Effect of prior exposure to herpes simplex virus 1 on viral vector-mediated tumor therapy in immunocompetent mice

A Broad Influenza Vaccine Based on a Heat-Activated, Tissue-Restricted Replication-Competent Herpesvirus

Vaccination with transiently activated replication-competent controlled herpesviruses (RCCVs) expressing influenza A virus hemagglutinins broadly protects mice against lethal influenza virus challenges. The non-replicating RCCVs can be activated to transiently replicate with high efficiency. Activation involves a brief heat treatment to the epidermal administration site in the presence of a drug. The drug co-control is intended as a block to inadvertent reactivation in the nervous system and, secondarily, viremia under adverse conditions. While the broad protective effects observed raise an expectation that RCCVs may be developed as universal flu vaccines, the need for administering a co-activating drug may dampen enthusiasm for such a development. To replace the drug co-control, we isolated keratin gene promoters that were active in skin cells but inactive in nerve cells and other cells in vitro. In a mouse model of lethal central nervous system (CNS) infection, the administration of a recombinant that had the promoter of the infected cell protein 8 (ICP8) gene of a wild-type herpes simplex virus 1 (HSV-1) strain replaced by a keratin promoter did not result in any clinical signs, even at doses of 500 times wild-type virus LD50. Replication of the recombinant was undetectable in brain homogenates. Second-generation RCCVs expressing a subtype H1 hemagglutinin (HA) were generated in which the infected cell protein 4 (ICP4) genes were controlled by a heat switch and the ICP8 gene by the keratin promoter. In mice, these RCCVs replicated efficiently and in a heat-controlled fashion in the epidermal administration site. Immunization with the activated RCCVs induced robust neutralizing antibody responses against influenza viruses and protected against heterologous and cross-group influenza virus challenges.

Safety of non-replicative and oncolytic replication-selective HSV vectors

Herpes simplex virus type 1 (HSV-1) is a DNA virus and human pathogen used to construct promising therapeutic vectors. HSV-1 vectors fall into two classes: replication-selective oncolytic vectors for cancer therapy and defective non-replicative vectors for gene therapy. Vectors from each class can accommodate ≥30 kb of inserts, have been approved clinically, and demonstrate a relatively benign safety profile. Despite oncolytic HSV (oHSV) replication in tumors and elicited immune responses, the virus is well tolerated in cancer patients. Current non-replicative vectors elicit only limited immune responses. Seropositivity and immune responses against HSV-1 do not eliminate either the vector or infected cells, and the vectors can therefore be re-administered. In this review we highlight vectors that have been translated to the clinic and host-virus immune interactions that impact on the safety and efficacy of HSVs.

Very Broadly Effective Hemagglutinin-Directed Influenza Vaccines with Anti-Herpetic Activity

A universal vaccine that generally prevents influenza virus infection and/or illness remains elusive. We have been exploring a novel approach to vaccination involving replication-competent controlled herpesviruses (RCCVs) that can be deliberately activated to replicate efficiently but only transiently in an administration site in the skin of a subject. The RCCVs are derived from a virulent wild-type herpesvirus strain that has been engineered to contain a heat shock promoter-based gene switch that controls the expression of, typically, two replication-essential viral genes. Additional safety against inadvertent replication is provided by an appropriate secondary mechanism. Our first-generation RCCVs can be activated at the administration site by a mild local heat treatment in the presence of an antiprogestin. Here, we report that epidermal vaccination with such RCCVs expressing a hemagglutinin or neuraminidase of an H1N1 influenza virus strain protected mice against lethal challenges by H1N1 virus strains representing 75 years of evolution. Moreover, immunization with an RCCV expressing a subtype H1 hemagglutinin afforded full protection against a lethal challenge by an H3N2 influenza strain, and an RCCV expressing a subtype H3 hemagglutinin protected against a lethal challenge by an H1N1 strain. Vaccinated animals continued to gain weight normally after the challenge. Protective effects were even observed in a lethal influenza B virus challenge. The RCCV-based vaccines induced robust titers of in-group, cross-group and even cross-type neutralizing antibodies. Passive immunization suggested that observed vaccine effects were at least partially antibody-mediated. In summary, RCCVs expressing a hemagglutinin induce robust and very broad cross-protective immunity against influenza.

Preexisting immunity: Barrier or bridge to effective oncolytic virus therapy?

Oncolytic viruses (OVs) represent a highly promising treatment strategy for a wide range of cancers, by mediating both the direct killing of tumor cells as well as mobilization of antitumor immune responses. As many OVs circulate in the human population, preexisting OV-specific immune responses are prevalent. Indeed, neutralizing antibodies (NAbs) are abundantly present in the human population for commonly used OVs, such as Adenovirus type 5 (Ad5), Herpes Simplex Virus-1 (HSV-1), Vaccinia virus, Measles virus, and Reovirus. This review discusses (pre)clinical evidence regarding the effect of preexisting immunity against OVs on two distinct aspects of OV therapy; OV infection and spread, as well as the immune response induced upon OV therapy. Combined, this review provides evidence that consideration of preexisting immunity is crucial in realizing the full potential of the highly promising therapeutic implementation of OVs. Future investigation of current gaps in knowledge highlighted in this review should yield a more complete understanding of this topic, ultimately allowing for better and more personalized OV therapies.

Recent oncolytic virotherapy clinical trials outline a roadmap for the treatment of high-grade glioma

Adult and pediatric high-grade gliomas (HGGs) are aggressive cancers of the central nervous system that confer dismal clinical prognoses. Standard radiation and chemotherapy have demonstrated only limited efficacy in HGGs, motivating the accelerated investigation of novel modalities such as oncolytic virus (OV) therapies. OV centered therapies work through a mixed mechanism centered on oncolysis and the stimulation of an antitumor immune response. Three recent clinical trials utilizing herpes simplex virus-1 and adenovirus-based oncolytic virotherapy demonstrated not only the safety and efficacy of OVs but also novel dosing strategies that augment OV response potential. Considering these recent trials, herein we present a roadmap for future clinical trials of oncolytic immunovirotherapy in both adult and pediatric HGG, as well as persistent roadblocks related to the assessment of OV efficacy within and between trials.

Pre-Existing HSV-1 Immunity Enhances Anticancer Efficacy of a Novel Immune-Stimulating Oncolytic Virus

Oncolytic viruses (OVs) can specifically replicate in the host and cause cancer cell lysis while inducing an antitumor immune response. The aim of this study is to investigate the impact of either pre-existing immunity against herpes simplex virus type-1 (HSV-1) or multicycle treatment with OVs on anticancer efficacy of VG161, an HSV-1 OV in phase 2 clinical trial. VG161 efficacy was tested in CT26 mouse models by comparing the efficacy and immune response in naïve mice or in mice that were immunized with VG161. Moreover, VG161 efficacy in HLA-matched CD34+ humanized intrahepatic cholangiocarcinoma (ICC) patient-derived xenograft (PDX) models was also tested in multicycle treatment and was compared to standard chemotherapy for this type of cancer (gemcitabine). The HSV-1-immunized mice significantly inhibited tumor growth in VG161-treated mice compared to control naïve treated mice. RNA expression profiling and ELISPOT analyses indicated changes in the tumor's immune profile in the immunized and treated group compared to naïve and treated mice, as well as enhanced T cell function depicted by higher numbers of tumor specific lymphocytes, which was enhanced by immunization. In the ICC PDX model, repeated treatment of VG161 with 2 or 3 cycles seemed to increase the anticancer efficacy of VG161. In conclusion, the anticancer efficacy of VG161 can be enhanced by pre-immunization with HSV-1 and multicycle administration when the virus is given intratumorally, indicating that pre-existing antiviral immunity might enhance OV-induced antitumor immunity. Our results suggest potential clinical benefits of HSV-1-based OV therapy in HSV-1-seropositive patients and multicycle administration of VG161 for long-term maintenance treatment.

