B-myb promoter retargeting of herpes simplex virus gamma34.5 gene-mediated virulence toward tumor and cycling cells

Breaking the barriers in cancer care: The next generation of herpes simplex virus-based oncolytic immunotherapies for cancer treatment

Since the US Food and Drug Administration first approved talimogene laherparepvec for the treatment of melanoma in 2015, the field of oncolytic immunotherapy (OI) has rapidly evolved. There are numerous ongoing clinical studies assessing the clinical activity of OIs across a wide range of tumor types. Further understanding of the mechanisms underlying the anti-tumor immune response has led to the development of OIs with improved immune-mediated preclinical efficacy. In this review, we discuss the key approaches for developing the next generation of herpes simplex virus-based OIs. Modifications to the viral genome and incorporation of transgenes to promote safety, tumor-selective replication, and immune stimulation are reviewed. We also review the advantages and disadvantages of intratumoral versus intravenous administration, summarize clinical evidence supporting the use of OIs as a strategy to overcome resistance to immune checkpoint blockade, and consider emerging opportunities to improve OI efficacy in the combination setting.

Current Status and Challenges of Oncolytic Virotherapy for the Treatment of Glioblastoma

Despite decades of research and numerous clinical trials, the prognosis of patients diagnosed with glioblastoma (GBM) remains dire with median observed survival at 8 months. There is a critical need for novel treatments for GBM, which is the most common malignant primary brain tumor. Major advances in cancer therapeutics such as immune checkpoint inhibitors and chimeric antigen receptor (CAR) T-cell therapy have not yet led to improved outcomes for GBM. Conventional therapy of surgery followed by chemoradiation with or without tumor treating fields remains the standard of care. One of the many approaches to GBM therapy currently being explored is viral therapies. These typically work by selectively lysing target neoplastic cells, called oncolysis, or by the targeted delivery of a therapeutic transgene via a viral vector. In this review, we discuss the underlying mechanisms of action and describe both recent and current human clinical trials using these viruses with an emphasis on promising viral therapeutics that may ultimately break the field's current stagnant paradigm.

Progress of oncolytic virotherapy for neuroblastoma

As a neuroendocrine tumor derived from the neural crest, neuroblastoma (NB) is the most common extracranial solid tumor in children. The prognosis in patients with low- and intermediate-risk NB is favorable while that in high-risk patients is often detrimental. However, the management of the considerably large proportion of high-risk patients remains challenging in clinical practice. Among various new approaches, oncolytic virus (OV) therapy offers great advantages in tumor treatment, especially for high-risk NB. Genetic modified OVs can target NB specifically without affecting normal tissue and avoid the widespread drug resistance issue in anticancer monotherapy. Meanwhile, its safety profile provides great potential in combination therapy with chemo-, radio-, and immunotherapy. The therapeutic efficacy of OV for NB is impressive from bench to bedside. The effectiveness and safety of OVs have been demonstrated and reported in studies on children with NB. Furthermore, clinical trials on some OVs (Celyvir, Pexa-Vec (JX-594) and Seneca Valley Virus (NTX-010)) have reported great results. This review summarizes the latest evidence in the therapeutic application of OVs in NB, including those generated in cell lines, animal models and clinical trials.

Oncolytic HSV: Underpinnings of Tumor Susceptibility

Oncolytic herpes simplex virus (oHSV) is a therapeutic modality that has seen substantial success for the treatment of cancer, though much remains to be improved. Commonly attenuated through the deletion or alteration of the γ₁34.5 neurovirulence gene, the basis for the success of oHSV relies in part on the malignant silencing of cellular pathways critical for limiting these viruses in healthy host tissue. However, only recently have the molecular mechanisms underlying the success of these treatments begun to emerge. Further clarification of these mechanisms can strengthen rational design approaches to develop the next generation of oHSV. Herein, we review our current understanding of the molecular basis for tumor susceptibility to γ₁34.5-attenuated oHSV, with particular focus on the malignant suppression of nucleic acid sensing, along with strategies meant to improve the clinical efficacy of these therapeutic viruses.

Microorganisms in chemotherapy for pancreatic cancer: An overview of current research and future directions

Pancreatic cancer is a malignant tumor of the digestive system with a very high mortality rate. While gemcitabine-based chemotherapy is the predominant treatment for terminal pancreatic cancer, its therapeutic effect is not satisfactory. Recently, many studies have found that microorganisms not only play a consequential role in the occurrence and progression of pancreatic cancer but also modulate the effect of chemotherapy to some extent. Moreover, microorganisms may become an important biomarker for predicting pancreatic carcinogenesis and detecting the prognosis of pancreatic cancer. However, the existing experimental literature is not sufficient or convincing. Therefore, further exploration and experiments are imperative to understanding the mechanism underlying the interaction between microorganisms and pancreatic cancer. In this review, we primarily summarize and discuss the influences of oncolytic viruses and bacteria on pancreatic cancer chemotherapy because these are the two types of microorganisms that are most often studied. We focus on some potential methods specific to these two types of microorganisms that can be used to improve the efficacy of chemotherapy in pancreatic cancer therapy.

The Current Landscape of Oncolytic Herpes Simplex Viruses as Novel Therapies for Brain Malignancies

Despite advances in surgical resection and chemoradiation, high-grade brain tumors continue to be associated with significant morbidity/mortality. Novel therapeutic strategies and approaches are, therefore, desperately needed for patients and their families. Given the success experienced in treating multiple other forms of cancer, immunotherapy and, in particular, immunovirotherapy are at the forefront amongst novel therapeutic strategies that are currently under investigation for incurable brain tumors. Accordingly, herein, we provide a focused mini review of pertinent oncolytic herpes viruses (oHSV) that are being investigated in clinical trials.

