Adding Mercaptopurine and Methotrexate to Alternate Week ATRA Maintenance Therapy Does Not Improve the Outcome for Adults with Acute Promyelocytic Leukemia (APL) in First Remission: Results From North American Leukemia Intergroup Trial C9710

Abstract ²⁵⁸

This randomized phase III clinical trial was designed to evaluate the potential benefit and toxicity of (a) arsenic trioxide (ATO) as initial consolidation therapy and (b) maintenance therapy with oral tretinoin (ATRA) either alone or together with 6-mercaptopurine (MP) and methotrexate (MTX) in newly diagnosed patients with APL. All patients received induction therapy with ATRA, daunorubicin (DNR) and cytarabine. Adults (≥ 15 years) were randomized at study entry to receive a standard consolidation with 2 courses of ATRA plus DNR, or 2 courses of ATO as initial consolidation followed by 2 courses of ATRA plus DNR. Patients who remained in complete remission (CR; n=331) were then randomized (stratified by consolidation arm and age group) to one year of maintenance with ATRA alone (45 mg/m²/d) for 7 days repeated every other week (n=166) or in combination with MP 60 mg/m²/daily plus oral MTX 20 mg/m²/weekly (n=161). The target number of maintenance events was 146, and the study had 80% power to detect a hazard ratio of 1.6 at 5 years. We previously reported that the addition of ATO consolidation markedly improved event-free (EFS) and disease-free (DFS) survival (Blood 2010; 116:3751–3757). We now report the results of the maintenance randomization after a median follow up of 6.2 years. The two groups were well balanced by pretreatment characteristics. DFS, the primary endpoint, and overall survival (OS) were not statistically different for the two maintenance arms (log-rank p=0.14 and p=0.33, respectively). Evaluation by consolidation arm (by intention-to-treat, ITT) and by APL risk group also failed to demonstrate a significant advantage for either maintenance treatment. There was no interaction effect between consolidation and maintenance arms (p=0.78). Age, gender, CD56 expression and FLT3-ITD or TKD mutations at diagnosis did not have an impact on outcome by maintenance arm.

No treatment-related deaths were reported during maintenance therapy. Hematologic adverse events were more common in the combination arm (maximum grade 3/4, 18% vs 4%; p< 0.0001), as were non-hematologic adverse events (maximum grade 3/4, 36% vs 25%; p=0.033). Only 71 DFS events have occurred to date. Although the 3-yr DFS favors the combination arm, the differences in DFS and OS with the addition of MP and MTX to ATRA maintenance do not reach statistical significance. The addition of ATO consolidation therapy remains the most important determinant of DFS and OS for APL patients in first remission on this randomized trial. Among patients who were randomized to maintenance, only 5 patients who received ATO consolidation have relapsed – 2 from the combination arm and 3 from the ATRA alone arm. Relapse of APL is uncommon in patients who received ATO consolidation, and the need for any maintenance therapy in these patients has yet to be determined.