A Novel Three-Pronged Approach to Kill Cancer Cells Selectively: Concomitant Viral, Double Suicide Gene, and Radiotherapy

Abstract

Two obstacles limiting the efficacy of nearly all cancer gene therapy trials are low gene transduction efficiencies and the lack of tumor specificity. Recently, a replication-competent, E1B-attenuated adenovirus (ONYX-015) was developed that could overcome these limitations, because it was capable of efficiently and selectively destroying tumor cells lacking functional p53. In an attempt to improve both the efficacy and safety of this approach, we constructed a similar adenovirus (FGR) containing a cytosine deaminase (CD)/herpes simplex virus type-1 thymidine kinase (HSV-1 TK) fusion gene, thereby allowing for the utilization of double-suicide gene therapy, which has previously been demonstrated to produce significant antitumor effects and potentiate the therapeutic effects of radiation. The FGR virus exhibited the same tumor cell specificity and replication kinetics as the ONYX-015 virus in vitro. Importantly, both the CD/5-FC and HSV-1 TK/GCV suicide gene systems markedly enhanced the tumor cell-specific cytopathic effect of the virus, and, as expected, sensitized tumor cells to radiation. By contrast, neither the FGR virus nor either suicide gene system showed significant toxicity to normal human cells. Both suicide gene systems could be used to suppress viral replication effectively, thereby providing a means to control viral spread. The results support the thesis that the three-pronged approach of viral therapy, suicide gene therapy, and radiotherapy may represent a powerful and safe means of selectively destroying tumor cells in vivo.

Comparison of HSV-1 thymidine kinase-dependent and -independent inhibition of replication-competent adenoviral vectors by a panel of drugs

“…The intrinsic oncolytic effects of E1B-55K-deleted adenoviruses could be significantly enhanced in several solid xenograft tumor models when prodrug (GCV alone or in combination with 5-fluorocytosine (5-FC)) was withheld until maximum vector replication and gene expression occurred. [68][69][70] However, GCV administration was not beneficial to the intrinsic oncolytic activity of an adenovirus (Ad.OW34) with a much more robust replication than the E1B-55K-deleted vector Ad.TK RC , expressing the entire adenovirus E1 region as well as HSV-tk under conditions that favor active viral replication and spread, 14 probably because the potential HSV-tk/GCV-mediated enhancement of the intrinsic viral oncolytic activity was balanced out by the virostatic effects of GCV.…”

Adenovirus-mediated lymphotactin gene transfer improves therapeutic efficacy of cytosine deaminase suicide gene therapy in established murine colon carcinoma

“…Some of them, including fusion of CD and HSVtk genes, combination with radiation, or utilization of tumor-or tissue-specific promoter, are effective in increasing the killing of tumor cells and reducing its toxicity. [25][26][27][28] Among these approaches, co-transfection of suicide gene and cytokine gene has been found to elicit potent antitumor effects and induce potent antitumor immunity efficiently. Various cytokine genes, including GM-CSF, IL-2, IL-4, IFN-␣ and IFN-␥ have been transferred into tumor cells in combination with HSVtk or CD suicide gene therapy, and more potent antitumor effects and more efficient induction of antitumor immune response have been observed with these combination therapies.…”

Clinical development directions in oncolytic viral therapy

“…Adenoviruses are the most widely studied engineered oncolytic viruses clinically. Adenoviral constructs include Onyx 015, 1,2 CG7060, 3 CG7870, 4 dl922-947, 5 Ad5-CD/tk-rep, 6 Ad-delta24, 7 Ad DF3-E1, 8 Onyx 411, 9 OAV001, 10 KD3, 11 01/PEME 12 and Telomelysin. 13,14 Historically, evidence of viral oncolytic activity was published in case reports as early as 1912.…”