Gastric cancer is the fourth most common malignancy and the second most frequent cause of cancer-related death world-wide [1, 14]. Recurrence or distant metastasis is one of the most common complications and often the cause of death . While chemotherapy is a useful adjuvant therapy compared to surgical therapy alone, its therapeutic potential is limited . Most gastric cancers are resistant to currently available chemotherapy regimens. Therefore, novel therapeutic agents are needed to improve outcomes for gastric cancer patients who are not responsive to conventional therapies. Oncolytic viral therapy is a promising approach to cancer treatment that depends on the ability of viruses to infect, replicate within, and lyse a host cell [17, 18]. In this study, we described the cytotoxic effects of GLV-1 h153, a novel recombinant VACV carrying the hNIS gene, on gastric cancer cells in vitro. We further demonstrated that GLV-1 h153-infected gastric cancer xenografts expressed functioning hNIS protein that allowed for non-invasive imaging of the tumor and also efficient tumor regression in vivo.
A variety of viruses have shown oncolytic properties including adenovirus, herpes simplex virus, Newcastle disease virus, vesicular stomatitis virus, and reovirus . Among a variety of oncolytic viral agents, vaccinia virus has several advantages. VACV exclusively replicates in the cytoplasm without using the host’s DNA-synthesis machinery, thereby lowering the risk of integration of the viral genome into the host genome . A large amount of foreign DNA (up to 25 kb) can be incorporated without significantly reducing the viral replication efficiency . Moreover, vaccinia has been proven to have a good safety profile as it has been historically given to millions during the smallpox vaccination. It also demonstrates efficient replication and a broad range of host cell tropisms . Several preclinical studies have shown that systemic injection of recombinant VACV into xenografts resulted in high viral titers in tumors only, indicating tumor-specific colonization [11, 20, 21]. There is a small concern that patients who have received smallpox vaccination in the past have neutralizing antibody against the virus. This could potentially result in compromised treatment efficacy. However, in the blood, complement plays a more important role in inactivating VACV than neutralizing antibodies. We therefore predict that the presence of neutralizing antibodies in patients should not hinder VACV treatment; however, a higher treatment dose might be required.
Genetically engineered VACVs have shown efficacy in the treatment of a wide range of human cancers . GLV-1 h168 has already shown to be an effective diagnostic and therapeutic vector in several human tumor models, including breast tumor, mesothelioma, pancreatic cancers, and squamous cell carcinoma  The hNIS protein, which is an intrinsic membrane glycoprotein with 13 putative transmembrane domains, actively transports both Na+ and I- ions across the cell membrane . Functioning hNIS protein can uptake several commercially available radio-nucleotides, including 123I, 124I, 125I, 131I, 99mTc and 188Re [22, 23]. In this study, GLV-1 h153-mediated expression of hNIS protein in infected MKN-74 xenografts resulted in a localized 99mTc and 124I radiotracer uptake. Our results suggest that hNIS gene expression via viral vector can be used as a non-invasive imaging modality to monitor tumor progression and treatment effects.
A single intratumoral injection of GLV-1 h153 in MKN-74 xenografts exhibited localized intratumoral GFP and hNIS expression. Moreover, there was no evidence of viral spread to any other organs based on GFP imaging, 99mTc scintigraphy, and 124I PET, indicating tumor-specific viral infection and activity. We also demonstrated that GLV-1 h153 is effective and safe in treating gastric tumors in a murine xenograft model. The GLV-1 h153-treated group was continuously followed until day 35 and there was no tumor regrowth (data not shown between day 28 and 35). The control group had to be sacrificed in accordance to our approved animal protocol on day 28. Expressing the hNIS gene in an otherwise non-hNIS-expressing tissue is exciting. It could potentially make use of the well-established radioiodine imaging and therapy in other non-thyroid originated cancers. Several studies have shown promising results in a variety of tumors using radioiodine treatment via tumor-specific expression of the hNIS gene, including medullary thyroid carcinoma , prostate cancer , colon cancer , and breast cancer . Tumor-specific hNIS expression using GLV-1 h153 can maximize localized radioiodine accumulation and minimize non-specific uptake in other organs. Based on our promising results, it would be of significant clinical importance to evaluate the effect of combination therapy of GLV-1 h153 and radioiodine.