Adenovirus-mediated gene therapy is a potential complement to standard cancer treatments. Advantages are that vectors can be used to target tumors and that replicating viruses lead to increased therapeutic dosage. In this thesis, an oncolytic serotype 5 adenovirus (Ad5), Ad[i/PPT-E1A, E3], was developed where viral replication is controlled by the insulator-shielded (i) prostate-specific PPT promoter. The adenoviral E3 region was inserted for its immune regulatory and lysis functions. Ad[i/PPT-E1A, E3] had improved cytotoxic abilities both in vitro and in a prostate cancer xenograft mouse model compared to a virus lacking the E3 region. To further improve adenoviral vectors, the histone deacetylase inhibitor (HDACi) FK228 was studied. FK228 has been proposed to enhance the effect of adenoviral therapy by upregulation of CAR, the primary receptor for Ad5 infection. In the present study, we observed that FK228 promotes transgene expression even better when administered after viral transduction, indicating a post-transductional enhancement of transgene expression. Another interesting finding was that FK228 reduced transgene expression from the PPT promoter in the prostate cancer cell line LNCaP. This is explained by the fact that different HDACi have the ability to provoke a neuroendocrine phenotype of LNCaP. A potential drawback with adenoviral gene therapy is the rapid clearance of the virus from the circulation. Viral particles have been coated with polyethylene glycol (PEG) to evade immune recognition, a strategy that works well in mouse models. However, less is known about the effects of adenoviral PEGylation in human blood. We have studied cell interactions and immune responses to PEGylated and uncoated Ad5 vectors in human whole blood using a blood loop model with constant blood flow. Limited effects of PEGylation were observed in human blood, which were associated with the neutralizing ability of the donor blood. An important finding that donors with high neutralizing ability in whole blood do not necessarily have neutralizing antibodies against the virus strongly implies that neutralization should be measured in whole blood.