Gene therapy of experimental malignant mesothelioma using adenovirus vectors encoding the HSVtk gene

Replication-defective adenovirus vectors were generated in which the gene of interest (lacZ, luciferase or HSV-tk) is driven by the adenovirus major late promoter (MLP) or the human cytomegalovirus immediate-early gene promoter/enhancer (CMV). In vitro experiments with rat (II-45) and human (MERO 25) mesothelioma cell lines revealed that the CMV promoter was stronger than the MLP promoter regarding levels of expression of the luciferase reporter gene and ganciclovir (GCV) killing efficiency after tk gene transfer. Following administration of IG.Ad.CMV.lacZ recombinant adenovirus (Introgene, IG) into the pleural cavity of Fischer rats with established mesothelioma, a widespread distribution of infectious virus particles through the thorax contents was demonstrated. However, a relatively small proportion of tumor cells were transduced. Nevertheless, a strong tumor growth inhibition was observed following treatment with IG.Ad.CMV.TK recombinant adenovirus and GCV. Separate groups of rats inoculated on day 0 with 10(5) II-45 cells in the pleural cavity, received 7 x 10(9) infectious particles of IG.Ad. CMV.TK on day 1, day 2, day 4 or day 8. One day after virus administration, 25 mg/kg GCV or PBS (controls) was injected i.p. (intraperitoneally) twice daily. On day 15, all animals were killed. Significant tumor regression, equivalent to 5 log cell kill, occurred in the treated rats suggesting an impressive bystander effect. In a survival study, animals were treated 9 days after inoculation of 10(5) tumor cells with IG.Ad.CMV.TK and a 14 days course of GCV. This treatment prolonged symptom-free survival time from 19 days in the controls to 33 days in the treated group. These responses can be best explained by assuming continued tk expression in or around the tumor tissue during GCV treatment. Our results confirm and extend earlier findings with the same model and demonstrate the potential of the herpes simplex virus thymidine kinase suicide gene therapy as a local treatment for malignant mesothelioma.