RNA viruses as virotherapy agents

Construction of an MUC-1 promoter driven, conditionally replicating adenovirus that expresses the sodium iodide symporter for gene therapy of breast cancer

Introduction

The sodium iodide symporter (NIS) directs the uptake and concentration of iodide in thyroid cells. This in turn allows radioiodine imaging and therapy for thyroid cancer. To extend the use of NIS-mediated radioiodine therapy to other types of cancer, we successfully transferred and expressed the sodium-iodide symporter (NIS) gene in prostate, colon, and breast cancer cells both in vivo and in vitro by using non-replicating adenoviral vectors.

Methods

To improve virotherapy efficiency, we developed a conditionally replicating adenovirus (CRAd) in which the transcriptional cassette RSV promoter-human NIScDNA-bGH polyA was also inserted at the E3 region. The E1a gene is driven by the tumor-specific promoter MUC-1 in the CRAd Ad5AMUCH_RSV-NIS.

Results

In vitro infection of the MUC-1-positive breast cell line T47D resulted in virus replication, cytolysis, and release of infective viral particles. Conversely, the MUC-1-negative breast cancer cell line MDA-MB-231 was refractory to the viral cytopathic effect and did not support viral replication. The data indicate that Ad5AMUCH_RSV-NIS activity is stringently restricted to MUC-1-positive cancer cells. Radioiodine uptake was readily measurable in T47 cells infected with Ad5AMUCH_RSV-NIS 24 hours after infection, thus confirming NIS expression before viral-induced cell death.

Conclusions

This construct may allow multimodal therapy, combining virotherapy with radioiodine therapy to be developed as a novel treatment for breast and other MUC1-overexpressing cancers.

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.

Adenoviral virotherapy for malignant brain tumors

Glioblastoma multiforme is the most common form of primary brain cancer. In the past decade, virotherapy of tumors has gained credence, particularly in glioma management, as these tumors are not completely resectable and tend to micro-metastasize. Adenoviral vectors have an advantage over other viral vectors in that they are relatively non-toxic and do not integrate in the genome. However, the lack of coxsackie and adenovirus receptors on surface of gliomas provides for inefficient transduction of wild-type adenoviral vectors in these tumors. By targeting receptors that are overexpressed in gliomas, modified adenoviral constructs have been shown to efficiently infect glioma cells. In addition, by taking advantage of tumor-specific promoter elements, oncolytic adenoviral vectors offer the promise of selective tumor-specific replication. This dual targeting strategy has enabled specificity in both laboratory and pre-clinical settings. This review examines current trends in adenoviral virotherapy of gliomas, with an emphasis on targeting modalities and future clinical applications.

Tumor and vascular targeting of a novel oncolytic measles virus retargeted against the urokinase receptor

Oncolytic measles virus (MV) induces cell fusion and cytotoxicity in a CD46-dependent manner. Development of fully retargeted oncolytic MVs would improve tumor selectivity. The urokinase-type plasminogen activator receptor (uPAR) is a tumor and stromal target overexpressed in multiple malignancies. MV-H glycoproteins fully retargeted to either human or murine uPAR were engineered and their fusogenic activity was determined. Recombinant human (MV-h-uPA) and murine (MV-m-uPA) uPAR-retargeted MVs expressing enhanced green fluorescent protein (eGFP) were rescued and characterized. Viral expression of chimeric MV-H was shown by reverse transcription-PCR and Western blot. In vitro viral replication was comparable to MV-GFP control. The receptor and species specificity of MV-uPAs was shown in human and murine cells with different levels of uPAR expression. Removal of the NH(2)-terminal fragment ligand from MV-uPA by factor X(a) treatment ablated the MV-uPA functional activity. Cytotoxicity was shown in uPAR-expressing human and murine cells. MV-h-uPA efficiently infected human endothelial cells and capillary tubes in vitro. I.v. administration of MV-h-uPA delayed tumor growth and prolonged survival in the MDA-MB-231 breast cancer xenograft model. Viral tumor targeting was confirmed by immunohistochemistry. MV-m-uPA transduced murine mammary tumors (4T1) in vivo after intratumor administration. MV-m-uPA targeted murine tumor vasculature after systemic administration, as shown by dual (CD31 and MV-N) staining of tumor capillaries in the MDA-MB-231 model. In conclusion, MV-uPA is a novel oncolytic MV associated with potent and specific antitumor effects and tumor vascular targeting. This is the first retargeted oncolytic MV able to replicate in murine cells and target tumor vasculature in a uPAR-dependent manner.

Newcastle disease virus: a promising vector for viral therapy, immune therapy, and gene therapy of cancer

This review deals with the avian paramyxovirus Newcastle disease virus (NDV) and describes properties that explain its oncolytic activity, its tumor-selective replication behavior, and its immune-stimulatory capacity with human cells. The strong interferon response of normal cells upon contact with NDV appears to be the basis for the good tolerability of the virus in cancer patients and for its immune stimulatory properties, whereas the weak interferon response of tumor cells explains the tumor selectivity of replication and oncolysis. Various concepts for the use of this virus for cancer treatment are pointed out and results from clinical studies are summarized. Reverse genetics technology has made it possible recently to clone the genome and to introduce new foreign genes thus generating new recombinant viruses. These can, in the future, be used to transfer new therapeutic genes into tumors and also to immunize against new emerging pathogens. The modular nature of gene transcription, the undetectable rate of recombination, and the lack of a DNA phase in the replication cycle make NDV a suitable candidate for the rational design of a safe and stable vaccine and gene therapy vector.

