The ability of OAds to replicate selectively within the tumor tissue has two main advantages (i) the lysis of cells will be tumor specific; (ii) the proteins encoded by OAds will accumulate within the tumor. The last mechanism has been exploited by several groups to foster the expression of suicide genes or immunomodulatory molecules into the tumor tissue.
Suicide Gene Therapy by Adenoviruses
Thymidine kinase (TK) is an enzyme of the pyrimidine salvage pathway, which phosphorylates thymidine to produce the dTMP. The TK enzyme of herpes simplex virus (HSV) is less specific, hence recognizes a very broad range of substrates including guanosine analogs such as ganciclovir or acyclovir. Once phosphorylated by the HSV-TK enzyme, these analogs can interfere with the DNA synthesis pathway leading to cell cycle arrest and cell death (Kokoris and Black 2002). The HSV-TK/ ganciclovir system represents one of the most common suicide gene/prodrug combinations, hence it has been used in combination with OAds. Freytag et al. developed an OAd expressing HSV-TK and cytosine deaminase (CD), two suicide genes. In addition, the vector featured the expression of interleukin 12 (IL-12) to modulate the immune system. In their preclinical studies the trivalent vector (HSV-TK, CD, and IL-12) was able to increase the survival of mice, proving that the combination of the oncolytic activity, suicide gene therapy, and production of IL-2 was effective (Freytag et al. 2013). A similar approach was used by Kostova et al., who engineered a glioblastoma-selective OAd (thus dependent on YB-1 protein) to express a more active form of the HSV-TK protein: sr39TK. Treatment with the oncolytic vector in combination with ganciclovir resulted in a more efficient killing of tumor cells compared to the controls (Kostova et al. 2015).
The CD from Escherichia coli is another well-known suicide gene. Similarly to HSV-TK mechanism, CD converts prodrugs like 5-fluorocytosine into 5-fluorouracil. The latter, induces thymidylate synthase inhibition and formation of RNA-DNA complexes leading to apoptosis. This suicide gene has also been successfully cloned into oncolytic vectors for cancer treatment. Zhang et al. (2010) constructed an OAd expressing CD gene and observed an increased cytotoxicity in vitro and anti-tumor activity in vivo. A similar approach has been developed by Tang et al., who armed a E1B-55 kDa deficient adenovirus with the multisubstrate deoxyribonucleoside kinase of Drosophila melanogaster (Dm-dNK). This enzyme is able to increase to susceptibility of cells to different cytotoxic nucleoside analogs. In vitro and in vivo studies showed that the combination of this vector with nucleoside analogs achieved an improved anticancer activity compared to controls (Tang et al. 2015).