1004. Enhanced Survival Uncovers Liver Toxicity in Mice with Orthotopic Advanced Ovarian Cancer Treated with Gemcitabine and Ad5/3-Δ24, a Serotype 3 Receptor Targeted Oncolytic Adenovirus

Abstract

Top of pageAbstract Oncolytic adenoviruses a.k.a. conditionally replicating adenoviruses (CRAds) represent a promising approach for treatment of cancers resistant to conventional therapies. Our hypothesis was that efficacy of CRAds might be further improved by using chemotherapeutic agents in a multimodal antitumor approach. Oncolytic tumor killing differs mechanistically from conventional therapies and therefore an additive or synergistic antitumor effect is possible. The toxicity profiles of the agents may be different and could result in enhanced efficacy without increased side effects. Finally, it may be possible to use lower treatment doses and still achieve a greater efficacy than with single agents. We evaluated Ad5/3-|[Delta]|24 in combination with deoxycytidine analogue gemcitabine against human ovarian adenocarcinoma. Ad5/3-|[Delta]|24 is a serotype 3 receptor targeted CRAd containing a 24 bp deletion in the E1A gene. As a result the E1A protein is unable to bind the retinoblastoma (Rb) protein for induction of viral replication. Therefore the virus replicates selectively in cells dysfunctional in the Rb/p16 pathway, including most if not all cancer cells. Most CRAds are based on adenovirus serotype 5 (Ad5). However, the expression of the Ad5 primary receptor, coxsackie-adenovirus receptor (CAR), is highly variable on ovarian cancer cells. The lack of CAR can be circumvented by substituting the knob domain of Ad5 with the corresponding domain of serotype 3 (Ad3). Ad3 has a distinct receptor which is expressed to high degree on ovarian cancer cells. For evaluation of the potential interaction between Ad5/3-|[Delta]|24 and gemcitabine in vitro, the cell killing effect of the combination treatment was compared to single treatments on established ovarian adenocarcinoma cell lines. The combination resulted in synergistic cell killing as determined by Chou and Talalay's synergy analysis. However, the effect was dependent on dose and sequencing of the agents. In an orthotopic murine model of aggressive peritoneally disseminated ovarian cancer, the combination of Ad5/3-|[Delta]|24 and gemcitabine increased the survival of mice over either agent alone, and almost 60 % of treated mice were cured. However, sequencing of the agents was critical for toxicity versus efficacy. Mice remained free from intraperitoneal disease, but some succumbed to treatment related toxicity. Possible reasons for the interaction between Ad5/3-|[Delta]|24 and gemcitabine include the chemosensitizing activity of E1A and/or altered replication kinetics. Our results indicate that gemcitabine reduces the initial rate of Ad5/3-|[Delta]|24 replication without affecting the total amount of virus produced.