Complete protection against aflatoxin B₁-induced liver cancer with a triterpenoid: DNA adduct dosimetry, molecular signature, and genotoxicity threshold

In experimental animals and humans, aflatoxin B₁ (AFB ₁) is a potent hepatic toxin and carcinogen. The synthetic oleanane triterpenoid 1-[2-cyano-3-,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole (CDDO-Im), a powerful activator of Keap1-Nrf2 signaling, protects against AFB₁-induced toxicity and preneoplastic lesion formation (GST-P-positive foci). This study assessed and mechanistically characterized the chemoprotective efficacy of CDDO-Imagainst AFB₁-induced hepatocellular carcinoma (HCC). A lifetime cancer bioassay was undertaken in F344 rats dosed with AFB₁ (200 μg/kg rat/day) for four weeks and receiving either vehicle or CDDO-Im(three times weekly), oneweek before and throughout the exposure period. Weekly, 24-hour urine samples were collected for analysis of AFB₁ metabolites. In a subset of rats, livers were analyzed for GST-P foci. The comparative response of a toxicogenomic RNA expression signature for AFB₁ was examined. CDDO-Im completely protected (0/20) against AFB₁-induced liver cancer compared with a 96% incidence (22/23) observed in the AFB₁ group. With CDDO-Im treatment, integrated level of urinary AFB₁-N⁷-guanine was significantly reduced (66%) and aflatoxin-N-acetylcysteine, a detoxication product, was consistently elevated (300%) after the first AFB₁ dose. In AFB₁-treated rats, the hepatic burden of GST-P-positive foci increased substantially (0%-13.8%) over the four weeks, but was largely absent with CDDO-Im intervention. The toxicogenomic RNA expression signature characteristic of AFB₁ was absent in the AFB₁ + CDDO-Im-treated rats. The remarkable efficacy of CDDO-Imas an anticarcinogen is established even in the face of a significant aflatoxin adduct burden. Consequently, the absence of cancer requires a concept of a threshold for DNA damage for cancer development.