A phase I/II study of triple-mutated oncolytic herpes virus G47∆ in patients with progressive glioblastoma

Here, we report the results of a phase I/II, single-arm study (UMIN-CTR Clinical Trial Registry UMIN000002661) assessing the safety (primary endpoint) of G47∆, a triple-mutated oncolytic herpes simplex virus type 1, in Japanese adults with recurrent/progressive glioblastoma despite radiation and temozolomide therapies. G47Δ was administered intratumorally at 3 × 10⁸ pfu (low dose) or 1 × 10⁹ pfu (set dose), twice to identical coordinates within 5–14 days. Thirteen patients completed treatment (low dose, n = 3; set dose, n = 10). Adverse events occurred in 12/13 patients. The most common G47Δ-related adverse events were fever, headache and vomiting. Secondary endpoint was the efficacy. Median overall survival was 7.3 (95%CI 6.2–15.2) months and the 1-year survival rate was 38.5%, both from the last G47∆ administration. Median progression-free survival was 8 (95%CI 7–34) days from the last G47∆ administration, mainly due to immediate enlargement of the contrast-enhanced area of the target lesion on MRI. Three patients survived >46 months. One complete response (low dose) and one partial response (set dose) were seen at 2 years. Based on biopsies, post-administration MRI features (injection site contrast-enhancement clearing and entire tumor enlargement) likely reflected tumor cell destruction via viral replication and lymphocyte infiltration towards tumor cells, the latter suggesting the mechanism for “immunoprogression” characteristic to this therapy. This study shows that G47Δ is safe for treating recurrent/progressive glioblastoma and warrants further clinical development.

G47Δ is a third-generation, triple-mutated oncolytic HSV-1 that has demonstrated anti-tumor efficacy in preclinical studies. Here the authors report the results of a phase I/II study of G47Δ in patients with recurrent or progressive glioblastoma.

Intratumoral oncolytic herpes virus G47∆ for residual or recurrent glioblastoma: a phase 2 trial

This investigator-initiated, phase 2, single-arm trial primarily assessed the efficacy of G47∆, a triple-mutated, third-generation oncolytic herpes simplex virus type 1, in 19 adult patients with residual or recurrent, supratentorial glioblastoma after radiation therapy and temozolomide (UMIN-CTR Clinical Trial Registry UMIN000015995). G47Δ was administered intratumorally and repeatedly for up to six doses. The primary endpoint of 1-yr survival rate after G47∆ initiation was 84.2% (95% confidence interval, 60.4–96.6; 16 of 19). The prespecified endpoint was met and the trial was terminated early. Regarding secondary endpoints, the median overall survival was 20.2 (16.8–23.6) months after G47∆ initiation and 28.8 (20.1–37.5) months from the initial surgery. The most common G47∆-related adverse event was fever (17 of 19) followed by vomiting, nausea, lymphocytopenia and leukopenia. On magnetic resonance imaging, enlargement of and contrast-enhancement clearing within the target lesion repeatedly occurred after each G47∆ administration, which was characteristic to this therapy. Thus, the best overall response in 2 yr was partial response in one patient and stable disease in 18 patients. Biopsies revealed increasing numbers of tumor-infiltrating CD4⁺/CD8⁺ lymphocytes and persistent low numbers of Foxp3⁺ cells. This study showed a survival benefit and good safety profile, which led to the approval of G47∆ as the first oncolytic virus product in Japan.

Results from a pivotal single-arm phase 2 trial show that the repeated intratumoral administration of the oncolytic herpes virus G47∆ in residual or recurrent glioblastoma exhibits survival benefit and a safe profile.

Beyond Immunotherapy: Seizing the Momentum of Oncolytic Viruses in the Ideal Platform of Skin Cancers

Simple Summary

Oncolytic viruses (OVs) are the most innovative and promising class of intratumoral immunotherapies. The broad immunogenic landscape of skin cancer, accessible to intralesional infusion and available for direct response assessment, seems to be an ideal platform to expand the role of OVs. The established efficacy of immune checkpoint inhibitors (ICIs) in this field and their hypothetical synergy with OVs have generated expectations for their combined use beyond the current immunotherapy achievements. Despite the recent negative phase III results of the MASTERKEY-265 trial for the combination of T-VEC plus pembrolizumab, such projects, including different ICIs and various natural or genetically modified OVs, continue to attract considerable interest, with numerous clinical trials underway for all the subtypes of skin cancer. To date, the majority of studies confirm the safety of tested OVs in patients with advanced skin cancers but cannot clearly prove whether these viral agents add any therapeutic benefit in the standard ICI-based approach. The aim of this overview is to present the main findings related to the examined OV-containing regimens at pre-clinical and clinical levels, and to discuss the previous failures as well as the future perspectives of oncolytic virotherapy.

Abstract

Despite the durable remissions induced by ICIs and targeted therapies in advanced melanoma and non-melanoma skin cancers, both subtypes usually relapse. Many systematic therapies have been tested to increase efficacy and delay relapse in ICIs, but their success has been limited. Due the feasibility of this approach, skin cancers have become the ideal platform for intralesional infusions of many novel agents, including oncolytic viruses (OVs). Talimogene laherparepvec (T-VEC) was the first FDA-approved OV for the treatment of unresectable melanoma and this virus opened up further potential for the use of this class of agents, especially in combination with ICIs, in order to achieve deeper and longer immune-mediated responses. However, the recently announced phase III MASTERKEY-265 trial was not able to confirm that the addition of T-VEC to pembrolizumab treatment improves progression-free or overall survival over the use of pembrolizumab alone. Despite these results, numerous studies are currently active, evaluating T-VEC and several other OVs as monotherapies or in regimens with ICIs in different subtypes of skin cancer. This overview provides a comprehensive update on the evolution status of all available OVs in melanoma and non-melanoma skin cancers and summarizes the more interesting preclinical findings, the latest clinical evidence, and the future insights in relation to the expected selective incorporation of some of these OVs into oncological practice.

Efficacy and safety of a third-generation oncolytic herpes virus G47Δ in models of human esophageal carcinoma

Treatment options are limited for esophageal carcinoma (EC). G47Δ, a triple-mutated, conditionally replicating herpes simplex virus type 1 (HSV-1), exhibits enhanced killing of tumor cells with high safety features. Here, we studied the efficacy of G47Δ using preclinical models of human EC. In vitro, G47Δ showed efficient cytopathic effects and replication capabilities in all eight human esophageal cancer cell lines tested. In athymic mice harboring subcutaneous tumors of human EC (KYSE180, TE8, and OE19), two intratumoral injections with G47Δ significantly inhibited the tumor growth. To mimic the clinical treatment situations, we established an orthotopic EC model using luciferase-expressing TE8 cells (TE8-luc). An intratumoral injection with G47Δ markedly inhibited the growth of orthotopic TE8-luc tumors in athymic mice. Furthermore, we evaluated the safety of applying G47Δ to the esophagus in mice. A/J mice inoculated intraesophageally or administered orally with G47Δ (10⁷ plaque-forming units [pfu]) survived for more than 2 months without remarkable symptoms, whereas the majority with wild-type HSV-1 (10⁶ pfu) deteriorated within 10 days. PCR analyses showed that the G47Δ DNA was confined to the esophagus after intraesophageal inoculation and was not detected in major organs after oral administration. Our results provide a rationale for the clinical use of G47Δ for treating EC.

