Both H2O2-resistant fibroblast (lines OC5 and OC14) and photodynamic therapy (PDT)-resistant fibrosarcoma (line RIF-8A) cells, known to be cross-resistant to cisplatin, were more transfectable in vitro and in vivo than their parental HA1 and RIF-1 cells, respectively. The transfection efficiencies of OC14 and the most cisplatin-resistant OC5 cells were similar, indicating that transfectability was not directly correlated to the level of cisplatin resistance. Irt situ lipofection was much greater in the cells resistant to H2O2 than to PDT. In situ CAT transgene expression was significantly elevated in the cisplatin-resistant variant 2008C13* cells grown as a subcutaneous tumor and was further increased when 2008C13* tumor-bearing SCID mice were resensitized by i.p. injection of cisplatin. These results suggest that the enhanced gene transfer by cationic liposomes in the cells resistant to cisplatin may be, in part, due to cellular resistance to oxidative stress.

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