Oncolytic Virotherapy in Glioma Tumors

Glioma tumors are one of the most devastating cancer types. Glioblastoma is the most advanced stage with the worst prognosis. Current therapies are still unable to provide an effective cure. Recent advances in oncolytic immunotherapy have generated great expectations in the cancer therapy field. The use of oncolytic viruses (OVs) in cancer treatment is one such immune-related therapeutic alternative. OVs have a double oncolytic action by both directly destroying the cancer cells and stimulating a tumor specific immune response to return the ability of tumors to escape the control of the immune system. OVs are one promising alternative to conventional therapies in glioma tumor treatment. Several clinical trials have proven the feasibility of using some viruses to specifically infect tumors, eluding undesired toxic effects in the patient. Here, we revisited the literature to describe the main OVs proposed up to the present moment as therapeutic alternatives in order to destroy glioma cells in vitro and trigger tumor destruction in vivo. Oncolytic viruses were divided with respect to the genome in DNA and RNA viruses. Here, we highlight the results obtained in various clinical trials, which are exploring the use of these agents as an alternative where other approaches provide limited hope.

The limiting factors of oncolytic virus immunotherapy and the approaches to overcome them

Oncolytic virus (OV) immunotherapy is characterized by viruses which specifically target cancer cells and cause their cytolysis. They provide a unique and promising new tool for the eradication of cancer as they interact with and affect the tumor microenvironment (TME), vasculature, and immune system. Advancements of genetic engineering have allowed for these viruses to be armed in such a way to have enhanced targeting, strong immunomodulation properties, and an ability to modify the TME. However, there are still major limitations in their use, mostly due to difficulties in delivering the viral particles to the tumors and in ensuring that the immunomodulatory properties are able to stimulate the host immune response to mount a complete response. Using novel delivery systems and using OVs as a complementary therapy in a combinatorial treatment have shown some significant successes. In this review, we discuss the major issues and difficulties in using OVs as anti-tumor agents and some of the strategies put in place so far to overcome these limitations. KEY POINTS: • Oncolytic viruses (OVs) infect cancer cells and cause their cytolysis. • The major limitations in using OVs as anti-tumor therapy were discussed. • The potential strategies to overcome these limitations were summarized.

Preclinical Toxicology of rQNestin34.5v.2: An Oncolytic Herpes Virus with Transcriptional Regulation of the ICP34.5 Neurovirulence Gene

rQNestin34.5v.2 is an oncolytic herpes simplex virus 1 (oHSV) that retains expression of the neurovirulent ICP34.5 gene under glioma-selective transcriptional regulation. To prepare an investigational new drug (IND) application, we performed toxicology and efficacy studies of rQNestin34.5v.2 in mice in the presence or absence of the immunomodulating drug cyclophosphamide (CPA). ICP34.5 allows HSV1 to survive interferon and improves viral replication by dephosphorylation of the eIF-2α translation factor. rQNestin34.5v.2 dephosphorylated eIF-2α in human glioma cells, but not in human normal cells, resulting in significantly higher cytotoxicity and viral replication in the former compared to the latter. In vivo toxicity of rQNestin34.5v.2 was compared with that of wild-type F strain in immunocompetent BALB/c mice and athymic mice by multiple routes of administration in the presence or absence of CPA. A likely no observed adverse effect level (NOAEL) dose for intracranial rQNestin34.5v.2 was estimated, justifying a phase 1 clinical trial in recurrent glioma patients (ClinicalTrials.gov: NCT03152318), after successful submission of an IND.

Transcriptional Regulation of Latency-Associated Transcripts (LATs) of Herpes Simplex Viruses

Herpes simplex viruses (HSVs) cause cold sores and genital herpes and can establish lifelong latent infection in neurons. An engineered oncolytic HSV (oHSV) has recently been approved to treat tumors in clinics. HSV latency-associated transcripts (LATs) are associated with the latent infection, but LAT transcriptional regulation was seldom reported. For a better treatment of HSV infection and tumors, here we sequenced the LAT encoding DNA and LAT transcription regulatory region of our recently isolated new strain HSV-1-LXMW and did comparative analysis of the sequences together with those of other four HSV-1 and two HSV-2 strains. Phylogenetic analysis of LATs revealed that HSV-1-LXMW is evolutionarily close to HSV-1-17 from MRC University, Glasgow, UK. For the first time, Using a weight matrix-based program Match and multi-sequences alignment of the 6 HSV strains, we identified HSV LAT transcription regulatory sequences that bind to 9 transcription factors: AP-1, C-REL, Comp1, E2F, Hairy, HFH-3, Kr, TCF11/MAFG, v-Myb. Interestingly, these transcription regulatory sequences and factors are either conserved or unique among LATs of HSV-1 and HSV-2, suggesting they are potentially functional. Furthermore, literature analysis found that the transcription factors v-myb and AP-1 family member JunD are functional in regulating HSV gene transcription, including LAT transcription. For the first time, we discovered seven novel transcription factors and their corresponding transcription regulatory sequences of HSV LATs. Based on our findings and other reports, we proposed potential mechanisms of the initiation and maintenance of HSV latent infection. Our findings may have significant implication in our understanding of HSV latency and engineering of better oncolytic HSVs.

Identification of Putative UL54 (ICP27) Transcription Regulatory Sequences Binding to Oct-1, v-Myb, Pax-6 and Hairy in Herpes Simplex Viruses

An oncolytic herpes simplex virus (oHSV) has proven amenable in oncolytic virotherapy and was approved to treat melanoma. The immediate-early (IE) protein ICP27 encoded by gene UL54 is essential for HSV infection. Post-transcriptional modification of UL54 would increase tumor targeting of oHSVs. However, UL54 gene transcription regulatory sequences and factors were not reported yet. Here we isolated a new strain LXMW of type 1 HSV (HSV-1-LXMW) in China and found it's closely related to HSV-1 strains Patton and H129 in the US by the first and next generation DNA sequencing viral DNA phylogenetic analysis. Using a weight matrix-based program Match, we found the UL54 transcription regulatory sequences binding to the transcription factors Oct-1, v-Myb and Pax-6 in HSV-1-LXMW, while the sequences binding to Oct-1 and Hairy in a HSV-2 strain. Further validation showed that HSV-1 and HSV-2 shared the common sequence binding to Oct-1, but had unique sequences to bind v-Myb and Pax-6, or Hairy, respectively, by DNA sequence alignment of total 11 HSV strains. The published results howed that the expression of transcription factors is consistent with the tissue tropism of HSV-1 and HSV-2. In the current article a new HSV-1 strain LXMW was isolated and its putative HSV UL54 transcription regulatory sequences and factors were identified for the first time. Our findings highlight the new understanding of the principles of transcriptional regulation in HSV biology and oncolytic virotherapy.

