Molecular MRD status as a predictor of outcome in patients with NPM1-mutated acute myeloid leukemia (AML)

Measurable residual disease (MRD) assessed prior to allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a predictor of relapse and outcome.¹ Thus, determination of MRD is of growing interest and its adoption in standard clinical practice could be useful to guide treatment decisions. At the 46^th annual meeting of the European Society for Blood and Marrow Transplantation (EBMT) the AML Hub will hold a Satellite Symposium (August 30, 2020, 8:30am, IFEMA – Feria de Madrid, ES) to discuss the latest developments of MRD assessments in the context of HSCT.

Mutations in the nucleophosmin (NPM1) gene are present in a large subgroup of patients with AML who have normal cytogenetics.² In this study, published in Blood, Richard Dillon and colleagues used reverse-transcription quantitative PCR (RT-qPCR) to assess pretransplant MRD in patients with NPM1-mutated AML enrolled in the UK National Cancer Research Institute (NCRI) AML17 study. They then examined the effects of MRD status on the outcomes of these patients.³

Study design³

• The study enrolled 3,215 patients (aged 16–77 years) eligible for intensive chemotherapy:
  • 861 had a mutated NPM1 and of these 530 provided samples for MRD assessment
  • In total 107 patients received allo-HSCT and were included in the study. These patients were transplanted:
    • In first remission (CR1), 52% (n= 56)
    • After molecular relapse (MR), 28% (n= 30)
    • In second remission (CR2) after morphological relapse, 20% (n= 21)
• Pre-HSCT peripheral blood (PB) and bone marrow (BM) samples were taken in the 60 days prior to HSCT (median time, 29 days; range, 5─57):
  • Patients MRD negative before HSCT, 54% (n= 58)
  • Of 48 patients who were receiving additional chemotherapy before HSCT for molecular or hematological relapse, 27 achieved MRD negativity
• As a risk factor, FMS-like tyrosine kinase 3 (FLT3) internal tandem duplication (ITD) status⁴ at diagnosis was also evaluated

Results³

After a median follow-up of 4.9 years (range, 1.0─8.4 years) after transplant:

• Forty-two (39%) patients died (21 because of relapse)
• 2-year overall survival (OS) was:
  • 68% for patients transplanted in CR1
  • 63% for those transplanted after MR
  • 57% for patients transplanted in CR2

Overall survival differences between MRD-negative and MRD-positive patients

The 2-year OS was evaluated in MRD-negative vs MRD-positive patients:

• Overall, 83% vs 45%; median OS (mOS) not reached (NR) vs 10.5 months (hazard ratio [HR], 3.60; 95% confidence interval [CI], 1.92─6.77; p < 0.0001)
• PB samples, 81% vs 30%; mOS NR vs 7.4 months (HR, 8.30; CI, 3.77─18.20; p < 0.0001)
• 5-year OS in BM samples, 84% vs 49%; mOS NR vs 13.1 months (HR, 3.17; CI, 1.54─6.54; p = 0.002)

Using a threshold of 200 copies of mutant NPM1 transcripts per 10⁵ ABL (control gene) in the PB sample or 1000 copies in the BM sample, patients were split into three groups: negative, low, and high number of copies. The 2-year OS was evaluated and results are reported in Table 1.

Table 1. Differences in overall survival according to pretransplant molecular MRD status

BM, bone marrow; CI, confidence interval; MRD, measurable residual disease; PB, peripheral blood
MRD status	2-year overall survival
	PB sample	BM sample	Combined
Negative, (n)	81% (73)	84% (37)	83% (58)
Low, (n)	54% (13)	56% (32)	63% (30)
High, (n)	12% (17)	22% (9)	13% (19)
Hazard ratio (CI)	2.81 (1.96─4.02)	2.87 (1.69─4.86)	2.83 (1.92─4.19)
p value	< 0.0001	< 0.0001	< 0.0001

Effect of FLT3-ITD status on outcomes

According to FLT3-ITD status, 34 patients were FLT3-ITD positive and 73 negative. FLT3-ITD status was associated with outcome only in patients with low levels of MRD, with a 2-year OS in patients FLT3-ITD positive vs FLT3-ITD negative of 25% (mOS, 7.1 months) vs 77% (mOS, NR) (HR, 6.14; CI, 1.50─25.13; p = 0.01)

A prognostic model incorporating MRD status and FLT3-ITD has been developed and 83 patients have been assigned to:

• High-risk group: patients with high levels of MRD, and patients with low levels of MRD plus FLT3-ITD at diagnosis (n= 27; 2-year OS, 17%; mOS, 6.5 months)
• Low risk group: all the other patients (n= 56; 2-year OS, 82%; mOS, NR; HR, 13.2; CI, 5.80─30.2; p < 0.0001)

Effects of donor source, conditioning regimen or T-cell depletion on outcome

Donor source or conditioning regimen do not correlate with outcome. On the contrary, in the MRD-positive group, T-cell depletion was significantly associated with inferior survival (2-year OS, 34% vs 100%, in patients who did undergo T-cell depletion vs who did not, respectively; HR, 3.78; CI, 1.57─19.2; p= 0.003).

Conclusions³

This study confirms the prognostic value of MRD assessment:

• Patients with NPM1-mutated AML, MRD negative by RT-qPCR pre-HSCT have a better outcome than MRD positive patients, regardless of FLT3 status
• MRD-positive patients with high levels of MRD and/or the presence of an FLT3-ITD mutation have a poor prognosis. In these high-risk patients might be useful reducing the levels of MRD before HSCT and further studies are required to investigate these approaches