In Situ Cancer Vaccination and Immunovirotherapy Using Oncolytic HSV

Herpes simplex virus (HSV) can be genetically altered to acquire oncolytic properties so that oncolytic HSV (oHSV) preferentially replicates in and kills cancer cells, while sparing normal cells, and inducing anti-tumor immune responses. Over the last three decades, a better understanding of HSV genes and functions, and improved genetic-engineering techniques led to the development of oHSV as a novel immunovirotherapy. The concept of in situ cancer vaccination (ISCV) was first introduced when oHSV was found to induce a specific systemic anti-tumor immune response with an abscopal effect on non-injected tumors, in the process of directly killing tumor cells. Thus, the use of oHSV for tumor vaccination in situ is antigen-agnostic. The research and development of oHSVs have moved rapidly, with the field of oncolytic viruses invigorated by the FDA/EMA approval of oHSV talimogene laherparepvec in 2015 for the treatment of advanced melanoma. Immunovirotherapy can be enhanced by arming oHSV with immunomodulatory transgenes and/or using them in combination with other chemotherapeutic and immunotherapeutic agents. This review offers an overview of the development of oHSV as an agent for ISCV against solid tumors, describing the multitude of different oHSVs and their efficacy in immunocompetent mouse models and in clinical trials.

The discovery and development of oncolytic viruses: are they the future of cancer immunotherapy?

Introduction: Despite diverse treatment modalities and novel therapies, many cancers and patients are not effectively treated. Cancer immunotherapy has recently achieved breakthrough status yet is not effective in all cancer types or patients and can generate serious adverse effects. Oncolytic viruses (OVs) are a promising new therapeutic modality that harnesses virus biology and host interactions to treat cancer. OVs, genetically engineered or natural, preferentially replicate in and kill cancer cells, sparing normal cells/tissues, and mediating anti-tumor immunity.Areas covered: This review focuses on OVs as cancer therapeutic agents from a historical perspective, especially strategies to boost their immunotherapeutic activities. OVs offer a multifaceted platform, whose activities are modulated based on the parental virus and genetic alterations. In addition to direct viral effects, many OVs can be armed with therapeutic transgenes to also act as gene therapy vectors, and/or combined with other drugs or therapies.Expert opinion: OVs are an amazingly versatile and malleable class of cancer therapies. They tend to target cellular and host physiology as opposed to specific genetic alterations, which potentially enables broad responsiveness. The biological complexity of OVs have hindered their translation; however, the recent approval of talimogene laherparepvec (T-Vec) has invigorated the field.

New viral vectors for infectious diseases and cancer

Since the discovery in 1796 by Edward Jenner of vaccinia virus as a way to prevent and finally eradicate smallpox, the concept of using a virus to fight another virus has evolved into the current approaches of viral vectored genetic vaccines. In recent years, key improvements to the vaccinia virus leading to a safer version (Modified Vaccinia Ankara, MVA) and the discovery that some viruses can be used as carriers of heterologous genes encoding for pathological antigens of other infectious agents (the concept of 'viral vectors') has spurred a new wave of clinical research potentially providing for a solution for the long sought after vaccines against major diseases such as HIV, TB, RSV and Malaria, or emerging infectious diseases including those caused by filoviruses and coronaviruses. The unique ability of some of these viral vectors to stimulate the cellular arm of the immune response and, most importantly, T lymphocytes with cell killing activity, has also reawakened the interest toward developing therapeutic vaccines against chronic infectious diseases and cancer. To this end, existing vectors such as those based on Adenoviruses have been improved in immunogenicity and efficacy. Along the same line, new vectors that exploit viruses such as Vesicular Stomatitis Virus (VSV), Measles Virus (MV), Lymphocytic choriomeningitis virus (LCMV), cytomegalovirus (CMV), and Herpes Simplex Virus (HSV), have emerged. Furthermore, technological progress toward modifying their genome to render some of these vectors incompetent for replication has increased confidence toward their use in infant and elderly populations. Lastly, their production process being the same for every product has made viral vectored vaccines the technology of choice for rapid development of vaccines against emerging diseases and for 'personalised' cancer vaccines where there is an absolute need to reduce time to the patient from months to weeks or days. Here we review the recent developments in viral vector technologies, focusing on novel vectors based on primate derived Adenoviruses and Poxviruses, Rhabdoviruses, Paramixoviruses, Arenaviruses and Herpesviruses. We describe the rationale for, immunologic mechanisms involved in, and design of viral vectored gene vaccines under development and discuss the potential utility of these novel genetic vaccine approaches in eliciting protection against infectious diseases and cancer.

Efficacy of a Third-Generation Oncolytic Herpes Virus G47Δ in Advanced Stage Models of Human Gastric Cancer

Advanced gastric cancer, especially scirrhous gastric cancer with peritoneal dissemination, remains refractory to conventional therapies. G47Δ, a third-generation oncolytic herpes simplex virus type 1, is an attractive novel therapeutic agent for solid cancer. In this study, we investigated the therapeutic potential of G47Δ for human gastric cancer. In vitro, G47Δ showed good cytopathic effects and replication capabilities in nine human gastric cancer cell lines tested. In vivo, intratumoral inoculations with G47Δ (2 × 10⁵ or 1 × 10⁶ plaque-forming units [PFU]) significantly inhibited the growth of subcutaneous tumors (MKN45, MKN74, and 44As3). To evaluate the efficacy of G47Δ for advanced-stage models of gastric cancer, we generated an orthotopic tumor model and peritoneal dissemination models of human scirrhous gastric cancer (MKN45-luc and 44As3Luc), which have features mimicking intractable scirrhous cancer patients. G47Δ (1 × 10⁶ PFU) was constantly efficacious whether administered intratumorally or intraperitoneally in the clinically relevant models. Notably, G47Δ injected intraperitoneally readily distributed to, and selectively replicated in, disseminated tumors. Furthermore, flow cytometric analyses of tumor-infiltrating cells in subcutaneous tumors revealed that intratumoral G47Δ injections markedly decreased M2 macrophages while increasing M1 macrophages and natural killer (NK) cells. These findings indicate the usefulness of G47Δ for treating human gastric cancer, including scirrhous gastric cancer and the ones in advanced stages.

Anti-viral immunity in the tumor microenvironment: implications for the rational design of herpes simplex virus type 1 oncolytic virotherapy

Purpose of review

The design of novel herpes simplex type I (HSV-1)-derived oncolytic virotherapies is a balancing act between safety, immunogenicity and replicative potential. We have undertaken this review to better understand how these considerations can be incorporated into rational approaches to the design of novel herpesvirus oncolytic virotherapies.

Recent findings

Several recent papers have demonstrated that enhancing the potential of HSV-1 oncolytic viruses to combat anti-viral mechanisms present in the tumor microenvironment leads to greater efficacy than their parental viruses.

