Parameters influencing the efficiency of the thymidine kinase/ganciclovir strategy in human glioblastoma cell lines

The effect of DFMO induced uptake of [3H] putrescine on human glioma cells

Polyamine synthesis inhibitors, such as a-difluoromethylornithine (DFMO), inhibit tumor cell growth in vitro and in vivo. However, upon cessation of treatment, tumor growth resumes. We hypothesized that incorporation of radioactive polyamines might kill the growth-arrested cells. This hypothesis was previously tested in rat 9L brain tumor cells in which DFMO increased both the uptake and the retention of [3H] putrescine. In these rat cells, DFMO-induced retention of high-specific-activity [3H] putrescine for 20 days resulted in several logs killing. In the present studies all of the 5 different human glioma cell lines tested with DFMO treatment also showed enhanced uptake of exogenous [3H] putrescine, reduced cell counts and enhanced killing of colony forming cells (CSF). Extending the time of DFMO treatment of cells that had taken up high-specific-activity (80 Ci/mmol) [3H] putrescine further increased the killing. A 10-day extension resulted in a 10,000-fold reduction in cumulative cell growth. A 5-day extension resulted in a 2-3 log decrease in numbers of surviving CFC. These data further support the hypothesis and suggest that DFMO-induced cell cycle arrest enhances cellular retention of [3H] putrescine, increasing the effective internal radiation dose enough to cause proliferative death. In a clinical setting, the short (approximately 1 microm) path-length of the tritium beta particle should limit effects to the tumor cells and spare adjacent normal cells. These results support the concept that treatment with the combination of polyamine inhibitors and radioactive polyamines might be a useful adjunct to current therapies for glioblastoma multiforme.

Nucleoside binding site of herpes simplex type 1 thymidine kinase analyzed by X-ray crystallography

The crystal structures of the full-length Herpes simplex virus type 1 thymidine kinase in its unligated form and in a complex with an adenine analogue have been determined at 1.9 A resolution. The unligated enzyme contains four water molecules in the thymidine pocket and reveals a small induced fit on substrate binding. The structure of the ligated enzyme shows for the first time a bound adenine analogue after numerous complexes with thymine and guanine analogues have been reported. The adenine analogue constitutes a new lead compound for enzyme-prodrug gene therapy. In addition, the structure of mutant Q125N modifying the binding site of the natural substrate thymidine in complex with this substrate has been established at 2.5 A resolution. It reveals that neither the binding mode of thymidine nor the polypeptide backbone conformation is altered, except that the two major hydrogen bonds to thymidine are replaced by a single water-mediated hydrogen bond, which improves the relative acceptance of the prodrugs aciclovir and ganciclovir compared with the natural substrate. Accordingly, the mutant structure represents a first step toward improving the virus-directed enzyme-prodrug gene therapy by enzyme engineering.