Ralf W. Suchomel

MULTIDRUG RESISTANCE IN LEUKEMIAS AND ITS REVERSAL

Drug resistance often results in failure of anticancer chemotherapy in leukemias. Several mechanisms of drug resistance are known with multidrug resistance (MDR) being the best characterized one. MDR can be due to enhanced expression of certain genes (MDR1, MRP or LRP), alterations in glutathione-S-transferase activity or GSH levels and to reduction of the amount or the activity of topoisomerase II. Here we review the current status of the clinical significance of the various mechanisms of MDR in leukemias and also discuss possibilities for the reversal of MDR. MDR1 gene expression has been seen in many leukemias, notably in acute myeloid leukemia (AML) and blast crisis of chronic myeloid leukemia. Both MDR1 RNA and P-glycoprotein expression of the leukemic cells have been shown to correlate with poor clinical outcome in AML. However, preliminary results indicate that the MRP gene as well as the LRP gene can be expressed in AML. Thus, drug resistance in leukemias appears to be multifactorial. P-glycoprotein-mediated MDR can be reversed by several drugs. These resistance modifiers are currently evaluated with regard to their clinical efficacy. Despite some encouraging results, reversal of drug resistance and subsequent improvement in clinical outcome remains to be shown.

MRP Expression in Acute Myeloid Leukemia

To determine the clinical significance of the multidrug resistance protein (MRP) in patients with de novo AML, we have studied MRP expression of leukemic cells at diagnosis and its association with clinical outcome in 127 patients. MRP expression was determined by immunocytochemistry by means of monoclonal antibodies QCRL-l/QCRL-3. MRP expression was low, intermediate and high in 30%, 46% and 24% of the patients, respectively. MRP expression was independent of age and sex of the patients, white blood cell count, FAB subtype, serum lactate dehydrogenase levels and karyotype aberrations. MRP expression had no impact on response to induction chemotherapy. The complete remission rates were 75%, 70% and 64% for patients with low, intermediate and high expression, respectively. Patients with intermediate or high MRP expression showed a trend toward shorter overall survival (p=0.09) as compared to patients with low MRP expression. MRP does not predict for response to induction chemotherapy but intermediate or high MRP expression might be associated with shorter overall survival of the patients.

Immunocytochemical detection of the multidrug resistance-associated protein and P-glycoprotein in acute myeloid leukemia: impact of antibodies, sample source and disease status

Immunocytochemical detection of the expression of the MRP gene and the MDR1 gene in clinical specimens might be affected by several factors. Thus, we studied the impact of monoclonal antibodies, sample source (peripheral blood vs bone marrow) and disease status on the expression of multidrug resistance-associated protein (MRP) as well as P-glycoprotein (P-gp) in leukemic cells of patients with acute myeloid leukemia (AML). MRP expression was determined by means of anti-MRP antibodies (QCRL-1, QCRL-3, QCRL-1/QCRL-3 or MRPr1). In the case of P-gp, monoclonal antibodies C219 and MRK16 were used. High MRP expression ranged from 5 to 35% and high P-gp expression from 5 to 14% of the specimens. A fair correlation between results obtained with QCRL-1/QCRL-3 and those obtained with MRPr1, as well as a moderate correlation between C219 and MRK16, were seen. MRP and P-gp expression of peripheral blood blasts were similar to those of bone marrow blasts in the majority of cases. The degrees of MRP expression at the time of diagnosis were also similar to the degrees of expression at relapse, albeit an analysis of sequential MRP expression in 13 patients indicated an increase of expression at relapse in six patients as compared to the time of diagnosis.

The lung resistance protein (LRP) predicts poor outcome in acute myeloid leukemia.

To determine the clinical significance of the lung resistance protein (LRP) in acute myeloid leukemia (AML), we have studied LRP expression of leukemic blasts and its association with clinical outcome in patients with de novo AML. LRP expression of leukemic blasts was determined by immunocytochemistry by means of monoclonal antibody LRP-56. LRP expression at diagnosis was detected in 31 out of 86 (36%) patients and correlated with white blood cell count (p = 0.01). The complete remission rate of induction chemotherapy was 72% for all treated patients (n = 82). The complete remission rate was 81% for patients without LRP expression but only 55% for patients with LRP expression (p = 0.01). Overall survival and disease-free survival were estimated according to Kaplan-Meier in 82 and 59 patients, respectively. At a median follow-up of 16 months, median overall survival was 17 months for LRP-negative patients but only 8 months for LRP-positive patients (p = 0.006). Disease-free survival was 9 months for LRP-negative patients and 6 months for LRP-positive patients (p = 0.078). Thus LRP predicts for poor outcome indicating that the LRP gene is a clinically relevant drug resistance gene in AML.

92 P - MRP and MDR1 Gene expression in primary breast carcinomas

To evaluate the clinically important mechanisms of drug resistance in breast cancer, the expression of the MRP gene and the corresponding one for the MDR1 gene were determined in primary breast carcinoma specimens by both reverse transcription-PCR (n = 134) and immunohistochemistry (n = 63). Expression of MRP RNA was observed in all breast cancer specimens. MDR1 RNA was detected in 80 (60%) of the carcinomas. Staining with monoclonal antibodies QCRL-1 and QCRL-3, which both recognize MRP, was strong in 15 (24%) and weak in the remaining 48 specimens (76%). Staining with C219, which recognizes P-glycoprotein, was strong in 6 (9%), weak in 30 (48%), and negative in 27 (43%) of the samples. Strong MRP staining was more frequent in T3 and T4 tumors than in T1 and T2 tumors and in the primary tumors of patients with distant metastases but was independent of age, menopausal status, histology, histological grade, estrogen receptor, progesterone receptor, and lymph node involvement. No correlation between MRP staining and expression of MDR1 RNA or P-glycoprotein was observed. Thus, these results indicate expression of both the MRP gene and the MDR1 gene in primary breast carcinomas and suggest that clinical drug resistance in breast cancer is most likely multifactorial.

152 P - MDR1 transcripts do not indicate long-term prognosis in primary colorectal carcinomas

The MDRI gene, a multidrug resistance gene, is frequently expressed in colorectal carcinomas. To determine whether this expression is of prognostic value, we determined the relationship between MDRI gene expression of primary colorectal carcinomas and the long-term outcome of the patients. RNA was isolated from tumor specimens by standard techniques. MDRI gene expression was determined by slot blot analysis by means of a radiolabeled cDNA (probe 5A). Survival durations of the patients were calculated according to Kaplan-Meier. MDRl RNA was detected in 65% of the primary carcinomas. At a median follow-up of 80 months, the durations of both relapse-free survival and overall survival were not different between patients with MDRI RNA positive tumors and those with MDRl RNA negative tumors. Thus MDRI gene expression of colorectal carcinomas does not predict the prognosis of the patients, although it might be involved in the well known drug resistance of these tumors.