Oncolytic Semliki forest virus vector as a novel candidate against unresectable osteosarcoma

Oncolytic viruses are a promising tool for treatment of cancer. We studied an oncolytic Semliki Forest virus (SFV) vector, VA7, carrying the enhanced green fluorescent protein gene (EGFP), as a novel virotherapy candidate against unresectable osteosarcoma. The efficiency and characteristics of the VA7-EGFP treatment were compared with a widely studied oncolytic adenovirus, Ad5Delta24, both in vitro and in vivo. VA7-EGFP resulted in more rapid oncolysis and was more efficient at low multiplicities of infection (MOI) when compared with Ad5Delta24 in vitro. Yet, in MG-63 cells, a subpopulation resistant to the VA7-EGFP vector emerged. In subcutaneous human osteosarcoma xenografts in nude mice treatment with either vector reduced tumor size, whereas tumors in control mice expanded quickly. The VA7-EGFP-treated tumors were either completely abolished or regressed to pinpoint size. The efficacy of VA7-EGFP vector was studied also in an orthotopic osteosarcoma nude mouse model characterized by highly aggressive tumor growth. Treatment with oncolytic SFV extended survival of the animals significantly (P < 0.01), yet none of the animals were finally cured. Sera from SFV-treated mice contained neutralizing antibodies, and as nude mice are not able to establish IgG response, the result points out the role of IgM class antibodies in clearance of virus from peripheral tumors. Furthermore, biodistribution analysis at the survival end point verified the presence of virus in some of the brain samples, which is in line with previous studies demonstrating that IgG is required for clearance of SFV from central nervous system.

Development of targeted oncolytic virotherapeutics through translational research

BACKGROUND

Oncolytic virotherapeutics is a promising platform for cancer treatment but the product class has yet been successful. The key to success is integration of bidirectional translational research to rapidly address issues encountered in the laboratory and the clinics.

OBJECTIVE

We highlight the hurdles identified for the targeted oncolytic virotherapy approach, specifically those identified in clinical trials with wild-type viruses and first-generation targeted agents. We also analyze the translational research and development that has been applied to overcome these hurdles, including virus engineering and design improvements for next-generation virotherapeutics.

RESULTS/CONCLUSION

The iterative loop between the clinic and the lab can function as a major driving force to optimize products from this platform.

Oncolytic Viruses: A New Weapon to Fight Cancer

Remission from cancer after viral infection was first noted in the beginning of the 20th century, and with advances in virotherapy and genetic engineering, the advent of an approved viral therapeutic in North America is fast approaching. Mechanisms of tumour selectivity and killing, along with information obtained from clinical trials are reviewed here. Although oncolytic viruses are generally safe and well tolerated, their overall anti-tumour efficacy has varied. This article outlines strategies to improve the efficacy of the oncolytic platform without compromising its impressive safety profile. It will highlight new methods being developed to quantify the activity of oncolytic viruses in real time. Harnessing the factors that control the tumour microenvironment and the immune system are the key to enhancing the oncolytic activity. The purpose of this article is to introduce and provide an overview of the current state of cancer killing of oncolytic viruses. The reader will acquire knowledge of the basic principles of oncolytic viruses and their use in the clinical setting. This review summarizes articles retrieved from Medline using key words such as "virus," "oncolytic virus," "virotherapy," "cancer," and "clinical trials." Review articles published in the English language from 2005 onward were read and corroborating data and conclusions were summarized. When appropriate, cited references were also reviewed and incorporated. The reader is directed to references we found most concise.

A hyperfusogenic F protein enhances the oncolytic potency of a paramyxovirus simian virus 5 P/V mutant without compromising sensitivity to type I interferon

Viral fusogenic membrane proteins have been proposed as tools to increase the potency of oncolytic viruses, but there is a need for mechanisms to control the spread of fusogenic viruses in normal versus tumor cells. We have previously shown that a mutant of the paramyxovirus simian virus 5 (SV5) that harbors mutations in the P/V gene from the canine parainfluenza virus (P/V-CPI(-)) is a potent inducer of type I interferon (IFN) and apoptosis and is restricted for spread through normal but not tumor cells in vitro. Here, we have used the cytopathic P/V-CPI(-) as a backbone vector to test the hypothesis that a virus expressing a hyperfusogenic glycoprotein will be a more effective oncolytic vector but will retain sensitivity to IFN. A P/V mutant virus expressing an F protein with a glycine-to-alanine substitution in the fusion peptide (P/V-CPI(-)-G3A) was more fusogenic than the parental P/V-CPI(-) mutant. In two model prostate tumor cell lines which are defective in IFN production (LNCaP and DU145), the hyperfusogenic P/V-CPI(-)-G3A mutant had normal growth properties at low multiplicities of infection and was more effective than the parental P/V-CPI(-) mutant at cell killing in vitro. However, in PC3 cells which produce and respond to IFN, the hyperfusogenic P/V-CPI(-)-G3A mutant was attenuated for growth and spread. Killing of PC3 cells was equivalent between the parental P/V-CPI(-) mutant and the hyperfusogenic P/V-CPI(-)-G3A mutant. In a nude mouse model using LNCaP cells, the hyperfusogenic P/V-CPI(-)-G3A mutant was more effective than P/V-CPI(-) at reducing tumor burden. In the case of DU145 tumors, the two vectors based on P/V-CPI(-) were equally effective at limiting tumor growth. Together, our results provide proof of principle that a cytopathic SV5 P/V mutant can serve as an oncolytic virus and that the oncolytic effectiveness of P/V mutants can be enhanced by a fusogenic membrane protein without compromising sensitivity to IFN. The potential advantages of SV5-based oncolytic vectors are discussed.

Reovirus activates human dendritic cells to promote innate antitumor immunity

Oncolytic viruses can exert their antitumor activity via direct oncolysis or activation of antitumor immunity. Although reovirus is currently under clinical investigation for the treatment of localized or disseminated cancer, any potential immune contribution to its efficacy has not been addressed. This is the first study to investigate the ability of reovirus to activate human dendritic cells (DC), key regulators of both innate and adaptive immune responses. Reovirus induced DC maturation and stimulated the production of the proinflammatory cytokines IFN-alpha, TNF-alpha, IL-12p70, and IL-6. Activation of DC by reovirus was not dependent on viral replication, while cytokine production (but not phenotypic maturation) was inhibited by blockade of PKR and NF-kappaB signaling. Upon coculture with autologous NK cells, reovirus-activated DC up-regulated IFN-gamma production and increased NK cytolytic activity. Moreover, short-term coculture of reovirus-activated DC with autologous T cells also enhanced T cell cytokine secretion (IL-2 and IFN-gamma) and induced non-Ag restricted tumor cell killing. These data demonstrate for the first time that reovirus directly activates human DC and that reovirus-activated DC stimulate innate killing by not only NK cells, but also T cells, suggesting a novel potential role for T cells in oncolytic virus-induced local tumor cell death. Hence reovirus recognition by DC may trigger innate effector mechanisms to complement the virus's direct cytotoxicity, potentially enhancing the efficacy of reovirus as a therapeutic agent.

