Tumor-Targeted Delivery of 6-Diazo-5-oxo-L-norleucine (DON) Using Substituted Acetylated Lysine Prodrugs

6-Diazo-5-oxo-L-norleucine (DON) is a glutamine antagonist with robust anticancer efficacy; however, its therapeutic potential was hampered by its biodistribution and toxicity to normal tissues, specifically gastrointestinal (GI) tissues. To circumvent DON's toxicity, we synthesized a series of tumor-targeted DON prodrugs designed to circulate inert in plasma and preferentially activate over DON in tumor. Our best prodrug 6 (isopropyl 2-(6-acetamido-2-(adamantane-1-carboxamido)hexanamido)-6-diazo-5-oxohexanoate) showed stability in plasma, liver, and intestinal homogenates yet was readily cleaved to DON in P493B lymphoma cells, exhibiting a 55-fold enhanced tumor cell-to-plasma ratio versus that of DON and resulting in a dose-dependent inhibition of cell proliferation. Using carboxylesterase 1 knockout mice that were shown to mimic human prodrug metabolism, systemic administration of 6 delivered 11-fold higher DON exposure to tumor (target tissue; AUC(0-t) = 5.1 nmol h/g) versus GI tissues (toxicity tissue; AUC(0-t) = 0.45 nmol h/g). In summary, these studies describe the discovery of a glutamine antagonist prodrug that provides selective tumor exposure.