CHEMOGENE THERAPY - OSTEOCALCIN PROMOTER-BASED SUICIDE GENE-THERAPY IN COMBINATION WITH METHOTREXATE IN A MURINE OSTEOSARCOMA MODEL

We previously reported that the recombinant adenovirus (Ad) vector con taining the thymidine kinase (TK) gene driven by the osteocalcin (OC) promoter (Ad-OC-TK), when delivered concurrently with acyclovir (ACV), is highly selective in blocking the growth of osteosarcoma in experim ental models (Cancer Res. 1996;56:4614-4619). To investigate the possi ble additive effects of the combined treatment of gene therapy and con ventional chemotherapy (chemogene therapy), we compared the effect of low dose (IC10) methotrexate (MTX) and OC promoter-based toxic gene th erapy with either of these single modalities alone. We choose low dose MTX with the intent of determining whether chemosensitization of the osteosarcoma may be possible in combination with gene therapy with an overall reduced toxicity profile and enhanced therapeutic efficacy whe n compared to a single agent alone. In vitro, the combined treatments of MTX (3 ng/mL) and Ad-OC-TK (20 multiplicity of infection (MOI)/targ et cell) plus ACV (10 mg/mL) had an additive therapeutic effect over t hat of either MTX (P < 0.05) or Ad-OC-TK plus ACV treatment alone (P < 0.05). In vivo, nude mice with subcutaneous tumors of either human os teosarcoma (MG-63) or rat osteosarcoma (ROS) received three intratumor al injections of Ad-OC-TK (5 X 10(8) PFU) plus daily intraperitoneal A CV (40 mg/kg body weight) for 2 weeks in combination with five weekly bolus intraperitoneal MTX (87.5 mg/kg). Osteosarcoma tumor growth was inhibited more efficiently than by either Ad-OC-TK plus ACV (P < 0.05) or MTX treatment (P < 0.005) alone. At day 45 in the ROS group, 100% of the animals survived when treated with chemogene therapy, whereas 8 0% survived with gene therapy and no animals survived in the MTX-treat ed or untreated controls. In summary, we developed a novel therapeutic strategy for the treatment of osteosarcoma employing both chemotherap y and gene therapy. Chemogene therapy could potentially achieve better antitumor effects with reduced toxicity than the conventional chemoth erapy or gene therapy protocols alone.