To obtain cisplatin (CDDP)-resistant cells possessing the clinically induced resistance phenotype, H-460 nonsmall cell lung cancer cells (NSCLC) were pulse treated with 20, 60, 80 microM CDDP for 1 h every week, respectively. Twelve months later, three CDDP-resistant cell lines (H-460/CDDP20, H-460/CDDP60, H-460/CDDP80) were obtained that exhibit different levels of CDDP resistance (6- to 22-fold), and the possible mechanisms of resistance were studied. These resistant cells were cross-resistant to carboplatin and melphalan, but not to adriamycin or 5-fluorouracil. CDDP resistance in these cell lines appeared to be stable even after 6 months of growth in cisplatin-free medium. There was no evidence of drug accumulation differences between parental and resistant cells. Although both intracellular glutathione (GSH) content and glutathione S-transferase (GST) activity increased 1.5- to 2.5-fold in the resistant cells, only a minimal reversal of drug resistance was observed after buthionine sulfoximine (BSO) treatment, which depleted intracellular GSH levels. An enhancement of DNA repair activity was found in the resistant cell lines and played the major role in the cisplatin resistance phenotype. Using H-460/CDDP80 cells as a model, addition of a nontoxic concentration of pentoxifylline (PTX) significantly enhanced CDDP-induced cytotoxicity in a synergistic manner. Furthermore, more prominent reversal of CDDP resistance could be achieved when the resistant cells were pretreated with BSO, followed by PTX and CDDP combined treatment. This provides a rationale for combination therapy in refractory lung cancer using CDDP and two resistance modulators.