Accurate determination of the expression of P-glycoprotein and its encoding mRNA,MDR-1, is key to determining the importance of this mechanism of drug resistance in the clinic. This understanding is critical to development of P-glycoprotein antagonists to block drug efflux and to overcome drug resistance. Reversal studies are best performed in diseases in which this mechanism represents the major component of drug resistance. The clearest approach to identifying these is to measureMDR-1 mRNA or P-glycoprotein in tumors at diagnosis, at the time of disease recurrence, and before and after treatment with P-glycoprotein reversal agents.
