Midostaurin in Adults with Newly Diagnosed FLT3-Mutation-Positive Acute Myeloid Leukemia Eligible for Standard Chemotherapy: Update from the Radius-X Midostaurin Expanded Access Program

Introduction: Midostaurin was the first multikinase inhibitor approved in combination with daunorubicin and cytarabine induction and high-dose cytarabine (HiDAC) consolidation chemotherapy for the treatment of adults with newly diagnosed FLT3-mutation-positive (mut+) acute myeloid leukemia (AML). Approval was largely based on the results from the phase 3 RATIFY trial; patients who received midostaurin had significantly improved overall and event-free survival than those who received placebo (Stone et al, N Engl J Med, 2017). RADIUS-X (NCT02624570) is an expanded treatment protocol (ETP) designed to provide access to midostaurin during the US Food and Drug Administration's review process and to extend the understanding of the safety and tolerability of midostaurin in patients with newly diagnosed FLT3-mut+ AML. The safety profile of midostaurin in preliminary data from RADIUS-X was consistent with that in the RATIFY study (Roboz et al, Blood, 2017 [abstract 1338]). Here we report updated safety data for midostaurin during induction and consolidation and safety data during the maintenance phase.

Methods: In this open-label, single-arm ETP, patients (aged ≥18 years) received 1-2 cycles of induction therapy (cytarabine plus daunorubicin [60-90 mg/m²/day] or idarubicin [12 mg/m²/day]) and up to 4 cycles of HiDAC consolidation chemotherapy plus midostaurin (50 mg twice daily [bid] on days 8-21 of each 28-day cycle), followed by up to 12 months of single-agent midostaurin (50 mg bid on days 1-28). Patients could enroll at any point before completion of a second cycle of consolidation. Patients achieving complete remission (CR) or CR with incomplete hematologic recovery (CRi) after induction proceeded to consolidation; patients who maintained a response were eligible to proceed to maintenance. The primary endpoints were safety and tolerability of midostaurin.

Results: Of 111 patients screened, 103 were enrolled in the study: 47 during induction (46%) and

56 during consolidation (54%) (Figure). The median age was 58 y (range, 19-79 y); all patients were FLT3-mut+ (Table). Of 47 patients enrolled during induction, 15 received daunorubicin and 32 received idarubicin as the anthracycline. Of 35 patients who completed consolidation and entered maintenance, 9 had completed the protocol treatment and 3 remained on therapy at the data cutoff date (March 30, 2018). The CR/CRi rate for the induction phase was 74% (57% CR, 17% CRi). The relapse rate was 14% overall.

The most common reason for study discontinuation was proceeding to transplant (overall, 52%; induction, 11%; consolidation, 42%; maintenance, 34%). The median duration of midostaurin exposure was 35 days (range, 3-426 days). Dose adjustment or interruption due to adverse events (AEs) occurred in 26 patients, most commonly due to febrile neutropenia (n=9) and gastrointestinal disorders (n=6). No new safety events were observed with longer follow-up. Most patients (99%) experienced ≥1 any-grade AE, mostly during induction and/or consolidation. Due to the timing of patient enrollment (up to the second cycle of consolidation), hematologic AEs were lower than reported in comparable studies. The most common AEs occurring in ≥20% of patients were febrile neutropenia (53%), nausea (42%), diarrhea (37%), anemia (36%), platelet count decreased (31%), fatigue (23%), headache (22%), and vomiting (22%). Serious AEs occurred in 50% of patients overall, most commonly febrile neutropenia (37%). AEs during induction were generally similar, regardless of anthracycline received. Overall, 9 patients discontinued due to AEs: 5 during induction (febrile neutropenia, blood bilirubin increased, electrocardiogram QT prolonged, renal impairment, and respiratory distress), 1 during consolidation (sepsis), and 1 during maintenance (leukocytosis). During maintenance, 16 of 35 patients (46%) reported any-grade AEs with midostaurin monotherapy; the most common any-grade and grade 3/4 AEs occurring in >1 patient were platelet count decrease (11% and 3%), nausea (9% and 0%), and oropharyngeal pain (6% and 0%).The rate of death during the study was low, with 1 death reported (disease progression).

Conclusions: Midostaurin continued to demonstrate a manageable safety profile with longer follow-up and was associated with high transplant and low relapse rates. Maintenance therapy with midostaurin was well tolerated; no new safety signals were observed.

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