Oncolytic virotherapy for multiple myeloma

BACKGROUND

Current therapies for multiple myeloma (MM) are not curative, thus novel targeted therapeutics are being developed. One such targeted therapy is oncolytic virotherapy, wherein viruses specifically infect and kill the malignant plasma cells, leaving normal cells intact.

OBJECTIVE

This review provides an overview of the mechanisms and results of the oncolytic viruses being used to date and discusses the recent advances in the field of virotherapy for MM.

METHODS

All papers using viruses to treat MM were identified and screened. Only papers describing replicating, oncolytic viruses were reviewed.

RESULTS/CONCLUSIONS

Several viruses are currently being developed preclinically and clinically to treat MM, including measles virus, vesicular stomatitis virus, coxsackievirus A21 and vaccinia virus. Other viruses are being used preclinically to purge myeloma cells from autologous bone marrow transplants. Efforts to improve myeloma-specific targeting, avoid the antiviral immune response and evaluate combination therapies for MM are ongoing.

Oncolytic viruses: do they have a role in anti-cancer therapy?

Oncolytic viruses are replication competent, tumor selective and lyse cancer cells. Their potential for anti-cancer therapy is based upon the concept that selective intratumoral replication will produce a potent anti-tumor effect and possibly bystander or remote cell killing, whilst minimizing normal tissue toxicity. Viruses may be naturally oncolytic or be engineered for oncolytic activity, and possess a host of different mechanisms to provide tumor selectivity. Clinical use of live replicating viruses is associated with a unique set of safety issues. Clinical experience has so far provided evidence of limited efficacy and a favourable toxicity profile. The interaction with the host immune system is complex. An anti-viral immune response may limit efficacy by rapidly clearing the virus. However, virally-induced cell lysis releases tumor associated antigens in a 'dangerous' context, and limited evidence suggests that this can lead to the generation of a specific anti-tumor immune response. Combination therapy with chemotherapy or radiotherapy represents a promising avenue for ongoing translation of oncolytic viruses into clinical practice. Obstacles to therapy include highly effective non-specific host mechanisms to clear virus following systemic delivery, immune-mediated clearance, and intratumoral barriers limiting virus spread. A number of novel strategies are now under investigation to overcome these barriers. This review provides an overview of the potential role of oncolytic viruses, highlighting recent progress towards developing effective therapy and asks if they are a realistic therapeutic option at this stage.

Modeling of cancer virotherapy with recombinant measles viruses

The Edmonston vaccine strain of measles virus has potent and selective activity against a wide range of tumors. Tumor cells infected by this virus or genetically modified strains express viral proteins that allow them to fuse with neighboring cells to form syncytia that ultimately die. Moreover, infected cells may produce new virus particles that proceed to infect additional tumor cells. We present a model of tumor and virus interactions based on established biology and with proper accounting of the free virus population. The range of model parameters is estimated by fitting to available experimental data. The stability of equilibrium states corresponding to complete tumor eradication, therapy failure and partial tumor reduction is discussed. We use numerical simulations to explore conditions for which the model predicts successful therapy and tumor eradication. The model exhibits damped, as well as stable oscillations in a range of parameter values. These oscillatory states are organized by a Hopf bifurcation.

Augmented transgene expression in transformed cells using a parvoviral hybrid vector

Autonomous parvoviruses possess an intrinsic oncotropism based on viral genetic elements controlling gene expression and genome replication. We constructed a hybrid vector consisting of the H1 parvovirus-derived expression cassette comprising the p4 promoter, the ns1 gene and the p38 promoter flanked by the adeno-associated viruses 2 (AAV2) inverted terminal repeats and packaged into AAV2 capsids. Gene transduction using this vector could be stimulated by coinfection with adenovirus, by irradiation or treatment with genotoxic agents, similar to standard AAV2 vectors. However, the latter were in most cases less efficient in gene transduction than the hybrid vector. With the new vector, tumor cell-selective increase in transgene expression was observed in pairs of transformed and non-transformed cells, leading to selective killing of the transformed cells after expression of a prodrug-converting enzyme. Preferential gene expression in tumor versus normal liver tissue was also observed in vivo in a syngeneic rat model. Comparative transduction of a panel of different tumor cell lines with the H1 and the H1/AAV hybrid vector showed a preference of each vector for distinct cell types, probably reflecting the dependence of the viral tropism on capsid determinants.

Intratumoral injection of inactivated Sendai virus particles elicits strong antitumor activity by enhancing local CXCL10 expression and systemic NK cell activation

We have already demonstrated that inactivated, replication-defective Sendai virus particles (HVJ-E) have a powerful antitumor effect by both the generation of tumor-specific cytotoxic T cells and inhibition of regulatory T cell activity. Here, we report that HVJ-E also has an antitumor effect through non-T cell immunity. Microarray analysis revealed that direct injection of HVJ-E induced the expression of CXCL10 in established Renca tumors. CXCL10 was secreted by dendritic cells in the tumors after HVJ-E injection. Quantitative real-time RT-PCR and immunohistochemistry revealed that CXCR3+ cells (predominantly NK cells) infiltrated the HVJ-E-injected tumors. Moreover, HVJ-E injection caused systemic activation of NK cells and enhanced their cytotoxity against tumor cells. In an in vivo experiment, approximately 50% of tumors were eradicated by HVJ-E injection, and this activity of HVJ-E against Renca tumors was largely abolished by NK cell depletion using anti-asialo GM1 antibody. Since HVJ-E injection induced systemic antitumor immunity by enhancing or correcting the chemokine-chemokine receptor axis, it might be a potential new therapy for cancer.

Targeting of interferon-beta to produce a specific, multi-mechanistic oncolytic vaccinia virus

BACKGROUND

Oncolytic viruses hold much promise for clinical treatment of many cancers, but a lack of systemic delivery and insufficient tumor cell killing have limited their usefulness. We have previously demonstrated that vaccinia virus strains are capable of systemic delivery to tumors in mouse models, but infection of normal tissues remains an issue. We hypothesized that interferon-beta (IFN-beta) expression from an oncolytic vaccinia strain incapable of responding to this cytokine would have dual benefits as a cancer therapeutic: increased anticancer effects and enhanced virus inactivation in normal tissues. We report the construction and preclinical testing of this virus.

