Molecular Imaging and Treatment of Malignant Gliomas Following Adenoviral Transfer of the Herpes Simplex Virus-Thymidine Kinase Gene and the Somatostatin Receptor Subtype 2 Gene

Patients suffering from malignant glioma have a very poor prognosis. New therapy approaches for gliomas are necessary; these tumors are attractive targets for gene therapy. Our research concentrated on evaluation of the use of the Herpes Simplex Virus-thymidine kinase (tk) enzyme and the somatostatin receptor subtype 2 (sst2). DOTA-Tyr³-octreotate is an analog of somatostatin with high affinity for sst₂. It shows rapid internalization in sst₂-positive tumor cells in vitro and in vivo. For gene therapy, we used the adenoviral vector Ad5.tk.sstr, which carries the tk gene and the sst₂ gene. The aim of our study was to compare uptake of the tk substrate 1-(2-fluoro-2-deoxy-β -D-ribofuranosyl)-5-[*]iodouracil (FIRU) labeled with ¹²⁵I and the somatostatin analog ¹¹¹In-DOTA-Tyr³-octreotate in several glioma cell lines after infection with Ad5.tk.sstr. Uptake of ¹²⁵I-FIRU was measured in rat 9L-tk glioma cells without infection with Ad5.tk.sstr. Results showed that the uptake of ¹²⁵I-FIRU was concentration and time dependent. We also used several rat and human glioma cell lines for infection with Ad5.tk.sstr. Forty-eight hours after infection, uptake studies were performed using ¹²⁵I-FIRU and ¹¹¹In-DOTA-Tyr³-octreotate. In all cell lines, the uptake of ¹²⁵I-FIRU and In-DOTA-Tyr³-octreotate increased with increasing multiplicity of infection of virus and showed that the uptake of ¹¹¹In-DOTA-Tyr³-octreotate was higher than that of ¹²⁵I-FIRU in all cell lines. We conclude that the sst₂ imaging and therapy modality is most promising for glioma gene therapy, either alone or in combination with HSV-tk suicide gene therapy. Therapy can be performed using combinations of DOTA-Tyr³-octreotate radiolabeled with ¹⁷⁷Lu or ⁹⁰Y, ¹³¹I-FIRU and/or the prodrug ganciclovir.