Preclinical studies demonstrate that intratumoral delivery of adenovirus expressing IL-2 eradicates pre-established tumors in mice and confers immune protection from rechallenge. To explore the activity of AdCAIL-2 in prostate cancer, a Phase I clinical trial was conducted in patients with localized disease and Gleason score >7 or prostate-specific antigen (PSA) >10 plus Gleason score 7. A total of 12 patients were injected 4 weeks prior to prostatectomy in a dose-escalation study at doses of 10(9), 5 x 10(9) and 10(10) PFU of virus. No dose-limiting toxicity was observed. Side effects included perineal discomfort, hematuria, flu-like symptoms in two patients and urinary hesitancy in one patient. Pathology demonstrated an inflammatory response consisting predominantly of CD3+CD8+ T lymphocytes with areas of tumor necrosis. Intracellular cytokine staining of tumor-infiltrating lymphocytes demonstrated increases in both gamma-interferon and IL-4 secreting T cells after vaccination. PSA levels fell in five of five evaluable patients treated at the lowest dose (mean decline of 33.3%, range 17-69%). At higher doses, PSA values initially increased after injection, then fell to baseline prior to surgery. This trial demonstrates the feasibility and safety of intraprostatic adenovector-mediated IL-2 gene delivery.