Real-World Use of FLT3 Tyrosine Kinase Inhibitors in Patients with Relapsed/Refractory FLT3 mutation-Positive Acute Myeloid Leukemia in the United States

Background: FLT3 tyrosine kinase inhibitors (TKIs) have improved outcomes in clinical trials for patients with FLT3 mutation-positive acute myeloid leukemia (FLT3^mut+ AML). Gilteritinib is the first FDA-approved (11/28/2018) targeted therapy for relapsed/refractory (R/R) FLT3^mut+ AML in adults, although two other multikinase inhibitors, midostaurin and sorafenib, are used off-label in this population. This analysis characterized treatment duration for these three drugs in R/R FLT3^mut+ AML, as little is known about the treatment of patients receiving a FLT3 inhibitor in the real world.

Aim/Objective: To describe real-world treatment patterns of patients newly initiating a FLT3 TKI for R/R AML following the launch of gilteritinib.

Methods: This was a post hoc analysis of a US-based retrospective cohort study using IBM MarketScan claims data (1/1/2007-10/30/2020) to evaluate real-world treatment patterns. Adult patients (≥18 years) with R/R AML who newly initiated a FLT3 TKI (ie, ≥1 claim for gilteritinib, midostaurin, or sorafenib) on or after 12/1/2018 were eligible for inclusion. Included patients were required to have continuous enrollment starting 180 days prior to first AML diagnosis through first FLT3 TKI initiated for R/R AML on or after 12/1/2018 (index date). FLT3 TKI treatment duration was calculated as the difference between the first claim and the last day of supply or end of enrollment/study, whichever occurred first. Treatments that were not discontinued prior to the end of the study period (10/30/2020) were censored. Treatment duration was estimated using Kaplan-Meier analysis, and the hazard ratio for discontinuation was estimated with a Cox proportional hazards model. Additional subgroup analyses were conducted based on prior FLT3 TKI exposure for R/R AML and use of FLT3 TKI therapy alone or in combination with chemotherapy. Data from the ProMetrics specialty pharmacy database (12/6/2018-3/3/2021) were also analyzed to establish a benchmark for gilteritinib and validate the treatment duration in the MarketScan results.

Results: A total of 65 patients newly initiating FLT3 TKIs for R/R AML were identified in the MarketScan database. Mean patient age was 53.4 years and mean Quan-Charlson Comorbidity index score at baseline was 5.1. Most patients initiating FLT3 TKI therapy received gilteritinib (n=44 [68%]). Patients initiating gilteritinib compared with other FLT3 TKIs had the highest prior history of high-intensity chemotherapy (n=24 [47%]) and hematopoietic stem cell transplantation (n=16 [31%]). Midostaurin was the most common prior TKI for patients who initiated sorafenib (n=6 [50%]) or gilteritinib (n=28 [55%]).

The median (95% CI) treatment duration was 150 (73-260) days for gilteritinib (n=51), 60 (15-210) days for sorafenib (n=12), and 54 (28-268) days for midostaurin (n=12). Treatment duration was significantly longer for patients receiving gilteritinib compared with midostaurin (P=.0018) and sorafenib (P=.0016) in the Cox proportional hazards model (Table) and the Kaplan-Meier analysis (P=.0021) (Figure). Differences in gilteritinib duration in patients with prior TKI treatment versus no prior TKI treatment and patients who used a FLT3 TKI alone versus in combination with chemotherapy were not statistically significant (Table). The median gilteritinib treatment duration in the MarketScan data (150 days) was aligned with the median duration in the ProMetrics specialty pharmacy data (154 days), which validates the MarketScan results.

Conclusions: This early look at treatment patterns suggests a median duration of therapy for gilteritinib of 150 days. Importantly, gilteritinib treatment duration does not appear to differ based on prior TKI treatment or overlapping use with chemotherapy. Small sample sizes precluded adjusted comparisons between the treatments. Gilteritinib was the most commonly used treatment. The availability of new targeted therapies such as gilteritinib is promising for patients with R/R FLT3^mut+ AML and is changing the therapeutic landscape for this aggressive AML subtype.

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