Taxol-resistant cells lose the resistant characteristics after injection into nude mice

To obtain an in vivo Taxol-resistant MCF-7 xenograft model (MCF-7/T) for studying resistance mechanisms frequently observed in a human therapeutic setting, MCF-7 cells were injected subcutaneously into the left flanks of the athymic nude mice (Balb/c-nu/nu females, 6–8 weeks old) which were purchased from Vital River Laboratory Animal Technology, Co., Ltd. (Beijing, China) and housed in the controlled environment at 25°C on a 12-h light/dark cycle. Mice were maintained following the rules of the National Institute of Health Guide for the Care and Use of Laboratory Animals. Taxol-resistant MCF-7 xenograft tumors were achieved after ten passages of Taxol treatment. For each passage, mice were treated with 30.0 mg/kg Taxol 24 h before sacrifice. Then, xenograft tumors were collected and transplanted into the new athymic nude mice. After ten passages (~12 months), drug-resistant characteristics of the xenografts were determined by the absence of tumor regression after treatment of Taxol.