Limitations of adenovirus-mediated interleukin-2 gene therapy for oral cancer

Objective/Hypothesis: Adenoviral interleukin-2 (AdV-IL-2) gene therapy has previously not proven effective in treating established murine oral cancer. We hypothesize that the intratumoral level of IL-2 expression is a major l imiting factor in treatment outcome. Methods: A microscopic disease and est ablished oral cancer murine model was used to test this hypothesis. IL-2 ge ne transfer was performed with a recombinant adenovirus vector. Results: Tu mor cells were transduced in vitro with AdV-IL-2 and subsequently implanted into the floor of the mouth in C3H/HeJ mice. IL-2 expression in vitro rang ed from 990 to 1,050 pg/10(6) tumor cells. This microscopic disease treatme nt resulted in either complete tumor regression or a dramatic decrease in t umor progression. Cytolytic T-cell (CTL) assays demonstrated a predominance of CD8-specific, T-cell-mediated tumor killing. Reducing IL-2 expression b y half with a mixture of 1:1 transduced to nontransduced tumor cells elimin ated the antitumor effect and decreased the CTL response. These findings su pport the presence of a critical "threshold" of IL-2 expression. Adenovirus repurification and amplification allowed isolation of a twofold-higher-tit er AdV-IL-2 vector, Treatment of established tumors with the higher-titer A dV-IL-2 at a new maximal dose of 1.4 x 10(9) plaque-forming units (pfu) inc reased in vivo IL-2 expression to 1,127 pg/10(6) cells and generated a sign ificant antitumor response, Complete regression of established tumors, howe ver, could not be achieved, and we noted a decrease in IL-2 expression well below the threshold at 1 week after treatment, Upon repeat maximal AdV-IL- 2 injection in vivo, a greater antitumor effect and increased CTL response was seen, but also, 28% of the animals died of IL-2 toxicity, Conclusion: A lthough limited by expression and toxicity as a single-treatment strategy f or established tumors, AdV-IL-2 gene therapy should be considered a potenti al component of combination therapy strategies.