Oncolytic virotherapy using herpes simplex virus: how far have we come?

Oncolytic virotherapy exploits the properties of human viruses to naturally cytolysis of cancer cells. The human pathogen herpes simplex virus (HSV) has proven particularly amenable for use in oncolytic virotherapy. The relative safety of HSV coupled with extensive knowledge on how HSV interacts with the host has provided a platform for manipulating HSV to enhance the targeting and killing of human cancer cells. This has culminated in the approval of talimogene laherparepvec for the treatment of melanoma. This review focuses on the development of HSV as an oncolytic virus and where the field is likely to head in the future.

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.

Targeted oncolytic herpes simplex virus type 1 eradicates experimental pancreatic tumors

As many other cancers, pancreatic ductal adenocarcinoma (PDAC) progression is associated with a series of hallmark changes for cancer cells to secure their own growth success. Yet, these very changes render cancer cells highly sensitive to viral infection. A promising strategy may rely on and exploit viral replication for tumor destruction, whereby infection of tumor cells by a replication-conditional virus may lead to cell destruction and simultaneous release of progeny particles that can spread and infect adjacent tumor cells, while sparing healthy tissues. In the present study, we used Myb34.5, a second-generation replication-conditional herpes simplex virus type 1 (HSV-1) mutant in which ICP6 gene expression is defective and expression of the HSV-1 γ134.5 gene is regulated by the cellular B-myb promoter. We found that B-myb is present in experimental PDAC and tumors, and is overexpressed in patients' tumors, as compared with normal adjacent pancreas. Myb34.5 replicates to high level in human PDAC cell lines and is associated with cell death by apoptosis. In experimental models of PDAC, mice receiving intratumoral Myb34.5 injections appeared healthy and tumor progression was inhibited, with evidence of tumor necrosis, hemorrhage, viral replication, and cancer cell death by apoptosis. Combining standard-of-care chemotherapy with Myb34.5 successfully led to a very impressive antitumoral effect that is rarely achieved in this experimental model, and resulted in a greater reduction in tumor growth than chemotherapy alone. These promising results warrant further evaluation in early phase clinical trial for patients diagnosed with PDAC for whom no effective treatment is available.

From scourge to cure: tumour-selective viral pathogenesis as a new strategy against cancer

Tumour mutations corrupt cellular pathways, and accumulate to disrupt, dysregulate, and ultimately avoid mechanisms of cellular control. Yet the very changes that tumour cells undergo to secure their own growth success also render them susceptible to viral infection. Enhanced availability of surface receptors, disruption of antiviral sensing, elevated metabolic activity, disengagement of cell cycle controls, hyperactivation of mitogenic pathways, and apoptotic avoidance all render the malignant cell environment highly supportive to viral replication. The therapeutic use of oncolytic viruses (OVs) with a natural tropism for infecting and subsequently lysing tumour cells is a rapidly progressing area of cancer research. While many OVs exhibit an inherent degree of tropism for transformed cells, this can be further promoted through pharmacological interventions and/or the introduction of viral mutations that generate recombinant oncolytic viruses adapted to successfully replicate only in a malignant cellular environment. Such adaptations that augment OV tumour selectivity are already improving the therapeutic outlook for cancer, and there remains tremendous untapped potential for further innovation.

Modification of HSV-1 to an oncolytic virus

Cancer-permissive viruses or oncolytic viruses consist of either genetically engineered or naturally occurring strains that possess relatively selective replicative and/or infection abilities for cancer vs. normal cells (Chiocca, Nat Rev Cancer 2: 938-950, 2002). They can also be armed with additional anticancer cDNAs (e.g., cytokines, prodrug-activating, anti-angiogenesis genes, and others) to extend therapeutic effects (Kaur et al., Curr Gene Ther 9: 341-355, 2009). Herpes simplex virus type 1 (HSV-1) possesses several advantages as an oncolytic virus such as a rapid lytic cycle and a large capacity for insertion of heterologous DNA sequences (Wade-Martins et al., Nat Biotechnol, 19: 1067-1070, 2001). However, the technical nuances of genetic manipulation of the HSV-1 genome may still be relatively challenging. Here, we describe a system that has been durable and consistent in providing the ability to generate multiple recombinant HSV-1. The HsvQuik technology utilizes an HSV-1 genome cloned in a bacterial artificial chromosome to recombine heterologous cDNAs in a relatively rapid and reliable manner (Terada et al., Gene Ther 13: 705-714, 2006).

New viruses for cancer therapy: meeting clinical needs

Early-stage clinical trials of oncolytic virotherapy have reported the safety of several virus platforms, and viruses from three families have progressed to advanced efficacy trials. In addition, preclinical studies have established proof-of-principle for many new genetic engineering strategies. Thus, the virotherapy field now has available a diverse collection of viruses that are equipped to address unmet clinical needs owing to improved systemic administration, greater tumour specificity and enhanced oncolytic efficacy. The current key challenge for the field is to develop viruses that replicate with greater efficiency within tumours while achieving therapeutic synergy with currently available treatments.