METHODS AND FINDINGS

In vitro screening of viral strains by cytotoxicity and replication assay was coupled to cellular characterization by phospho-flow cytometry in order to select a novel oncolytic vaccinia virus. This virus was then examined in vivo in mouse models by non-invasive imaging techniques. A vaccinia B18R deletion mutant was selected as the backbone for IFN-beta expression, because the B18R gene product neutralizes secreted type-I IFNs. The oncolytic B18R deletion mutant demonstrated IFN-dependent cancer selectivity and efficacy in vitro, and tumor targeting and efficacy in mouse models in vivo. Both tumor cells and tumor-associated vascular endothelial cells were targeted. Complete tumor responses in preclinical models were accompanied by immune-mediated protection against tumor rechallenge. Cancer selectivity was also demonstrated in primary human tumor explant tissues and adjacent normal tissues. The IFN-beta gene was then cloned into the thymidine kinase (TK) region of this virus to create JX-795 (TK-/B18R-/IFN-beta+). JX-795 had superior tumor selectivity and systemic intravenous efficacy when compared with the TK-/B18R- control or wild-type vaccinia in preclinical models.

CONCLUSIONS

By combining IFN-dependent cancer selectivity with IFN-beta expression to optimize both anticancer effects and normal tissue antiviral effects, we were able to achieve, to our knowledge for the first time, tumor-specific replication, IFN-beta gene expression, and efficacy following systemic delivery in preclinical models.

Seneca Valley virus, a systemically deliverable oncolytic picornavirus, and the treatment of neuroendocrine cancers

Background

Numerous clinical trials have demonstrated that oncolytic viruses can elicit antitumor responses when they are administered directly into localized cancers. However, the treatment of metastatic disease with oncolytic viruses has been challenging due to the inactivation of viruses by components of human blood and/or to inadequate tumor selectivity.

Methods

We determined the cytolytic potential and selectivity of Seneca Valley Virus-001 (SVV-001), a newly discovered native picornavirus, in neuroendocrine and pediatric tumor cell lines and normal cells. Suitability of the virus for intravenous delivery in humans was assessed by blood inactivation assays. Safety was evaluated in vivo using an immune-competent mouse model, and efficacy was evaluated in vivo in athymic mice bearing tumors derived from human small-cell lung cancer and retinoblastoma cell lines.

Results

Cell lines derived from small-cell lung cancers and solid pediatric cancers were at least 10000-fold more sensitive to the cytolytic activity of SVV-001 than were any of the adult normal human cells tested. Viral infectivity was not inhibited by human blood components. Intravenous doses up to 1 × 10¹⁴ virus particles (vp) per kg were well tolerated, and no dose-limiting toxicity was observed in immune-competent mice. A single intravenous dose of 1 × 10⁸ vp per kg into athymic mice bearing preestablished small-cell lung or retinoblastoma tumors resulted in complete, durable responses in ten of ten and five of eight mice, respectively.

Conclusions

SVV-001 has potent cytolytic activity and high selectivity for tumor cell lines having neuroendocrine properties versus adult normal cells. Systemically administered SVV-001 has potential for the treatment of metastatic neuroendocrine cancers.

Gene therapy of gynaecological diseases

Gene therapy represents a potentially useful approach for the treatment of diseases refractory to conventional therapies. Various preclinical and clinical strategies have been explored for treatment of gynaecological diseases. Given the most severe unmet clinical need, much of the work has been performed with gynaecological cancers and ovarian cancer in particular. Although the safety of many treatment strategies has been demonstrated in early phase clinical trials, efficacy has been mostly limited heretofore. Major challenges include improving the vectors used with the aim of more effective and selective delivery. In addition, effective penetration into and spreading within advanced and complex tumour masses and metastases remains challenging. This review focuses on existing and developmental gene transfer applications for gynaecological diseases.

A new therapy for highly effective tumor eradication using HVJ-E combined with chemotherapy

BACKGROUND

Inactivated HVJ (hemagglutinating virus of Japan; Sendai virus) particles (HVJ envelope vector; HVJ-E can incorporate and deliver plasmid DNA, siRNA, antibody and peptide and anti-cancer drugs to cells both in vitro and in vivo. We attempted to eradicate tumors derived from mouse colon cancer cells, CT26, by combining bleomycin (BLM)-incorporated HVJ-E (HVJ-E/BLM) with cisplatin (CDDP) administration.

METHODS

CT-26 tumor mass was intradermally established in Balb/c mice. HVJ-E/BLM was directly injected into the tumor mass with or without intraperitoneal administration of CDDP. The anti-tumor effect was evaluated by measuring tumor size and cytotoxic T cell activity against CT26. Re-challenge of tumor cells to treated mice was performed 10 days or 8 months after the initial tumor inoculation.

RESULTS

We found that three intratumoral injections of HVJ-E/BLM along with a single intraperitoneal administration of CDDP eradicated CT26 tumors with more than 75% efficiency. When tumor cells were intradermally re-injected on day 10 after the initial tumor inoculation, tumors on both sides disappeared in most of the mice that received the combination therapy of HVJ-E/BLM and CDDP. Eight months after the initial tumor eradication, surviving mice were re-challenged with CT26 cells. The re-challenged tumors were rejected in all of the surviving mice treated with the combination therapy. Cytotoxic T lymphocytes specific for CT26 were generated in these surviving mice.

CONCLUSION

Combination therapy consisting of HVJ-E and chemotherapy completely eradicated the tumor, and generated anti-tumor immunity. The combination therapy could therefore be a promising new strategy for cancer therapy.

Radioiodide imaging and radiovirotherapy of multiple myeloma using VSV(Delta51)-NIS, an attenuated vesicular stomatitis virus encoding the sodium iodide symporter gene

Multiple myeloma is a radiosensitive malignancy that is currently incurable. Here, we generated a novel recombinant vesicular stomatitis virus [VSV(Delta51)-NIS] that has a deletion of methionine 51 in the matrix protein and expresses the human sodium iodide symporter (NIS) gene. VSV(Delta51)-NIS showed specific oncolytic activity against myeloma cell lines and primary myeloma cells and was able to replicate to high titers in myeloma cells in vitro. Iodide uptake assays showed accumulation of radioactive iodide in VSV(Delta51)-NIS-infected myeloma cells that was specific to the function of the NIS transgene. In bg/nd/xid mice with established subcutaneous myeloma tumors, administration of VSV(Delta51)-NIS resulted in high intratumoral virus replication and tumor regression. VSV-associated neurotoxicity was not observed. Intratumoral spread of the infection was monitored noninvasively by serial gamma camera imaging of (123)I-iodide biodistribution. Dosimetry calculations based on these images pointed to the feasibility of combination radiovirotherapy with VSV(Delta51)-NIS plus (131)I. Immunocompetent mice with syngeneic 5TGM1 myeloma tumors (either subcutaneous or orthotopic) showed significant enhancements of tumor regression and survival when VSV(Delta51)-NIS was combined with (131)I. These results show that VSV(Delta51)-NIS is a safe oncolytic agent with significant therapeutic potential in multiple myeloma.

