P459: DISEASE MONITORING OF NPM1-MUTANT (MUT) ACUTE MYELOID LEUKEMIA (AML) USING MEASURABLE RESIDUAL DISEASE (MRD) ASSESSMENTS DURING ORAL AZACITIDINE (ORAL-AZA) TREATMENT (TX): A QUAZAR AML-001 SUBANALYSIS

Background:NPM1 is mutated in 20–30% of patients (pts) with AML at diagnosis (Dx), with greater frequency among pts with normal karyotype. As NPM1 mutations are generally stable at relapse and AML-specific, dynamic PCR-based measurement of residual disease provides a reliable and quantifiable marker of prognosis. In QUAZAR (NCT01757535), Oral-AZA significantly prolonged survival vs placebo (PBO) in pts with AML in remission after intensive chemotherapy (IC), including pts who were MRD+ by multiparameter flow cytometry (MFC). The impact of Oral-AZA on NPM1mut MRD via PCR-based methods and concordance with MFC in NPM1mut AML is not known. Aims: To examine changes in NPM1mut MRD status by PCR vs MFC during Oral-AZA Tx, and associations between molecular NPM1mut MRD and clinical outcome. Methods: Quantitation of NPM1mut variants by PCR (types A/B/D by QuantStudio 6 or 7500 Fast qRT-PCR; types G/I by QX200 ddPCR; sensitivity range: 0.001–0.023%) in QUAZAR was performed on bone marrow (BM) mononuclear cell RNA collected at screening post IC (81/472 pts), during Tx (end of cycle [C] 6/C12; 14 days [d] Tx per 28d cycle) and morphologic relapse (≥5% BM blasts) from pts with investigator-reported NPM1mut AML at Dx. NPM1 transcript copy number was normalized to ABL (%). MFC MRD was analyzed centrally using 22 cell surface markers (MRD+: ≥0.1% blasts). A multivariable Cox regression model evaluated associations of specific covariates (NPM1mut status, cytogenetic risk, MFC MRD) with RFS. Results: In pts with NPM1mut AML reported at Dx pre IC and in remission per protocol requirements post IC (n=81; CR: 87.7%, CRi: 12.3%; ≥1 consolidation cycles: 86.4%, median 2 cycles), NPM1mut+ MRD (PCR) was present in 38.3% (31/81; type A, 90.3% [28/31]). MRD+ by MFC was reported in 37.0% (30/81) of pts. At the end of C6, MRD was present in 39.1% (25/64) by PCR and 27.4% (17/62) by MFC. Median NPM1:ABL % among pts with NPM1mut MRD by PCR at screening was 0.064 vs 164.5 at relapse (Mann-Whitney test P<0.0001). At relapse, NPM1mut+ was observed in 80% (35/44) by PCR. At all cycles, NPM1 PCR and MFC MRD were generally concordant (Spearman r=0.5, P<0.0001); NPM1mut+/MFC MRD+ 25.4% (61/240) and NPM1mut–/MFC MRD– 44.2% (106/240). In outlier analyses, 17.5% of cases were NPM1mut+ by PCR (42/240) and MRD– by MFC (NPM1:ABL % 0.24 vs 44.3 for NPM1mut+/MRD+, n=61), whereas 12.9% (31/240) were NPM1mut–/MFC MRD+. Post IC, both MRD+ and – segments (n=15/80 vs 34/80) aligned with inferior or favorable RFS, respectively. Discordant MRD +/– and –/+ segments (n=16/80 vs 15/80) were not clearly associated with RFS. With Oral-AZA, 57% (8/14) of NPM1mut+ pts converted to NPM1mut– status or achieved log reduction in NPM1 (at C6/C12) vs 30% (3/10) with PBO. Of 44 paired NPM1mut– samples available at screening and relapse, 20.5% (9/44) remained NPM1mut– at relapse by PCR (no difference between Tx arms) and were characterized by other mutations detected by NGS including EZH2 (3 cases) and TET2, TP53 and PHF6 (2 cases). After adjusting for prognostic variables post IC, Oral-AZA remained independently associated with favorable RFS, while NPM1mut+ status by PCR post IC was associated with inferior RFS (Table). Summary/Conclusion: For pts with NPM1mut AML at Dx, NPM1 PCR was positive in 38.3% at screening. NPM1mut was not detected in 20.5% of cases at morphologic relapse. For pts with NPM1mut AML at Dx, PCR is informative and concordant with MFC MRD. NPM1 monitoring has prognostic value in AML and, importantly, achievement of NPM1 negativity post IC is associated with better clinical outcomes with Oral-AZA. Table. Multivariable analysis of NPM1+ ptsKeywords: Acute myeloid leukemia, Clinical trial, Maintenance

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