Functional interaction between fluorodeoxyuridine-induced cellular alterations and replication of a ribonucleotide reductase-negative herpes simplex virus

Modulation of Reoviral Cytolysis (I): Combination Therapeutics

Patients with stage IV gastric cancer suffer from dismal outcomes, a challenge especially in many Asian populations and for which new therapeutic options are needed. To explore this issue, we used oncolytic reovirus in combination with currently used chemotherapeutic drugs (irinotecan, paclitaxel, and docetaxel) for the treatment of gastric and other gastrointestinal cancer cells in vitro and in a mouse model. Cell viability in vitro was quantified by WST-1 assays in human cancer cell lines treated with reovirus and/or chemotherapeutic agents. The expression of reovirus protein and caspase activity was determined by flow cytometry. For in vivo studies, athymic mice received intratumoral injections of reovirus in combination with irinotecan or paclitaxel, after which tumor size was monitored. In contrast to expectations, we found that reoviral oncolysis was only poorly correlated with Ras pathway activation. Even so, the combination of reovirus with chemotherapeutic agents showed synergistic cytopathic effects in vitro, plus enhanced reovirus replication and apoptosis. In vivo experiments showed that reovirus alone can reduce tumor size and that the combination of reovirus with chemotherapeutic agents enhances this effect. Thus, we find that oncolytic reovirus therapy is effective against gastric cancer. Moreover, the combination of reovirus and chemotherapeutic agents synergistically enhanced cytotoxicity in human gastric cancer cell lines in vitro and in vivo. Our data support the use of reovirus in combination with chemotherapy in further clinical trials, and highlight the need for better biomarkers for reoviral oncolytic responsiveness.

Current clinical landscape of oncolytic viruses as novel cancer immunotherapeutic and recent preclinical advancements

Oncolytic viruses (OVs) have been gaining attention in the pharmaceutical industry as a novel immunotherapeutic and therapeutic adjuvant due to their ability to induce and boost antitumor immunity through multiple mechanisms. First, intrinsic mechanisms of OVs that enable exploitation of the host immune system (e.g., evading immune detection) can nullify the immune escape mechanism of tumors. Second, many types of OVs have been shown to cause direct lysis of tumor cells, resulting in an induction of tumor-specific T cell response mediated by release of tumor-associated antigens and danger signal molecules. Third, armed OV-expressing immune stimulatory therapeutic genes could be highly expressed in tumor tissues to further improve antitumor immunity. Last, these OVs can inflame cold tumors and their microenvironment to be more immunologically favorable for other immunotherapeutics. Due to these unique characteristics, OVs have been tested as an adjuvant of choice in a variety of therapeutics. In light of these promising attributes of OVs in the immune-oncology field, the present review will examine OVs in clinical development and discuss various strategies that are being explored in preclinical stages for the next generation of OVs that are optimized for immunotherapy applications.

Triple-mutated oncolytic herpes virus for treating both fast- and slow-growing tumors

Oncolytic virus therapy has emerged as a promising treatment option against cancer. To date, oncolytic viruses have been developed for malignant tumors, but the need for this new therapeutic modality also exists for benign and slow‐growing tumors. G47∆ is an oncolytic herpes simplex virus type 1 (HSV‐1) with an enhanced replication capability highly selective to tumor cells due to genetically engineered, triple mutations in the γ34.5, ICP6 and α47 genes. To create a powerful, but safe oncolytic HSV‐1 that replicates efficiently in tumors regardless of growth speed, we used a bacterial artificial chromosome system that allows a desired promoter to regulate the expression of the ICP6 gene in the G47∆ backbone. Restoration of the ICP6 function in a tumor‐specific manner using the hTERT promoter led to a highly capable oncolytic HSV‐1. T‐hTERT was more efficacious in the slow‐growing OS‐RC‐2 and DU145 tumors than the control viruses, while retaining a high efficacy in the fast‐growing U87MG tumors. The safety features are also retained, as T‐hTERT proved safe when inoculated into the brain of HSV‐1 sensitive A/J mice. This new technology should facilitate the use of oncolytic HSV‐1 for all tumors irrespective of growth speed.

Chemovirotherapy of Pancreatic Adenocarcinoma by Combining Oncolytic Vaccinia Virus GLV-1h68 with nab-Paclitaxel Plus Gemcitabine

Oncolytic viruses have proven their therapeutic potential against a variety of different tumor entities both in vitro and in vivo. Their ability to selectively infect and lyse tumor cells, while sparing healthy tissues, makes them favorable agents for tumor-specific treatment approaches. Particularly, the addition of virotherapeutics to already established chemotherapy protocols (so-called chemovirotherapy) is of major interest. Here we investigated the in vitro cytotoxic effect of the oncolytic vaccinia virus GLV-1h68 combined with dual chemotherapy with nab-paclitaxel plus gemcitabine in four human pancreatic adenocarcinoma cell lines (AsPc-1, BxPc-3, MIA-PaCa-2, and Panc-1). This chemovirotherapeutic protocol resulted in enhanced tumor cell killing in two tumor cell lines compared to the respective monotherapies. We were thereby able to show that the combination of oncolytic vaccinia virus GLV-1h68 with nab-paclitaxel and gemcitabine has great potential in the chemovirotherapeutic treatment of advanced pancreatic adenocarcinoma. However, the key to a successful combinatorial chemovirotherapeutic treatment seems to be a profound viral replication, as tumor cell lines that were non-responsive to the combination therapy exhibited a reduced viral replication in the presence of the chemotherapeutics. This finding is of special significance when aiming to achieve a virus-mediated induction of a profound and long-lasting antitumor immunity.

Designing Herpes Viruses as Oncolytics

Oncolytic herpes simplex virus (oHSV) was one of the first genetically-engineered oncolytic viruses. Because herpes simplex virus (HSV) is a natural human pathogen that can cause serious disease, it is incumbent that it be genetically-engineered or significantly attenuated for safety. Here we present a detailed explanation of the functions of HSV-1 genes frequently mutated to endow oncolytic activity. These genes are non-essential for growth in tissue culture cells but are important for growth in post-mitotic cells, interfering with intrinsic antiviral and innate immune responses or causing pathology, functions dispensable for replication in cancer cells. Understanding the function of these genes leads to informed creation of new oHSVs with better therapeutic efficacy. Virus infection and replication can also be directed to cancer cells through tumor-selective receptor binding and transcriptional- or post-transcriptional miRNA-targeting, respectively. In addition to the direct effects of oHSV on infected cancer cells and tumors, oHSV can be 'armed' with transgenes that are: reporters, to track virus replication and spread; cytotoxic, to kill uninfected tumor cells; immune modulatory, to stimulate anti-tumor immunity; or tumor microenvironment altering, to enhance virus spread or to inhibit tumor growth. In addition to HSV-1, other alphaherpesviruses are also discussed for their oncolytic activity.