Engineering oncolytic measles virus to circumvent the intracellular innate immune response

The innate antiviral responses of tumor cells are often impaired but may still be sufficient to impede the intratumoral spread of an oncolytic virus. Here, we establish that the oncolytic measles virus (MV-eGFP) induces interferon (IFN) production in human myeloma and ovarian cancer cells. In addition, MV gene expression and virus progeny production were inhibited by IFN treatment of these tumor cells. The P gene of wild-type measles virus encodes P/V/C proteins known to antagonize IFN induction and/or response. We therefore engineered MV-eGFP for IFN evasion and more efficient intratumoral spread by arming it with the P gene from wild-type IC-B strain MV, thus generating MV-eGFP-Pwt. The chimeric virus exhibited reduced IFN sensitivity and diminished capacity to induce IFN in BJAB lymphoma, ARH-77 myeloma cells, and activated peripheral blood mononuclear cells. Interestingly, unlike the wild-type MV, MV-eGFP-Pwt was unable to shut down IFN induction completely. In immunocompromised mice bearing human myeloma xenografts, intravenously administered MV-eGFP-Pwt showed significantly enhanced oncolytic potency compared to MV-eGFP. These results indicate that oncolytic viruses are subject to control by the innate immune defenses of human tumor cells and may therefore be more effective if their natural ability to combat innate immunity is maintained.

Engineered measles virus as a novel oncolytic viral therapy system for hepatocellular carcinoma

The oncolytic measles virus Edmonston strain (MV-Edm), a nonpathogenic virus targeting cells expressing abundant CD46, selectively destroys neoplastic tissue. Clinical development of MV-Edm would benefit from noninvasive monitoring strategies to determine the speed and extent of the spread of the virus in treated patients and the location of virus-infected cells. We evaluated recombinant MV-Edm expressing carcinoembryonic antigen (CEA) or the human sodium iodide symporter (hNIS) for oncolytic potential in hepatocellular carcinoma (HCC) and efficiency in tracking viruses in vivo by noninvasive monitoring. CD46 expression in human HCC and primary hepatocytes was assessed by flow cytometry and immunohistochemistry. Infectivity, syncytium formation, and cytotoxicity of recombinant MV-Edm in HCC cell lines were evaluated by fluorescence microscopy, crystal violet staining, and the MTS assay. Transgene expression in HCC cell lines after infection with recombinant MV-Edm in vitro and in vivo was assessed by CEA concentration, 125I-uptake, and 123I-imaging studies. Toxicology studies were performed in Ifnar(KO)xCD46 transgenic mice. The CD46 receptor was highly expressed in HCC compared to nonmalignant hepatic tissue. Recombinant MV-Edm efficiently infected HCC cell lines, resulting in extensive syncytium formation followed by cell death. Transduction of HCC cell lines and subcutaneous HCC xenografts with recombinant MV-Edm resulted in high-level expression of transgenes in vitro and in vivo. MV-Edm was nontoxic in susceptible mice. Intratumoral and intravenous therapy with recombinant MV-Edm resulted in inhibition of tumor growth and prolongation of survival with complete tumor regression in up to one third of animals. In conclusion, engineered MV-Edm may be a potent and novel cancer gene therapy system for HCC. MV-Edm expressing CEA or hNIS elicited oncolytic effects in human HCC cell lines in vitro and in vivo, enabling the spread of the virus to be monitored in a noninvasive manner.

The use of a tropism-modified measles virus in folate receptor-targeted virotherapy of ovarian cancer

PURPOSE

Attenuated measles viruses are promising experimental anticancer agents currently being evaluated in a phase I dose escalation trial for ovarian cancer patients. Virus attachment, entry, and subsequent intercellular fusion between infected and uninfected neighboring cells are mediated via the two measles receptors (CD46 and SLAM). To minimize potential toxicity due to measles virus-associated immunosuppression and infection of nontarget tissues, we sought to develop an ovarian cancer exclusive fully retargeted measles virus.

EXPERIMENTAL DESIGN AND RESULTS

Interactions of measles virus with its natural receptors were ablated, and a single-chain antibody (scFv) specific for alpha-folate receptor (FRalpha), a target overexpressed on 90% of nonmucinous ovarian cancer, was genetically engineered on the viral attachment protein (MV-alphaFR). Specificity of virus tropism was tested on tumor and normal cells. Biodistribution of measles virus infection was evaluated in measles-susceptible CD46 transgenic mice, whereas antitumor activity was monitored noninvasively by bioluminescence imaging in xenograft models. Tropism and fusogenic activity of MV-alphaFR was redirected exclusively to FRalpha without compromise to virus infectivity. In contrast to the parental virus, MV-alphaFR has no background infectivity on normal human cells. The antitumor activity of MV-alphaFR, as assessed by tumor volume reduction and overall survival increase, was equal to the parental virus in two models of human ovarian cancer (s.c. and i.p.).

CONCLUSIONS

A FR-exclusive ovarian cancer targeted oncolytic virus was generated and shown to be therapeutically effective, thus introducing a new modality for FR targeting and a candidate measles virus for clinical testing.

