Significance A challenge in cancer therapy is delivering high, consistent levels of therapeutics to tumors. Protein-based therapeutics (e.g., antibodies) are typically delivered intravenously and require multiple doses to get sufficient levels to traffic into tumors to exert an effect. Yet healthy tissues are also exposed to similar drug levels, which can lead to significant side effects. This study uses sophisticated three-dimensional imaging of transparent tumors to characterize a versatile gene therapy platform using adenovirus that solves this problem by producing drugs directly in the tumor. This approach increases the tumor-to-bloodstream level of a model antibody 1,800-fold in comparison to direct administration. Thus, this system could allow for the local production of highly potent drugs with greatly reduced risk of systemic toxicities.