Summary

It is not entirely clear how the immunosuppressive tumor microenvironment affects oncolytic viral replication and spread within tumors. Recent work has shown that the manipulation of specific cellular and molecular mechanisms of immunosuppression operating within the tumor microenvironment can enhance the efficacy of oncolytic virotherapy. We anticipate that future work will integrate greater knowledge of immunosuppression in tumor microenvironments with design of oncolytic virotherapies.

Talimogene laherparepvec (T-Vec) for the treatment of melanoma and other cancers

Talimogene laherparepvec (T-Vec) is the first live virus to be approved by the US Food and Drug Administration for the treatment of cancer. This engineered version of herpes simplex virus type 1 (HSV-1) is the product of decades of preclinical work aimed at identifying and modifying aspects of the viral genome involved in virulence and immunogenicity. T-Vec preferentially infects and lyses tumor cells and, in some cases, induces a systemic immune response against the tumor. These properties have translated into significant and durable clinical responses, particularly in advanced melanoma. Many unanswered questions remain, including how to augment these clinical responses and which other tumor types may respond to oncolytic therapy. Here, we review the development of T-Vec, our current understanding of its impact on the tumor immune micro-environment, and its safety and efficacy in clinical trials for melanoma and other cancers.

CD8(+) T-cell Immune Evasion Enables Oncolytic Virus Immunotherapy

Although counteracting innate defenses allows oncolytic viruses (OVs) to better replicate and spread within tumors, CD8(+) T-cells restrict their capacity to trigger systemic anti-tumor immune responses. Herpes simplex virus-1 (HSV-1) evades CD8(+) T-cells by producing ICP47, which limits immune recognition of infected cells by inhibiting the transporter associated with antigen processing (TAP). Surprisingly, removing ICP47 was assumed to benefit OV immuno-therapy, but the impact of inhibiting TAP remains unknown because human HSV-1 ICP47 is not effective in rodents. Here, we engineer an HSV-1 OV to produce bovine herpesvirus UL49.5, which unlike ICP47, antagonizes rodent and human TAP. Significantly, UL49.5-expressing OVs showed superior efficacy treating bladder and breast cancer in murine models that was dependent upon CD8(+) T-cells. Besides injected subcutaneous tumors, UL49.5-OV reduced untreated, contralateral tumor size and metastases. These findings establish TAP inhibitor-armed OVs that evade CD8(+) T-cells as an immunotherapy strategy to elicit potent local and systemic anti-tumor responses.

To Infection and Beyond: The Multi-Pronged Anti-Cancer Mechanisms of Oncolytic Viruses

Over the past 1–2 decades we have witnessed a resurgence of efforts to therapeutically exploit the attributes of lytic viruses to infect and kill tumor cells while sparing normal cells. We now appreciate that the utility of viruses for treating cancer extends far beyond lytic cell death. Viruses are also capable of eliciting humoral and cellular innate and adaptive immune responses that may be directed not only at virus-infected cells but also at uninfected cancer cells. Here we review our current understanding of this bystander effect, and divide the mechanisms into lytic, cytokine, innate cellular, and adaptive phases. Knowing the key pathways and molecular players during virus infection in the context of the cancer microenvironment will be critical to devise strategies to maximize the therapeutic effects of oncolytic viroimmunotherapy.

Pediatric cancer gone viral. Part I: strategies for utilizing oncolytic herpes simplex virus-1 in children

Progress for improving outcomes in pediatric patients with solid tumors remains slow. In addition, currently available therapies are fraught with numerous side effects, often causing significant life-long morbidity for long-term survivors. The use of viruses to kill tumor cells based on their increased vulnerability to infection is gaining traction, with several viruses moving through early and advanced phase clinical testing. The prospect of increased efficacy and decreased toxicity with these agents is thus attractive for pediatric cancer. In part I of this two-part review, we focus on strategies for utilizing oncolytic engineered herpes simplex virus (HSV) to target pediatric malignancies. We discuss mechanisms of action, routes of delivery, and the role of preexisting immunity on antitumor efficacy. Challenges to maximizing oncolytic HSV in children are examined, and we highlight how these may be overcome through various arming strategies. We review the preclinical and clinical evidence demonstrating safety of a variety of oncolytic HSVs. In Part II, we focus on the antitumor efficacy of oncolytic HSV in pediatric tumor types, pediatric clinical advances made to date, and future prospects for utilizing HSV in pediatric patients with solid tumors.

Replication-Competent Controlled Herpes Simplex Virus

We present the development and characterization of a replication-competent controlled herpes simplex virus 1 (HSV-1). Replication-essential ICP4 and ICP8 genes of HSV-1 wild-type strain 17syn+ were brought under the control of a dually responsive gene switch. The gene switch comprises (i) a transactivator that is activated by a narrow class of antiprogestins, including mifepristone and ulipristal, and whose expression is mediated by a promoter cassette that comprises an HSP70B promoter and a transactivator-responsive promoter and (ii) transactivator-responsive promoters that drive the ICP4 and ICP8 genes. Single-step growth experiments in different cell lines demonstrated that replication of the recombinant virus, HSV-GS3, is strictly dependent on an activating treatment consisting of administration of a supraphysiological heat dose in the presence of an antiprogestin. The replication-competent controlled virus replicates with an efficiency approaching that of the wild-type virus from which it was derived. Essentially no replication occurs in the absence of activating treatment or if HSV-GS3-infected cells are exposed only to heat or antiprogestin. These findings were corroborated by measurements of amounts of viral DNA and transcripts of the regulated ICP4 gene and the glycoprotein C (gC) late gene, which was not regulated. Similar findings were made in experiments with a mouse footpad infection model.

IMPORTANCE

The alphaherpesviruses have long been considered vectors for recombinant vaccines and oncolytic therapies. The traditional approach uses vector backbones containing attenuating mutations that restrict replication to ensure safety. The shortcoming of this approach is that the attenuating mutations tend to limit both the immune presentation and oncolytic properties of these vectors. HSV-GS3 represents a novel type of vector that, when activated, replicates with the efficiency of a nonattenuated virus and whose safety is derived from deliberate, stringent regulation of multiple replication-essential genes. By directing activating heat to the region of virus administration, replication is strictly confined to infected cells within this region. The requirement for antiprogestin provides an additional level of safety, ensuring that virus replication cannot be triggered inadvertently. Replication-competent controlled vectors such as HSV-GS3 may have the potential to be superior to conventional attenuated HSV vaccine and oncolytic vectors without sacrificing safety.

A novel approach for addressing diseases not yielding to effective vaccination? Immunization by replication-competent controlled virus

Vaccination involves inoculation of a subject with a disabled disease-causing microbe or parts thereof. While vaccination has been highly successful, we still lack sufficiently effective vaccines for important infectious diseases. We propose that a more complete immune response than that elicited from a vaccine may be obtained from immunization with a disease-causing virus modified to subject replication-essential genes to the control of a gene switch activated by non-lethal heat in the presence of a drug-like compound. Upon inoculation, strictly localized replication of the virus would be triggered by a heat dose administered to the inoculation site. Activated virus would transiently replicate with an efficiency approaching that of the disease-causing virus and express all viral antigens. It may also vector heterologous antigens or control co-infecting microbes.

