Dose-dependent accumulation of clofarabine triphosphate. Blood samples were obtained at the end of clofarabine infusion to isolate leukemia cells. Normal and analog nucleotides were extracted using perchloric acid, and the triphosphate was measured using HPLC and was calculated as described in “Patients, materials, and methods.” The number of patients at each dose level was 1 each at 11.5, 15, 35, and 70 mg/m², 2 at 40 mg/m², and 5 at 52 mg/m².