Preclinical evaluation of engineered oncolytic herpes simplex virus for the treatment of neuroblastoma

Despite intensive research efforts and therapeutic advances over the last few decades, the pediatric neural crest tumor, neuroblastoma, continues to be responsible for over 15% of pediatric cancer deaths. Novel therapeutic options are needed for this tumor. Recently, investigators have shown that mice with syngeneic murine gliomas treated with an engineered, neuroattenuated oncolytic herpes simplex virus-1 (oHSV), M002, had a significant increase in survival. M002 has deletions in both copies of the γ₁34.5 gene, enabling replication in tumor cells but precluding infection of normal neural cells. We hypothesized that M002 would also be effective in the neural crest tumor, neuroblastoma. We showed that M002 infected, replicated, and decreased survival in neuroblastoma cell lines. In addition, we showed that in murine xenografts, treatment with M002 significantly decreased tumor growth, and that this effect was augmented with the addition of ionizing radiation. Importantly, survival could be increased by subsequent doses of radiation without re-dosing of the virus. Finally, these studies showed that the primary entry protein for oHSV, CD111 was expressed by numerous neuroblastoma cell lines and was also present in human neuroblastoma specimens. We concluded that M002 effectively targeted neuroblastoma and that this oHSV may have potential for use in children with unresponsive or relapsed neuroblastoma.

Questing for an optimal, universal viral agent for oncolytic virotherapy

One of the most promising strategies to treat cancer is attacking it with viruses designed to exploit specific altered pathways. Here, the effects of oncolytic virotherapy on tumors having compact, papillary, and disconnected morphologies are investigated through computer simulations of a multiscale model coupling macroscopic reaction-diffusion equations for the nutrients with microscopic stochastic rules for the actions of individual cells and viruses. The interaction among viruses and tumor cells involves cell infection, intracellular virus replication, and the release of new viruses in the tissue after cell lysis. The evolution over time of both the viral load and cancer cell population, as well as the probabilities for tumor eradication, were evaluated for a range of multiplicities of infection, viral entries, and burst sizes. It was found that in immunosuppressed hosts, the antitumor efficacy of a virus is primarily determined by its entry efficiency, its replicative capacity within the tumor, and its ability to spread over the tissue. However, the optimal traits for oncolytic viruses depend critically on the tumor growth dynamics and do not necessarily include rapid replication, cytolysis, or spreading, currently assumed as necessary conditions for a successful therapeutic outcome. Our findings have potential implications on the design of new vectors for the viral therapy of cancer.

Cancer stem cells: the final frontier for glioma virotherapy

Cancer stem cells (CSC) are a very small subset of all cancer cells and possess characteristics very similar to normal stem cells, in particular, the capacity for self-renewal, multipotency and relative quiescence. These chemo- and radiation resistant cells are responsible for maintaining tumor volume leading to therapy failure and recurrence. In glioblastoma multiforme (GBM), the most common primary intracranial malignancy, glioma stem cells have been implicated as one of the key players in treatment failure. Many novel treatment modalities are being investigated to specifically target this small group of cells. In this review, we shed light on one such targeted therapy, specifically, oncolytic virotherapy, and review the literature to highlight the advances and challenges in designing effective oncolytic virotherapy for glioma stem cells.

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.

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.

Development of a regulatable oncolytic herpes simplex virus type 1 recombinant virus for tumor therapy

Oncolytic viruses are genetically modified viruses that preferentially replicate in host cancer cells, leading to the production of new viruses and, ultimately, cell death. Currently, no oncolytic viruses that are able to kill only tumor cells while leaving normal cells intact are available. Using T-REx (Invitrogen, Carlsbad, CA) gene switch technology and a self-cleaving ribozyme, we have constructed a novel oncolytic HSV-1 recombinant, KTR27, whose replication can be tightly controlled and regulated by tetracycline in a dose-dependent manner. Infection of normal replicating cells as well as multiple human cancer cell types with KTR27 in the presence of tetracycline led to 1,000- to 250,000-fold-higher progeny virus production than in the absence of tetracycline, while little viral replication and virus-associated cytotoxicity was observed in infected growth-arrested normal human cells. We show that intratumoral inoculation with KTR27 markedly inhibits tumor growth in a xenograft model of human non-small-cell lung cancer in nude mice. It is shown further that replication of KTR27 in the inoculated tumors can be efficiently controlled by local codelivery of tetracycline to the target tumors at the time of KTR27 inoculation. Collectively, KTR27 possesses a unique pharmacological feature that can limit its replication to the targeted tumor microenvironment with localized tetracycline delivery, thus minimizing unwanted viral replication in distant tissues following local virotherapy. This regulatory mechanism would also allow the replication of the virus to be quickly shut down should adverse effects be detected.

Spontaneous and Engineered Compensatory HSV Mutants that Counteract the Host Antiviral PKR Response

A virulent recombinant HSV lacking the diploid γ₁34.5 gene (Δγ₁34.5) have been investigated over the last two decades both for anti-tumor therapy and as vaccine vectors. The first generation vectors, while safe, are incapable of sustained replication in the majority of treated patients. An interferon inducible host antiviral kinase, protein kinase R (PKR), limits late viral protein synthesis and replication of Δγ₁34.5 viruses. This review describes the development of new Δγ₁34.5 vectors, through serial passage selection and direct viral genome engineering, which demonstrate selective PKR evasion in targeted cells and improved viral replication without restoring neurovirulence.

Virotherapy against malignant glioma stem cells

Glioblastoma multiforme, the most common primary intracranial malignancy, is associated with very poor outcome despite advances in surgical techniques and chemo- and radiation therapy. Many novel treatment modalities are being investigated with varying amount of success. Evolution of cancer stem cell hypothesis provides a new venue for developmental therapeutics. In this review, we highlight the literature regarding the existence of glioma stem cells and their characteristics. We also discuss the potential for virotherapy, a novel therapeutic approach utilizing conditionally replicative viruses, to directly target this population of self-renewing cancer stem cells.

