Direct Intratumoral Gene Transfer of the Herpes Simplex Virus Thymidine Kinase Gene with DNA‐liposome Complexes: Growth Inhibition of Tumors and Lack of Localization in Normal Tissues

To constitute the site‐specific expression of the herpes simplex virus thymidine‐kinase (HSV‐TK) gene in tumor cells, we have assessed the promoter function of the simian virus 40 (SV40) promoter and the 5’flanking region of c‐erbB‐2 gene using a luciferase‐expressing reporter plasmid. After the transfec‐tion of the luciferase plasmid directed by the promoter region of c‐erbB‐2 gene, a large amount of luciferase activity was observed in c‐erbB‐2‐expressing cells (Colo201, MCF‐7, and HEC1‐A), while none was detected in cells with no expression of c‐erbB‐2 protein (HRA and KF cells). On the other hand, a high level of luciferase activity was detected in all tumor cell lines tested, when the transfection was performed with SV40 promoter. The repeated transfection of the liposome‐conjugated HSV‐TK gene regulated by the SV40 promoter or by the promoter region of c‐erbB‐2 gene with cultivation in 100 μg/ml of aciclovir for 5 days in vitro resulted in growth inhibition for all four cell lines examined or for only c‐erbB‐2‐expressing cells in the presence of SV40 promoter or c‐erbB‐2 promoter, respectively. Finally, direct injection of the DNA‐liposome complex into established tumors in the presence of 50 mg/kg of aciclovir led to significant tumor volume reduction in all three tumors tested when SV40 promoter was employed. However, this anti‐tumor effect was noted only in c‐erbB‐2‐positive cells (Colo201 cells) upon intratumoral injection of HSV‐TK gene regulated by c‐erbB‐2 promoter. In the case of intratumoral gene transfer, foreign DNA was detected in only one of seven mice by polymerase chain reaction (PCR) analysis performed 7 days following injection. When PCR analysis was carried out at 14 or 21 days following injection, no DNA signal was found at all. However, DNA was detected in several normal tissues at all three times tested in the case of intravenous injection. No abnormalities were seen in histologic examinations of normal tissues or in serum biochemical parameters following DNA liposome delivery. These results suggest that the direct gene transfer of HSV‐TK gene regulated by tumor‐specific transcriptional units may be one of the most clinically promising of the selective genetic strategies against cancer.