Gene therapy of thyroid cancer via retrovirally-driven combined expression of human interleukin-2 and Herpes Simplex Virus thymidine kinase

Based on our clinical experience with combined gene therapy of glioblastoma, we developed a retroviral vector expressing two therapeutic genes (i.e. thymidine kinase of herpes simplex virus, HSV-TK, and interleukin-2, IL-2) and evaluated its efficiency in vitro and in vivo. METHODS: Expression of therapeutic genes in transduced thyroid carcinoma cells was analyzed by real-time RT-PCR. Ganciclovir sensitivity of infected cells was assessed in vitro in thyroid carcinoma cell lines and in vivo in nude mice bearing xenografted thyroid cancers. The combined effect of IL-2/HSV-TK was compared with the effect of IL-2 alone. RESULTS: Expression of therapeutic genes was higher in differentiated than in anaplastic thyroid carcinoma cells. Ganciclovir treatment led to dose- and time-dependent killing of transduced cells in vitro. A bystander effect was demonstrated by using mixtures of infected and non-infected cells. In vivo studies showed a significant reduction of growth and the presence of an inflammatory infiltrate in transduced thyroid tumors expressing IL-2 alone, as compared with non-infected tumors. By using the retroviral vector expressing IL-2/HSV-TK, treatment with ganciclovir led to complete eradication of anaplastic tumors and a >80% reduction of the size of differentiated thyroid carcinomas. Histological analysis of tumor specimens showed extensive necrosis and inflammatory cell infiltrates. The combination of IL-2/HSV-TK plus ganciclovir was significantly more efficient than IL-2 alone in eradicating tumor masses. The bystander effect was also obtained in vivo