Targeting cancer by transcriptional control in cancer gene therapy and viral oncolysis

Cancer-specificity is the key requirement for a drug or treatment regimen to be effective against malignant disease--and has rarely been achieved adequately to date. Therefore, targeting strategies need to be implemented for future therapies to ensure efficient activity at the site of patients' tumors or metastases without causing intolerable side-effects. Gene therapy and viral oncolysis represent treatment modalities that offer unique opportunities for tumor targeting. This is because both the transfer of genes with anti-cancer activity and viral replication-induced cell killing, respectively, facilitate the incorporation of multiple mechanisms restricting their activity to cancer. To this end, cellular mechanisms of gene regulation have been successfully exploited to direct therapeutic gene expression and viral cell lysis to cancer cells. Here, transcriptional targeting has been the role model and most widely investigated. This approach exploits cellular gene regulatory elements that mediate cell type-specific transcription to restrict the expression of therapeutic genes or essential viral genes, ideally to cancer cells. In this review, we first discuss the rationale for such promoter targeting and its limitations. We then give an overview how tissue-/tumor-specific promoters are being identified and characterized. Strategies to apply and optimize such promoters for the engineering of targeted viral gene transfer vectors and oncolytic viruses-with respect to promoter size, selectivity and activity in the context of viral genomes-are described. Finally, we discuss in more detail individual examples for transcriptionally targeted virus drugs. First highlighting oncolytic viruses targeted by prostate-specific promoters and by the telomerase promoter as representatives of tissue-targeted and pan-cancer-specific virus drugs respectively, and secondly recent developments of the last two years.

Gene therapy: the end of the rainbow?

The increased understanding of the molecular basis of oral cancer has led to expectations that correction of the genetic defects will lead to improved treatments. Nevertheless, the first clinical trials for gene therapy of oral cancer occurred 20 years ago, and routine treatment is still not available. The major difficulty is that genes are usually delivered by virus vectors whose effects are weak and temporary. Viruses that replicate would be better, and the field includes many approaches in that direction. If any of these are effective in patients, then gene therapy will become available in the next few years. Without significant advances, however, the treatment of oral cancer by gene therapy will remain as remote as the legendary pot of gold at the end of the rainbow.

Histone deacetylase inhibitors augment antitumor efficacy of herpes-based oncolytic viruses

Replication-conditional (oncolytic) mutants of herpes simplex virus (HSV), are considered promising therapeutic alternatives for human malignancies, and chemotherapeutic adjuvants are increasingly sought to augment their efficacy. Histone deacetylase (HDAC) inhibitors are a new class of antineoplastic agents because of their potent activity in growth arrest, differentiation, and apoptotic death of cancer cells. The ability of the HDAC inhibitors to upregulate exogenous transgene expression and inhibit interferon (IFN) responses prompted our exploration of their use in improving the antitumor efficacy of oncolytic HSV. We discovered that the yield of viral progeny increased significantly when cultured glioma cells were treated with HDAC inhibitors before viral infection. Valproic acid (VPA), a commonly used antiepileptic agent with HDAC inhibitory activity, proved most effective when used to treat glioma cells before viral infection, but not concomitantly with viral infection. Pretreatment with VPA inhibited the induction of several IFN-responsive antiviral genes, augmented the transcriptional level of viral genes, and improved viral propagation, even in the presence of type I IFNs. Moreover, VPA pretreatment improved the propagation and therapeutic efficacy of oncolytic HSV in a human glioma xenograft model in vivo. These findings indicate that HDAC inhibitors can improve the efficacy of tumor virotherapies.

Reprogrammed viruses as cancer therapeutics: targeted, armed and shielded

Virotherapy is currently undergoing a renaissance, based on our improved understanding of virus biology and genetics and our better knowledge of many different types of cancer. Viruses can be reprogrammed into oncolytic vectors by combining three types of modification: targeting, arming and shielding. Targeting introduces multiple layers of cancer specificity and improves safety and efficacy; arming occurs through the expression of prodrug convertases and cytokines; and coating with polymers and the sequential usage of different envelopes or capsids provides shielding from the host immune response. Virus-based therapeutics are beginning to find their place in cancer clinical practice, in combination with chemotherapy and radiation.

Neurovirulent factor ICP34.5 uniquely expressed in the herpes simplex virus type 1 Delta gamma 1 34.5 mutant 1716

The herpes simplex virus type 1 (HSV-1) diploid gene gamma(1)34.5 encodes a neurovirulent factor, infected cell protein 34.5 (ICP34.5). The promoter to gamma(1)34.5 is located within the HSV-1 genome where there are repeated sequences. This region of the genome also contains important overlapping transcripts involved with the virus's ability to establish lytic and latent infections and reactivation. These transcripts include the latency-associated transcripts and regulator proteins ICP0 and ICP4. This study aimed to separate ICP34.5 from these overlapping transcripts and test if its expression from a single gene could restore wild-type HSV-1 strain 17+ virulence. To address these aims, different recombinant viruses were constructed using the Delta gamma(1)34.5 mutant 1716. Immunoblots probed with different ICP34.5 antisera demonstrated that one of the newly generated recombinant viruses, 1622, overexpresses ICP34.5 relative to a panel of wild-type viruses. Interestingly, the overexpression of ICP34.5 does not yield a more virulent virus. The onset of ICP34.5 expression from 1622-infected cells in vitro matched that of 17+, and its expression restored the function of maintaining protein synthesis in human neuroblastoma cells. Replication of 1622, however, was only partially restored to 17+ levels in vivo. Additionally, plaque morphology from 1622-infected cells indicates there is an additional defect. The authors report that the mutant virus 1622 can express ICP34.5 from a single gamma(1)34.5 gene and restore most (but not all) wild-type function. These findings are discussed with respect to the use of the gamma(1)34.5 deleted mutant, 1716, in oncolytic viral vector therapies and future studies for ICP34.5.

Feasibility of herpes simplex virus type 1 mutants labeled with radionuclides for tumor treatment

For over one hundred years, viruses have been recognized as capable of killing tumor cells. At present, people are still researching and constructing more suitable oncolytic viruses for treating different malignant tumors. Although extensive studies have demonstrated that herpes simplex virus type 1 (HSV-1) is the most potential oncolytic virus, therapies based on herpes simplex virus type 1 vectors still arouse bio-safety and risk management issues. Researchers have therefore introduced the new idea of treating cancer with HSV-1 mutants labeled with radionuclides, combining radionuclide and oncolytic virus therapies. This overview briefly summarizes the status and mechanisms by which oncolytic viruses kill tumor cells, discusses the application of HSV-1 and HSV-1 derived vectors for tumor therapy, and demonstrates the feasibility and prospect of HSV-1 mutants labeled with radionuclides for treating tumors.

