Results of part 1 of a two-part study evaluating the combination of tazemetostat and CPX-351 (part 1) and palbociclib with CPX-351 (part 2) for the treatment of relapsed or refractory Acute Myeloid Leukemia

Abstract INTRODUCTION For fit patients (pts) with acute myeloid leukemia (AML), conventional chemotherapy (chemotx) with DNA-damaging agents remains the mainstay of treatment. These agents damage DNA by targeting cells during DNA replication. Chromatin structure is highly influential in the efficacy of chemotx agents, as areas of compacted chromatin are more resistant to DNA-damaging effects. Enhancer of Zeste Homolog 2 (EZH2) is the catalytic subunit of the polycomb repressive complex 2 which methylates H3K27 to produce a di- or trimethylated state (H3K27me2/3). Areas of DNA marked with H3K27me3 are transcriptionally repressed and exhibit reduced accessibility to DNA-damaging agents. In preclinical studies, AML cell lines treated with the combination of an EZH2 inhibitor (EZH2i) and doxorubicin exhibited increased DNA damage and apoptosis compared to those treated with doxorubicin alone. This effect was enhanced by temporary pre-treatment of AML cells with palbociclib (PALB), a CDK4/6 inhibitor, by significantly increasing the number of cells in S phase prior to treatment with the EZH2i and doxorubicin combination. These findings were reproduced in vivo in mice injected with AML cell lines and primary AML cells. Mice treated with PALB followed by EZH2i + chemotx had prolonged survival compared to those treated with EZH2i + chemotx, PALB + chemotx and each drug as monotherapy. METHODS This is a single-center, 2-part, phase 1 dose escalation study of EZH2i tazemetostat (TAZ) in combination with CPX-351 induction chemotx (Part 1) and of PALB pre-treatment followed by CPX-351 (Part 2) in pts with relapsed or refractory (R/R) AML. Pending completion of Parts 1 and 2, the protocol will be amended to add Part 3 to evaluate safety of the 3-drug combination. All pts will receive CPX-351 at standard dose on days 1, 3 and 5. Dose escalation of TAZ (Part 1) and PALB (Part 2) follows a 3+3 design. In Part 1, escalating doses of TAZ are administered on days -1 to 6. Key eligibility criteria include age ≥18 years, non-acute promyelocytic R/R AML who have received at least 1 prior line of therapy and have adequate performance status and organ function and who are not at excess risk of anthracycline-induced cardiotoxicity. The primary endpoint of Part 1 is to determine the maximum tolerated dose and recommended Part 3 dose of TAZ in combination with CPX-351. Key secondary endpoint for Parts 1 is preliminary assessment of efficacy. Correlative studies to evaluate the on-target effect of TAZ on H3K27me3 and DNA-damage are performed at protocol-specified time points. RESULTS Ten pts were treated in Part 1, 7 at dose level 1 (1 subject was not evaluable due to not taking the required number of doses of TAZ), and 3 at dose level 2. A total of 1 DLT was observed in the 6 evaluable pts in dose level 1. One DLT was observed in the 3 pts treated at dose level 2. The study was then suspended to review the aggregate safety, efficacy and correlative data. Regarding clinical efficacy, the overall response rate with TAZ + CPX-351 is 2/10 (20%). Of the 2 responding pts, 1 pt at dose level 2 achieved a complete response (CR) and 1 pt at dose level 1 initially achieved a morphologic leukemia free state and subsequently achieved a CR with incomplete hematologic recovery. Six pts had refractory disease and 2 pts were not evaluable. Of the 2 responding pts, both went on to receive consolidative cellular therapies (1 allogeneic stem cell transplant and 1 donor lymphocyte infusion) and 1 is still alive over 1 year after study enrollment. Evaluation of levels of DNA damage (as measured by γH2AX foci) and H3K27me3, the histone H3 mark added by EZH2, (as measured by single-cell proximity ligation amplification) in pts treated with TAZ + CPX-351 were performed. The available samples from 9 evaluable pts revealed similar measures of DNA damage and H3K27me3 between those treated at dose level 1 and dose level 2. CONCLUSIONS The combination of TAZ + CPX-351 yielded similar tolerability compared to CPX-351 alone. Similar levels of H3K27me3 and DNA-damage were observed at TAZ dose levels 1 and 2, suggesting that no additional biologic activity was observed at the higher dose level. Dose level 1 has been selected as the recommended Part 3 dose.