PR-104 is a phosphate ester dinitrobenzamide mustard pre-prodrug that is rapidly hydrolyzed systemically to PR-104A, a bioreductive prodrug. PR-104A is in turn activated via reduction by NADPH:cytochrome P450 oxidoreductase and other one-electron reductases in hypoxic cells, and by aldo-keto reductase 1C3 (AKR1C3) independently of oxygen, to the corresponding hydroxylamine (PR-104H) and amine (PR-104M) metabolites. Subsequently, these reactive nitrogen mustards crosslink DNA and cause cytotoxicity in cells. PR-104 is known to have preclinical anti-tumor activity in human tumor xenograft models as mono-therapy and in combination with radiotherapy and chemotherapy. Thrombocytopenia, and to a lesser extent neutropenia, was the dose-limiting toxicity of weekly PR-104. Combination of PR-104 with docetaxel or gemcitabine caused dose-limiting and severe myelotoxicity in patients with advanced solid tumors. PR-104 had been in phase II clinical trial for the treatment of acute myeloid leukemia.