Discovery of Novel Glutamine Antagonist Prodrugs: A Story From Bench 2 Bedside

6-diazo-5-oxo-l-norleucine (DON) is a glutamine antagonist with robust efficacy in preclinical and clinical cancer studies as well as preclinical neurological disorders with aberrant glutamatergic transmission. However, its therapeutic potential has been hampered by its biodistribution and toxicity to normal tissues, specifically gastrointestinal (GI) tissues as the GI system is highly dependent on glutamine utilization. Given DON’s promising efficacy, we identified a strategy to deliver it selectively to the tumor and/or brain while minimizing its peripheral toxicity. Our cell-targeted glutamine antagonist prodrugs were designed to circulate intact as inert prodrugs in plasma and be preferentially biotransformed to DON at the target sites (i.e. CNS, tumor), permitting significant dose reduction and improved therapeutic index. Our prodrug design for tumor targeting was based on the elevated activities of specific tumor associated proteases (e.g. HDAC, Cathepsins) to serve as “triggers” for prodrug activation. Further for CNS delivery, we utilized promoieties with bulky, lipophilic substituents to increase it cLogP and improve its CNS penetration. We identified several prodrugs that were stable in plasma, gut and liver, yet were readily cleaved to DON at its target sites. When directly compared to DON, our best tumor targeted prodrug exhibited a > 6-fold tumor to plasma and tumor to gut ratio. Our best CNS targeted prodrug showed a >10-fold enhancement in brain penetration index. The best tumor targeted glutamine antagonist prodrug is currently being tested clinical trials.