6-diazo-5-oxo-L-norleucine (DON), a non-natural amino acid with structural similarity to glutamine, was first isolated from Streptomyces bacteria in the 1950s. It has shown promissing antitumor activity in preclinical and several clinical studies. DON acts as an irreversible inhibitor of many glutamine utilizing enzymes critical for the synthesis of nucleic acids, proteins and the generation of α-ketoglutarate for energy metabolism. However, its high toxicity leading to gastrointestinal side effects prevented its further development. We hypothesized that a novel cell-directed prodrug of DON which could deliver the drug selectively to cells and would permit significant dose reduction, greatly alleviating the GI adverse events.
Herein we report the design, synthesis, and evaluation of several novel DON prodrugs targeted to Peripheral Blood Mononuclear Cells (PBMC’s). Using whole blood from mouse, pig, dog, monkey and human, we found that several of the new prodrugs selectively delivered DON into PBMC‘s versus plasma by >10-fold. These findings open an opportunity to develop therapeutics active at lower dose circumventing dose limiting toxicities.
 Rais, R.; Jančařík, A.; Tenora, L.; Nedelcovych, M.; Alt, J.; Englert, J.; Rojas, C.; Le, A.; Elgogary, A.; Tan, J.; Monincová, L.; Pate, K.; Adams, R.; Ferraris, D.; Powell, J.; Majer, P.; Slusher, B. S. J. Med. Chem. 2016, 59, 8621.
 Nedelcovych, M.; Tenora, L.; Kim, B.-H.; Kelschenbach, J.; Chao, W.; Hadas, E.; Jančařík, A.; Prchalová, E.; Zimmermann, S. C.; Gadiano, A.; Garrett, C.; Furtmüller, G.; Oh, B.; Brandacher,G.; Alt, J.; Majer, P.; Volsky, D.J.; Rais, R.; Slusher, B.S. J. Med. Chem. 2017, 60, 7186.