Combination therapy and noninvasive imaging with a dual therapeutic vector expressing MDR1 short hairpin RNA and a sodium iodide symporter

We investigated the feasibility of using combination gene therapy and noninvasive nuclear imaging after expression of the human sodium iodide symporter (hNIS) and inhibition of the multidrug resistance (MDR1) gene in colon cancer cells. Methods: HCT-15 cells were stably transfected with a dual expression vector, in which the hNIS gene, driven by a constitutive cytomegalovirus promoter, has been coupled to an MDR1 short hairpin RNA (shRNA) cassette. Cell lines stably expressing the hNIS gene and MDR1 shRNA (designated MN-61 and MN-62) were produced, and the expression of the NIS gene and MDR1 shRNA was examined by Western blotting, reverse transcription-polymerase chain reaction, and immunostaining. The functional activities of MDR1 shRNA were determined by paclitaxel uptake and sensitivity to doxorubicin. Functional NIS expression was confirmed by the uptake and efflux of ¹²⁵I and the cytotoxicity of ¹³¹I. The effect of the combination of ¹³¹I and doxorubicin was determined by an in vitro clonogenic assay. In vivo NIS expression was examined by small-animal PET with ¹²⁴I. Results: The shMDRNIS-expressing cells showed a significant decrease in the expression of MDR1 messenger RNA and its translated product, P-glycoprotein. The inhibition of P-glycoprotein expression by shRNA enhanced the intracellular accumulation of paclitaxel, the cellular retention of which is mediated by P-glycoprotein, thereby increasing sensitivity to the anticancer drug. The shMDR-NIS-expressing cells showed a significant increase of ¹²⁵I uptake, which was completely inhibited by KClO₄. Although the iodide efflux rate was rapid, the cell survival rate was markedly reduced by ¹³¹I treatment. Interestingly, the combination of doxorubicin and a radioiodide (¹³¹I) displayed synergistic cytotoxicity that correlated with MDR1 inhibition and NIS expression in shMDR-NIS-expressing cells. Furthermore, in mice with shMDR-NIS-expressing tumor xenografts, small-animal PET with ¹²⁴I clearly visualized shMDR1-NIS-expressing tumors. Conclusion: We developed a dual expression vector with the NIS gene and MDR1 shRNA. This study represents a promising first step in investigations of the potential use of a combination of the NIS gene and MDR1 shRNA as a new therapeutic strategy allowing RNA interference-based gene therapy, NIS-based radio-iodine therapy, and in vivo monitoring based on NIS imaging.