Fludarabine impacts on the growth of MEC1 cells in vitro and in vivo. (A) MEC1 cells were plated in vitro in 48-well plates with increasing concentrations of fludarabine and a luminescent assay was performed 72 hours later to demonstrate MEC1 cells' sensitivity to the drug. EC₅₀ indicates half maximal effective concentration (11.9 g/mL). (B) Tumor growth curves obtained in Rag2^−/−γ_c^−/− male mice that received a transplant subcutaneously in the left flank of MEC1 cells (10 × 10⁶). Twenty-one days later, mice bearing MEC1 tumor were randomly assigned to one of the following intraperitoneal treatments (5 animals/group): saline solution (PBS), fludarabine alone (34 mg/kg) daily on days 21 to 25, or fludarabine (0.625 mg/kg) + cyclophosphamide (6.25 mg/kg) daily on days 21 to 23. The treatment was repeated 3 times every 2 weeks. Tumor size was evaluated by measuring perpendicular diameters by a caliper. Animals were killed when the tumor volume reached 1000 mm³. Measurements were stopped when 75% of originally treated mice were still surviving. *Statistically significant differences calculated using the Student t test: day 23, PBS versus fludarabine, P = .01; day 29, PBS versus fludarabine, P = .012, and PBS versus fludarabine + cyclophosphamide, P = .038. Data are representative of at least 2 independent experiments. (C) Kaplan-Meier survival curves for Rag2^−/−γ_c^−/− male mice challenged subcutaneously in the left flank with MEC1 cells. Twenty-one days later, mice bearing MEC1 tumor were randomly assigned to one of the following intraperitoneal treatments (5 animals/group): PBS, fludarabine alone (34 mg/kg) daily on days 21 to 25, or fludarabine (0.625 mg/kg) + cyclophosphamide (6.25 mg/kg) daily on days 21 to 23. The treatment was repeated 3 times every 2 weeks. Tumor size was evaluated by measuring perpendicular diameters by a caliper. Animals were killed when the tumor volume reached 1000 mm³.