Genetically modified cancer models: the value of luciferase-tagged tumors and in vivo luciferase-based reporter assays in oncology drug discovery

Recent technological advances in visible light imaging allowed us to use this technique to non-invasively detect and quantify tumor cells, and to follow responses to drug candidates at molecular level in mouse models of human cancers. Human prostate tumor cells tagged with a firefly luciferase (PC-3M2AC6) were used in an orthotopic model, and in a model of experimental bone metastasis in athymic nude mice. Upon intravenous injection of D-luciferin these cells emitted light, that could be non-invasively registered with a charged-coupled device camera. Light emission was proportional to the tumor burden. The assay had sufficient throughput to allow for screening of advanced drug candidates for efficacy. Several compounds demonstrated significant and reproducible activity against tumors growing in prostates and in bones. These findings would not be possible without the non-invasive method of quantitation of bone tumor burden. A reporter gene, where luciferase is expressed from the p21waf-1 promoter, was introduced into the H1299 human lung cancer cell line. So constructed reporter cells were encapsulated in hollow fibers and the fibers were implanted subcutaneously into athymic, nude mice. The walls of hollow fibers are permeable to molecules up to 0.5 MDa in size, but not to cells. The animals were then treated intravenously with various histone deacetylase inhibitors twice (24 and 32 hours post implantation); the fibers were retrieved from the animals 18 hours after the last treatment, and were processes for light emission ex vivo. Histone deacetylase inhibitors induced expression of luciferase and thus light emission in the presence of D-luciferin. Results of this pharmacodynamic 40 hour assay were predictive of anti-tumor activity of histone deacetylase inhibitors in mouse xenograft tumor models. Therefore 5 week efficacy studies could be replaced with 2 day pharmacodynamic assays. Preliminary results for the in vivo Caspase 3 induction assay will be presented.