Oncolytic virotherapy: molecular targets in tumor-selective replication and carrier cell-mediated delivery of oncolytic viruses

Tremendous advances have been made in developing oncolytic viruses (OVs) in the last few years. By taking advantage of current knowledge in cancer biology and virology, specific OVs have been genetically engineered to target specific molecules or signal transduction pathways in cancer cells in order to achieve efficient and selective replication. The viral infection and amplification eventually induce cancer cells into cell death pathways and elicit host antitumor immune responses to further help eliminate cancer cells. Specifically targeted molecules or signaling pathways (such as RB/E2F/p16, p53, IFN, PKR, EGFR, Ras, Wnt, anti-apoptosis or hypoxia) in cancer cells or tumor microenvironment have been studied and dissected with a variety of OVs such as adenovirus, herpes simplex virus, poxvirus, vesicular stomatitis virus, measles virus, Newcastle disease virus, influenza virus and reovirus, setting the molecular basis for further improvements in the near future. Another exciting new area of research has been the harnessing of naturally tumor-homing cells as carrier cells (or cellular vehicles) to deliver OVs to tumors. The trafficking of these tumor-homing cells (stem cells, immune cells and cancer cells), which support proliferation of the viruses, is mediated by specific chemokines and cell adhesion molecules and we are just beginning to understand the roles of these molecules. Finally, we will highlight some avenues deserving further study in order to achieve the ultimate goals of utilizing various OVs for effective cancer treatment.

An oncolytic mutant of herpes simplex virus type-1 in which replication is governed by a promoter/enhancer of human papillomavirus type-16

Although herpes simplex virus type-1 (HSV-1) can be used as an oncolytic virus it has the undesirable side effect of neurotoxicity. To create a virus with improved specificity for oral cancer we used a fragment of human papillomavirus type-16, which is frequently found in oral and cervical cancers, but not elsewhere. The upstream regulatory region, URR16, was shown to have a high level of transcriptional activity in three of four oral cancer cell lines but low activity in three cell lines derived from brain cancers. URR16 was therefore placed in HSV-1, replacing the promoter of the essential gene ICP4, and the resulting virus was named HSPV-1. When cells were infected with HSPV-1, ICP4 was expressed at levels that were not associated with the level of activity of URR16. The virus replicated in each type of cell to a final titer that showed a correlation with the level of expression of ICP4, but with no correlation to either the tumor of origin or the presence of HPV sequences in the cells. To find if some function of HSV-1 was affecting the activity of URR16, oral cancer cells were transfected with a URR-reporter construct and were then infected with virus. This induced transcription, which was attributed to immediate-early viral genes other than ICP4. A promoter/enhancer from a papillomavirus therefore has the potential to regulate the functions of an oncolytic strain of HSV-1, and is affected by functions of both the host cell and of HSV-1 itself.

Effects of tumor selective replication-competent herpes viruses in combination with gemcitabine on pancreatic cancer

PURPOSE

Pancreatic cancer still has a poor prognosis, even if aggressive therapy is pursued. Currently, new modalities of oncolytic virus therapy are being tested against this cancer. The combination of one of two representative mutant herpes simplex viruses (R3616: gamma(1)34.5 inactivated, hrR3: UL39 inactivated) with a standard anti-pancreatic cancer chemotherapy drug (gemcitabine), was investigated in this study.

EXPERIMENTAL DESIGN

The intracellular concentration of ribonucleotide reductase was estimated by Western blotting. The effect of gemcitabine on viral replication and the total cytotoxic effect of the combination therapy were investigated on pancreatic cancer cell lines. We compared the results of two oncolytic viruses, R3616 and hrR3. A mouse model of pancreatic cancer with peritoneal dissemination was used to evaluate the in vivo effect of the combination therapy.

RESULTS

Although the replication of both viruses was inhibited by gemcitabine, the combination caused more tumor cell cytotoxicity than did virus alone in vitro. The results with R3616 were more striking. Although the difference was not statistically significant, R3616 with gemcitabine had a greater effect than did R3616 alone, while hrR3 with gemcitabine had a weaker effect than did hrR3 alone in vivo experiments.

CONCLUSION

The combination of oncolytic virus with gemcitabine is a promising new strategy against advanced pancreatic cancer. Each virus has different functional characteristics, and can affect the results of the combination of viruses and chemotherapy drugs. The results indicate that there is a complicated interaction among viruses, cells, and chemotherapy drugs and that the best combination of oncolytic virus and chemotherapeutic agents should be studied more extensively before embarking on a clinical trial.

The status of gene therapy for brain tumors

The advent of gene therapy in the early 1990's raised expectations for brain tumor therapies; however, whereas clinical trials in patients with malignant gliomas provided evidence of safety, therapeutic benefit was not convincing. These early forays resembled the historical introductions of other therapies that seemed promising, only to fail in human trials. Nevertheless, re-study in the laboratory and retesting in iterative laboratory-clinic processes enabled therapies with strong biological rationales to ultimately show evidence of success in humans and become accepted. Examples, such as organ transplantation, monoclonal antibody therapy and antiangiogenic therapy, provide solace that a strategy's initial lack of success in humans provides an opportunity for its further refinement in the laboratory and development of solutions that will translate into patient success stories. The authors herein summarize results from clinical trials of gene therapy for malignant gliomas, and discuss the influence of these results on present thought in preclinical research.

Viruses as anticancer drugs

Oncolytic viruses are being developed as anticancer drugs. They propagate selectively in tumor tissue and destroy it without causing excessive damage to normal non-cancerous tissues. When used as drugs, they must meet stringent criteria for safety and efficacy and be amenable to pharmacological study in human subjects. Specificity for neoplastic tissue is the key to safety, and this goal can be achieved through a variety of ingenious virus-engineering strategies. Antiviral immunity remains a significant barrier to the clinical efficacy of oncolytic viruses but this is being addressed by using novel immune-evasive delivery strategies and immunosuppressive drugs. Noninvasive pharmacokinetic monitoring is facilitated by engineering marker genes into the viral genome. Clinical data on the pharmacokinetics of oncolytic viruses will be the key to accelerating their development and approval as effective anticancer drugs. This review introduces concepts relevant to the use of viruses as anticancer drugs, emphasizing targeting mechanisms as well as safety and efficacy issues that are currently limiting their clinical success.