Establishment of a novel foreign gene delivery system combining an HSV amplicon with an attenuated replication-competent virus, HSV-1 HF10

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have been used widely in genetic engineering with many advantages for gene delivery, being easily constructed. An attenuated and replication-competent HSV-1 HF10 clone demonstrating an oncolytic effect on cancer cells in vitro and in vivo has been applied recently for clinical virotherapy of breast cancers and the present studies were conducted to test its efficacy in combination with an HSV-1 amplicon. For this purpose, a new system was developed to produce high titers of the HSV-1 amplicon vector and the results showed that its package efficiency and the titer ratio to HF10 were improved by passage through two cell lines. A high ratio of amplicon/helper virus HF10 (A/H) (>1) was required to express the foreign gene efficiently. Furthermore, in order to express the foreign gene conditionally, an HSV-1 ICP8 promoter was introduced in place of the human cytomegalovirus MIE promoter, this driving expression of the transgene when replication of HF10 progressed. The methodology for simple preparation of mixtures of viruses containing the amplicon with the oncolytic virus is documented. This system should find application for studies of cancer therapy.

Gene therapy for malignant glioma: current clinical status

Glioblastoma is an aggressive brain tumor with a dismal prognosis. Gene therapy may offer a new option for the treatment of these patients. Several gene therapy approaches have shown anti-tumor efficiency in experimental studies, and the first clinical trials for the treatment of malignant glioma were conducted in the 1990s. HSV-tk gene therapy has been the pioneering and most commonly used approach, but oncolytic conditionally replicating adenoviruses and herpes simplex virus mutant vectors, p53, interleukins, interferons, and antisense oligonucleotides have also been used. During the past few years, adenoviruses have become the most popular gene transfer vectors, and some recent randomized, controlled trials have shown significant anti-tumor efficacy in clinical use. However, efficient gene delivery into the brain still presents a major problem, and there is a lack of definitive phase III trials, which would avoid potential problems associated with a small number of patients, inadvertent patient selection, and overinterpretation of results based on a few long-time survivors. For clinical efficacy, median survival is one of the most rigorous endpoints. It is used here to evaluate the usefulness of various treatment approaches and current clinical status of gene therapy for malignant glioma.

Use of double-stranded RNA templates by the tombusvirus replicase in vitro: Implications for the mechanism of plus-strand initiation

Plus-stranded RNA viruses replicate efficiently in infected hosts producing numerous copies of the viral RNA. One of the long-standing mysteries in RNA virus replication is the occurrence and possible role of the double-stranded (ds)RNA formed between minus- and plus-strands. Using the partially purified Cucumber necrosis virus (CNV) replicase from plants and the recombinant RNA-dependent RNA polymerase (RdRp) of Turnip crinkle virus (TCV), in this paper, we demonstrate that both CNV replicase and the related TCV RdRp can utilize dsRNA templates to produce viral plus-stranded RNA in vitro. Sequence and structure of the dsRNA around the plus-strand initiation site had a significant effect on initiation, suggesting that initiation on dsRNA templates is a rate-limiting step. In contrast, the CNV replicase could efficiently synthesize plus-strand RNA on partial dsRNAs that had the plus-strand initiation promoter "exposed", suggesting that the polymerase activity of CNV replicase is strong enough to unwind extended dsRNA regions in the template during RNA synthesis. Based on the in vitro data, we propose that dsRNA forms might have functional roles during tombus- and carmovirus replication and the AU-rich nature of the terminus could be important for opening the dsRNA structure around the plus-strand initiation promoter for tombus- and carmoviruses and possibly many other positive-strand RNA viruses.

Development of a rapid method to generate multiple oncolytic HSV vectors and their in vivo evaluation using syngeneic mouse tumor models

Replication-conditional herpes simplex virus (HSV)-based vectors have great potential in the treatment of various types of cancers including brain tumors. HSV mutants lacking the U(L)39 gene and both copies of the gamma(1)34.5 gene (e.g. MGH1, G207) have been demonstrated to possess oncolytic effects as well as potent anticancer vaccination effects without compromising safety. Such mutants thus provide optimal templates to produce novel oncolytic HSV vectors for cancer gene therapy applications. In order to accomplish quick and efficient construction of oncolytic HSV vectors, a novel BAC-based method designated as 'HSVQuik system' was developed. This system sequentially utilizes two different site-specific recombination systems to introduce virtually any transgene cassettes of interest into the deleted U(L)39 locus (Flp-FRT in Escherichia coli) and to release the vector genome sequence from the procaryotic plasmid backbone (Cre-loxP in Vero cells). Taking advantage of the HSVQuik system, we constructed three oncolytic HSV vectors that express mouse IL4, CD40 ligand and 6CK, respectively. In vivo therapeutic experiments using two luciferase-labeled syngeneic mouse brain tumor models revealed that expression of these immunomodulators significantly enhanced antitumor efficacy of oncolytic HSV. The HSVQuik system, together with luciferase-labeled tumor models, should expedite the process of generating and evaluating oncolytic HSV vectors for cancer gene therapy applications.

Imaging immediate-early and strict-late promoter activity during oncolytic herpes simplex virus type 1 infection and replication in tumors

An increasing number of oncolytic viruses have been developed and studied for cancer therapy. In response to needs for non-invasive monitoring and imaging of oncolytic virotherapy, several different approaches, including a positron emission tomography-based method, a method using secreted marker peptides, and optical imaging-based methods, have been reported. Among these modalities, we utilized the luciferase-based bioluminescent assay/imaging systems to determine the kinetics and dynamics of a productive viral infection. The replication cycle of herpes simplex virus type 1 (HSV-1) is punctuated by a temporal cascade of three classes of viral genes: immediate-early (IE), early (E) and late (L) genes. U(L)39- and gamma(1)34.5-deleted, replication-conditional HSV-1 mutants that express firefly luciferase under the control of the IE4/5 or strict-late gC promoters were generated. These oncolytic viruses were examined in cultured cells and a mouse tumor model. IE promoter- and strict-late promoter-mediated luciferase expression was confirmed to indicate viral infection and replication, respectively. Incorporation of a strict-late promoter-driven luciferase cassette into oncolytic HSV-1 vectors would be useful for assessing tumor oncolysis in preclinical tumor treatment studies.

Oncolytic measles virus targets high CD46 expression on multiple myeloma cells

OBJECTIVE

Multiple myeloma (MM) is an incurable B cell malignancy and novel therapeutics are urgently needed. Live attenuated measles virus (MV) has potent oncolytic activity against MM tumor xenografts. The virus is tumor selective and preferentially targets cells that express high levels of CD46 receptors. However, CD46 levels on MM have not previously been evaluated. In this study, we investigated the potential of CD46 as a target for MM therapy and correlated surface levels of CD46 on MM cells with their susceptibility to MV-induced cytopathic effects.