A novel oncolytic viral therapy and imaging technique for gastric cancer using a genetically engineered vaccinia virus carrying the human sodium iodide symporter

Background

Gastric cancers have poor overall survival despite recent advancements in early detection methods, endoscopic resection techniques, and chemotherapy treatments. Vaccinia viral therapy has had promising therapeutic potential for various cancers and has a great safety profile. We investigated the therapeutic efficacy of a novel genetically-engineered vaccinia virus carrying the human sodium iodide symporter (hNIS) gene, GLV-1 h153, on gastric cancers and its potential utility for imaging with ^99mTc pertechnetate scintigraphy and ¹²⁴I positron emission tomography (PET).

Methods

GLV-1 h153 was tested against five human gastric cancer cell lines using cytotoxicity and standard viral plaque assays. In vivo, subcutaneous flank tumors were generated in nude mice with human gastric cancer cells, MKN-74. Tumors were subsequently injected with either GLV-1 h153 or PBS and followed for tumor growth. ^99mTc pertechnetate scintigraphy and ¹²⁴I microPET imaging were performed.

Results

GFP expression, a surrogate for viral infectivity, confirmed viral infection by 24 hours. At a multiplicity of infection (MOI) of 1, GLV-1 h153 achieved > 90% cytotoxicity in MNK-74, OCUM-2MD3, and AGS over 9 days, and >70% cytotoxicity in MNK- 45 and TMK-1. In vivo, GLV-1 h153 was effective in treating xenografts (p < 0.001) after 2 weeks of treatment. GLV-1 h153-infected tumors were readily imaged by ^99mTc pertechnetate scintigraphy and ¹²⁴I microPET imaging 2 days after treatment.

Conclusions

GLV-1 h153 is an effective oncolytic virus expressing the hNIS protein that can efficiently regress gastric tumors and allow deep-tissue imaging. These data encourages its continued investigation in clinical settings.

Oncolytic herpes viruses, chemotherapeutics, and other cancer drugs

Oncolytic viruses are emerging as a potential new way of treating cancers. They are selectively replication-competent viruses that propagate only in actively dividing tumor cells but not in normal cells and, as a result, destroy the tumor cells by consequence of lytic infection. At least six different oncolytic herpes simplex viruses (oHSVs) have undergone clinical trials worldwide to date, and they have demonstrated an excellent safety profile and intimations of efficacy. The first pivotal Phase III trial with an oHSV, talimogene laherparepvec (T-Vec [OncoVex(GM-CSF)]), is almost complete, with extremely positive early results reported. Intuitively, therapeutically beneficial interactions between oHSV and chemotherapeutic and targeted therapeutic drugs would be limited as the virus requires actively dividing cells for maximum replication efficiency and most anticancer agents are cytotoxic or cytostatic. However, combinations of such agents display a range of responses, with antagonistic, additive, or, perhaps most surprisingly, synergistic enhancement of antitumor activity. When synergistic interactions in cancer cell killing are observed, chemotherapy dose reductions that achieve the same overall efficacy may be possible, resulting in a valuable reduction of adverse side effects. Therefore, the combination of an oHSV with "standard-of-care" drugs makes a logical and reasonable approach to improved therapy, and the addition of a targeted oncolytic therapy with "standard-of-care" drugs merits further investigation, both preclinically and in the clinic. Numerous publications report such studies of oncolytic HSV in combination with other drugs, and we review their findings here. Viral interactions with cellular hosts are complex and frequently involve intracellular signaling networks, thus creating diverse opportunities for synergistic or additive combinations with many anticancer drugs. We discuss potential mechanisms that may lead to synergistic interactions.

Advance in herpes simplex viruses for cancer therapy

Oncolytic virotherapy is an attractive approach that uses live viruses to selectively kill cancer cells. Oncolytic viruses can be genetically engineered to induce cell lyses through virus replication and cytotoxic protein expression. Herpes simplex virus (HSV) has become one of the most widely clinically used oncolytic agent. Various types of HSV have been studied in basic or clinical research. Combining oncolytic virotherapy with chemotherapy or radiotherapy generally produces synergic action with unclear molecular mechanisms. Arming HSV with therapeutic transgenes is a promising strategy and can be used to complement conventional therapies. As an efficient gene delivery system, HSV has been successfully used to deliver various immunomodulatory molecules. Arming HSV with therapeutic genes merits further investigation for potential clinical application.

An oncolytic herpes simplex virus vector, G47Δ, synergizes with paclitaxel in the treatment of breast cancer

Paclitaxel-containing treatment regimens are standard chemotherapy schemes for breast cancer patients. The use of oncolytic herpes simplex virus (oHSV) vectors has been shown to be a safe and effective therapeutic approach for different types of cancer. We hypothesized that paclitaxel in combination with an oHSV vector would present an enhanced killing effect when used against breast cancer cells. In the present study, we demonstrated that the combined use of the oHSV vector G47Δ and paclitaxel produced a synergistic effect against breast cancer cells both in vitro and in vivo. In vitro studies demonstrated that paclitaxel and G47Δ both caused dose-dependent cytotoxicity against the human breast cancer cell lines MCF-7 and MDA-MB-468. G47Δ and paclitaxel also demonstrated synergistic cytotoxicity when applied together, with Chou-Talalay combination indices ranging from 0.44 to 0.77 for MCF-7 cells and 0.68 to 0.83 for MDA-MB‑468 cells. Paclitaxel did not enhance viral replication or viral spread among tumor cells. However, G47Δ increased the antitumor ability of paclitaxel by inducing mitotic arrest and apoptosis. In vivo studies indicated that when combined with G47Δ, the dose of paclitaxel could be reduced at least 5-fold while maintaining levels of tumor reduction similar to those achieved with the administration of paclitaxel alone. Combination therapy resulted in no morbidity in vivo. Our data demonstrated that G47Δ and paclitaxel combination therapy had synergistic effects in the treatment of breast cancer. This combination therapy may be promising for breast cancer patients.