A phase 1 trial of oncolytic HSV-1, G207, given in combination with radiation for recurrent GBM demonstrates safety and radiographic responses

G207, a mutant herpes simplex virus (HSV) type 1, is safe when inoculated into recurrent malignant glioma. We conducted a phase 1 trial of G207 to demonstrate the safety of stereotactic intratumoral administration when given 24 hours prior to a single 5 Gy radiation dose in patients with recurrent malignant glioma. Nine patients with progressive, recurrent malignant glioma despite standard therapy were included. Patients received one dose of G207 stereotactically inoculated into the multiple sites of the enhancing tumor margin and were then treated focally with 5 Gy radiation. Treatment was well tolerated, and no patient developed HSV encephalitis. The median interval between initial diagnosis and G207 inoculation was 18 months (mean: 23 months; range: 11-51 months). Six of the nine patients had stable disease or partial response for at least one time point. Three instances of marked radiographic response to treatment occurred. The median survival time from G207 inoculation until death was 7.5 months (95% confidence interval: 3.0-12.7). In conclusion, this study showed the safety and the potential for clinical response of single-dose oncolytic HSV therapy augmented with radiation in the treatment of malignant glioma patients. Additional studies with oncolytic HSV such as G207 in the treatment of human glioma are recommended.

Molecular network pathways and functional analysis of tumor signatures associated with development of resistance to viral gene therapy

Replication-competent attenuated herpes simplex viruses have proven effective in killing many cancer cell lines. However, determinants of resistance to oncolytic therapy are mostly unknown. We developed viral therapy-resistant cells and examined changes in gene-expression pattern compared with therapy-sensitive parental cells. Colon cancer cell line HT29 and hepatoma cell line PLC5 were exposed to increasing concentrations of virus G207. Therapy-resistant cells were isolated and grown in vitro. Tumorigenicity was confirmed by ability of cell lines to form tumors in mice. Human Genome U133A complementary DNA microarray chips were used to determine gene-expression patterns, which were analyzed in the context of molecular network interactions, pathways and gene ontology. In parental cell lines, 90-100% of cells were killed by day 7 at 1.0 multiplicity of infection. In resistant cell lines, cytotoxicity assay confirmed 200- to 400-fold resistance. Microarray analysis confirmed changes in gene expressions associated with resistance: cell surface proteins affecting viral attachment and entry, cellular proteins affecting nucleotide pools and proteins altering apoptotic pathways. These changes would decrease viral infection and replication. Our study identifies gene-expression signatures associated with resistance to oncolytic viral therapy. These data provide potential targets to overcome resistance, and suggest that molecular assays may be useful in selecting patients for trial with this novel treatment.

Oncolytic virotherapy reaches adolescence

Lytic viruses kill cells as a consequence of their normal replication life cycle. The idea of harnessing viruses to kill cancer cells arose over a century ago, before viruses were even discovered, from medical case reports of infections associated with cancer remissions. Since then, there has been no shortage of hype, hope, or fear regarding the prospect of oncolytic virotherapy for cancer. Early developments in the field included encouraging antitumor efficacy both in animal studies in the 1920s-1940s and in human clinical trials in the 1950s-1970s. Despite its long-standing history, oncolytic virotherapy was an idea ahead of its time. Without needed advances in molecular biology, virology, immunology, and clinical research ethics, early clinical trials resulted in infectious complications and were fraught with controversial research conduct, so that enthusiasm in the medical community waned. Oncolytic virotherapy is now experiencing a major growth spurt, having sustained numerous laboratory advances and undergone multiple encouraging adult clinical trials, and is now witnessing the emergence of pediatric trials. Here we review the history and salient biology of the field, including preclinical and clinical data, with a special emphasis on those agents now being tested in pediatric cancer patients.

Host responses to herpes simplex virus and herpes simplex virus vectors

Herpes simplex virus (HSV) is a well-known, ubiquitous pathogen of humans. Engineered mutants of HSV can also be exploited as vectors in gene therapy or for virotherapy of tumors. HSV has multiple abilities to evade and modulate the innate and adaptive responses of the host. The increasing knowledge on the mutual interactions of the invading HSV with the host defenses will contribute to our deeper understanding of the relationship between HSV and the host, and thereby lead to future development of more effective and specific HSV vectors for treatment of human diseases. The future advances of HSV vaccines and vaccine vectors are based on the knowlegde of the complex interplay between HSV and the host defenses.

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

HSV Recombinant Vectors for Gene Therapy

The very deep knowledge acquired on the genetics and molecular biology of herpes simplex virus (HSV), has allowed the development of potential replication-competent and replication-defective vectors for several applications in human healthcare. These include delivery and expression of human genes to cells of the nervous systems, selective destruction of cancer cells, prophylaxis against infection with HSV or other infectious diseases, and targeted infection to specific tissues or organs. Replication-defective recombinant vectors are non-toxic gene transfer tools that preserve most of the neurotropic features of wild type HSV-1, particularly the ability to express genes after having established latent infections, and are thus proficient candidates for therapeutic gene transfer settings in neurons. A replication-defective HSV vector for the treatment of pain has recently entered in phase 1 clinical trial. Replication-competent (oncolytic) vectors are becoming a suitable and powerful tool to eradicate brain tumours due to their ability to replicate and spread only within the tumour mass, and have reached phase II/III clinical trials in some cases. The progress in understanding the host immune response induced by the vector is also improving the use of HSV as a vaccine vector against both HSV infection and other pathogens. This review briefly summarizes the obstacle encountered in the delivery of HSV vectors and examines the various strategies developed or proposed to overcome such challenges.

Analysis of genetically engineered oncolytic herpes simplex viruses in human prostate cancer organotypic cultures

Oncolytic herpes simplex viruses type 1 (oHSVs) such as G47Delta and G207 are genetically engineered for selective replication competence in cancer cells. Several factors can influence the overall effectiveness of oHSV tropism, including HSV-1 receptor expression, extracellular matrix milieu and cellular permissiveness. We have taken advantage of human prostate organ cultures derived from radical prostatectomies to investigate oHSV tropism. In this study, we show that both G47Delta and G207 specifically replicate in epithelial cells of the prostatic glands but not in the surrounding stroma. In contrast, both the epithelial and stromal cell compartments were readily infected by wild-type HSV-1. Analysis of oHSV replication in prostate surgical specimens 3 days post infection showed that G47Delta generated approximately 30-fold more viral progeny than did G207. This correlated with the enhanced expression of G47Delta-derived glycoprotein gB protein levels as compared with G207. In benign prostate tissues, G207 and G47Delta titers were notably reduced, whereas strain F titers were maintained at similar levels compared with prostate cancer specimens. Overall, our results show that these oncolytic herpes vectors show both target specificity and replication competence in human prostate cancer specimens and point to the utility of using human prostate organ cultures in assessing oHSV tropism and cellular specificity.

Phase II Clinical Trial of a Granulocyte-Macrophage Colony-Stimulating Factor–Encoding, Second-Generation Oncolytic Herpesvirus in Patients With Unresectable Metastatic Melanoma

Purpose

Treatment options for metastatic melanoma are limited. We conducted this phase II trial to assess the efficacy of JS1/34.5-/47-/granulocyte-macrophage colony-stimulating factor (GM-CSF) in stages IIIc and IV disease.