Virotherapy with a type 2 herpes simplex virus-derived oncolytic virus induces potent antitumor immunity against neuroblastoma

PURPOSE

We recently constructed an oncolytic virus from type 2 herpes simplex virus (HSV-2) that selectively targets and kills tumor cells with an activated Ras signaling pathway. Designated FusOn-H2, this virus has shown several discrete killing mechanisms. Here, we evaluated the antitumor immune responses after FusOn-H2-mediated virotherapy in a syngeneic murine neuroblastoma model.

EXPERIMENTAL DESIGN

We directly injected FusOn-H2 into established tumors and then measured its antitumor effect and the accompanying tumor-specific immune responses. Several oncolytic HSVs constructed from HSV-1 were included in the same experiments for comparisons.

RESULTS

Our data show that tumor destruction by FusOn-H2 in vivo induces potent antitumor immune responses in this syngeneic neuroblastoma model. The elicited cellular immunity not only eradicated neuroblastoma cells in vitro but also inhibited the growth of tumors at sites distant from the virus injection site. Moreover, adoptive transfer of splenocytes from mice receiving virotherapy to naïve mice resulted in a measurable antitumor effect.

CONCLUSION

We conclude that the ability of FusOn-H2 to induce tumor-specific cellular immunity expands the oncolytic repertoire of this virus and increases the likelihood that its use in patients would produce significant therapeutic benefits.

Augmented therapeutic efficacy of an oncolytic herpes simplex virus type 1 mutant expressing ICP34.5 under the transcriptional control of musashi1 promoter in the treatment of malignant glioma

Although second-generation replication-conditional herpes simplex virus type 1 (HSV-1) vectors defective for both ribonucleotide reductase (RR) and the virulence factor gamma(1)34.5 have been proven safe through a number of animal experiments and clinical trials, their therapeutic efficacy was also markedly reduced. To overcome this situation, we concentrated on the use of a tumor-specific promoter in this study, to express ICP34.5 selectively in malignant glioma cells. As a molecular marker for malignant glioma, we focused on the neural RNA-binding protein, Musashi1. On the basis of the results of defective vector dvM345, as reported previously, we created, via homologous recombination, a novel HSV-1 vector termed KeM34.5, which expresses ICP34.5 under the transcriptional control of the musashi1 gene promoter (P/musashi1). Cytotoxicity mediated by KeM34.5 was significantly enhanced in human glioma cell lines (U87MG, U87MG-E6, U251, and T98G), resulting in an approximately 2-log increase in viral yield, compared with its parental vector G207. This virus also showed much higher therapeutic efficacy in the in vivo glioma model, while maintaining the desirable neuroattenuated phenotype. These results suggest that oncolytic HSV-1 expressing ICP34.5 under the transcriptional control of the musashi1 gene promoter could be a promising therapeutic agent for the treatment of malignant glioma.

Dominant-negative fibroblast growth factor receptor expression enhances antitumoral potency of oncolytic herpes simplex virus in neural tumors

PURPOSE

Oncolytic herpes simplex viruses (HSV) appear to be a promising platform for cancer therapy. However, efficacy as single agents has thus far been unsatisfactory. Fibroblast growth factor (FGF) signaling is important for the growth and migration of endothelial and tumor cells. Here, we examine the strategy of arming oncolytic HSV with a dominant-negative FGF receptor (dnFGFR) that targets the FGF signaling pathway.

EXPERIMENTAL DESIGN

A mouse Nf1:p53 malignant peripheral nerve sheath tumor (MPNST) cell line expressing dnFGFR was generated by transfection. The effects of dnFGFR expression on cell growth and migration in vitro and tumor formation in vivo were determined. The dnFGFR transgene was then inserted into oncolytic HSV G47Delta using a bacterial artificial chromosome construction system. Antitumoral and antiangiogenic activities of bG47Delta-dnFGFR were examined.

RESULTS

MPNST 61E4 cells expressing dnFGFR grew less well than parental control cells. bG47Delta-dnFGFR showed enhanced killing of both tumor (human U87 glioma and F5 malignant meningioma cells and murine MPNST 61E4 and 37-3-18-4 cells) and proliferating endothelial cells (human umbilical vascular endothelial cell and Py-4-1) in vitro compared with the control vector bG47Delta-empty without inhibiting viral replication. In vivo, bG47Delta-dnFGFR was more efficacious than its nonexpressing parent bG47Delta-empty at inhibiting tumor growth and angiogenesis in both human U87 glioma and mouse 37-3-18-4 MPNST tumors in nude mice.

CONCLUSIONS

By using multiple therapeutic mechanisms, including destruction of both tumor cells and tumor endothelial cells, an oncolytic HSV encoding dnFGFR enhances antitumor efficacy. This strategy can be applied to other oncolytic viruses and for clinical translation.

Enhanced therapeutic efficacy of oncolytic herpes vector G207 against human non-small cell lung cancer--expression of an RNA-binding protein, Musashi1, as a marker for the tailored gene therapy

BACKGROUND

Oncolytic herpes vectors like G207 have shown considerable promise in the treatment of solid tumors, but their potency must be enhanced for the full achievement of therapeutic efficacy. Deletion of the innate gamma34.5 gene made these vectors extremely safe, but their efficacy was also severely attenuated. Use of tumor-specific promoters is one method to direct toxicity and enhance efficacy against tumors. Recently, Musashi1 has been shown expressed in some tumor tissues.