Oncolytic herpes simplex virus expressing yeast cytosine deaminase: relationship between viral replication, transgene expression, prodrug bioactivation

Yeast cytosine deaminase (yCD) is a well-characterized prodrug/enzyme system that converts 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU), and has been combined with oncolytic viruses. However, in vivo studies of the interactions between 5-FC bioactivation and viral replication have not been previously reported, nor have the kinetics of transgene expression and the pharmacokinetics of 5-FC and 5-FU. We constructed a replication-conditional HSV-1 expressing yCD and examined cytotoxicity when 5-FC was initiated at different times after viral infection, and observed that earlier 5-FC administration led to greater cytotoxicity than later 5-FC administration in vitro and in vivo. Twelve days of 5-FC administration was superior to 6 days in animal models, but dosing beyond 12 days did not further enhance efficacy. Consistent with the dosing schedule results, both viral genomic DNA copy number and viral titers were observed to peak on Day 3 after viral injection and gradually decrease thereafter. The virus is replication-conditional and was detected in tumors for as long as 2 weeks after viral injection. The maximum relative extent of yCD conversion of 5-FC to 5-FU in tumors was observed on Day 6 after viral injection and it decreased progressively thereafter. The observation that 5-FU generation within tumors did not lead to appreciable levels of systemic 5-FU (<10 ng/ml) is important and has not been previously reported. The approaches used in these studies of the relationship between the viral replication kinetics, transgene expression, prodrug administration and anti-tumor efficacy are useful in the design of clinical trials of armed, oncolytic viruses.

Synergistic effects of oncolytic reovirus and docetaxel chemotherapy in prostate cancer

Background

Reovirus type 3 Dearing (T3D) has demonstrated oncolytic activity in vitro, in in vivo murine models and in early clinical trials. However the true potential of oncolytic viruses may only be realized fully in combination with other modalities such as chemotherapy, targeted therapy and radiotherapy. In this study, we examine the oncolytic activity of reovirus T3D and chemotherapeutic agents against human prostate cancer cell lines, with particular focus on the highly metastatic cell line PC3 and the chemotherapeutic agent docetaxel. Docetaxel is the standard of care for metastatic prostate cancer and acts by disrupting the normal process of microtubule assembly and disassembly. Reoviruses have been shown to associate with microtubules and may require this association for efficient viral replication.

Methods

The effects of reovirus and chemotherapy on in vitro cytotoxicity were investigated in PC3 and Du 145 cells and the interactions between agents were assessed by combination index analysis. An Annexin V/propidium iodide fluorescence-activated cell sorting-based assay was used to determine mode of cell death. The effects of reovirus and docetaxel administered as single agent or combination therapy were tested in vivo in a murine model. The effects of docetaxel and reovirus, alone and together, on microtubule stabilisation were investigated by Western blot analysis.

Results

Variable degrees of synergistic cytotoxicity were observed in PC3 and Du 145 cells exposed to live reovirus and several chemotherapy agents. Combination of reovirus infection with docetaxel exposure led to increased late apoptotic/necrotic cell populations. Reovirus/docetaxel combined therapy led to reduced tumour growth and increased survival in a PC3 tumour bearing mouse model. Microtubule stabilization was enhanced in PC3 cells treated with reovirus/docetaxel combined therapy compared to other reovirus/chemotherapy combinations.

Conclusions

The co-administration of a variety of chemotherapeutic agents with live reovirus was able to enhance cytotoxicity synergistically in vitro. The combination of docetaxel with reovirus also delayed tumour growth and improved survival in vivo. Enhanced microtubule stabilisation following this combination treatment may, in part, explain the mechanism of synergy. These results provide evidence to support the ongoing clinical trials using these agents.

Molecular engineering and validation of an oncolytic herpes simplex virus type 1 transcriptionally targeted to midkine-positive tumors

BACKGROUND

Expression profile analyses of midkine (MDK), a multifunctional protein important in development but repressed postnataly, indicate that it is highly expressed in approximately 80% of adult carcinomas and many childhood cancers including malignant peripheral nerve sheath tumors (MPNST). In the present study, we sought to leverage its selective expression to develop a novel oncolytic herpes simplex virus (oHSV) capable of targeting developmentally primitive cancers that express MDK.

METHODS

We sought to increase the oncolytic efficacy of the virus by fusing the human MDK promoter to the HSV type 1 neurovirulence gene, gamma(1)34.5, whose protein product increases viral replication.

RESULTS

Tissue-specific MDK promoter activity in human tumor cells and transgene biological activity was confirmed in human MPNST tumor cells. In vitro replication and cytotoxicity in human fibroblasts and MPNST cells by plaque and MTT assays showed that oHSV-MDK-34.5 increased replication and cytotoxicity compared to oHSV-MDK-Luc. By contrast, no significant difference in cytotoxicity was detected between these viruses in normal human fibroblasts. oHSV-MDK-34.5 impaired in vivo tumor growth and increased median survival of MPNST tumor-bearing nude mice.

CONCLUSIONS

The transcriptional targeting of HSV lytic infection to MDK-expressing tumor cells is feasible. oHSV-MDK-34.5 shows enhanced anti-tumor effects both in vitro and in vivo. Further studies are warranted and may lead to its use in clinical trials.

REO-10: a phase I study of intravenous reovirus and docetaxel in patients with advanced cancer

PURPOSE

REOLYSIN (Oncolytics Biotech) consists of a wild-type oncolytic reovirus, which has selective cytotoxicity for tumor cells while sparing normal cells. In a phase I study as a single agent, repeated infusions of reovirus were safe with evidence of antitumor activity. Preclinical studies indicate potential for synergy between reovirus and chemotherapeutic agents. A multicenter, phase I dose escalation study was designed to assess the safety of combining reovirus with docetaxel chemotherapy in patients with advanced cancer.

EXPERIMENTAL DESIGN

Patients received 75 mg/m(2) docetaxel (day 1) and escalating doses of reovirus up to 3 × 10(10) TCID(50) (days 1-5) every 3 weeks.

RESULTS

Twenty-five patients were enrolled, and 24 patients were exposed to treatment, with 23 completing at least one cycle and 16 suitable for response assessment. Dose-limiting toxicity of grade 4 neutropenia was seen in one patient, but the maximum tolerated dose was not reached. Antitumor activity was seen with one complete response and three partial responses. A disease control rate (combined complete response, partial response, and stable disease) of 88% was observed. Immunohistochemical analysis of reovirus protein expression was observed in posttreatment tumor biopsies from three patients.

CONCLUSION

The combination of reovirus and docetaxel is safe, with evidence of objective disease response, and warrants further evaluation in a phase II study at a recommended schedule of docetaxel (75 mg/m(2), three times weekly) and reovirus (3 × 10(10) TCID(50), days 1-5, every 3 weeks).

Oncolytic herpes simplex virus vectors and chemotherapy: are combinatorial strategies more effective for cancer?