MATERIALS AND METHODS

CD46 expression on neoplastic plasma cells (PCs) and nonplasma cells (NPCs) from 38 MM patients was analyzed by flow cytometry and receptor numbers were quantitated using BD QuantiBRITE PE beads.

RESULTS

Results showed that malignant PCs expressed significantly higher levels of CD46 receptors compared to NPCs (p < 0.0001). The mean CD46 receptor numbers on PCs and NPCs were 49,130/cell and 7,340/cell, respectively. Potent cytopathic effects of extensive intercellular fusion were observed in measles-infected PCs but not in NPCs. The extent of MV-induced cytopathic effects of cell fusion correlated with CD46 expression levels on the MM cells. Normal plasma cells do not overexpress CD46 and colony-forming assays demonstrated that MV was not cytotoxic to normal bone marrow progenitor cells.

CONCLUSION

The present study establishes CD46 as a surface antigen that is expressed more abundantly on primary MM cells compared to normal hematopoietic cells of various lineages in the bone marrow, making CD46 a promising surface marker for targeted cytoreductive therapy of MM.

Tumor selective replication of Newcastle disease virus: association with defects of tumor cells in antiviral defence

To investigate tumor-selective viral replication, we compared several tumorigenic human cell lines to nontumorigenic human cells from the blood for the sensitivity to become infected by a recombinant lentogenic strain of Newcastle Disease Virus (NDV) with incorporated transgene EGFP (NDFL-EGFP). Although fluorescence signals in nontumorigenic cells were only weak or missing completely, a massive and long-lasting transgene-expression was observed in all tumor cell lines. The majority of tumor cells (50-95%) could be infected, and viral replication was associated with an increase in the cell surface density of viral antigens. To clarify the underlying mechanism of the observed difference in virus susceptibility we examined the kinetics of interferon-induced antiviral enzymes because NDV is a strong type-I interferon inducer. This analysis revealed several defects of tumor cells in their antiviral defence responses: They showed no response to UV-inactivated NDV, whereas nontumorigenic cells reacted with induction of high-levels of the antiviral enzymes PKR and MxA. Upon coincubation with live NDV, tumor cells showed a delayed response in the increased expression of the antiviral enzymes in comparison with PBMC. In nontumorigenic cells the replication cycle of NDV stopped after the production of positive-strand RNA, while tumor cells continued in the replication cycle and copied viral genomes 10-50 hr after infection. These results can explain the tumor selective replication behavior of this interesting antineoplastic virus.

Generation of measles virus with a segmented RNA genome

Viruses classified in the order Mononegavirales have a single nonsegmented RNA molecule as the genome and employ similar strategies for genome replication and gene expression. Infectious particles of Measles virus (MeV), a member of the family Paramyxoviridae in the order Mononegavirales, with two or three RNA genome segments (2 seg- or 3 seg-MeV) were generated using a highly efficient reverse genetics system. All RNA segments of the viruses were designed to have authentic 3' and 5' self-complementary termini, similar to those of negative-stranded RNA viruses that intrinsically have multiple RNA genome segments. The 2 seg- and 3 seg-MeV were viable and replicated well in cultured cells. 3 seg-MeV could accommodate up to six additional transcriptional units, five of which were shown to be capable of expressing foreign proteins efficiently. These data indicate that the MeV genome can be segmented, providing an experimental insight into the divergence of the negative-stranded RNA viruses with nonsegmented or segmented RNA genomes. They also illustrate a new strategy to develop mononegavirus-derived vectors harboring multiple additional transcriptional units.

Viral gene therapy strategies: from basic science to clinical application

A major impediment to the successful application of gene therapy for the treatment of a range of diseases is not a paucity of therapeutic genes, but the lack of an efficient non-toxic gene delivery system. Having evolved to deliver their genes to target cells, viruses are currently the most effective means of gene delivery and can be manipulated to express therapeutic genes or to replicate specifically in certain cells. Gene therapy is being developed for a range of diseases including inherited monogenic disorders and cardiovascular disease, but it is in the treatment of cancer that this approach has been most evident, resulting in the recent licensing of a gene therapy for the routine treatment of head and neck cancer in China. A variety of virus vectors have been employed to deliver genes to cells to provide either transient (eg adenovirus, vaccinia virus) or permanent (eg retrovirus, adeno-associated virus) transgene expression and each approach has its own advantages and disadvantages. Paramount is the safety of these virus vectors and a greater understanding of the virus-host interaction is key to optimizing the use of these vectors for routine clinical use. Recent developments in the modification of the virus coat allow more targeted approaches and herald the advent of systemic delivery of therapeutic viruses. In the context of cancer, the ability of attenuated viruses to replicate specifically in tumour cells has already yielded some impressive results in clinical trials and bodes well for the future of this approach, particularly when combined with more traditional anti-cancer therapies.

Soluble receptor-mediated targeting of mouse hepatitis coronavirus to the human epidermal growth factor receptor

The mouse hepatitis coronavirus (MHV) infects murine cells by binding of its spike (S) protein to murine CEACAM1a. The N-terminal part of this cellular receptor (soR) is sufficient for S binding and for subsequent induction of the conformational changes required for virus-cell membrane fusion. Here we analyzed whether these characteristics can be used to redirect MHV to human cancer cells. To this end, the soR domain was coupled to single-chain monoclonal antibody 425, which is directed against the human epidermal growth factor receptor (EGFR), resulting in a bispecific adapter protein (soR-425). The soR and soR-425 proteins, both produced with the vaccinia virus system, were able to neutralize MHV infection of murine LR7 cells. However, only soR-425 was able to target MHV to human EGFR-expressing cancer cells. Interestingly, the targeted infections induced syncytium formation. Furthermore, the soR-425-mediated infections were blocked by heptad repeat-mimicking peptides, indicating that virus entry requires the regular S protein fusion process. We conclude that the specific spike-binding property of the CEACAM1a N-terminal fragment can be exploited to direct the virus to selected cells by linking it to a moiety able to bind a receptor on those cells. This approach might be useful in the development of tumor-targeted coronaviruses.

