopenia, anemia, or fibrosis – there have been favorable data, but they have been modest. It’s not quite clear that there is a combination that is ready for prime time, nor is there yet a combination that we have recommended through the treatment guidelines to be utilized for these patients,” he said.
Dr. Mesa has received consulting fees, honoraria, and/or grant/research support from ARIAD Pharmaceuticals; Celgene Corporation, CTI BioPharma, the maker of pacritinib; Galena Biopharma; Gilead, the maker of momelotinib; Incyte, the maker of ruxolitinib; Novartis, the maker of panobinostat and BKM120; and Promedior, the maker of PRM-151.