METHODS

Eleven human cancer cell lines including five non-small cell lung cancers (NSCLCs) were investigated. Musashi1 mRNA expression was examined by RT-PCR analysis. Western blotting was also performed. Transcriptional activity of P/musashi1 in NSCLCs was assayed by GFP reporter plasmids. Then we constructed a defective amplicon vector containing musashi1 promoter/ICP34.5 with G207 as helper virus (dvM345). In vitro cytotoxicity against NSCLCs and growth characteristics of helper virus were examined. A Lu-99 subcutaneous tumor model was used in an animal study. The tumor volume treated with G207 alone or dvM345 was measured.

RESULTS

Musashi1 mRNA was detected in four cell lines. Two in five NSCLCs were positive, and P/musashi1 was proved functional within them. Against these cell lines, dvM345 showed enhanced cytotoxicity, and helper viral growth was augmented. A subcutaneous tumor study confirmed the enhanced therapeutic efficacy of G207 by dvM345 without compromising safety.

CONCLUSIONS

These results suggest that Musashi1 might be involved in the development of several carcinomas including NSCLC. In the context of oncolytic herpes vector strategy, the P/musashi1-ICP34.5 method could be used for the treatment of cancers expressing Musashi1.

Nectin-1 Expression by Squamous Cell Carcinoma is a Predictor of Herpes Oncolytic Sensitivity

Oncolytic viruses based on herpes simplex virus type 1 (HSV-1) are able to infect and lyse a variety of malignant cell lines. However, there is variability in the degree of tumor susceptibility, and the cancer cell determinants of HSV sensitivity are poorly defined. Nectin-1 is a cell surface adhesion molecule that functions as a cellular receptor to HSV envelope glycoprotein D (gD). We assessed tumor nectin-1 expression as a predictor of oncolytic HSV sensitivity. A panel of human squamous carcinoma cell lines was evaluated for viral entry, replication, and cytotoxicity to an attenuated, replication-competent, oncolytic HSV (NV1023). Potential tumor determinants of HSV sensitivity were assessed, including nectin-1, herpes viral entry mediator, total gD receptor expression, S-phase fraction, and doubling time. Significant correlations between nectin-1 expression measured by quantitative fluorescence-activated cell sorting and viral sensitivity measures were identified using Pearson's coefficients. Cancer cell nectin-1 receptor blockade and nectin-1 transfection led to inhibition and enhancement of NV1023 viral entry, respectively. Cell lines with varying nectin-1 expression showed corresponding sensitivity to NV1023 therapy in vivo. Immunohistochemistry for nectin-1 was inversely related to E-cadherin staining, suggesting increased herpes sensitivity of E-cadherin-deficient tumors. These results suggest that nectin-1 may be used as a marker to predict the sensitivity of a tumor to herpes oncolytic therapy.

Radiation-induced cellular DNA damage repair response enhances viral gene therapy efficacy in the treatment of malignant pleural mesothelioma

BACKGROUND

Malignant pleural mesothelioma (MPM) treated with radiotherapy (RT) has incomplete responses as a result of radiation-induced tumoral stress response that repairs DNA damage. Such stress response is beneficial for oncolytic viral therapy. We hypothesized that a combination of RT and NV1066, an oncolytic herpes virus, might exert an additive or synergistic effect in the treatment of MPM.

METHODS

JMN, a MPM cell line, was infected with NV1066 at multiplicities of infection of .05 to .25 in vitro with and without radiation (1 to 5 Gy). Virus replication was determined by plaque assay, cell kill by lactate dehydrogenase assay, and GADD34 (growth arrest and DNA damage repair 34, a DNA damage-repair protein) by real-time reverse transcriptase-polymerase chain reaction and Western blot test. Synergistic cytotoxicity dependence on GADD34 upregulation was confirmed by GADD34 small inhibitory RNA (siRNA).

RESULTS

Synergism was demonstrated between RT and NV1066 across a wide range of doses. As a result of such synergism, a dose-reduction for each agent (up to 5500-fold) can be accomplished over a wide range of therapeutic-effect levels without sacrificing tumor cell kill. This effect is correlated with increased GADD34 expression and inhibited by transfection of siRNA directed against GADD34.

CONCLUSIONS

RT can be combined with oncolytic herpes simplex virus therapy in the treatment of malignant pleural mesothelioma to achieve synergistic efficacy while minimizing dosage and toxicity.

Radiation therapy potentiates effective oncolytic viral therapy in the treatment of lung cancer

BACKGROUND

Replication-competent oncolytic herpes simplex viruses with deletion of the gamma(1)34.5 gene preferentially replicate in and kill malignant cells. The gamma(1)34.5 gene codes for ICP 34.5, a protein that enhances viral replication, and is homologous to growth arrest and DNA damage protein 34 (GADD34), a radiation-inducible DNA repair gene. We hypothesized that radiation therapy may potentiate efficacy of oncolytic viral therapy by upregulating GADD34 and promoting viral replication.

METHODS

The A549 and H1299 lung cancer cell lines were infected with NV1066, an oncolytic herpes simplex virus, at multiplicities of infection (number of viral particles per tumor cell) of 0.1 to 0.5 in vitro with radiation (2 to 10 Gy) or without radiation. Viral replication was determined by plaque assay, cell-to-cell spread was determined by flow cytometry, cell kill was determined by lactate dehydrogenase assay, and GADD34 induction was determined by real-time reverse transcription-polymerase chain reaction and Western blot method. Evidence of synergistic cytotoxicity dependence with GADD34 induction is further confirmed by small inhibitory RNA inhibition of GADD34 expression.

RESULTS

Using both the isobologram method and combination index method of Chou and Talalay, significant synergism was demonstrated between radiation therapy and NV1066 both in vitro and in vivo. As a result of such synergism, a dose reduction for each agent (2- to 6,000-fold) can be accomplished for a wide range of therapeutic effect levels without sacrificing tumor cell kill. This effect is correlated with increased GADD34 expression and inhibited by transfection of small inhibitory RNA directed against GADD34.

CONCLUSIONS

These data provide the cellular basis for the clinical investigation of combined use of radiation therapy with oncolytic herpes simplex virus therapy in the treatment of lung cancer to achieve synergistic efficacy while minimizing dosage and toxicity.