Adenoviral vectors for prodrug activation-based gene therapy for cancer

Abstract

Cancer cell heterogeneity is a common feature - both between patients diagnosed with the same cancer and within an individual patient's tumor - and leads to widely different response rates to cancer therapies and the potential for the emergence of drug resistance. Diverse therapeutic approaches have been developed to combat the complexity of cancer, including individual treatment modalities designed to target tumor heterogeneity. This review discusses adenoviral vectors and how they can be modified to replicate in a cancer-specific manner and deliver therapeutic genes under multi-tiered regulation to target tumor heterogeneity, including heterogeneity associated with cancer stem cell-like subpopulations. Strategies that allow for combination of prodrug-activation gene therapy with a novel replication-conditional, heterogeneous tumor-targeting adenovirus are discussed, as are the benefits of using adenoviral vectors as tumor-targeting oncolytic vectors. While the anticancer activity of many adenoviral vectors has been well established in preclinical studies, only limited successes have been achieved in the clinic, indicating a need for further improvements in activity, specificity, tumor cell delivery and avoidance of immunogenicity.

Figure 1. Targeting tumor heterogeneity

A) Core DF3/Mucin1 (DF3/MUC1) and human telomerase (hTERT) promoters utilized to regulate adenoviral replication. These promoters use different transcription factor binding sites and distinct mechanisms of transcriptional activation, i.e., the presence or absence of a canonical TATA box or CpG methylation. Nucleotide +1, transcription start site. B) Multiple promoters regulating adenoviral replication may help ensure viral replication and increased anti-tumor activity in a heterogeneous tumor cell population. Since DF3/MUC1 is overexpressed in ~75% of all human solid tumors [–66] and the hTERT gene is activated in 85–90% of tumor tissues [45], the combined use of the corresponding two promoters to express adenoviral E1A will help ensure successful induction of adenoviral replication in a heterogeneous tumor cell population, where either one, or both promoters is active.

Figure 2. Gene-directed prodrug-activating enzyme therapy (GDEPT) using adenoviral vectors

A) Traditional chemotherapy involves systemic drug delivery, often resulting in non-specific, toxic side effects to multiple host tissues. B) Introduction of the capacity for intratumoral prodrug activation, via adenovirus-mediated gene therapy, may allow for use of a lower dose of the anti-cancer prodrug, thereby decreasing non-specific systemic side effects while increasing local, intratumoral anti-tumor activity.

Figure 3. Enhancing GDEPT bystander activity by concomitant delivery of the pan-caspase inhibitor p35

A) Adenoviral gene-directed enzyme-prodrug therapy (GDEPT) facilitates chemotherapeutic prodrug activation and tumor cell kill. However, the adenovirally transduced tumor cell may die quickly due to its exposure to high local concentrations of active drug metabolites. B) The adenovirally transduced tumor factory cell may be protected from caspase-dependent apoptotic cell death by delivery of the pan-caspase inhibitor p35. The increased longevity of these factory cells that results prolongs their ability to metabolize an inactive chemotherapeutic prodrug into its active, cytotoxic metabolites, thereby enhancing anti-tumor activity. p35-infected tumor cells eventually die by a caspase-independent mechanism.

Figure 4. Adenoviral helper virus effect

A) Administration of a replication-deficient adenovirus leads to successful gene transfer to those (few) tumor cells directly infected by the virus. B) Infection of tumor cells with a replication-defective virus carrying a prodrug-activating gene (as in A) in combination with a replication-competent adenovirus facilitates spread of the viral infection to include secondary sites of infection. The replicating virus serves as a helper virus that supplies all of the required replication and repackaging machinery in trans, leading to spread of both adenoviral vectors in the treated tumor cell population.