Biology of multiple drug resistance in acute leukemia.
Since the early 1970s, multiple drug resistance (MDR) has been known to exist in cancer cells and is thought to be attributable to a membrane-bound, energy-dependent pump protein (P-glycoprotein [P-gp]) capable of extruding various related and unrelated chemotherapeutic drugs. P-gp is coded for by the MDR1 gene, which in the human genome is located on the long arm of chromosome 7 (7q21-31). At the cellular level, the function of P-gp has been extensively investigated in human cancer. Although innumerable reports have been published in which P-gp has been shown to confer MDR to malignant (including leukemia) cells, so far, large-scale studies in the clinical setting have not convincingly proven that MDR1 plays a major role in clinical drug resistance when the influence of other known prognostic factors in human leukemia are taken into account. At present, results from phase 3 clinical trials evaluating the efficiency of inhibiting (or reversing) the function of P-gp in hematologic malignancies are eagerly awaited. Moreover, the horizon of cellular drug resistance in human cancer has during recent years widened dramatically. Thus, an array of different molecules and mechanisms by which resistant cells can escape the cytotoxic effect of anticancer drugs has now been identified. These molecules and mechanisms include apoptosis-related proteins. In this article, we review the different methods for determining MDR and, in particular, methods for determining P-gp/MDR1, with special reference to their potential importance for therapeutic strategies in human acute leukemia.