Despite aggressive treatments, including chemotherapy and radiotherapy, cancers often recur owing to resistance to conventional therapies. Oncolytic viruses such as oncolytic herpes simplex virus (oHSV) represent an exciting biological approach to cancer therapy. A range of viral mutations has been engineered into HSV to engender oncolytic activity. While oHSV as a single agent has been tested in a number of cancer clinical trials, preclinical studies have demonstrated enhanced efficacy when it is combined with cytotoxic anticancer drugs. Among the strategies that will be discussed in this article are combinations with standard-of-care chemotherapeutics, expression of prodrug-activating enzymes to enhance chemotherapy and small-molecule inhibitors. The combination of oHSV and chemotherapy can achieve much more efficient cancer cell killing than either single agent alone, often through synergistic interactions. This can be clinically important not just for improving efficacy but also for permitting lower and less toxic chemotherapeutic doses. The viral mutations in an oHSV vector often determine the favorability of its interactions with chemotherapy, just as different cancer cells, due to genetic alterations, vary in their response to chemotherapy. As chemotherapeutics are often the standard of care, combining them with an investigational new drug, such as oHSV, is clinically easier than combining multiple novel agents. As has become clear for most cancer therapies, multimodal treatments are usually more effective. In this article, we will discuss the recent progress of these combinatorial strategies between virotherapy and chemotherapy and future directions.

Identification of the ENT1 antagonists dipyridamole and dilazep as amplifiers of oncolytic herpes simplex virus-1 replication

Oncolytic herpes simplex virus-1 (oHSV) vectors selectively replicate in tumor cells, where they kill through oncolysis while sparing normal cells. One of the drawbacks of oHSV vectors is their limited replication and spread to neighboring cancer cells. Here, we report the outcome of a high-throughput chemical library screen to identify small-molecule compounds that augment the replication of oHSV G47Delta. Of the 2,640-screened bioactives, 6 compounds were identified and subsequently validated for enhanced G47Delta replication. Two of these compounds, dipyridamole and dilazep, interfered with nucleotide metabolism by potently and directly inhibiting the equilibrative nucleoside transporter-1 (ENT1). Replicative amplification promoted by dipyridamole and dilazep were dependent on HSV mutations in ICP6, the large subunit of ribonucleotide reductase. Our results indicate that ENT1 antagonists augment oHSV replication in tumor cells by increasing cellular ribonucleoside activity.

Synergistic effects of oncolytic reovirus and cisplatin chemotherapy in murine malignant melanoma

PURPOSE

To test combination treatment schedules of reovirus and cisplatin chemotherapy in human and murine melanoma cell lines and murine models of melanoma and to investigate the possible mechanisms of synergistic antitumor effects.

EXPERIMENTAL DESIGN

The effects of reovirus +/- chemotherapy on in vitro cytotoxicity and viral replication were assessed using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay and plaque assay. Interactions between agents were assessed by combination index analysis. Mode of cell death was assessed by Annexin V/propidium iodide fluorescence-activated cell sorting-based assays; gene expression profiling of single versus combination treatments was completed using the Agilent microarray system. Single agent and combination therapy effects were tested in vivo in two immunocompetent models of murine melanoma.

RESULTS

Variable degrees of synergistic cytotoxicity between live reovirus and several chemotherapy agents were observed in B16.F10 mouse melanoma cells, most significantly with cisplatin (combination index of 0.42 +/- 0.03 at ED(50)). Combination of cisplatin and reovirus exposure led to increased late apoptotic/necrotic cell populations. Cisplatin almost completely abrogated the inflammatory cytokine gene up-regulation induced by reovirus. Combination therapy led to significantly delayed tumor growth and improved survival in vivo (P < 0.0001 and P = 0.0003, respectively). Cisplatin had no effect on the humoral response to reovirus in mice. However, cisplatin treatment suppressed the cytokine and chemokine response to reovirus in vitro and in vivo.

CONCLUSION

The combination of reovirus and several chemotherapeutic agents synergistically enhanced cytotoxicity in human and murine melanoma cell lines in vitro and murine tumors in vivo. The data support the current reovirus/chemotherapy combination phase I clinical studies currently ongoing in the clinic.

Enhanced cytotoxicity with a novel system combining the paclitaxel-2'-ethylcarbonate prodrug and an HSV amplicon with an attenuated replication-competent virus, HF10 as a helper virus

We previously demonstrated that HF10, which is a natural, non-engineered HSV-1, has potent oncolytic activity in the treatment of solid malignant tumors in vitro and in vivo [H. Takakuwa, F. Goshima, N. Nozawa, T. Yoshikawa, H. Kimata, A. Nakao, et al., Oncolytic viral therapy using a spontaneously generated herpes simplex virus type 1 variant for disseminated peritoneal tumor in immunocompetent mice, Arch. Virol. 148 (2003) 813-825; S. Kohno, C. Lou, F. Goshima, Y. Nishiyama, T. Sata, Y. Ono, Herpes simplex virus type 1 mutant HF10 oncolytic viral therapy for bladder cancer, Urology 66 (2005) 1116-1121; D. Watanabe, F. Goshima, I. Mori, Y. Tamada, Y. Matsumoto, Y. Nishiyama, Oncolytic virotherapy for malignant melanoma with herpes simplex virus type 1 mutant HF10, J. Dermatol. Sci. 50 (2008) 185-196; A. Nawa, C. Luo, L. Zhang, Y. Ushijima, D. Ishida, M. Kamakura, et al., Non-engineered, naturally oncolytic herpes simplex virus HSV1 HF10: applications for cancer gene therapy, Curr. Gene. Ther. 8 (2008) 208-221]. Previous reports have also shown that a combination of HF10 and paclitaxel (TAX) was more efficacious than either regimen alone for some types of malignant tumors [S. Shimoyama, F. Goshima, O. Teshigahara, H. Kasuya, Y. Kodera, A. Nakao, et al., Enhanced efficacy of herpes simplex virus mutant HF10 combined with paclitaxel in peritoneal cancer dissemination models, Hepatogastroenterology 54 (2007) 1038-1042]. In this study, we investigated the efficacy of gene-directed enzyme prodrug therapy (GDEPT) using a novel system that combines the paclitaxel-2'-ethylcarbonate prodrug (TAX-2'-Et) and an HSV amplicon expressing rabbit-carboxylesterase (CES) with HF10 as a helper virus. This GDEPT system aims to produce high level of CES at the tumor site, resulting in efficient local conversion of the TAX-2'-Et prodrug into the active drug TAX [A. Nawa, T. Tanino, C. Lou, M. Iwaki, H. Kajiyama, K. Shibata, et al., Gene directed enzyme prodrug therapy for ovarian cancer: could GDEPT become a promising treatment against ovarian cancer?, Anti-Cancer Agents Med Chem 8 (2008) 232-239]. We demonstrated that the green fluorescent protein (GFP) gene, as a trace maker, was more efficiently introduced by the HSV amplicon compared to the expression vector, pHGCX, and that the HSV amplicon system expressed an active CES enzyme that could convert TAX-2'-Et to TAX in Cos7 cells. Furthermore, although the cytotoxicity of this amplicon system was not enhanced in virus-sensitive tumor cells, it was significantly enhanced in low virus-sensitive tumor cells in the presence of the prodrug in a concentration-dependent manner, compared to the control virus alone (p<0.05). These results indicate that the addition of a prodrug converting enzyme may be a feasible approach to further enhance the efficacy of HF10 as a cancer therapeutics in low HF10-sensitive malignancies.

