Accumulation of dGTP at the end of infusion with different courses of forodesine. Patient no. 5 was treated with 40, 40, 60, and 90 mg/m²/d during course 1 (white bars), course 2 (gray bars), course 3 (hatched bars), and course 4 (black bars) of therapy. Intracellular levels of dGTP were quantitated by DNA polymerase assay after first infusion on every day of therapy.