A Retrospective Cohort Analysis of Minimal Residual Disease (MRD) across Therapy in FLT3 Acute Myeloid Leukemia (AML)
Authors: Arora Arshia , Tsai Hua-Ling , Kaduluri Venkata Preetam Sandeep , Levis Mark J. , Ambinder Alexander J. 
Published: December 2024

BACKGROUND
FLT3-ITD measurable residual disease (MRD), as assessed by next generation sequencing (NGS), is increasingly used as a prognostic and predictive marker. Patients with FLT3-ITD mutated AML who are in remission have a higher risk of relapse and reduced overall survival if they have detectable FLT3-ITD MRD,1 even in the immediate pre-transplant setting.2 The use of post-transplant gilteritinib maintenance in patients with pre-transplant FLT3-ITD MRD largely mitigates the negative impact of FLT3-ITD MRD on relapse-free survival. FLT3-ITD MRD may therefore be a useful surrogate marker for clinical outcomes to guide drug development and clinical practice; Yet, there are few studies that have charted the trajectory of FLT-ITD MRD with commonly used, intensive therapies. Herein, we characterize the trajectory of FLT3-ITD MRD in patients with FLT3-ITD AML treated with intensive chemotherapy combined with FLT3-inhibitors.

METHODS
We performed a retrospective analysis of a cohort of patients with FLT3-ITD AML evaluated at a single center. Eligible patients were ≥18 years old at the time of AML diagnosis, achieved first remission after induction with intensive chemotherapy in combination with a FLT3 inhibitor, and had at least one measurement of FLT3-ITD MRD using the Invivoscribe NGS-PCR assay after induction or cycle 1 of consolidation.1 Medical record chart review included patient demographics, AML karyotypes, cytogenetic profiles, NGS testing, induction and consolidation treatment, MRD assay at various time intervals following treatment, and final outcomes. Frequencies of conversion to MRD negativity after induction or cycle 1 consolidation were tabulated by treatment cycle. The association between NPM1 mutation status on achievement of MRD negativity after cycle 1 consolidation was determined using Fisher's exact test.

RESULTS
83 patients diagnosed between February 2016 and October 2023 were identified. Of these, 53 patients were excluded due to MRD testing outside of the window of interest, receipt of less intensive therapy, absence of a FLT3 inhibitor, or use of an experimental FLT3 inhibitor. The remaining 30 patients received intensive induction with conventional 7+3 or liposomal cytarabine + daunorubicin plus midostaurin or gilteritinib. Consolidation consisted of liposomal cytarabine + daunorubicin or high dose cytarabine plus midostaurin or gilteritinib. The median age of patients was 53.5 (Range 19-73). 15 (50%) had a concomitant NPM1 mutation. 17 (56.7%) patients received induction with intensive chemotherapy plus midostaurin followed by intensive consolidation + midostaurin. 11 (36.6%) received intensive induction plus midostaurin followed by intensive consolidation + gilteritinib. Two (6.6%) received intensive induction plus gilteritinib and consolidation plus gilteritinib. Of the 18 patients who had FLT3-ITD MRD measured after induction, four (22.2%) had no evidence of FLT3-ITD MRD (threshold 1 x 10^-6). Of the 25 who had FLT3-ITD MRD measured after cycle 1 consolidation, 15 (60%) had no evidence of FLT3-ITD MRD. Of the 15 who had FLT3-ITD measured after induction and cycle 1 consolidation, three (20%, 95% CI 7%-45%) had no evidence of FLT3-ITD MRD after induction and 10 (67%, 95% CI 42-85%) had no evidence of FLT3-ITD MRD after cycle 1 consolidation. Thus, 58% of patients who were FLT3-ITD MRD positive after induction converted to FLT3-ITD MRD negativity after cycle 1 consolidation. Ten out of 13 patients (76.9%) with concomitant NPM1 mutations achieved FLT3-ITD MRD negativity after cycle 1 consolidation compared to five out of 13 patients (38.6%) with wildtype NPM1, but this was not statistically significant in this small patient population (p = .11).

CONCLUSION
In this retrospective analysis, we characterize the trajectory of FLT3-ITD MRD in patients who achieve remission with commonly used intensive induction regimens plus midostaurin or gilteritinib. FLT3-MRD negativity after induction is uncommon but increases dramatically after cycle 1 consolidation. These data suggest that patients with concomitant NPM1 mutations may have higher rates of conversion to FLT3-ITD MRD negativity. These data provide a benchmark for the efficacy of commonly used intensive therapies in MRD eradication that can be used for comparison in assessing the efficacy of future, novel treatment regimens.