A novel recombinant vaccinia virus expressing the human norepinephrine transporter retains oncolytic potential and facilitates deep-tissue imaging

Noninvasive and repetitive monitoring of a virus in target tissues and/or specific organs of the body is highly desirable for the development of safe and efficient cancer virotherapeutics. We have previously shown that the oncolytic vaccinia virus GLV-1h68 can target and eradicate human tumors in mice and that its therapeutic effects can be monitored by using optical imaging. Here, we report on the development of a derivative of GLV-1h68, a novel recombinant vaccinia virus (VACV) GLV-1h99, which was constructed to carry the human norepinephrine transporter gene (hNET) under the VACV synthetic early promoter placed at the F14.5L locus for deep-tissue imaging. The hNET protein was expressed at high levels on the membranes of cells infected with this virus. Expression of the hNET protein did not negatively affect virus replication, cytolytic activity in cell culture, or in vivo virotherpeutic efficacy. GLV-1h99-mediated expression of the hNET protein in infected cells resulted in specific uptake of the radiotracer [131I]-meta-iodobenzylguanidine (MIBG). In mice, GLV-1h99-infected tumors were readily imaged by [124I]-MIBG positron emission tomography. To our knowledge, GLV-1h99 is the first oncolytic virus expressing the hNET protein that can efficiently eliminate tumors and simultaneously allow deep-tissue imaging of infected tumors.

Regression of human pancreatic tumor xenografts in mice after a single systemic injection of recombinant vaccinia virus GLV-1h68

Oncolytic virotherapy of tumors has shown promising results in both preclinical and clinical studies. Here, we investigated the therapeutic efficacy of a replication-competent vaccinia virus, GLV-1h68, against human pancreatic carcinomas in cell cultures and in nude mice. We found that GLV-1h68 was able to infect, replicate in, and lyse tumor cells in vitro. Virus-mediated marker gene expressions were readily detected. Moreover, s.c. PANC-1 pancreatic tumor xenografts were effectively treated by a single i.v. dose of GLV-1h68. Cancer killing was achieved with minimal toxicity. Viral titer analyses in homogenized organs and PANC-1 tumors showed that the mutant virus resides almost exclusively in the tumors and not in healthy organs. Except mild spleen enlargements, no histopathology changes were observed in any other organs 2 months after virus injection. Surprisingly, s.c. MIA PaCa-2 pancreatic tumors were treated with similar efficiency as PANC-1 tumors, although they differ significantly in sensitivity to viral lysis in cell cultures. When GLV-1h68 oncolytic viral therapy was used together with cisplatin or gemcitabine to treat PANC-1 tumors, the combination therapy resulted in enhanced and accelerated therapeutic results compared with the virus treatment alone. Profiling of proteins related to immune response revealed a significant proinflammatory immune response and marked activation of innate immunity in virus-colonized tumors. In conclusion, the GLV-1h68 strain showed outstanding therapeutic effects and a documented safety profile in mice, with great promise for future clinical development.

Oncolytic herpes simplex virus vectors and taxanes synergize to promote killing of prostate cancer cells

Genetically engineered oncolytic herpes simplex virus-1 (HSV-1) vectors selectively replicate in tumor cells causing direct killing whereas sparing normal cells. One clinical limitation of using oncolytic HSV vectors is their attenuated growth. We hypothesized that the appropriately chosen chemotherapeutic agent combined with an oncolytic HSV could be an effective means to promote augmented prostate cancer cell killing both in vitro and in vivo. Here we have identified that G47Delta synergizes with the microtubule-stabilizing taxane agents docetaxel and paclitaxel to enhance the in vitro killing of prostate cancer cells. In vivo efficacy studies show that when combined with docetaxel, G47Delta could be reduced at least 10-fold. Immunoblot analysis revealed that docetaxel-induced accumulation of the phospho-specific mitotic markers op18/stathmin or histone-H3 was markedly reduced by G47Delta, which correlated with enhanced apoptosis and required active viral replication. Furthermore, cell-cycle analysis demonstrated that in the presence of G47Delta, the majority of 4N cells arrested in mitosis were MPM-2-negative, indicative of cells exiting mitosis prematurely. These findings suggest that G47Delta may act in part, on mitotically blocked cells to enhance cell death, which may account for the enhanced antitumor efficacy observed in vivo.

A herpes oncolytic virus can be delivered via the vasculature to produce biologic changes in human colorectal cancer

Genetically engineered herpes simplex viruses (HSVs) can selectively infect and replicate in cancer cells, and are candidates for use as oncolytic therapy. This long-term report of a phase I trial examines vascular administration of HSV as therapy for cancer. Twelve subjects with metastatic colorectal cancer within the liver failing first-line chemotherapy were treated in four cohorts with a single dose (3 x 10(6) to 1 x 10(8) particles) of NV1020, a multimutated, replication-competent HSV. After hepatic arterial administration, subjects were observed for 4 weeks before starting intra-arterial chemotherapy. All patients exhibited progression of disease before HSV injection. During observation, levels of the tumor marker carcinoembryonic antigen (CEA) decreased (median % drop = 24%; range 13-74%; P < 0.02). One of three individuals at the 10(8) level showed a 39% radiologic decrease in tumor size by cross-section and 75% by volume. HSV infection was documented from liver tumor biopsies. After beginning regional chemotherapy, all patients demonstrated a further decrease in CEA (median 96%; range 50-98%; P < 0.008) and a radiologic partial response. Median survival for this group was 25 months. During follow-up, no signs of virus reactivation were found. Multimutated HSV can be delivered safely into the human bloodstream to produce selective infection of tumor tissues and biologic effects.

Synergy of a herpes oncolytic virus and paclitaxel for anaplastic thyroid cancer

PURPOSE

Novel therapeutic regimens are needed to improve the dismal outcomes of patients with anaplastic thyroid cancer (ATC). Oncolytic herpes simplex virus have shown promising activity against human ATC. We studied the application of oncolytic herpes simplex virus (G207 and NV1023) in combination with currently used chemotherapeutic drugs (paclitaxel and doxorubicin) for the treatment of ATC.

