LIVER-ASSOCIATED TOXICITY OF THE HSV-TK GCV APPROACH AND ADENOVIRAL VECTORS/

Intravenous injection of immunodeficient mice with adenoviral vectors carrying the HSV-tk gene led to preferential transduction of liver tis sue. Subsequent application of ganciclovir (GCV) resulted in extremely high toxicity with negligible survival rates. This toxicity was only seen when GCV was applied in addition to the adenoviral vector and not if combined with the Ad-beta gal control vector. This indicates a spe cific Ad-tk/GCV-related toxicity. Low survival rates were seen not onl y after intravenous administration but also after injection of the vec tors into the portal vein, the liver tissue and liver tumors, but not during the treatment of subcutaneous tumors. This, together with exten sive signs of liver degeneration occurring as early as one day after t he initiation of GCV treatment, strongly suggests a specific liver-ass ociated toxicity. Severe toxicity was observed when animals received G CV treatment as late as 7 weeks after vector administration. Moreover, in vitro survival rates of resting primary hepatocytes treated with A d-tk and GCV were very low. We therefore suppose a mechanism of toxici ty of phosphorylated GCV which is independent from cellular proliferat ion. Since our results indicate that the Ad-HSV-tk/GCV approach is tox ic for the resting liver, additional safely features such as tumor-res tricted transgene expression should be included in adenoviral vectors.