The Enhanced Tumor Specificity of TG6002, an Armed Oncolytic Vaccinia Virus Deleted in Two Genes Involved in Nucleotide Metabolism

Abstract

Oncolytic vaccinia viruses are currently in clinical development. However, the safety and the tumor selectivity of these oncolytic viruses must be improved. We previously constructed a first-generation oncolytic vaccinia virus by expressing the suicide gene FCU1 inserted in the J2R locus that encodes thymidine kinase. We demonstrated that the combination of this thymidine-kinase-deleted vaccinia virus and the FCU1/5-fluocytosine system is a potent vector for cancer therapy. Here, we developed a second generation of vaccinia virus, named TG6002, expressing FCU1 and with targeted deletions of the J2R gene and the I4L gene, which encodes the large subunit of the ribonucleotide reductase. Compared to the previously used single thymidine-kinase-deleted vaccinia virus, TG6002 is highly attenuated in normal cells, yet it displays tumor-selective replication and tumor cell killing. TG6002 replication is highly dependent on cellular ribonucleotide reductase levels and is less pathogenic than the single-deleted vaccinia virus. Tumor-selective viral replication, prolonged therapeutic levels of 5-fluorouracil in tumors, and significant antitumor effects were observed in multiple human xenograft tumor models after systemic injection of TG6002 and 5-fluorocytosine. TG6002 displays a convincing safety profile and is a promising candidate for treatment of cancer in humans.

Engineering and Characterization of Oncolytic Vaccinia Virus Expressing Truncated Herpes Simplex Virus Thymidine Kinase

“…It is known that other mechanisms, such as extracellular signal-regulated kinase (ERK) [32] and ribonucleotide reductase (RR) [33] expression, can also influence viral replication in cancer cells. Consequently, different double-gene deletion strategies have been applied to further improve the tumor selectivity of OVV [8,22].…”

“…This distinction cumulatively explains why HSV is sensitive to GCV, a purine analogue, while VACV is not [21]. However, the insertion of the wild-type HSV-tk gene into the VV-tk region of an OVV may raise concern for impaired tumor selectivity, while considering that the HSV-tk can also catalyze thymidine (a pyrimidine deoxynucleoside), since VV-tk gene deletion results in tumor selectivity [11,12,22]. Therefore, in this study, an artificial selection procedure while using bromodeoxyuridine (BrdU) was adopted to exclude any HSV-tk incorporated recombinant viruses expressing thymidine kinase activity.…”

Development of oncolytic virotherapy: from genetic modification to combination therapy

“…However, some tumor cells are resistant to 5-FU, reducing its therapeutic efficiency [147]. TG6002 is an oncolytic VACV expressing the suicide gene FCU1 [148]. FCU1 is a bifunctional gene comprising FCY1 and FUR1.…”

Safety studies and viral shedding of intramuscular administration of oncolytic vaccinia virus TG6002 in healthy Beagle dogs

“…It is derived from a VACV strain engineered to express GM-CSF and has successfully entered Phase III clinical trials [10]. An oncolytic VACV, designated as TG6002, has been developed with deletion of the thymidine kinase (TK) and the ribonucleotide reductase (RR) loci in its genome resulting in attenuated virulence and enhanced tumorspeci c targeting [11]. To enhance therapeutic e cacy, the chimeric gene FCU1 was inserted in the TG6002 genome.…”

“…FCU1 converts the non-toxic prodrug 5-uorocytosine (5-FC) into the chemotherapeutic compound 5-uorouracil (5-FU), and further into 5-uorouracil-monophosphate, which inhibits DNA and protein synthesis [12]. In murine xenograft mice treated by TG6002 followed by per os 5-FC administration, high levels of 5-FU were detected in tumors [11]. In this model, TG6002 in combination with 5-FC has signi cant antitumor e cacy against a large range of human tumors [11].…”