EXPERIMENTAL DESIGN AND RESULTS

All four agents showed dose-response cytotoxicity in vitro for the human ATC cell lines KAT4 and DRO90-1. G207, combined with paclitaxel, showed synergistic cytotoxicity. Chou-Talalay combination indices ranged from 0.56 to 0.66 for KAT4, and 0.68 to 0.74 for DRO90-1 at higher affected fractions. Paclitaxel did not enhance G207 viral entry and early gene expression or G207 viral replication. Paclitaxel combined with G207 compared with single-agent treatment or controls showed significantly increased microtubule acetylation, mitotic arrest, aberrant chromatid separation, inhibition of metaphase to anaphase progression, and apoptosis. A single i.t. injection of G207 combined with biweekly i.p. paclitaxel injections in athymic nude mice bearing KAT4 flank tumors showed significantly reduced mean tumor volume (74 +/- 38 mm(3)) compared with G207 alone (388 +/- 109 mm(3)), paclitaxel alone (439 +/- 137 mm(3)), and control (520 +/- 160 mm(3)) groups at 16 days. There was no morbidity in vivo attributable to therapy.

CONCLUSIONS

Mechanisms of paclitaxel antitumoral activity, including microtubule acetylation, mitotic block, and apoptosis, were enhanced by G207, which also has direct oncolytic effects. Combination of G207 and paclitaxel therapy is synergistic in treating ATC and holds promise for patients with this fatal disease.

Myxoma virus is oncolytic for human pancreatic adenocarcinoma cells

BACKGROUND

Viral oncolytic therapy, which seeks to exploit the use of live viruses to treat cancer, has shown promise in the treatment of cancers resistant to conventional anticancer therapies. Among the most difficult to treat cancers is advanced pancreatic adenocarcinoma. Our study investigates the ability of a novel oncolytic agent, myxoma virus, to infect, productively replicate in, and kill human pancreatic cancer cells in vitro.

METHODS

The myxoma virus vMyxgfp was tested against a panel of human pancreatic adenocarcinoma cell lines. Infectivity, viral proliferation, and tumor cell kill were assessed.

RESULTS

Infection of tumor cells was assessed by expression of the marker gene enhanced green fluorescent protein (e-GFP). vMyxgfp had the ability to infect all pancreatic cancer cell lines tested. Killing of tumor cells varied among the 6 cell lines tested, ranging from >90% cell kill at 7 days for the most sensitive Panc-1 cells, to 39% in the most resistant cell line Capan-2. Sensitivity correlated to replication of virus, and was found to maximally exhibit a four-log increase in foci-forming units for the most sensitive Panc-1 cells within 72 h.

CONCLUSION

Our study demonstrates for the first time the ability of the myxoma virus to productively infect, replicate in, and lyse human pancreatic adenocarcinoma cells in vitro. These data encourage further investigation of this virus, which is pathogenic only in rabbits, for treatment of this nearly uniformly fatal cancer.

Efficacy of oncolytic herpesvirus NV1020 can be enhanced by combination with chemotherapeutics in colon carcinoma cells

NV1020, an oncolytic herpes simplex virus type 1, can destroy colon cancer cells by selectively replicating within these cells, while sparing normal cells. NV1020 is currently under investigation in a clinical phase I/II trial as an agent for the treatment of colon cancer liver metastases, in combination with conventional chemotherapeutic agents such as 5-fluorouracil (5-FU), SN38 (the active metabolite of irinotecan), and oxaliplatin. To study the synergy of NV1020 and chemotherapy, cytotoxicity and viral replication were evaluated in vitro by treating various human and murine colon carcinoma cell lines, using a colorimetric viability assay, a clonogenic assay, and a plaque-forming assay. In vivo experiments, using a subcutaneous syngeneic CT-26 tumor model in BALB/c mice, were performed to determine the efficacy of combination therapy. In vitro studies showed that the efficacy of NV1020 on human colon carcinoma cell lines HT-29, WiDr, and HCT-116 was additively or synergistically enhanced in combination with 5-FU, SN38, or oxaliplatin. The sequence of application was not important and effects were still apparent after a 21-day incubation period. Three intra-tumoral treatments with NV1020 (1 x 10(7) plaque-forming units), followed by three subcutaneous treatments with 5-FU (50 mg/kg), resulted in substantially higher inhibition of tumor growth and prolongation of survival compared with monotherapies (NV1020/5-FU vs. NV1020, p = 0.027). On WiDr cells, reduced replication of NV1020, in combination with 5-FU, indicated that additive and synergistic effects of combination therapy must be independent from viral replication. These results suggest that NV1020, in combination with chemotherapy, is a promising therapy for treating patients with metastatic colorectal cancer of the liver. We hypothesize that infection of cells with NV1020 sensitizes the infected cells for the cytotoxic effect of the chemotherapeutics.

Effect of chemotherapy-induced DNA repair on oncolytic herpes simplex viral replication

BACKGROUND

Gliomas treated with the alkylating agent temozolomide have incomplete responses in part because of tumoral repair of chemotherapy-induced DNA damage. Data from phase I trials suggest that G207, an oncolytic herpes simplex virus (HSV) with mutated ribonucleotide reductase (RR) and gamma34.5 genes, is safe but needs greater viral oncolysis to be effective. We hypothesized that temozolomide and G207 treatment limitations could be jointly addressed using temozolomide-induced tumor-protective DNA repair pathways to enhance viral replication.

METHODS

Human glioblastoma cells (U87, T98, and U373) and U87 cells transfected with the gene for the DNA repair enzyme O6-methylguanine DNA methyltransferase (MGMT) were treated with G207 and/or temozolomide. Drug interactions, expression of the growth arrest DNA damage 34 (GADD34) and RR transcripts before and after their knockdown with short interfering RNAs, DNA strand breaks, and apoptosis were measured using Chou-Talalay analysis, real-time reverse transcription-polymerase chain reaction, the comet assay, and flow cytometry, respectively. Survival of mice (groups of ten) with intracranial U87 xenograft tumors treated with temozolomide and/or G207 was analyzed using Kaplan-Meier analysis.

RESULTS

Temozolomide exhibited strong synergy with G207 in both MGMT-negative and the MGMT inhibitor O6-benzylguanine-treated MGMT-expressing gliomas (Chou-Talalay combination indices = 0.005 to 0.39) and induced GADD34 expression primarily in nonapoptotic MGMT-negative U87 glioma cells (fold difference = 16, 95% confidence interval [CI] = 12.6 to 20.4, compared with untreated cells). MGMT-expressing T98 and U87/MGMT cells treated with temozolomide plus O6-benzylguanine had higher RR expression than untreated cells (fold difference =14.9, 95% CI = 10.1 to 22.0 [T98]; 9.9, 95% CI = 7.0 to 13.8 [U87/MGMT]). GADD34 and RR knockdown increased temozolomide-induced DNA damage and inhibited the synergy of G207 and temozolomide in U87 and O6-benzylguanine-treated U87/MGMT cells. Mice bearing intracranial U87 tumors survived longer after combination therapy (100% survival at 90 days) than after single-agent therapy (median survival = 46 and 48 days with G207 and temozolomide treatment, respectively).

