Molecular Minimal Residual Disease After Induction Is Related to ABC Proteins' Activity and Associated with Survival in 26 Patients with NPM1 Mutated Acute Myeloid Leukemia

A major issue in the treatment of acute myeloid leukemia (AML) is resistance to chemotherapeutic drugs. The role of ABC proteins, and specially ABCB1 (PgP/mdr1), in this resistance has been well established, and higher ABC proteins' activity, assessed with functional tests, has been associated with poorer complete remission rates and poorer overall prognosis (Marzac et al, Haematologica, 2011). Furthermore, the evaluation of molecular minimal residual disease (MRD), using mutated nucleophosmin (NPM1)expression quantification has been related to patients' global prognosis (Krönke et al, J. Clin. Oncol., 2011), and to response to treatments. In this study, we evaluate the impact of ABC proteins' activity on MRD after one course of induction chemotherapy, in 26 patients with NPM1 mutated AML.

We retrospectively identified 26 AML patients with NPM1 mutation treated in our center and with MRD data. MRD was evaluated as the ratio of NPM1 mutated allele and total NPM1, using PCR DNA quantification and the delta delta Ct method. MRD was measured at the time of diagnosis and after one course of anthracycline-based induction chemotherapy. ABC proteins' activity was evaluated at the time of diagnosis using JC1 +/− cyclosporine A assay (Legrand et al, Blood, 2001). Correlations between ABC proteins' activity and the level of post induction MRD were evaluated with the Mann-Whitney test. Survival was evaluated using the Cox model. For all analyses, P values were considered significant when lower than 0. 05.

Median age at diagnosis was 53 years old. Twenty-two patients had normal cytogenetic analysis at diagnosis, and the other 4 patients had intermediate prognosis cytogenetic analysis. Nine patients harboured FLT3-ITD mutation.

Median ABC proteins' activity was 0. 11 (0 – 0. 77). After one course of induction chemotherapy, 3 patients did not reach cytological complete remission. In 17 patients MRD level after induction therapy was inferior to 1 %, in 11 patients MRD was inferior to 0. 1 % and in 7 patients MRD was inferior to 0. 01 %. Overall, higher MRD level after induction (defined by MRD level higher than 0. 1 %) was associated with poorer prognosis for disease free survival (HR= 4. 25 [95% CI 1. 049–17. 27]; p=0. 04), and for overall survival HR=11. 25 [95% CI 1. 22–103. 23]; p=0. 03). Higher ABC proteins' activity was associated with higher MRD levels post induction, and patients who did not reach MRD level lower than 0. 1 % had significantly higher ABC proteins' activity than other patients (p=0. 008). ABC proteins' activity was also associated with overall survival in our patients (p=0. 04).

Higher ABC proteins' activity is associated with higher MRD levels after one course of induction chemotherapy in 26 NPM1 mutated AML patients, and is also associated with poorer overall survival. The poorer prognosis associated with high ABC proteins' activity in AML seems to be in part related to direct resistance to chemotherapy. These data should be confirmed in larger studies.

No relevant conflicts of interest to declare.