Expression of Escherichia coli B nitroreductase in established human tumor xenografts in mice results in potent antitumoral and bystander effects upon systemic administration of the prodrug CB1954.

Expression of the Escherichia coli enzyme nitroreductase (NTR) in mammalian cells enables them to activate the prodrug 5-(aziridin-1-yl)-2,4-dinitrobenzamide (CB1954), leading to interstrand DNA cross-linking and apoptosis in both proliferating and quiescent cells. In the work reported here, we used human hepatocellular carcinoma and squamous carcinoma cell lines constitutively expressing NTR to demonstrate that the ntr/CB1954 system results in potent, long-lasting antitumoral effects in mice. We also demonstrate that this enzyme/prodrug combination results in antitumoral effects in vivo when only a minority of tumor cells express the enzyme, using either cells constitutively expressing NTR or ntr gene delivery in situ.

High-level, beta-catenin/TCF-dependent transgene expression in secondary colorectal cancer tissue.

There is an urgent need for improved therapies for inoperable metastatic colon cancer. Gene-directed enzyme prodrug therapy (GDEPT) using adenovirus vectors works well in preclinical models of this disease, but successful clinical application is hampered by an inability to construct vectors that express at high levels in infected tumor cells but not in infected normal cells. Constitutive activation of beta-catenin-dependent gene expression is almost certainly a key causative event in the genesis of colon and some other cancers. Here we have exploited this oncogenic defect to design a synthetic promoter, CTP1, that, in contrast to currently available tumor-selective promoters, is both highly active in cancer cells and highly cancer-cell-specific. CTP1 directs high-level beta-galactosidase expression in freshly isolated biopsies of secondary colon cancer, but is not detectably active in associated normal liver tissue. We also demonstrate that CTP1 can direct high-level, tumor-specific therapeutic gene expression in vivo. Intratumoral injection of an adenovirus vector encoding Escherichia coli nitroreductase driven by CTP1 efficiently sensitized SW480 xenografts to the prodrug CB1954, whereas systemic vector and prodrug administration produced no apparent signs of toxicity. CTP1 may form the basis for effective, targeted gene therapy of metastatic colon cancer and other tumors with deregulated beta-catenin/T cell factor.

Tumour-specific therapeutic adenovirus vectors: repression of transgene expression in healthy cells by endogenous p53.

Approximately 50% of human tumours lack functional p53 suppressor protein. A promoter that is repressed by p53 in healthy cells could thus provide tumour-specific gene expression for a huge subset of tumours. In this report we describe a double recombinant adenovirus vector, 'Ad.p53R', encoding a therapeutic gene that is indirectly repressed by endogenous wild-type p53. Ad.p53R contains two independent expression cassettes; (1) the E. coli nitroreductase gene (NTR) driven by the human hsp70 promoter containing LacI binding sites (hsp70lacO-NTR) and (2) a p53-inducible lac repressor gene (tkGC3-lacI). In p53 null cells (Hep3B), Ad.p53R directed the same level of NTR expression as Ad.p53NR which lacks the tkGC3-lacI cassette. Moreover, injection of SW480 xenografts (mutated p53) with Ad.p53R resulted in a clear inhibition of growth in response to the prodrug CB1954. In cells retaining wt p53 (HepG2 and primary human endothelial cells), Ad.p53R expressed significantly less NTR (approximately 70%) than Ad.p53NR. Ad.p53R administered by i.v. injection also produced significantly less NTR than Ad.p53NR in normal tissues in vivo. Finally, adenovirus infection per se of cultured HepG2 cells at low MOI induced p53 stabilisation suggesting that adenovirus-mediated gene delivery may contribute to p53-based selectivity.

Combined adenovirus-mediated nitroreductase gene delivery and CB1954 treatment: a well-tolerated therapy for established solid tumors.

Gene-directed enzyme prodrug therapy (GDEPT) is a refinement of cancer chemotherapy that generates a potent cell-killing drug specifically in tumor cells by enzymatic activation of an inert prodrug. We describe in vivo studies that evaluate the efficacy and safety of intratumoral (i.t.) injection of an adenovirus vector (CTL102) expressing Escherichia coli nitroreductase (NTR) combined with systemic prodrug (CB1954) treatment. A single i.t. injection of CTL102 (7.5 x 10(9) to -2 x 10(10) particles) followed by CB1954 treatment produced clear anti-tumor effects in subcutaneous (s.c.) xenograft models of four cancers that are likely candidates for GDEPT (i.e., primary liver, head and neck, colorectal and prostate). Virus dose-response studies (s.c. liver model) revealed a steep increase and subsequent rapid plateauing of both NTR gene delivery and anti-tumor efficacy. Evidence of minor virus spread (toxicity) was observed in a s.c. head and neck xenograft model. This was eliminated by passive immunization with neutralizing anti-Ad5 antibodies prior to virus injection without reducing the magnitude of the anti-tumor effect. Preexisting anti-Ad5 neutralizing antibodies may therefore be an advantage rather than an issue in the clinical use of this new therapy.