Evaluation of Prospective Detection of PML-RARA and RARA-PML Fusion Transcripts by Real-Time Quantitative PCR (RQ-PCR) To Direct Pre-Emptive Therapy with Arsenic Trioxide (ATO) in Acute Promyelocytic Leukemia (APL) Patients Treated in the UK MRC AML15 Trial.
Authors: Grimwade David J. , Jovanovic J. , Hills R. , Nugent E. , Patel Y. , Flora R. , Diverio D. , Jones K. , Aslett H. , Sellwood E. , Solomon E. , Coco F. Lo , Wheatley K. , Burnett A. 
Published: October 2019

Abstract
A key challenge in the management of APL is to improve upon survival rates now achieved with ATRA and anthracycline-based chemotherapy. One approach involves minimal residual disease (MRD) monitoring to identify patients otherwise destined to relapse, whose outcome may be improved by pre-emptive therapy. Patients with PML-RARA+ APL (n=261) were prospectively monitored by RQ-PCR in the MRC AML15 trial, comparing a MRC treatment schedule comprising 4 courses of combination chemotherapy (ADE+ATRA,ADE,MACE,MidAC) with a less intensive PETHEMA schedule involving anthracycline monotherapy+ATRA followed by maintenance (ATRA/6MP/MTX). Paired peripheral blood (PB) and bone marrow (BM) samples were analyzed after each course, then 3-monthly for 2 years. Patients with persistent PCR positivity or molecular relapse (defined by successive PCR positive results with rising PML-RARA transcript level) received ATO and proceeded to autologous/allogeneic transplant according to molecular response; patients unfit for transplant received 3 ATO courses. 4813 samples were analyzed (median 16/pt), including 1507 paired samples. No difference in kinetics of disease response was observed between schedules (median time to molecular CR (CRm): 70d MRC v 67d PETHEMA,p=0.3). Furthermore, neither response kinetics nor MRD status during or immediately after consolidation identified those most at risk of relapse. 6/182 (3%) patients tested PCR positive after course 4. While this has been considered an indication for allogeneic transplant, serial monitoring showed progression in only 2 (given ATO pre-emptively); the other 4 remain in CRm 3–37mo later. Most patients subject to relapse tested PCR negative post-consolidation (10/12,83%), with fusion transcripts first detected at a median of 8mo (3–33mo) later. While concordance between BM and PB MRD results was high in early phases of treatment (r>.46,p<0.0001), in relapsing patients molecular conversion in BM preceded that in PB in 4/5 evaluable patients by a median of 29d, correlating with a 1.5log greater sensitivity for the former sample type. Six patients received pre-emptive ATO and remain in CR at a median follow-up of 22mo. However, half the remaining patients with recurrent disease were not treated pre-emptively; in 3 although relapse was successfully predicted by RQ-PCR, BM blasts exceeded 5% by the time ATO was commenced 23–84d following molecular conversion, 1 patient developed early CNS relapse, while in 2 patients relapse could not be predicted due to omission of scheduled MRD samples. Three of 6 patients subject to frank relapse have died, including two of hemorrhage/sepsis shortly after presentation. In 5 patients, suspected relapse of APL was excluded by PML-RARA negativity and were diagnosed as t-MDS/AML. At 3 years the overall rate of frank relapse is 3%, which compares favorably with the previous MRC AML12 trial (13% at 3 yrs) in which the same MRC treatment was used, but routine MRD monitoring was not performed, suggesting that rigorous sequential RQ-PCR monitoring of BM coupled with pre-emptive therapy provides a valid strategy to reduce rates of frank relapse in APL.