Human tumor cell-to-plasma partitioning. DON (1), 2, 6, 10, and 13 were incubated for 1 h in 10 million P493B lymphoma cells suspended in 1 mL of human plasma. (A) 2, 6, and 10 showed DON release in tumor cells similar to equimolar DON, whereas 13 showed minimal DON release. (B) All prodrugs showed minimal release of DON in human plasma. (C) DON exhibited a human tumor cell-to-plasma ratio of 0.4, whereas the DON tumor cell-to-plasma partitioning ratio from 2, 6, 10, and 13 was 13, 22, 11, and 46, respectively. Although 13 afforded the best tumor cell-to-plasma partitioning ratio, 6 delivered enhanced DON tumor levels compared with 13 (8.81 ± 0.02 vs 0.12 ± 0.04 μM) and thus was selected for further evaluation in a time course experiment.