Dose of Adenoviral Vectors Expressing Interleukin-2 Plays an Important Role in Combined Gene Therapy with Cytosine Deaminase/5–Fluorocytosine: Preclinical Consideration

Using a syngeneic murine model, we investigated the therapeutic efficacy of combined gene therapy using adenoviral vectors expressing murine interleukin-2 (AdmIL-2) and Escherichia coli cytosine deaminase (AdCD). In a subcutaneous tumor model, tumor-bearing mice were treated with an intratumoral injection of adenoviral vectors and received an intraperitoneal administration of 5–fluorocytosine (5–FC). Only the mice treated with AdCD (2×10⁸ pfu) and an intermediate dose of AdmIL-2 (1×10⁶ pfu) survived significantly longer than mice treated with AdCD alone ( P <0.01). Moreover, 40% of these treated mice obtained complete remission from tumor-bearing status. The cytotoxicity of splenocytes obtained from the treated mice was related to the survival period. Tumor-specific cytotoxic T lymphocyte assay showed that the cell-mediated cytotoxic response was specific for parental tumor cells. In a hepatic metastasis model, mice treated with an intravenous administration of both AdCD (2×l0⁸ pfu) and an intermediate dose of AdmIL-2 (1×10⁶ pfu) demonstrated the most significant reduction of metastatic foci and the longest survival following a 5–FC administration. These results suggest that gene therapy combined with AdmIL-2 and AdCD may be a promising strategy for clinical application and, in addition, that translation of combined gene therapy from murine models into the clinical setting will require careful attention to the variables of cytokine expression levels in the design of clinical trials and in the evaluation of treatment efficacy.