Background and Significance: Patients (pts) with primary or secondary myelofibrosis (MF) experience variable degrees of constitutional symptoms, cytopenias and risk of progression to acute myeloid leukemia leading to a reduced life-expectancy compared to age-matched controls. Although several oral Janus Kinase (JAK) inhibitors such as ruxolitinib (RUX) have been approved by the US FDA based on improvements in symptom burden and spleen volume compared to physician's choice of best available therapy, the duration of response to these agents is generally time-limited and treatment with JAK inhibitors is not considered to have disease-modifying potential. Furthermore, the median overall survival after RUX discontinuation is only 11-14 months highlighting the need for novel mechanism-based therapies for pts with disease progression on RUX.

JAK2 V617F mutations promote transition from the G1 to S-phase of the cell cycle via increased expression of CDC25A, which is also upregulated in primary MPN patient samples. Additional cell cycle regulators such as CDK6 and Cyclin D have also been implicated in MF pathogenesis and contribute to myeloproliferation. Finally, we and others have shown synergistic effects of the combination of CDK4/6 inhibitors and RUX in murine models of MF (Rampal et al., CCR 2021) including improvement in bone marrow fibrosis suggesting the potential for disease-modifying activity. Several CDK4/6 inhibitors have been approved by the US FDA for the treatment of hormone receptor-positive metastatic breast cancer establishing a robust record of safety data. These preclinical data and known safety profile of CDK4/6 and RUX as single agents, provide rationale for combining CDK4/6 and JAK inhibition in pts with MF.

Methods: This multicenter, phase I dose-escalation trial (NCT05714072) evaluates the safety of RUX + the CDK4/6 inhibitor abemaciclib in primary or secondary MF pts with intermediate-1/2 or high-risk disease by DIPSS who require treatment and had an inadequate response to RUX. Inadequate response to RUX is defined by (I) palpable splenomegaly ≥5 cm below the left costal margin at study entry AND/OR (II) active MPN symptoms, as defined by the presence of one symptom score ≥5 or two symptom scores ≥3 using the screening MPN-SAF TSS. Pts must have been on a stable dose of 10 mg or 15 mg BID of RUX for at least 4 weeks prior to enrollment with adequate platelet (≥75 x 10 9/L) and neutrophil counts (≥1.5 x 10 9/L). Key exclusion criteria include prior treatment with a CDK4/6 inhibitor, accelerated- or blast-phase MPN, platelet count <50 x 10 9/L in the 4 weeks before screening or platelet transfusion(s) within 8 weeks before screening, and absolute neutrophil count <0.5 x 10 9/L in the 4 weeks before screening. Among the currently FDA-approved CDK4/6 inhibitors, abemaciclib was chosen for combination therapy due to less myelosuppressive effects compared with other CDK4/6 inhibitors.

This trial uses a conventional “3+3” dose-escalation design ( Figure), pts will be treated with increasing doses of abemaciclib added to a stable dose of RUX (10 mg or 15 mg BID). The pre-study dose of RUX will be maintained to determine the maximum tolerated dose of the combination of RUX and abemaciclib. The primary endpoint is to determine the maximum tolerated dose of RUX + abemaciclib. Secondary endpoints include measures of efficacy such as overall response rate, duration of response and survival endpoints. Exploratory endpoints include changes in allele fraction of driver mutations, gene expression profiling, and clonal architecture to identify biomarkers of response, mechanisms of resistance, and to assess for any disease-modifying effects of the combination therapy. At this time, 2 pts have been enrolled at dose level 0 without any dose-limiting toxicities observed. In line with the known adverse event profile of abemaciclib, grade 1 diarrhea occurred in one patient. One patient developed grade 3 neutropenia which promptly resolved with holding abemaciclib. No unexpected safety signal was noted and enrollment continues. This trial is open to enrollment at Memorial Sloan Kettering Cancer Center and will commence at MD Anderson Cancer Center later this year.

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2023
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