Overexpression of P-glycoprotein in human lung carcinoma xenografts after fractionated irradiation in vivo.

In vivo exposure of a human epidermoid lung carcinoma xenograft to seven irradiation treatments of 10 Gy in consecutive passages resulted in expression of resistance to vincristine. This about threefold drug resistance was detectable with a single dose of 1 mg/kg vincristine. Characterization of the radiation-pretreated subline showed that overexpression of P-glycoprotein, as determined by immunofluorescence and Mabs C219 and 265/F4, occurred in this tumor. After six X-ray fractions, only single positive cells were observed, whereas seven fractions produced an intense immunofluorescent reaction with both antibodies. Southern blot analyses indicated that no gene amplification had occurred. This result shows that irradiation can influence expression of P-glycoprotein and in this way influences drug resistance.