6-Diazo-5-oxo-L-norleucine

6-Diazo-5-oxo-L-norleucine (DON) is a glutamine antagonist, which was isolated originally from Streptomyces in a sample of Peruvian soil. It is a non-standard amino acid. The diazo compound was characterized in 1956 by Henry W Dion et al., who suggested a possible use in cancer therapy. This antitumoral efficacy was confirmed in different animal models. DON was tested as chemotherapeutic agent in different clinical studies, but was never approved. The last clinical results were published in 2008, though not as DON monotherapy but in combination with a recombinant glutaminase.