Results of a Phase 1 Study of Quizartinib (AC220) As Maintenance Therapy in Subjects with Acute Myeloid Leukemia in Remission Following Allogeneic Hematopoietic Cell Transplantation

FMS-like tyrosine kinase 3 internal tandem duplications (FLT3-ITD) in acute myeloid leukemia (AML) are associated with early relapse after standard chemotherapy and poor survival. Hematopoietic cell transplantation (HCT) is a recommended treatment for patients with FLT3-ITD(+) AML. However, a high rate of relapse after a HCT is observed when compared to FLT3-ITD(-) patients with the 2 year relapse rate of 30% vs. 16% in FLT3-ITD(+) and FLT3-ITD(-) patients, respectively (Brunet, J. Clin. Oncol. 2012). Quizartinib (AC220) is an oral FLT3 receptor tyrosine kinase inhibitor that has shown a high level of single agent activity in nearly 500 patients with FLT3(+) relapsed or refractory AML and is currently in a Phase 3 study.

This Phase 1 study examined maintenance therapy with quizartinib in AML patients (aged 18 years or older) in remission after receipt of an allogeneic HCT. Dose escalation was conducted using a modified 3+3 design, where 6 subjects were enrolled at each dose level. Two dose levels were tested; dose level 1 (DL1) at 40 mg and dose level 2 (DL2) at 60 mg given daily in continuous 28 day cycles. Dose limiting toxicities (DLTs) were assessed for the first 2 cycles and subjects were allowed to continue up to a maximum of 24 cycles.

Thirteen subjects were recruited and enrollment was completed in June 2013. The median age was 43 (range 23 to 61) years. All subjects had received an HLA-matched allogeneic HCT (3 related and 10 unrelated) a median of 56 (range 41 – 70) days before study enrollment. All subjects were positive for the FLT3-ITD mutation by local testing at the time of diagnosis.

Seven subjects were enrolled at DL1, including one who relapsed after 22 days on study and was replaced per protocol as deemed not evaluable for DLTs. There was 1 DLT of Grade 3 gastric hemorrhage which resolved and the subject was able to continue on a reduced dose (30 mg) of quizartinib. Six subjects were enrolled at DL2, including one subject who experienced 1 DLT of Grade 3 anemia but was able to continue on a reduced dose (30 mg).

Of the 13 total subjects, 10 (77%) received quizartinib for more than a year. Six (45%) subjects are continuing to receive quizartinib currently (15, 16, 18, 18, 23, and 23 cycles) and 2 (15%) subjects have completed 24 cycles. Three subjects (23%) discontinued due to AE: Grade 4 neutropenia (Cycle 4), Grade 2 corneal epithelium defect (Cycle 9), and Grade 3 autoimmune hemolysis (Cycle 15); 1(8%) discontinued for disease relapse (Cycle 1) and 1 (8%) for protocol non-compliance (Cycle 13).

The most common (≥20%) treatment-related adverse events (AEs) were diarrhea (38%; 5 subjects), neutropenia (31%; 4 subjects), nausea (23%; 3 subjects) and leukopenia (23%; 3 subjects).

This is the first study conducted to determine whether or not quizartinib can be safely administered as post-transplant maintenance therapy in patients with AML. The data from this study support the use of post-transplant maintenance therapy with quizartinib and show early evidence indicating a reduced relapse rate with only 1 relapse reported out of 13 subjects, which compares favorably to historical reference data. No MTD was identified but 60 mg daily was selected as the highest dose for continuous daily administration based on Phase 2 study data in relapsed or refractory subjects (J Cortes, Blood (ASH Annual Meeting Abstracts) 2013 [Abstract]). Quizartinib maintenance has been included in the current Phase 3 QUANTUM-R study comparing quizartinib vs. salvage chemotherapy in relapsed or refractory FLT3-ITD(+) AML.