CONCLUSIONS

Temozolomide-induced DNA repair pathways vary with MGMT expression and enhance HSV-mediated oncolysis in glioma cells. These findings unveil the potential of HSV to target cells surviving temozolomide treatment.

Treatment of Schwannomas with an Oncolytic Recombinant Herpes Simplex Virus in Murine Models of Neurofibromatosis Type 2

Gene therapy for schwannomas was evaluated in two mouse models of neurofibromatosis type 2 (NF2): (1) a transgenic model in which mice express a dominant mutant form of merlin and spontaneously develop schwannomas, and (2) a xenograft model in which human schwannoma tissue is implanted subcutaneously into immune- compromised mice. In both models, schwannoma volumes were monitored by magnetic resonance imaging (MRI) and showed strong gadolinium enhancement typical of these tumors in humans. Both types of tumor were positive for the Schwann cell marker S100, and highly infectable with herpes simplex virus (HSV) vectors. Schwannomas were injected with an oncolytic HSV-1 recombinant virus vector, G47Delta, which has deletions in genes for ribonucleotide reductase (ICP6), gamma34.5, and ICP47. In the NF2 transgenic model, schwannomas were reduced by more than half their original size by 10 days after infection. In the case of subcutaneous schwannoma xenografts, reduction in size after infection occurred more slowly, with a mean reduction of onethird by 42 days after treatment. Schwannomas injected with control vehicles continued to grow slowly over time in both schwannoma models. These studies demonstrate the ability of an oncolytic recombinant HSV vector to reduce the volume of schwannoma tumors in NF2 tumor models in mice and extend the possible therapeutic applications of oncolytic vectors for benign tumors to reduce mass while minimizing nerve damage.

Enhanced efficacy of conditionally replicating herpes simplex virus (G207) combined with 5-fluorouracil and surgical resection in peritoneal cancer dissemination models

BACKGROUND

The therapeutic efficacy of G207, a replication-competent herpes simplex virus, for malignancies is increased when combined with certain chemotherapies, but the mechanism is unclear and the interaction between G207 and surgical resection has not been extensively studied. The goals of the current study were to examine the performance of combination treatments for peritoneal disseminated cancers and to explore the mechanism of effective combinations.

METHODS

Hamsters and SCID and BALB/c mice harboring peritoneal dissemination of gallbladder, gastric or colon cancer cells were treated with G207, 5-fluorouracil (5FU), or surgical resection alone, or G207 combined with 5FU or surgery. Animal survival, antiviral immunity, intratumoral ribonucleotide reductase activity, and viral spread were compared between the groups.

RESULTS

The combination of G207 and 5FU prolonged the survival of hamsters bearing peritoneal dissemination of gallbladder cancer compared with the controls, G207 alone and 5FU alone. 5FU did not suppress the production of neutralizing antibodies against G207, but increased ribonucleotide reductase activity and viral spread in subcutaneous gallbladder tumors. The enhanced efficacy of the combination treatment was also observed in immunodeficient mice with disseminated gastric cancer. Although surgical resection did not significantly prolong animal survival or increase the intratumoral activity of ribonucleotide reductase, long-term survivors emerged from groups of animals treated with surgical resection and G207 for gallbladder and colon disseminated cancers.

CONCLUSIONS

These results indicate that the increased activity of ribonucleotide reductase in tumors mediated by 5FU and the decreased tumor burden resulting from surgical resection may enhance the therapeutic efficacy of oncolytic herpes virus for peritoneal disseminated cancer.

Multimutated herpes simplex virus g207 is a potent inhibitor of angiogenesis

The mode of the antitumoral activity of multimutated oncolytic herpes simplex virus type 1 G207 has not been fully elucidated yet. Because the antitumoral activity of many drugs involves the inhibition of tumor blood vessel formation, we determined if G207 had an influence on angiogenesis. Monolayers of human umbilical vein endothelial cells and human dermal microvascular endothelial cells, but not human dermal fibroblasts, bronchial epithelial cells, and retinal glial cells, were highly sensitive to the replicative and cytotoxic effects of G207. Moreover, G207 infection caused the destruction of endothelial cell tubes in vitro. In the in vivo Matrigel plug assay in mice, G207 suppressed the formation of perfused vessels. Intratumoral treatment of established human rhabdomyosarcoma xenografts with G207 led to the destruction of tumor vessels and tumor regression. Ultrastructural investigations revealed the presence of viral particles in both tumor and endothelial cells of G207-treated xenografts, but not in adjacent normal tissues. These findings show that G207 may suppress tumor growth, in part, due to inhibition of angiogenesis.

Infection with oncolytic herpes simplex virus-1 induces apoptosis in neighboring human cancer cells: a potential target to increase anticancer activity

PURPOSE

The antitumor efficacy of a herpes simplex virus (HSV)-1 oncolytic virus depends on the cytotoxic effect of the virus, but also on viral replication and spread within the tumor. Apoptosis is considered a defense mechanism of infected cells that minimizes the spread of viral progeny by limiting cellular production of virus. We sought to determine whether oncolytic HSV-1 infection induces apoptosis in neighboring, uninfected cells and whether manipulation of apoptosis can increase viral replication and cytotoxicity.

EXPERIMENTAL DESIGN

NV1066 is an oncolytic HSV-1 mutant that contains the marker gene for enhanced green fluorescent protein. OCUM human gastric cancer cells were infected with NV1066 in vitro and inspected for apoptosis by Hoechst and terminal deoxynucleotidyltransferase-mediated nick end labeling staining and for infection by expression of green fluorescence.

RESULTS

A significant increase in apoptosis was seen in cells infected by NV1066. More interestingly, a significant percentage (10%) of uninfected cells also proceeded to apoptosis. After NV1066 infection, cells were also treated with N-acetylcysteine (NAC), an inhibitor of apoptosis. By day 4 after infection, 2.7x more NV1066 was produced in cells exposed to NAC than in those not exposed to NV1066 (P = 0.04). NAC also increased tumor kill when administered with virus.

CONCLUSIONS

These data suggest that NV1066 induces apoptosis in uninfected cocultured cells, potentially hindering propagation of viral progeny and concomitant tumor kill. Inhibition of apoptosis may improve the efficacy of oncolytic HSV-1 therapy.

