Combinatory gene therapy with electrotransfer of midkine promoter-HSV-TK and interleukin-21

We examined the effect of the combination of herpes simplex virus type 1-thymidine kinase (HSV-TK)-suicide gene therapy and interleukin-21 (IL-21) immune gene therapy. Materials and Methods: To improve tumor specificity and the safety of gene therapy, we designed the suicide gene (HSV-TK) to be driven by midkine (MK) minimal promoter (MKp-TK). Plasmid DNA containing HSV-TK-suicide gene or IL-21 gene was injected into TE2 and Colon26 tumors developed in nude mice and electric pulses were then delivered. Results: Tumors transduced with the MKp-TK gene demonstrated increased sensitivity to ganciclovir (GCV) in vitro and in vivo. MK minimal promoter conferred efficient transcriptional activity to the HSV-TK-suicide gene and in vivo electroporation was an effective method for transducing the MKp-TK gene. IL-21-transduced tumors disappeared completely in syngeneic BALB/c mice. However, the tumors were not suppressed completely in T-cell-depleted nude mice, and antitumor effects were absent in NK-cell-depleted mice. These data suggest that IL-21 induces T- and NK-cell-dependent antitumor effects. Furthermore, the growth of IL-21-producing rumors subsequently transduced with MKp-TK by electroporation was significantly retarded compared with control groups. Conclusion: Using the minimal promoter region of MK to drive the HSV-TK gene and in vivo electroporation to transduce IL-21 DNA into the tumors produced an efficient gene therapy with improved safety. To our knowledge, this is the first report of a combination gene therapy using HSV-TK/GCV and IL-21.