A novel E1B-55kD-deleted oncolytic adenovirus carrying mutant KRAS-regulated hdm2 transgene exerts specific antitumor efficacy on colorectal cancer cells

E1B-55kD-deleted adenoviruses have been used as conditionally replicative adenoviruses (CRAds) for therapeutic purposes in tumors with loss-of-function p53 mutation. To target cancer cells that harbor activating mutant KRAS (KRAS(aMut)) but spare p53(wild) normal cells, we constructed and examined by reporter assays a KRAS(aMut) but not p53-responsive promoter, the Deltap53REP2 promoter. The Deltap53REP2 promoter, derived from human double minute 2 (hdm2) P2 promoter with its p53 response elements being deleted, was used to regulate the expression of the hdm2 transgene in a novel E1B-55kD-deleted CRAd, the Ad-KRhdm2. The Ad-KRhdm2 selectively replicated in and exerted cytopathic effects on KRAS(aMut) colorectal cancer cell lines (HCT116, LoVo, LS174T, LS123, and SW620), regardless of their p53 gene statuses, by forming plaques and exhibiting cytopathic effect in cultured cells. Ad-KRhdm2, like other E1B-55kD-deleted adenoviruses, also exerted selective cytopathic effects on tumor cells with loss-of-function p53 mutant. The multiplicities of infection of Ad-KRhdm2 required to decrease 50% viability of KRAS(aMut) tumor cells cultured for 7 days were 440 to 3,400 times less than those of MRC5 normal fibroblasts and KRAS(wild)/p53(wild) RKO tumor cells. Intratumoral injection of Ad-KRhdm2 vectors exhibited specific lytic activities in nude mouse xenografts of KRAS(aMut) cell lines (LoVo, SW620, and LS174T) but not in xenografts of RKO cells. Transduction of KRAS(aMut)/p53(wild) HCT116, LoVo, and LS174T cells by Ad-KRhdm2 significantly increased Hdm2 expression, decreased p53 level, and abolished the p53-transactivating p21(Cip1) promoter activity. Ad-KRhdm2 has shown its therapeutic potential in KRAS(aMut) cancer cells and warrants further clinical trials.