[Resistance to chemotherapeutic agents in the treatment of cancer. Emphasizing the multiresistant phenotype].

Despite considerable efforts, rather limited progress has been achieved in the treatment of disseminated cancer with chemotherapeutic agents during the last decade. Most tumours, especially those of epithelial origin, are more or less resistant to cytostatics right from initiation of therapy. Other tumours, on the other hand, like leukemias and lymphomas, are highly responsive initially but may become resistant during the course of therapy (secondary resistance). This type of resistance is due to random mutations in the malignant cells. Several mechanisms of cellular resistance have been described, the most important being characterized by simultaneous resistance to several unrelated chemotherapeutic agents (pleiotropic drug resistance or multidrug resistance). This mechanism is due to a membrane-associated efflux pump which transports the cytostatic agents out of the cells. The clinical importance of multidrug resistance and the possibility of inhibiting the function of the pump by pharmacological agents are currently being examined.