Enhanced therapeutic effect of multiple injections of HSV-TK plus GCV genetherapy in combination with ionizing radiation in a mouse mammary tumor model

Purpose: Standard therapies for breast cancer lack tumor specificity and ha ve significant risk for recurrence and toxicities. Herpes simplex virus-thy midine kinase (HSV-tk) gene therapy combined with radiation therapy (XRT) m ay be effective because of complementary mechanisms and distinct toxicity p rofiles. HSV-tk gene therapy followed by systemic administration of gancicl ovir (GCV) enhances radiation-induced DNA damage by generating high local c oncentrations of phosphorylated nucleotide analogs that increase radiation- induced DNA breaks and interfere with DNA repair mechanisms. In addition, r adiation-induced membrane damage enhances the "bystander effect" by facilit ating transfer of nucleotide analogs to neighboring nontransduced cells and by promoting local and systemic immune responses. This study assesses the effect of single and multiple courses of HSV-tk gene therapy in combination with ionizing radiation in a mouse mammary cancer model. Methods and Materials: Mouse mammary TM40D tumors transplanted s.c. in syng eneic immunocompetent BALB-c mice were treated with either adenoviral-media ted HSV-tk gene therapy or local radiation or the combination of gene and r adiation therapy. A vector consisting of a replication-deficient (El-delete d) adenovirus type 5 was injected intratumorally to administer the HSV-tk g ene, and GCV was initiated 24 h later for a total of 6 days. Radiation was given as a single dose of 5 Gy 48 h after the HSV-tk injection. A metastati c model was developed by tail vein injection of TM40D cells on the same day that the s.c. tumors were established. Systemic antitumor effect was evalu ated by counting the number of lung nodules after treating only the primary tumors with gene therapy, radiation, or the combination of gene and radiat ion therapy. To assess the therapeutic efficacy of multiple courses of this combinatorial approach, one, two, and three courses of HSV-tk + GCV gene t herapy, in combination with radiation, were compared to HSV-tk or XRT alone and to sham-treated animals. (Treatments were repeated at 7-day intervals from the HSV-tk injection.) Results: Both single-therapy modalities reduced tumor growth by 11% compare d to controls, while the combined therapy resulted in a decrease of 29%. Me dian survival was 36 days in the combined therapy group, compared to 33 day s in the monotherapy groups and 26 days in the control group. In the metast atic model, the number of lung nodules was reduced by 59.5% after RSV-tk ge ne therapy, whereas radiotherapy had no effect on metastatic growth. Combin ed therapy led to an additional 66.7% reduction in lung colonization. Compa red to controls, local tumor growth was maximally suppressed by three cours es of combined therapy (51.5%), followed by two courses of combined therapy (37.2%), and three sessions of XRT alone (35.6%). Median survival was also significantly prolonged to 58 days with the three courses of combined ther apy, followed by two courses, to 45 days. All other treatment groups demons trated median survival times between 26 and 35 days, while controls had a m edian survival of 24 days. Conclusions: These results indicate that multiple courses of HSV-tk therapy in combination with radiation improve the therapeutic efficacy of this app roach and may provide therapeutic implications for the treatment of human b reast cancer and other solid tumors. (C) 2001 Elsevier Science Inc.