Development of an immunotherapeutic adenovirus targeting hormone-independent prostate cancer

BACKGROUND To develop a targeting therapy for hormone-independent prostate cancer, we constructed and characterized conditionally replicating oncolytic adenovirus (Ad) equipped with mRFP (monomeric red fluorescence protein)/ttk (modified herpes simplex virus thymidine kinase). This construct was then further modified to express both mRFP/ttk and a soluble form of cytokine FLT3L (fms-related tyrosine kinase 3 ligand) simultaneously. METHODS To construct the recombinant oncolytic adenovirus, E1a and E4 genes, which are necessary for adenovirus replication, were controlled by the prostate-specific enhancer sequence (PSES) targeting prostate cancer cells expressing prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA). Simultaneously, it expressed the mRFP/ttk fusion protein in order to be able to elicit the cytotoxic effect. RESULTS The Ad5/35PSES.mRFP/ttk chimeric recombinant adenovirus was generated successfully. When replication of Ad5/35PSES.mRFP/ttk was evaluated in prostate cancer cell lines under fluorescence microscopy, red fluorescence intensity increased more in LNCaP cells, suggesting that the mRFP/ttk fusion protein was folded functionally. In addition, the replication assay including wild-type adenovirus as a positive control showed that PSES-positive cells (LNCaP and CWR22rv) permitted virus replication but not PSES-negative cells (DU145 and PC3). Next, we evaluated the killing activity of this recombinant adenovirus. The Ad5/35PSES. mRFP/ttk killed LNCaP and CWR22rv more effectively. Unlike PSES-positive cells, DU145 and PC3 were resistant to killing by this recombinant adenovirus. Finally, in order to potentiate therapeutic efficacy, we developed a recombinant adenovirus expressing multiexogenous genes, mRFP/ttk and sFLT3L. CONCLUSION In the present study, a replication-competent adenovirus was successfully designed to replicate conditionally in PSA-positive and PSMA-positive prostate cancer cells. This recombinant adenovirus is equipped with the fusion protein of suicidal and red-fluorescence fusion protein together with sFLT3L. This construct would be expected to have potent antitumor effects and deserves more extensive investigation.