The First Targeted Therapy Directed at FLT3 Mutations in Patients With AML

Richard M. Stone, MD

Mutations in FLT3 have long been recognized in a portion of patients with acute myeloid leukemia (AML). Yet it took more than 15 years until an agent targeting FLT3 mutations came to fruition with the FDA approval of midostaurin (Rydapt) in April 2017, marking about 40 years since the last new agent was approved to treat patients with AML.

Midostaurin is approved for the treatment of adult patients with newly diagnosed FLT3-positive AML in combination with standard cytarabine, daunorubicin induction, and cytarabine consolidation. The drug has been approved for patients with advanced systemic mastocytosis (SM), including aggressive systemic mastocytosis (ASM), SM with associated hematological neoplasm (SM-AHN), and mast cell leukemia.

AML is a genetically complex malignancy that researchers are still seeking to elucidate. Researchers believe that AML is driven by somatic alterations in a “2-hit” process: a proliferative mutation in a class I gene such as FLT3, and an aberration in a class II gene that prevents cells from maturing.¹

FLT3 mutations are among the epigenetic drivers of AML in normal karyotypes.²Investigators have found somatic mutations in FLT3 genes in several spots, notably in 27% to 34% of samples in the internal tandem duplication (ITD) domain in various studies.¹The presence of a FLT3-ITD mutation confers a poor risk for patients with AML, with worse outcomes in disease-free and overall survival (OS).³

In recently updated joint guidelines, the College of American Pathologists and the American Society of Hematology strongly recommend testing all pediatric and adult patients with suspected or confirmed AML for FLT3-ITD mutations.³Midostaurin was approved along with a companion diagnostic, the LeukoStrat CDx FLT3 Mutation Assay, to test for FLT3 mutations in DNA extracted from mononuclear cells obtained from peripheral blood or bone marrow aspirates.

Midostaurin is a small molecule that inhibits multiple receptor tyrosine kinases; in vitro biochemical and cellular assays have also suggested that it can inhibit wild-type FLT3 activity.⁴The drug inhibits FLT3 receptor signaling and cell proliferation, and induces apoptosis in leukemic cells expressing ITD and tyrosine kinase domain mutant FLT3 receptors.⁴

The midostaurin approval has generated excitement in the field. Several FLT3 inhibitors have entered the later stages of clinical development and the prospects for more targeted agents are building.

“The good news is that AML might be dealt with in a better way than we’ve done in the past by using targeted therapies,” said Richard M. Stone, MD, of Dana-Farber Cancer Institute, and lead investigator of the RATIFY trial, in an interview withTargeted Therapies in Oncology.

MIDOSTAURIN FINDINGS LEADING TO APPROVAL

The midostaurin approval is based on the phase III RATIFY trial in AML and 2 single-arm, open-label studies of patients with SM. In RATIFY, the addition of midostaurin to standard chemothera- py reduced the risk of death by 23% compared with chemother- apy alone in patients with FLT3-mutant AML.⁵After ltering for patients who received stem cell transplants, the OS benefit with midostaurin remained steady at 25%.

In the phase II trial considered for the SM approvals, among patients receiving 6 cycles of midostaurin, the rates of con rmed complete remission (CR) plus incomplete remission by modified Valent criteria were 38% for ASM and 16% for SM-AHN. One patient with mast cell leukemia had a CR.

In the phase III RATIFY trial, also known as CALGB 10603, 717 patients with newly diagnosed FLT3-mutant AML were random- ized to standard induction and consolidation chemotherapy plus midostaurin (n = 360) or placebo (n = 357). Hydroxyurea was allowed for up to 5 days prior to beginning therapy.

During induction therapy, daunorubicin was given at 60 mg/m2 on days 1 to 3 with cytarabine at 200 mg/m2 on days 1 to 7. Oral midostaurin was administered at 50 mg twice daily on days 8 to 22. If patients achieved a CR, consolidation therapy was given with cytarabine at 3 g/m2 for 3 hours every 12 hours on days 1, 3, and 5 plus either placebo or midostaurin. After 4 cycles of consolidation, maintenance therapy was administered for up to 1 year.

The 2 treatment arms were balanced for age, race, FLT3 subtype, and baseline complete blood counts. The primary endpoint of the study was OS, with secondary outcome measures such as event-free survival (EFS) and safety.

In uncensored data, median OS was 74.7 months with midostaurin versus 25.6 months with chemotherapy alone (HR, 0.77; 95% CI, 0.63-0.95; P = .016). The 5-year OS rate for patients in the midostaurin arm was 50.9% versus 43.9% with placebo. Median EFS with midostaurin was 8.2 versus 3.0 months with placebo (HR, 0.78; 95% CI, 0.66-0.93; P = .004). The 5-year EFS with midostaurin was 27.5% versus 19.3%, respectively.

Median OS seen in the midostaurin arm was well beyond investigator expectations of 20.9 months. A possible explanation for this could be the rates of stem cell transplantation or incomplete data. The confidence intervals for OS were not fully attained for the midostaurin arm (95% CI, 31.7 to not attained).

Overall, 57% of patients received an allogeneic stem cell transplant at some time during the trial, more commonly in the midostaurin arm versus placebo (58% vs 54%). Median time to transplant was 5.0 months with midostaurin and 4.5 months with placebo. Twenty- ve percent of transplants occurred during the rst CR. Overall, 59% of patients in the midostaurin arm and 54% in the placebo group experienced a CR (P = .18).

Median OS data were not obtained in the censored population. The 4-year censored OS rate with midostaurin was 63.8% versus 55.7% for placebo (HR, 0.75; P = .04). In those censored for transplant, median EFS with midostaurin was 8.2 versus 3.0 months with placebo (HR, 0.84; P = .025).

Grade ≥3 AEs were similar between the midostaurin and pla- cebo arms. Overall, 37 grade 5 AEs occurred in the study, which were similar between the 2 arms, at 5.3% with midostaurin versus 5.0% with placebo. A statistically significant difference was not observed for treatment-related grade 5 AEs (P = .82).

References:

1. Seegmiller A, Jagasia M, Wheeler S, Vnencak-Jones C. Molecular profiling of acute myeloid leukemia. My Cancer Genome website. www.mycancergenome.org/content/disease/acute-myeloid-leukemia. Updated January 26, 2016. Accessed May 31, 2017.
2. Mehdipour P, Santoro F, Minucci S. Epigenetic alterations in acute myeloid leukemias. FEBSJournal. 2015;282(9):1786-1800. doi: 10.1111/febs.13142.
3. Arber DA, Borowitz MJ, Cessna M, et al. Initial diagnostic workup of acute leukemia: guideline from the College of American Pathologists and the American Society of Hematology [published online February 22, 2017]. Arch Pathol Lab Med. doi: 10.5858/arpa.2016-0504-CP.
4. Stone RM, Mandrekar SJ, Sanford BL, et al. Midostaurin plus chemotherapy for acute myeloid leukemia with a FLT3 mutation. N Engl J Med. 2017. doi: 10.1056/NEJMoa1614359.