THE EFFECT OF THYMIDINE KINASE TRANSDUCTION AND GANCICLOVIR THERAPY ON TUMOR VASCULATURE AND GROWTH OF 9L GLIOMAS IN RATS

Eradication of malignant brain tumors by in situ intratumoral, retrovi rally mediated transfer of the herpes simplex virus thymidine kinase ( HSVtk) gene, which sensitizes the tumor cells to ganciclovir, has rece ntly been demonstrated in animal models. The observation that tumors s tudied in vitro and in animals can be completely eliminated despite on ly partial transduction of the tumor suggests a bystander mechanism th at affects nontransduced tumor cells. Such a bystander effect is not c ompletely understood and may represent a combination of several factor s that lead to tumor eradication. Endothelial cells of the tumor blood vessels were shown to occasionally integrate the retroviral vector an d thus become sensitized to ganciclovir. In the presence of vector-pro ducer cells, which continuously release infectious viral particles, di ffuse multifocal hemorrhages occurred during ganciclovir administratio n. When the tumor was composed of cells that had been transduced with the thymidine kinase gene before inoculation, no infectious viral part icles were present within the tumor, no transduction of endothelial ce lls occurred, and no hemorrhages were observed during ganciclovir ther apy. These observations suggest that tumor regression may be due, in p art, to destruction of in vivo HSVtk-transduced endothelial cells afte r exposure to ganciclovir, resulting in tumor ischemia as one possible bystander mechanism. The authors investigated this hypothesis using t he subcutaneous 9L gliosarcoma tumor model in Fischer rats. The tumors were evaluated with Doppler color-flow and ultrasound imaging during the various phases of the study. Twenty rats received intratumoral inj ections of HSVtk retroviral vector-producer cells (6 x 10(7) cells/ml) 21 days after bilateral flank tumor inoculation. Ten rats were subseq uently treated with intraperitoneal ganciclovir (15 mg/kg/ml twice a d ay) for 14 days starting on Day 7 after producer cell injection; 10 co ntrol rats received intraperitoneal saline injections (1 ml twice a da y) instead of ganciclovir. Ultrasound and flow images were obtained be fore cell injection, before and during ganciclovir or saline administr ation, and after cessation of treatment. The number, location, and ult rasonographic appearance of tumor vessels and the tumor volumes were r ecorded. The number of blood vessels in the tumors increased over time in both groups before treatment. Intratumoral cell injection without ganciclovir administration did not influence tumor growth or intratumo ral vasculature. However, tumor vasculature decreased after initiation of ganciclovir therapy in the HSVtk-transduced tumors (p < 0.05). Ear ly patchy or diffuse necrotic changes associated with ultrasonographic evidence of scattered intratumoral hemorrhage occurred in tumors trea ted with ganciclovir. Reduction of the tumor blood supply may be an im portant feature of HSVtk transduction-mediated tumor regression and ma y, at least partially, account for the degree of tumor destruction tha t occurs despite the lack of transduction of all tumor cells.