Parallel Molecular and Immunological Monitoring of Minimal Residual Disease in Adult Acute Lymphoblastic Leukemia (ALL) Patients.

Though the initial response to induction therapy, indicating rapid tumor clearance, is currently considered to be one of the most important prognostic factors, detection of submicroscopic levels of minimal residual disease (MRD), by means of PCR or immunophenotyping after induction and during different stages of treatment, has also become important in predicting outcome in ALL patients. We initiated a study using parallel PCR and flow cytometry to monitor MRD in adult acute lymphoblastic leukemia (ALL) patients treated by standard approach. Bone marrow samples were analyzed at diagnosis, after pre-phase (one week of steroid therapy to determine steroid responsiveness), at 8-weeks post induction, at consolidation, before maintenance (+7–8 mo), and during maintenance for the detection of clonal IgH, TCR-gamma rearrangements by PCR (The level of detection sensitivity is 10⁻³ to 10⁻⁴). Specific markers were identified for 88% of the patients (30/34); prolonged monitoring was carried out in 22 patients with follow up from 2 to 24 months. Fifteen of those patients were analyzed by three-color flow cytometry analysis with a panel of 20 monoclonal antibodies for patient-specific aberrant leukemia-associated phenotype (LAP) at diagnosis and before maintenance (10⁻⁴). Contrary to PCR analysis, LAPs were detected in all patients. The most common LAPs were CD19/34/10, CD 19/34/TdT, and CD19/34/IgM. Clinical CR was achieved in 85% of patients. Prednisone resistance was detected in 71% of patients. MRD was detected by PCR in all patients after pre-phase, in 21/22 (95%) after 1^st induction phase, in 11/15 (73%) after 2^nd induction phase, in 10/15 (67%) after consolidation, and in 8/14 (57%) before maintenance. Flow cytometry data coincides with molecular parameters: only 4 of 10 patients examined before maintenance (+7–8 mo) had no cells with LAP. Comparing our parallel measurements we have noted discrepancies in 3 of 10 cases. Two immunologically positive patients were PCR-negative, and one PCR-positive patient was immunologically negative.

All patients (n=3) whose PCR probes were positive more than twice, relapsed at different time points; no relapses were noted in once positive patients (n=3), and 1of 8 PCR-negative patients relapsed. Our data indicate that both methods are effective and relevant in detection of MRD. They show very slow tumor clearance in ALL patients. MRD positivity reflects the higher relapse probability. More patients can be followed for MRD with flow-cytometry, due to higher detection rate of LAP.