Effect of murine liver cell proliferation on herpes viral behavior: implications for oncolytic viral therapy

Replication-competent herpes simplex oncolytic viruses are promising anticancer agents that partly target increased DNA synthesis in tumor cells. Investigators have proposed that these DNA viruses may be combined with liver resection to enhance killing of liver malignancies. Whether or not the cellular alterations associated with hepatic regeneration affect the efficacy and toxicity of these promising anticancer agents is unknown. This study examined the behavior of two oncolytic viruses, NV1020 and G207, during liver regeneration. When delivered during the peak of liver regeneration, replication and appearance of both G207 and NV1020 in hepatic tissue are enhanced as demonstrated by histochemical staining for the marker gene lac Z, immunohistochemical staining, and quantitative polymerase chain reaction. This increased appearance of virus in liver tissue correlates with increases in cellular ribonucleotide reductase activity and DNA synthesis and is also associated with increased viral binding. However, increased viral presence is transient, and viral detection declines to baseline within 7 days. When these viruses were delivered to animals even as early as 7 days after hepatectomy, there proved to be no measurable viral replication in any organ and no increased morbidity or mortality. In conclusion, the early stages of hepatic regeneration after resection provide an environment suitable for viral replication. Administration of replication-competent herpes simplex virus during the peak of hepatocyte regeneration (24-48 hours) permits viral productivity in tissue that otherwise does not support viral growth. The increase in hepatotoxicity after hepatectomy is short-lived and can be predicted by peak hepatocyte DNA synthesis.

Oncolytic herpes viruses as a potential mechanism for cancer therapy

Oncolytic viruses have been considered as a potential form of cancer treatment throughout the last century because of their ability to lyse and destroy tumor cells both in tissue culture and in animal models of cancer. However, it is only during the past decade that new molecular technologies have become available and understanding of genetic and molecular components of these viruses has increased to the point that they can be manipulated and made safe for use in treatment in humans. Thus there has been a revival of the concepts of conditionally replication-competent viruses and suicide gene therapy to supplement currently existing cancer therapies. While a wide variety of viruses have been closely studied for this purpose, herpes simplex virus type-1 (HSV-1) has received particularly close attention. The inherent cytotoxicity of this virus, if harnessed and made to be selective in the context of a tumor microenvironment, makes this an ideal candidate for further development. Furthermore, its large genome size, ability to infect cells with a high degree of efficiency, and the presence of an inherent viral-specific thymidine kinase gene add to its potential capabilities. This review explores work performed in this field and its potential for application in the treatment of cancers in humans.

Oncolytic viruses

Although the cytotoxic effects of viruses are usually viewed in terms of pathogenicity, it is possible to harness this activity for therapeutic purposes. Viral genomes are highly versatile, and can be modified to direct their cytotoxicity towards cancer cells. These viruses are known as oncolytic viruses. How are viruses engineered to become tumour specific, and can they be used to safely treat cancer in humans?

Oncolytic herpes simplex virus vectors for cancer virotherapy

Oncolytic herpes simplex virus type 1 (HSV-1) vectors are emerging as an effective and powerful therapeutic approach for cancer. Replication-competent HSV-1 vectors with mutations in genes that affect viral replication, neuropathogenicity, and immune evasiveness have been developed and tested for their safety and efficacy in a variety of mouse models. Evidence to-date following administration into the brain attests to their safety, an important observation in light of the neuropathogenicity of the virus. Phase I clinical traits of three vectors, G207, 1716, and NV1020, are either ongoing or completed, with no adverse events attributed to the virus. These and other HSV-1 vectors are effective against a myriad of solid tumors in mice, including glioma, melanoma, breast, prostate, colon, ovarian, and pancreatic cancer. Enhancement of activity was observed when HSV-1 vectors were used in combination with traditional therapies such as radiotherapy and chemotherapy, providing an attractive strategy to pursue in the clinic. Oncolytic HSV-1 vectors expressing "suicide" genes (thymidine kinase, cytosine deaminase, rat cytochrome P450) or immunostimulatory genes (IL-12, GM-CSF, etc.) have been constructed to maximize tumor destruction through multimodal therapeutic mechanisms. Further advances in virus delivery and tumor specificity should improve the likelihood for successful translation to the clinic.

Friendly fire: redirecting herpes simplex virus-1 for therapeutic applications

Herpes simplex virus-1 (HSV-1) is a relatively large double-stranded DNA virus encoding at least 89 proteins with well characterized disease pathology. An understanding of the functions of viral proteins together with the ability to genetically engineer specific viral mutants has led to the development of attenuated HSV-1 for gene therapy. This review highlights the progress in creating attenuated genetically engineered HSV-1 mutants that are either replication competent (viral non-essential gene deleted) or replication defective (viral essential gene deleted). The choice between a replication-competent or -defective virus is based on the end-goal of the therapeutic intervention. Replication-competent HSV-1 mutants have primarily been employed as antitumor oncolytic viruses, with the lytic nature of the virus harnessed to destroy tumor cells selectively. In replacement gene therapy, replication-defective viruses have been utilized as delivery vectors. The advantages of HSV-1 vectors are that they infect quiescent and dividing cells efficiently and can encode for relatively large transgenes.

Ionizing radiation potentiates the antitumor efficacy of oncolytic herpes simplex virus G207 by upregulating ribonucleotide reductase

BACKGROUND

Replication-competent herpes simplex virus-1 (HSV-1) mutants have an oncolytic effect on human and animal cancers. The aim of this study was to determine whether G207, an HSV-1 mutant, can be combined with ionizing radiation (IR) to increase antitumor activity while decreasing treatment-associated toxicity.

METHODS

This study was performed by using G207, a replication-competent HSV-1 mutant deficient in viral ribonucleotide reductase (RR) and the gamma(1)34.5 neurovirulence protein. The antitumor activity of G207 or IR was tested against HCT-8 human colorectal cancer cells in vitro and in an in vivo mouse subcutaneous tumor model.

RESULTS

We demonstrated that G207 has significant oncolytic effect on HCT-8 cells in vitro in a cytotoxicity assay and in vivo in a mouse flank tumor model and that these effects are improved with low-dose IR. We further illustrated that the increased tumoricidal effect is dependent on the up-regulation of cellular RR by IR measured by a functional bioassay for RR activity. Chemical inhibition of RR by hydroxyurea abrogates the enhanced effect. In contrast to G207, R3616, the parent virus of G207 that expresses functional RR, does not exhibit enhanced oncolysis when combined with IR.

CONCLUSIONS

These data encourage clinical investigation of combination radiation therapy and HSV oncolytic therapy.