Multidrug resistance and P-glycoprotein in cancer cells

It is now clear that P170 is one of the determinants of multidrug resistance. This protein is physiologically present in normal tissues with an excreting and detoxifying action. A variety of tumours can express it at a different degree. It is relatively simple to detect it with specific monoclonal antibodies using the flow cytometric technique. Numerous drugs with no cytotoxic effect have beem employed with the aim of blocking P170 in order to restore the chemosensitivity of resistant tumour cells. Verapamil and other calcium channel blockers have been the most employed agents. Experimental data from both in vitro and in vivo studies have been very encouraging. But at present we have only a few clinical studies with no univocal results. We need more phase I-II studies in which different revertant agents should be studied at different dosages, closely monitoring their toxicity. However, only future phase III trials will be able to define the real role of all revertant agents in cancer chemotherapy.