Patients and Methods

Treatment involved intratumoral injection of up to 4 mL of 106pfu/mL of JS1/34.5-/47-/GM-CSF followed 3 weeks later by up to 4 mL of 108pfu/mL every 2 weeks for up to 24 treatments. Clinical activity (by RECIST [Response Evaluation Criteria in Solid Tumors]), survival, and safety parameters were monitored.

Results

Fifty patients (stages IIIc, n = 10; IVM1a, n = 16; IVM1b, n = 4; IVM1c, n = 20) received a median of six injection sets; 74% of patients had received one or more nonsurgical prior therapies for active disease, including dacarbazine/temozolomide or interleukin-2 (IL-2). Adverse effects were limited primarily to transient flu-like symptoms. The overall response rate by RECIST was 26% (complete response [CR], n = 8; partial response [PR], n = 5), and regression of both injected and distant (including visceral) lesions occurred. Ninety-two percent of the responses had been maintained for 7 to 31 months. Ten additional patients had stable disease (SD) for greater than 3 months, and two additional patients had surgical CR. On an extension protocol, two patients subsequently achieved CR by 24 months (one previously PR, one previously SD), and one achieved surgical CR (previously PR). Overall survival was 58% at 1 year and 52% at 24 months.

Conclusion

The 26% response rate, with durability in both injected and uninjected lesions including visceral sites, together with the survival rates, are evidence of systemic effectiveness. This effectiveness, combined with a limited toxicity profile, warrants additional evaluation of JS1/34.5-/47-/GM-CSF in metastatic melanoma. A US Food and Drug Administration–approved phase III investigation is underway.

Herpes simplex virus oncolytic therapy for pediatric malignancies

Despite improving survival rates for children with cancer, a subset of patients exist with disease resistant to traditional therapies such as surgery, chemotherapy, and radiation. These patients require newer, targeted treatments used alone or in combination with more traditional approaches. Oncolytic herpes simplex virus (HSV) is one of these newer therapies that offer promise for several difficult to treat pediatric malignancies. The potential benefit of HSV therapy in pediatric solid tumors including brain tumors, neuroblastomas, and sarcomas is reviewed along with the many challenges that need to be addressed prior to moving oncolytic HSV therapy from the laboratory to the beside in the pediatric population.

Considerations for intravascular administration of oncolytic herpes virus for the treatment of multiple liver metastases

PURPOSE

Oncolytic viral therapy is a newly developed modality for treating tumors. Many clinical trials using oncolytic virus have been performed worldwide, but most of them have used local injection in the tumor. Determination of the effect and safety of intravascular virus injection instead of local injection is necessary for clinical use against multiple liver metastases and systemic metastases.

METHODS

To evaluate the efficacy and safety of intravascular virus therapy, mice bearing multiple liver metastases were treated by intraportal or intravenous administration of the herpes simplex virus type 1 (HSV-1) mutant, hrR3. Mice treated with hrR3 were killed and organs were harvested for lacZ staining and PCR analysis. Inactivation of oncolytic virus in bloodstream was assessed by neutralization assay in vitro. Infectious activity of hrR3 with vascular endothelial cells was evaluated by replication and cytotoxicity assay.

RESULTS

The survival rate of animals treated by hrR3 was significantly improved compared with the untreated group. lacZ staining and PCR analysis demonstrated detectable virus in the tumor but not in normal tissue or other organs except for the adrenal glands. We also showed that vascular endothelial cells allowed virus replication, while normal hepatocytes did not, and human anti-HSV antibody revealed attenuation of the infectious activity of hrR3.

CONCLUSIONS

Intravascular delivery of hrR3 is effective in treating multiple liver metastases, however, several points must be kept in mind at the time of human clinical trials using intravascular virus administration in order to avoid critical side effects.

HIV-1(89.6) Gag expressed from a replication competent HSV-1 vector elicits persistent cellular immune responses in mice

We have constructed a replication competent, gamma(1)34.5-deleted herpes simplex virus type-1 (HSV-1) vector (J200) that expresses the gag gene from human immunodeficiency virus type-1, primary isolate 89.6 (HIV-1(89.6)), as a candidate vaccine for HIV-1. J200 replicates in vitro, resulting in abundant Gag protein production and accumulation in the extracellular media. Immunization of Balb/c mice with a single intraperitoneal injection of J200 elicited strong Gag-specific CD8 responses, as measured by intracellular IFN-gamma staining and flow cytometry analysis. Responses were highest between 6 weeks and 4 months, but persisted at 9 months post-immunization, the last time-point evaluated. These data highlight the potential utility of neuroattenuated, replication competent HSV-1 vectors for delivery of HIV-1 immunogens.

Induction of strong antitumor immunity by an HSV-2-based oncolytic virus in a murine mammary tumor model

Oncolytic viruses have shown considerable promise for the treatment of solid tumors. In previous studies, we demonstrated that a novel oncolytic virus (FusOn-H2), constructed by replacing the serine/threonine protein kinase (PK) domain of the ICP10 gene of type 2 herpes simplex virus (HSV-2) with the gene encoding the green fluorescent protein, can selectively replicate in and thus lyse tumor cells. 4T1 tumor cells are weakly immunogenic and the mammary tumors derived from them aggressively metastasize to different parts of body, thus providing an attractive model for evaluating anticancer agents. We thus tested the antitumor effect of FusOn-H2 in this tumor model, in comparisons with several other oncolytic HSVs derived from HSV-1, including a nonfusogenic HSV-1 (Baco-1) and a doubly fusogenic virus (Synco-2D). Our results show that FusOn-H2 and Synco-2D have greater oncolytic activity in vitro than Baco-1. Moreover, FusOn-H2 induced strong T cell responses against primary and metastatic mammary tumors in vivo, and splenocytes adoptively transferred from FusOn-H2-treated mice effectively prevented metastasis in naïve mice bearing implanted mammary tumors. We conclude that the HSV-2-based FusOn-H2 oncolytic virus may be an effective agent for the treatment of both primary and metastatic breast cancer.

Expression of human immunodeficiency virus type 1 tat from a replication-deficient herpes simplex type 1 vector induces antigen-specific T cell responses

Herpes simplex type-1 virus (HSV-1) based vectors have been widely used in different gene therapy approaches and also as experimental vaccines against HSV-1 infection. Recent advances in the HSV-1 technology do support the use of replication defective HSV-1 as vaccine vectors for delivery of foreign antigens. We have examined the ability of a recombinant replication-defective HSV-1 vector expressing the HIV-1 Tat protein to induce long-term Tat-specific immune responses in the Balb/c murine model. The results showed that vector administration by the subcutaneous route elicits anti-Tat specific T-cell mediated immune responses in mice characterized by the presence of the Tat-specific cytotoxic activity and production of high levels of IFN-gamma.

Herpes simplex virus 1 (HSV-1) for cancer treatment

Cancer remains a serious threat to human health, causing over 500 000 deaths each year in US alone, exceeded only by heart diseases. Many new technologies are being developed to fight cancer, among which are gene therapies and oncolytic virotherapies. Herpes simplex virus type 1 (HSV-1) is a neurotropic DNA virus with many favorable properties both as a delivery vector for cancer therapeutic genes and as a backbone for oncolytic viruses. Herpes simplex virus type 1 is highly infectious, so HSV-1 vectors are efficient vehicles for the delivery of exogenous genetic materials to cells. The inherent cytotoxicity of this virus, if harnessed and made to be selective by genetic manipulations, makes this virus a good candidate for developing viral oncolytic approach. Furthermore, its large genome size, ability to infect cells with a high degree of efficiency, and the presence of an inherent replication controlling mechanism, the thymidine kinase gene, add to its potential capabilities. This review briefly summarizes the biology of HSV-1, examines various strategies that have been used to genetically modify the virus, and discusses preclinical as well as clinical results of the HSV-1-derived vectors in cancer treatment.

