VP22 does not significantly enhance enzyme prodrug cancer gene therapy as a part of a VP22-HSVTk-GFP triple fusion construct

Background VP22 is a herpes simplex virus type 1 (HSV‐1) tegument protein that has been suggested to spread from cell to cell, alone or as a part of fusion proteins. Creating controversy, some reports indicate that VP22 cannot facilitate significant intercellular spreading. To study the capacity of VP22 to cause spreading and enhance thymidine kinase/ganciclovir cancer gene therapy, we constructed a novel triple fusion protein containing VP22, HSV thymidine kinase and green fluorescent protein (VP22‐Tk‐GFP). This fusion protein has three functional domains in the same polypeptide, thus making it possible to reliably compare the causality between transduction rate and cell killing efficiency in vitro and in vivo. Methods VP22‐Tk‐GFP was cloned into lenti‐ and adenoviral vectors and used for expression studies, analyses for VP22‐mediated protein spreading, and to study the effect of VP22 to thymidine kinase/ganciclovir‐mediated cytotoxicity. The function of VP22‐Tk‐GFP was also investigated in vivo. Results The triple fusion protein was expressed correctly in vitro, but intercellular trafficking was not observed in any of the studied cell lines. However, under certain conditions, VP22‐Tk‐GFP sensitized cells more efficiently to ganciclovir than Tk‐GFP. In vivo there was a trend for increased inhibition of tumor growth with VP22‐Tk‐GFP when ganciclovir was present, but the difference with Tk‐GFP was not statistically significant. Conclusions Based on our results, VP22 fusion proteins do not seem to traffic intercellularly at detectable levels in most tumor cell types. Even though VP22 enhanced cytotoxicity in one cell line in vitro, the effect in vivo was modest. Therefore, our results do not support the utility of VP22 as an enhancer of enzyme prodrug cancer gene therapy. Copyright © 2005 John Wiley & Sons, Ltd.