Assessment of PML-RARa Real-Time Quantitative PCR for Definition of Molecular Relapse in APL Patients (Results of the APL Study Group).

Although patients with Acute Promyelocytic Leukaemia (APL or AML M3) and the classical t(15;17) translocation can now benefit from efficient treatments, some 10 to 20% of patients still relapse at 5 years. In order to improve the clinical outcome of APL patients, the early detection of disease reactivation should allow treatment at molecular relapse rather than at complete haematological relapse. To this end accurately defining molecular relapse is compulsory. Recent studies suggest that RQ-PCR methods applied to the detection of minimal residual disease (MRD) could provide such criteria. Our laboratory has established a RQ-PCR method (Cassinat et al., 2001) using Taqman technology. Standard curves are established and the PML-RARa copy number is normalized according to the expression of the housekeeping gene PBGD. A total of 260 APL patients (APL study Group) have been followed from 2000 to 2004 with a total of 970 samples analyzed. Median follow up for all these patients was 24 mths (range: 1–83 mths) with a median number of samples per patient of 6 (range: 1–28). During this period, 38 patients presented at least one positive result after a median negative period of 13 mths and 13 of them relapsed (median follow-up of 17 mths, range from 3 to 43mths). To evaluate the value of this re-positivity, we tried to determine whether a predictive threshold of PML-RARa copy as assessed by RQ-PCR method could allow detecting patients with molecular relapse. At the time of the first positive result, patients were in hematological CR. Patients were grouped according to the normalized PML-RARa copy number and the risk of relapse was studied in each group. No relapse (n= 0/17; 0%) was noted in patients with less than 10 copies (group A). 38% relapses (n= 5/13) in patients with 10 to 10² copies (group B) and 100% relapses (n=8/8) in patients with more than 10² normalized copies (group C). Negative RQ-PCR was reached again in all patients that did not relapse. Interestingly in groups B and C, interval to hematological relapse appeared to be related to the number of PML-RARa copies at first positivity (median interval to relapse was 6 months in group B and 2 months in group C). For 3 positive patients of group B, 3 consecutive samples were analysed until relapse: copy numbers increased exponentially. From these results we conclude that RQ-PCR is a powerful tool to define the molecular relapse in APL patients as it is able to accurately identify two subgroups of patients with either a 100% or no risk of relapse wilts median follow up of 24 mths. These patients with true molecular relapse should probably receive salvage therapy without waiting for the hematological relapse.