Reduced immune responses after vaccination with a recombinant herpes simplex virus type 1 vector in the presence of antiviral immunity

Due to the continuous need for new vaccines, viral vaccine vectors have become increasingly attractive. In particular, herpes simplex virus type 1 (HSV-1)-based vectors offer many advantages, such as broad cellular tropism, large DNA-packaging capacity and the induction of pro-inflammatory responses. However, despite promising results obtained with HSV-1-derived vectors, the question of whether pre-existing virus-specific host immunity affects vaccine efficacy remains controversial. For this reason, the influence of pre-existing HSV-1-specific immunity on the immune response induced with a replication-defective, recombinant HSV-1 vaccine was investigated in vivo. It was shown that humoral as well as cellular immune responses against a model antigen encoded by the vaccine were strongly diminished in HSV-1-seropositive mice. This inhibition could be observed in mice infected with wild-type HSV-1 or with a replication-defective vector. Although these data clearly indicate that pre-existing antiviral host immunity impairs the efficacy of HSV-1-derived vaccine vectors, they also show that vaccination under these constraints might still be feasible.

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.

Variability in infectivity of primary cell cultures of human brain tumors with HSV-1 amplicon vectors

We investigated the variability in infectivity of cells in primary brain tumor samples from different patients using an HSV-1 amplicon vector. We studied the infectivity of HSV-1 amplicon vectors in tumor samples derived from neurosurgical resections of 20 patients. Cells were infected with a definite amount of HSV-1 amplicon vector HSV-GFP. Transduction efficiency in primary tumor cell cultures was compared to an established human glioma line. Moreover, duration of transgene expression was monitored in different tumor cell types. All primary cell cultures were infectable with HSV-GFP with variable transduction efficiencies ranging between 3.0 and 42.4% from reference human Gli36 Delta EGFR glioma cells. Transduction efficiency was significantly greater in anaplastic gliomas and meningiomas (26.7+/-17.4%) compared to more malignant tumor types (glioblastomas, metastases; 11.2+/-8.5%; P=0.05). To further investigate the possible underlying mechanism of this variability, nectin-1/HevC expression was analyzed and was found to contribute, at least in part, to this variability in infectability. The tumor cells expressed the exogenous gene for 7 to 61 days with significant shorter expression in glioblastomas (18+/-13 d) compared to anaplastic gliomas (42+/-24 d; P<0.05). Interindividual variability of infectivity by HSV-1 virions might explain, at least in part, why some patients enrolled in gene therapy for glioblastoma in the past exhibited a sustained response to HSV-1-based gene- and virus therapy. Infectivity of primary tumor samples from respective patients should be tested to enable the development of efficient and safe herpes vector-based gene and virus therapy for clinical application.

Spread and replication of and immune response to gamma134.5-negative herpes simplex virus type 1 vectors in BALB/c mice

We have previously shown that intracranial infection of herpes simplex virus type 1 (HSV-1) vector R8306 expressing interleukin-4 (IL-4) can abolish symptoms of experimental autoimmune encephalomyelitis, which is used as a model for human multiple sclerosis (Broberg et al., Gene Ther. 8:769-777, 2001). The aim of the current study was to search for means other than intracranial injection to deliver HSV-derived vectors to the central nervous system of mice. We also aimed to study the replication efficiency of these vectors in nervous system tissues and to elucidate the effects of the viruses on the immune response. We studied the spread and replication of the following viruses with deletions in neurovirulence gene gamma(1)34.5: R3616, R849 (lacZ transgene), R3659 (alpha-tk), R8306 (murine IL-4 transgene), and R8308 (murine IL-10 transgene). The samples were taken from trigeminal ganglia and brains of BALB/c mice after corneal, intralabial, and intranasal infection, and the viral load was examined by viral culture, HSV DNA PCR, and VP16 reverse transcription (RT)-PCR. The results show that (i) intranasal infection was the most efficient means of spread to the central nervous system (CNS) besides intracranial injection; (ii) the viruses did not grow in the culture from the brain samples, but the viral DNA persisted even until day 21 postinfection; (iii) viral replication, as observed by VP16 mRNA RT-PCR, occurred mainly on days 4 and 7 postinfection in trigeminal ganglia and to a low extent in brain; (iv) R3659, R8306, and R8308 showed reactivation from the trigeminal ganglia in explant cultures; (v) in the brain, the vectors spread to the midbrain more efficiently than to other brain areas; and (vi) the deletions in the R3659 genome significantly limited the ability of this virus to replicate in the nervous system. The immunological studies show that (i) the only recombinant to induce IL-4 mRNA expression in the brain was R8306, the gamma interferon response was very low in the brain for R3659 and R8306, and the IL-23p19 response to R8306 decreased by day 21 postinfection, unlike for the other viruses; (ii) Deltagamma(1)34.5 HSV vectors modulated the subsets of the splenocytes differently depending on the transgene; (iii) R3659 infection of the nervous system induces expression and production of cytokines from the stimulated splenocytes; and (iv) HSV vectors expressing IL-4 or IL-10 induce expression and production of both of the Th2-type cytokines from splenocytes. We conclude that the intranasal route of infection is a possible means of delivery of Deltagamma(1)34.5 HSV vectors to the CNS in addition to intracranial infection, although replication in the CNS remains minimal. The DNA of the HSV vectors is able to reside in the brain for at least 3 weeks. The features of the immune response to the vectors must be considered and may be exploited in gene therapy experiments with these vectors.

Augmentation of antitumor immune responses by multiple intratumoral inoculations of replication-conditional HSV and interleukin-12

Intratumoral inoculation with a herpes simplex virus (HSV) mutant, G207, as an in situ cancer vaccine has been shown to inhibit tumor growth by inducing tumor-specific immune responses. Here, as a step toward the clinical application of this therapeutic approach, we evaluated different protocols for enhancing the antitumor effect. First, in a bilaterally established tumor model with CT26 colon carcinoma, we demonstrated that multiple intratumoral inoculations with G207 induced a greater antitumor effect on both the inoculated and distant tumors than did 1 or 2 inoculations. Second, to boost this antitumor effect, we developed 2 strategies: multiple in situ cancer vaccines with G207 in combination with systemic administration of recombinant interleukin-12 (rIL-12) (G207/systemic rIL-12) or local administration of rIL-12 (G207/local rIL-12). The antitumor effects in both the inoculated and distant tumors by the combined treatments were significantly greater than by either G207 or rIL-12 treatment alone. G207/systemic rIL-12 and G207/local rIL-12 mediated the complete regression of both the inoculated and distant tumors in 67% and 79% of the animals, respectively. These results indicate that multiple intratumoral inoculations of G207 and systemic or local rIL-12 administration work synergistically to facilitate tumor regression and that this combination of treatments may have potential for treating cancer metastasis.

