Experimental Study On Cytosine Deaminase-thymidine Kinase Double Suicide Fusion Gene Therapy For Bladder Carcinoma

Suicide gene therapy is one of the most promising strategies in cancer gene therapy, which designed to improve the selectivity of cancer treatment. The suicide gene for a viral or bacterial enzyme is introduced into tumor cells by some gene engineering methods, enabling them to activate a normally innocuous prodrug to a cytotoxic form that causes tumor cell death.The most studied of these strategies involves transfer of the herpes simplex virus thymidine kinase (HSV-TK) gene or the bacterial cytosine deaminase (CD) gene into tumor cells, which then selectively sensitize them to ganciclovir (GCV) or 5-fluorocytosine (5-FC), respectively. Tumor cells expressing the viral HSV-TK are able to initially phosphorylate GCV, leading to an accumulation of its cytotoxic metabolite, GCV triphosphate (GCV-TP), and subsequent incorporation into DNA and cause tumor cell death. Tumor cells which express CD can selectively convert 5-FC to the anticancer drug 5-fluorouracil (5-FU), which is metabolized by cellular enzymes to 5-FdUMP, resulting in cell death due to inhibition of thymidylate synthase, and DNA double-strand breaks. Both of these therapies are made more effective by a phenomenon termed the "bystander effect," which is the ability of suicide enzyme-expressing cells to sensitize neighboring nonexpressing bystander cells to the prodrug.Since Moolten and colleagues initially demonstrated that tumor cells genetically modified with HSV-TK gene were killed by GCV both in vitro and in vivo in 1986, suicide genes therapy such as HSV-TK/GCV and CD/5-FC system represent a promising tool for treatment of the human cancer. HSV-TK/GCV and CD/5-FC suicide gene system had a significant therapeutic effect in vitro and in vivo for many malignant tumors and have been employed in clinical trial in some tumors. Although many studies have demonstrated that HSV-TK/GCV and CD/5-FC suicide gene system had a strong killing effect on tumor cells in vitro and inhibited tumor growth in vivo, the therapeutic effect in preliminary clinical trial had not so much. It was...