Use of standardized flow cytometric determinants of multidrug resistance to analyse response to remission induction chemotherapy in patients with acute myeloblastic leukaemia

We have used a combination of flow cytometric assays to define multidrug resistance (MDR) positive and negative blasts in cryopreserved samples from 47 MRC trial patients with acute myeloblastic leukaemia (AML). Our primary test is a standardized assay for daunorubicin accumulation. Confirmatory assays for MDR comprised the cyclosporin modulation assay for rhodamine-123 uptake as a measure of functional P-glycoprotein and the measurement of lung resistance protein and multidrug resistance associated protein (with LRP-56 and MRPr1 respectively). 57% of samples had both low accumulation and at least one positive confirmatory test. 32% were MDR negative in all four assays. 15% of patients had primary chemo-resistant disease. Resistant disease rates were 22% for confirmed. MDR-positive patients and 0% for confirmed MDR-negative patients (P=0.07). Complete remission was achieved in 74% of patients, with rates of 63% in confirmed MDR-positive patients and 93% in confirmed MDR-negative patients (P=0.06). The use of a standardized method for daunorubicin uptake, combined with the use of confirmatory tests, should reduce the uncertainty that is currently characteristic of MDR evaluation in leukaemia. In comparison with daunorubicin uptake, p-gp expression, measured using MRK-16 antibody, was more closely associated with remission rates (P=0.01). This suggests an additional role for p-glycoprotein in mediating drug resistance beyond that of a drug efflux pump