We evaluated the effectiveness of a replication-defective adenovirus-transducing
thymidine kinase (TK) gene under the control of the rat Tg (rTg) promoter (AdrTgtk) in
therapy of a human Hurthle
cancer (XTC-1 cell) in vitro and in vivo. The
ganciclovir (GCV) sensitivity of infected XTC-1 cells was assessed in vitro by H(3)-thymidine incorporation assay and
Trypan-blue exclusion, and by an in vivo
tumor development assay. Proliferation was strongly inhibited by adding GCV into the culture medium of infected cells, but not uninfected cells, proving cell
infection and expression of TK in the XTC-1 cells. AdrTgtk, and also viruses that have the noncell-specific cytomegalovirus (CMV) promoter-directing expression of TK (AdCMVtk), or
luciferase (AdCMVLuc), were used to transduce XTC-1 cells to evaluate killing effects. After
infection with AdCMVtk or AdrTgtk, followed by GCV treatment, 70% of infected cells were killed in the presence of GCV, compared with less than 20% of cells infected by AdCMVLuc and treated with GCV. In vivo toxicity was studied in BALB/c mice. When adenovirus is given iv, liver is the major organ infected. No significant changes of the serum
transaminase levels and no histological abnormalities were found in animals treated with AdrTgtk/GCV given iv, compared with control animals. High levels of serum
transaminases, lymphocyte infiltration, some Kupffer's cell prominence, and extensive single-cell hepatocyte death were found in AdCMVtk/GCV-treated animals, indicating severe liver damage induced, as expected, by the noncell-specific CMV promoter. XTL-1 cells (2 x 10(6)) were injected sc into BALB/c-severe combined immunodeficient mice (BALB/c-SCID), and the mice developed
tumors after 3 wk. After intratumoral injection of AdrTgtk and treatment with GCV,
tumors stabilized in 15 of 17 mice within 3 wk, 9
tumors remained stabilized after 5 wk of treatment, and 2 disappeared during observation. In AdCMVLuc/GCV-treated control mice, almost all
tumors grew continuously. The average
tumor size in AdrTgtk-treated mice was significantly smaller than that of control animals after 2 wk of treatment. Our data confirm the effectiveness and specificity of an adenovirus using rTg promoter to express TK, and support its future application to
thyroid cancer gene therapy in humans.