title: Prodrug activation gene therapy and external beam irradiation in the treatment of prostate cancer.
abstract: OBJECTIVE
  Gene therapy may represent a new avenue for the development of multimodal treatment for men with locally advanced prostate cancer. This study explores the potential benefits of combining adenovirus-mediated (ADV) herpes simplex virus thymidine kinase gene (HSV-tk) transduction and ganciclovir (GCV) therapy with external beam radiation therapy (XRT) to enhance the therapeutic efficacy of each treatment alone.
  
  
  METHODS
  ADV/HSV-tk-transduced mouse prostate cancer cells, RM-1, were irradiated as single-cell suspensions at escalating doses in a cesium source (4.4 Gy/min). HSV-tk-expressing cells were randomized to receive varying doses and varying chronologies of GCV therapy in relation to XRT to fully evaluate potential cooperative activities. End points were determined in a clonogenic assay by counting colonies with greater than 50 cells 7 days after replating. The potential role of apoptosis as a mediator of enhanced cell killing was addressed by a TUNEL assay 12 and 24 hours after therapy.
  
  
  RESULTS
  Neither ADV infection nor GCV alone affected XRT killing. However, the combination of ADV/HSV-tk+GCV plus XRT maintained the 1 log of cell kill from gene therapy alone through escalating doses of radiation. Radiation sensitization was noted at higher doses of radiation (8.8 Gy or more). Although decreasing the GCV dose had a profound negative influence on HSV-tk+GCV-mediated killing, combination therapy continued to maintain the degree of HSV-tk+GCV killing through escalating doses of XRT in an additive fashion but did not result in radiosensitization. Changing the chronology of GCV exposure in relation to XRT did not significantly alter the additive activities of combination therapy. Studies of apoptosis noted a doubling of apoptotic activity with HSV-tk+GCV compared with HSV-tk+PBS with or without XRT. However, there was no significant change in apoptotic activity in combination therapy over HSV-tk+GCV alone within the 24-hour period after GCV exposure.
  
  
  CONCLUSIONS
  The combination of ADV/HSV-tk+GCV and XRT appears to result in at least additive, and with higher doses of radiation, synergistic killing activities, indicating a potential usefulness of this treatment strategy for patients with prostate cancer.
authors.affiliation: Institute for Gene Therapy and Molecular Medicine and Department of Urology, Mount Sinai School of Medicine, New York, New York 10029, USA.
authors.forename: G
authors.initials: G
authors.lastname: Atkinson
authors.forename: S J
authors.initials: SJ
authors.lastname: Hall
mesh_terms: D000818:Animals; D000998:Antiviral Agents / Q000627:therapeutic use*; D003131:Combined Modality Therapy; D005240:Feasibility Studies; D015774:Ganciclovir / Q000627:therapeutic use; D015316:Genetic Therapy / Q000379:methods; D008297:Male; D051379:Mice; D011355:Prodrugs; D011471:Prostatic Neoplasms / Q000532:radiotherapy / Q000628:therapy*; D018139:Simplexvirus / Q000235:genetics; D013937:Thymidine Kinase / Q000235:genetics; D014161:Transduction, Genetic; D014407:Tumor Cells, Cultured
chemical_list: D000998:Antiviral Agents; D011355:Prodrugs; D013937:Thymidine Kinase; D015774:Ganciclovir