IL-4 is the key regulator in herpes simplex virus-based gene therapy of BALB/c experimental autoimmune encephalomyelitis

Local delivery of cytokines or other immunomodulatory components has been applied as a potential therapy for experimental autoimmune encephalomyelitis (EAE), which is used as a model of human multiple sclerosis. We have used herpes simplex virus-based vectors expressing Th2 cytokines IL-4 and IL-10 and have previously shown a significant abolishment of disease symptoms by the virus expressing IL-4 (R8306), but not by the one expressing IL-10 (R8308). In the present study, the aim was to investigate the local and systemic cytokine response after HSV-based gene therapy. We show that the local expression of IL-4 from an HSV vector delivered to the brain converts the cytokine environment from the disease-promoting Th1-prominent to the disease-limiting IL-4 expressing type. We measured the expression of cytokines IL-4, IL-10, IFN-gamma, IL-12p35, IL-12p40 and the novel IL-23p19 from the brain by quantitative LightCycler RT-PCR. We also investigated the systemic cytokine response from the mouse sera. The results indicate that an increase in the Th2 cytokine IL-4 is observed if the diseased mice are treated with IL-4-expressing virus R8306. Surprisingly, the IL-23 expression of R8306 treated mice was at the same level as in the untreated EAE mice. On the contrary, in the R8308 (IL-10 expression) treated mice, the expression of IL-23 was decreased (P < 0.05). We conclude that the favorable effect of IL-4 on the disease development is more important than the downregulation of the Th1 type cytokines (like IL-23), and that IL-4 would be the key mediator of disease abolishment during gene therapy using these vectors.

HSV-1 therapy of primary tumors reduces the number of metastases in an immune-competent model of metastatic breast cancer

The HSV-1 1716 mutant virus and similar oncolytic herpesviruses deficient in the gamma 34.5 neurovirulence gene are able to reduce the growth of tumors in mice. Here we demonstrate that HSV-1 1716 therapy moderately reduced the growth of tumors of the highly malignant, spontaneously metastasizing 4T1 mouse mammary carcinoma model. This moderate effect on 4T1 tumor growth was likely due to poor replication kinetics of HSV-1 1716 in 4T1 cells. Interestingly, HSV-1 therapy of the primary tumor increased the survival time of mice. Coincident with this increase was a reduction in metastases as determined by quantification of the number of metastatic cells in the lungs. HSV-1 therapy of the primary tumor was also able to reduce the establishment of a second challenge of 4T1 tumors. Moreover, infiltrates of both CD4(+) and CD8(+) T cells were detected in HSV-1 1716-treated tumors. An important role for the T cell infiltrates was confirmed when HSV-1 therapy did not reduce the growth of 4T1 tumors in SCID mice. Collectively, these results demonstrate that an HSV-dependent anti-tumor immune response is required for the reduction in primary 4T1 tumor growth and for the reduction in the establishment of metastases in this tumor model.

Effects of innate immunity on herpes simplex virus and its ability to kill tumor cells

Several clinical trials have or are being performed testing the safety and efficacy of different strains of oncolytic viruses (OV) for malignant cancers. OVs represent either naturally occurring or genetically engineered strains of viruses that exhibit relatively selective replication in tumor cells. Several types of OV have been derived from herpes simplex virus 1 (HSV1). Tumor oncolysis depends on the processes of initial OV infection of tumor, followed by subsequent propagation of OV within the tumor itself. The role of the immune responses in these processes has not been extensively studied. On the contrary, effects of the immune response on the processes of wild-type HSV1 infection and propagation in the central nervous system have been studied and described in detail. The first line of defense against a wild-type HSV1 infection in both naive and immunized individuals is provided by innate humoral (complement, cytokines, chemokines) and cellular (macrophages, neutrophils, NK cells, gammadelta T cells, and interferon-producing cells) responses. These orchestrate the lysis of virions and virus-infected cells as well as provide a link to effective adaptive immunity. The role of innate defenses in curtailing the oncolytic effect of genetically engineered HSV has only recently been studied, but several of the same host responses appear to be operative in limiting anticancer effects by the replicating virus. The importance of this knowledge lies in finding avenues to modulate such initial innate responses, in order to allow for increased oncolysis of tumors while minimizing host toxicity.

Genetic metamorphosis of herpes simplex virus-1 into a biological therapeutic for human cancer

Over the past decade, rapid progress has been made in engineering safe, replicating herpes simplex virus-1 (HSV-1) mutants for use as biological oncolytic agents in the treatment of human cancer. While initial efforts demonstrated the potential of HSV-1 mutants as antitumour agents, they relied on viruses that were not sufficiently attenuated. Following its identification as the major viral neurovirulence determinant, mutations in the gamma34.5 gene were subsequently incorporated into oncolytic strains. Despite the fact that gamma34.5 mutant derivatives can be safely administered to mice, non-human primates and humans, their efficacy is limited because, like many weakened viral strains, they replicate poorly in a number of cell types, including cancer cells. Strategies to improve the oncolytic properties of gamma34.5 mutant derivatives through further genetic manipulation are reviewed. In addition, traditional treatment modalities that incorporate viral inoculation, along with efforts to elicit an antitumour immune response following treatment with gamma34.5 derivatives, are discussed.

ICP34.5 deleted herpes simplex virus with enhanced oncolytic, immune stimulating, and anti-tumour properties

Herpes simplex virus type-1 (HSV1) in which the neurovirulence factor ICP34.5 is inactivated has been shown to direct tumour-specific cell lysis in several tumour models. Such viruses have also been shown to be safe in Phase I clinical trials by intra-tumoral injection in glioma and melanoma patients. Previous work has used serially passaged laboratory isolates of HSV1 which we hypothesized may be attenuated in their lytic capability in human tumour cells as compared to more recent clinical isolates. To produce ICP34.5 deleted HSV with enhanced oncolytic potential, we tested two clinical isolates. Both showed improved cell killing in all human tumour cell lines tested compared to a laboratory strain (strain 17+). ICP34.5 was then deleted from one of the clinical isolate strains (strain JS1). Enhanced tumour cell killing with ICP34.5 deleted HSV has also been reported by the deletion of ICP47 by the up-regulation of US11 which occurs following this mutation. Thus to further improve oncolytic properties, ICP47 was removed from JS1/ICP34.5-. As ICP47 also functions to block antigen processing in HSV infected cells, this mutation was also anticipated to improve the immune stimulating properties of the virus. Finally, to provide viruses with maximum oncolytic and immune stimulating properties, the gene for human or mouse GM-CSF was inserted into the JS1/34.5-/47- vector backbone. GM-CSF is a potent immune stimulator promoting the differentiation of progenitor cells into dendritic cells and has shown promise in clinical trials when delivered by a number of means. Combination of GM-CSF with oncolytic therapy may be particularly effective as the necrotic cell death accompanying virus replication should serve to effectively release tumour antigens to then induce a GM-CSF-enhanced immune response. This would, in effect, provide an in situ, patient-specific, anti-tumour vaccine. The viruses constructed were tested in vitro in human tumour cell lines and in vivo in mice demonstrating significant anti-tumour effects. These were greatly improved compared to viruses not containing each of the modifications described. In vivo, both injected and non-injected tumours showed significant shrinkage or clearance and mice were protected against re-challenge with tumour cells. The data presented indicate that JS1/ICP34.5-/ICP47-/GM-CSF acts as a powerful oncolytic agent which may be appropriate for the treatment of a number of solid tumour types in man.

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.