Mitoxantrone, Etoposide, and Cytarabine (MEC) Following Epigenetic Priming with Decitabine in Adults with Relapsed/Refractory Acute Myeloid Leukemia (AML) or High-Risk Myelodysplastic Syndrome (MDS): Final Results from a Phase 1/2 Study

Background: Standard chemotherapies for relapsed or refractory acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS) are often unsuccessful. Pre-treatment ("priming") with hypomethylating agents such as decitabine has been shown to sensitize AML cells to chemotherapeutics, prompting a phase 1/2 study (NCT01729845) of MEC preceded by decitabine priming (d/MEC) in relapsed/refractory AML/MDS.

Methods: Patients ≥18 years with relapsed/refractory AML or high-risk MDS (>10% blasts) requiring first or subsequent salvage therapy were eligible if they had adequate organ function and a treatment-related mortality (TRM) score of <9.2 (corresponding to an expected TRM of <9.2% with standard induction chemotherapy). Prior treatment with hypomethylating agents or MEC (but not combined) was allowed. Patients with prior hematopoietic stem cell transplant (HCT) were eligible. Excluded were patients with concomitant illness with expected survival <1 year and uncontrolled infection. In phase 1, cohorts of 6-12 patients were assigned to 1 of 3 dose levels of decitabine (20mg/m² for 5, 7, or 10 days) followed 5 days later by MEC (mitoxantrone 8mg/m²/day x 5 days; etoposide 100 mg/m²/day x 5 days; cytarabine 1 g/m²/day x 5 days). Patients with persistent AML were eligible for re-induction. Patients achieving a CR or CR with incomplete platelet recovery (CRp) could receive 2 more cycles of similarly dosed d/MEC. Dose-limiting toxicity (DLT) was defined as: 1) a grade 3 non-hematologic toxicity lasting >48 hours resulting in a >7-day delay of the subsequent treatment cycle; 2) a grade ≥4 non-hematologic toxicity, if recovery to grade ≤2 within 14 days. Both excluded febrile neutropenia/infection and constitutional symptoms. During phase 2, a Simon minimax 2-stage design was used to monitor whether a response rate of 0.25 was reached, with 80% power and a 1-sided alpha of 7%, assuming a historical CR rate of 10-15%.

Results: Fifty-two patients, median age 55 (range: 19-72) years, with primary refractory (n=19) or relapsed disease (n=33; median duration of prior CR: 5 [1-19] months) and a median TRM score of 3.15 (range: 0.07-9.05) were enrolled. They received a median of 2 (1-7) prior therapies; 14 had prior HCT. During dose escalation, 1 DLT occurred at each the 2^nd and 3^rd dose level, identifying a 10-day course of decitabine with MEC as MTD. Since no differences in response rates were noted between the 7 and 10-day course of decitabine, a 7-day course of decitabine was used in phase 2 for logistical reasons. Overall, 11/51 evaluable patients achieved a CR (21.6%; 95% confidence interval [CI]: 11.3-35.3%) of whom 9 had no measurable residual disease (MRD) by flow cytometry. This CR rate compared favorably to a historic control population with patient matching based on duration of prior remission and number of prior salvage therapies (Estey et al. Blood 1996; 88:756), with an observed/expected CR ratio of 1.77. Seven further patients achieved a CRp, and 2 had a CR with incomplete count recovery (CRi) for an overall response rate (ORR) of 20/51 (39.2%; 95% CI: 25.8-53.9%). Among the 45 evaluable patients treated with a 7 or 10-day course of d/MEC, the ORR was 37.8% (95% CI: 23.8-53.5%). Five patients achieved a morphologic leukemia-free state, 21 had resistant disease, and 5 died before response assessment. Median response duration (CR, CRi or CRp) was 130 (15-1,229) days, with 3 of these responses ongoing. Overall survival of these 20 patients was longer (median of 332 [62-1,000 days]) than that for patients who failed to achieve remission but did not experience TRM (median 119 [45-530] days). Inclusion of 6 eligible patients that were treated with this regimen outside the clinical trial did not change these results. Nine patients died within 28 days of treatment initiation for a TRM rate of 17.6% due to infection/septic shock (5), intracranial hemorrhage (1), respiratory failure (2), and progressive disease (1). Besides grade 3-4 cytopenias, infection/neutropenic fever, nausea, and mucositis were the most common adverse events.

Conclusion: D/MEC is feasible and has anti-leukemic activity in heavily pretreated relapsed/refractory AML and high-risk MDS. A comparative analysis of these results with other contemporary intensive salvage regimens is ongoing. A follow-up study is also being done to explore the relative value of pre-treatment vs. concomitant use of decitabine with intensive salvage chemotherapy.