Attendees at ASH 2017 Photo courtesy of ASH

ATLANTA—Results of a phase 1 study suggest patients with polycythemia vera (PV) can achieve extended treatment-free periods after receiving idasanutlin.

Six of 12 patients who received the drug were able to have a treatment holiday—4 patients for 1 month, 1 for 2 to 3 consecutive months, and 1 for more than 3 consecutive months.

Four patients achieved a complete response (CR) and 3 a partial response (PR), for an overall response rate (ORR) of 58%.

There were no dose-limiting toxicities (DLTs) in the trial.

John Mascarenhas, MD, of Icahn School of Medicine at Mount Sinai in New York, New York, reported these results at the 2017 ASH Annual Meeting as abstract 254.*

The study was supported by Roche and funded by the National Cancer Institute, PDRC, and a grant from the Leukemia and Lymphoma Society.

Study rationale

Patients with PV have higher levels of MDM2 in their CD34+ cells compared to normal CD34+ cells. Nutlins block the interaction between p53 and MDM2, thus activating the p53 pathway.

Investigators previously found that low doses of nutlin and pegylated IFNα 2a promoted apoptosis in PV CD34+ cells.

And treatment with the combination reduced the numbers of JAK2V617-positive cells transplanted into immune-deficient NOD/SCID mice.

So Dr Mascarenhas and his colleagues undertook a study (NCT02407080) to evaluate the toxicity, safety, and tolerability of the MDM2 antagonist idasanutlin in patients with PV and essential thrombocythemia (ET).

The investigators hypothesized that since overexpression of MDM2 negatively regulates wild-type p53 function in primary PV cells, idasanutlin therapy, either alone or in combination with low-dose peg-IFN, could result in selective reduction or elimination of the myeloproliferative neoplasm cells in PV patients.

Study design

The investigators evaluated 2 dose levels of idasanutlin—100 mg daily and 150 mg daily on days 1 to 5, repeated every 28 days. The first cycle was 56 days to allow investigators to evaluate any DLTs.

Dr Mascarenhas pointed out that the dose is 1/6 of that being evaluated in acute myeloid leukemia.

Investigators defined a DLT as a non-hematologic adverse event (AE) of grade 3 or higher or a hematologic AE of grade 2 or higher thrombocytopenia or grade 3 or higher neutropenia or anemia.

If patients did not achieve at least a PR by the end of cycle 3 of single-agent therapy, they could proceed to Part B of the study and receive idasanutlin with pegylated IFN.

After cycle 3, dosing was dependent upon patients hitting a hematocrit greater than 42% and/or platelet counts greater than 400,000.

“So you had to meet parameters, which means that if you did meet parameters, you could get a treatment holiday,” Dr Mascarenhas explained.

Patients were eligible if they had JAK2V617F-positive PV or ET confirmed by WHO diagnostic criteria.

They had to have high-risk disease, be older than 60, and have a history of thrombosis. They also had to be either intolerant or resistant to at least one prior treatment, including hydroxyurea, interferon, or anagrelide.

Patients were excluded if they had post-ET/PV myelofibrosis, blast phase disease, acute thrombosis within 3 months of screening, or uncontrolled inter-current illness.

Baseline patient characteristics

Eleven of the 12 patients enrolled had PV, and 1 had ET. Their median age was 63.5, and 7 were female. Their median duration of disease was 43.9 months (range, 14.9–154.3), 3 had previous thrombosis, and 10 had prior hydroxyurea therapy.

They had a median leukocyte count of 11.3 x 10⁹/L, median hemoglobin levels of 13.6 g/dL, median hematocrit of 42.3%, and median platelet levels of 443.5 x 10⁹/L.

They all had the JAK2V617F mutation. Some patients had additional mutations, including TET2, DNMT3A, ASXL1, CBL, and EZH2, among others.

“One patient had an inactivating p53 mutation,” Dr Mascarenhas said. “I didn’t know this when we put her on study. . . , but this is an interesting part of the study. So remember, one patient had an inactivating p53 mutation in a trial where I’m using a drug that interrupts wild-type p53-MDM2 interaction.”

Efficacy

Plasma MIC-1 levels were significantly increased (P=0.004) in PV patients following treatment with idasanutlin at day 5 compared to day 1. MIC-1 is a secreted protein strongly induced by activated p53.

“Some patients didn’t need to be treated every month,” Dr Mascarenhas said. “They got treatment holidays. That’s unique. I don’t usually see that in the treatments that we give. In fact, this one person here, after 3 cycles, didn’t need to be re-treated for 9 months.”

In the 100 mg cohort (n=6), patients received a median of 8 cycles of idasanutlin (range, 7-13) and were on study for a median of 34.1 weeks (range, 29.0–127.3). One patient experienced a treatment holiday of 1 month, and another patient had a treatment holiday of more than 3 consecutive months.

In the 150 mg cohort (n=6), patients received a median of 9.5 cycles of therapy (range, 5–17) and were on study for a median of 52.1 weeks (range, 23.1–72.9). Three patients experienced a 1-month treatment holiday, and 1 patient had a 2- to 3-month treatment holiday.

In total, 6 patients continued the single-agent regimen, and 4 proceeded to the combination treatment with pegylated IFN.

Reasons for discontinuation were patient refusal and investigator decision.

The ORR (CR + PR) for both dose cohorts with single-agent idasanutlin was 58%. One patient was not evaluable, 4 had no response, 3 had a PR, and 4 had a CR.

In the combination portion of the study, the ORR was 50%. One patient was not evaluable, 1 had no response, 1 had a PR, and 1 had a CR.

“The 1 non-responder in Part B,” Dr Mascarenhas noted, “was the p53-mutated patient. Makes sense.”

The ORR for both the single-agent and combination parts of the study was 75%.

Eight of 12 patients had a 50% reduction in total symptom score from baseline, which is considered clinically meaningful, according to Dr Mascarenhas.

“What’s also interesting,” he pointed out, “[ is that] patients who didn’t obtain a response also enjoyed symptom benefit.”

Patients had a median 43% reduction in JAK2 mutation from baseline.

“One patient had nearly 92% reduction in JAK2V617F,” Dr Mascarenhas said. “One patient had a 60% increase. But guess what? That was the p53-mutated patient. Makes sense.”

Bone morphology showed reduction in marrow hypercellularity and normalization of megakaryocyte atypia and clustering.

Safety

There were no DLTs with either dose of idasanutlin.

“This was a well-tolerated drug,” Dr Mascarenhas said.

Three patients experienced grade 3 non-hematologic treatment-emergent AEs, all at 100 mg, of fatigue (1 patient), headache (1 patient), and pain (1 patient).

No grade 4 non-hematologic treatment-emergent AEs occurred at either dose, and investigators observed no hematologic AE of any grade.

Investigators also observed no grade 3–4 gastrointestinal (GI) treatment-emergent AEs. Constipation (91.7%), nausea (75%), and diarrhea (66.7%) were the most frequent grade 1 or 2 events. Patients received GI prophylaxis upfront with ondansetron, lorazepam, or dexamethasone.

Because of the safety profile and manageable GI toxicity, the higher dose of idasanutlin was chosen as the recommended phase 2 dose.

A global, multicenter, single-arm, phase 2 trial with idasanutlin in patients with hydroxyurea-resistant or -intolerant PV is underway.

*Data in the presentation differ from the abstract.

Drug allows for treatment-free periods in PV