Evaluation of Molecular Response by Real-Time RT-PCR in Acute Promyelocytic Leukemia (APL): A Direct Comparison with Regular RT-PCR

PURPOSE: In this study, we systemically compare the molecular response by regular reverse transcript polymerase chain reaction (RT-PCR) and quantitative real-time RT-PCR in newly diagnosed patients with acute promyelocyte leukemia (APL). PATIENTS AND METHODS: A total of 31 newly diagnosed patients with APL who entered remission by induction therapy with ATRA plus As2O3 were included in this study with median follow-up of 38 months (27–46 months). Serial bone marrow samples were obtained before and after the induction, consolidation chemotherapy and every 3 to 6 months during maintenance therapy. Regular RT-PCR and real-time RT-PCR were carried out to evaluate the molecular response. The PML-RARa fusion transcript level was calculated in two ways: normalized PML-RARa level as DoseN and log-reduction compared to the pre-treatment level. RESULTS: A total of 29 patients remained disease free for a median of 27 to 46 months. Comparing two PCR assays, the sensitivity limit for real-time RT-PCR was 5.7 log DoseN reduction while the limit for regular RT-PCR was about 3 log. Furthermore, we established criteria for evaluation of molecular response by real-time RT-PCR: ≥3.0 log reduction of PML-RARa transcript level as minor molecular response; ≥5.0 log reduction as major molecular response and undetectable level of PML-RARa transcript as complete molecular responses. Minor molecular response by real-time RT-PCR was achieved in 22.6% and 96.8% of patients after induction and consolidation chemotherapy, which was equivalent to molecular response by regular RT-PCR (35.5% and 96.8% respectively). The major molecular response rate was 12.9% and 90.3% after induction and consolidation whereas the complete response was at 3.23% and 77.4% respectively. All patients achieved major and complete molecular remission during and after maintenance therapy. Loss of either major or complete molecular response occurred in two patients while both patients experienced subsequent loss of minor molecular response, which coincided with CNS leukemia and bone marrow relapse. CONCLUSION: Real-time RT-PCR was more sensitive than regular RT-PCR and the evaluation criteria for real-time RT-PCR was established and proved to be clinically relevant.