PR-104, currently in phase II clinical trials, is a
phosphate ester pre-
prodrug which is converted in vivo to its cognate alcohol,
PR-104A, a
prodrug designed to exploit tumor hypoxia. Bioactivation occurs via one-electron reduction to
DNA crosslinking metabolites in the absence of
oxygen. However, certain tumor cell lines activate
PR-104A in the presence of
oxygen, suggesting the existence of an aerobic
nitroreductase. Microarray analysis identified a cluster of five
aldo-keto reductase (AKR) family members whose expressions correlated with aerobic metabolism of
PR-104A. Plasmid-based expression of candidate genes identified
aldo-keto reductase 1C3 as a novel
nitroreductase.
AKR1C3 protein was detected by Western blot in 7 of 23 cell lines and correlated with oxic
PR-104A metabolism, an activity which could be partially suppressed by Nrf2 RNAi knockdown (or induced by Keap1 RNAi), indicating regulation by the ARE pathway. AKR1C3 was unable to sensitize cells to 10 other bioreductive
prodrugs and was associated with single-agent
PR-104 activity across a panel of 9 human
tumor xenograft models. Overexpression in two AKR1C3-negative
tumor xenograft models strongly enhanced
PR-104 antitumor activity. A population level survey of AKR1C3 expression in 2,490 individual cases across 19
cancer types using tissue microarrays revealed marked upregulation of AKR1C3 in a subset including hepatocellular, bladder, renal, gastric, and
non-small cell lung carcinoma. A survey of normal tissue AKR1C3 expression suggests the potential for
tumor-selective
PR-104A activation by this mechanism. These findings have significant implications for the clinical development of
PR-104.