Oncolytic virotherapy for cancer treatment: challenges and solutions

Advances in gene modification and viral therapy have led to the development of a variety of vectors in several viral families that are capable of replication specifically in tumor cells. Because of the nature of viral delivery, infection, and replication, this technology, oncolytic virotherapy, may prove valuable for treating cancer patients, especially those with inoperable tumors. Current limitations exist, however, for oncolytic virotherapy. They include the body's B and T cell responses, innate inflammatory reactions, host range, safety risks involved in using modified viruses as treatments, and the requirement that most currently available oncolytic viruses require local administration. Another important constraint is that genetically enhanced vectors may or may not adhere to their replication restrictions in long-term applications. Several solutions and strategies already exist, however, to minimize or circumvent many of these limitations, supporting viral oncolytic therapy as a viable option and powerful tool in the fight against cancer.

Fighting cancer with oncolytic viruses

Although gene therapy has huge potential for modern medicine, our enthusiasm for its powerful potential must not cloud our judgment about the dangers of using increasingly diverse, yet relatively untested, replicating viruses

Targeting non-human coronaviruses to human cancer cells using a bispecific single-chain antibody

To explore the potential of using non-human coronaviruses for cancer therapy, we first established their ability to kill human tumor cells. We found that the feline infectious peritonitis virus (FIPV) and a felinized murine hepatitis virus (fMHV), both normally incapable of infecting human cells, could rapidly and effectively kill human cancer cells artificially expressing the feline coronavirus receptor aminopeptidase N. Also 3-D multilayer tumor spheroids established from such cells were effectively eradicated. Next, we investigated whether FIPV and fMHV could be targeted to human cancer cells by constructing a bispecific single-chain antibody directed on the one hand against the feline coronavirus spike protein--responsible for receptor binding and subsequent cell entry through virus-cell membrane fusion--and on the other hand against the human epidermal growth factor receptor (EGFR). The targeting antibody mediated specific infection of EGFR-expressing human cancer cells by both coronaviruses. Furthermore, in the presence of the targeting antibody, infected cancer cells formed syncytia typical of productive coronavirus infection. By their potent cytotoxicity, the selective targeting of non-human coronaviruses to human cancer cells provides a rationale for further investigations into the use of these viruses as anticancer agents.

Mathematical modeling of cancer radiovirotherapy

Cancer virotherapy represents a dynamical system that requires mathematical modeling for complete understanding of the outcomes. The combination of virotherapy with radiation (radiovirotherapy) has been recently shown to successfully eliminate tumors when virotherapy alone failed. However, it introduces a new level of complexity. We have developed a mathematical model, based on population dynamics, that captures the essential elements of radiovirotherapy. The existence of corresponding equilibrium points related to complete cure, partial cure, and therapy failure is proved and discussed. The parameters of the model were estimated by fitting to experimental data. By using simulations we analyzed the influence of parameters that describe the interaction between virus and tumor cell on the outcome of the therapy. Furthermore, we evaluated relevant therapeutic scenarios for radiovirotherapy, and offered elements for optimization.

Combined I-124 Positron Emission Tomography/Computed Tomography Imaging of NIS Gene Expression in Animal Models of Stably Transfected and Intravenously Transfected Tumor

PURPOSE

With the advent of replication competent viruses for cancer gene therapy, it has become imperative to monitor the biodistribution, expression and replication of these vectors in living organisms. We evaluated the potential of I-124 positron emission tomography (PET)/computed tomography (CT) imaging in gene therapy animal models utilizing the sodium iodide symporter (NIS) and compared the findings to I-123 gamma camera imaging.

PROCEDURES

CB17 SCID mice were implanted with myeloma cell lines expressing NIS or infected by MV-NIS given systemically. Mice were imaged by both gamma camera (I-123) and PET/CT (I-124 ) and image quality assessed.

RESULTS

NIS expressing tumors concentrated 7.1% of the injected activity while tumors infected with the control virus had only 0.3% of the activity injected.

CONCLUSIONS

I-124 PET/CT in combination with NIS allows the tracking of stably transfected tumors or intravenously transfected tumors. Combined modality imaging using PET/CT allows accurate and non-invasive imaging of the distribution and gene expression of a replicating viral vector in living systems.

Gene therapy: twenty-first century medicine

Broadly defined, the concept of gene therapy involves the transfer of genetic material into a cell, tissue, or whole organ, with the goal of curing a disease or at least improving the clinical status of a patient. A key factor in the success of gene therapy is the development of delivery systems that are capable of efficient gene transfer in a variety of tissues, without causing any associated pathogenic effects. Vectors based upon many different viral systems, including retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses, currently offer the best choice for efficient gene delivery. Their performance and pathogenicity has been evaluated in animal models, and encouraging results form the basis for clinical trials to treat genetic disorders and acquired diseases. Despite some initial success in these trials, vector development remains a seminal concern for improved gene therapy technologies.

Viruses with deletions in antiapoptotic genes as potential oncolytic agents

Replication-selective oncolytic viruses have emerged as a new treatment platform for cancers. However, selectivity and potency need to be improved before virotherapy can become a standard treatment modality. In addition, mechanisms that can be incorporated to enable targeting a broad range of cancer types are highly desirable. Cancer cells are well known to have multiple blocks in apoptosis pathways. On the other hand, viruses have evolved to express numerous antiapoptotic genes to antagonize apoptosis induced upon infection. Viruses with deletions in antiapoptotic genes can therefore be complemented by antiapoptotic genetic changes in cancer cells for efficient replication and oncolysis. In this review, we summarize the recent development of this concept, the potential obstacles, and future directions for optimization.

Recombinant rhabdoviruses: vectors for vaccine development and gene therapy

The establishment of methods to recover rhabdoviruses from cDNA, so-called reverse genetics systems, has made it possible to genetically engineer rhabdoviruses and to study all aspects of the virus life cycle by introducing defined mutations into the viral genomes. It has also opened the way to make use of the viruses in biomedical applications such as vaccination, gene therapy, or oncolytic virotherapy. The typical gene expression mode of rhabdoviruses, a high genetic stability, and the propensity to tolerate changes in the virus envelope have made rhabdoviruses attractive, targetable gene expression vectors. This chapter provides an overview on the possibilities to manipulate biological properties of the rhabdoviruses that may be important for further development of vaccine vectors and examples of recombinant rhabdoviruses expressing foreign genes and antigens.

Conditionally replicative adenovirus for gastrointestinal cancers

The clinical outcome of advanced gastrointestinal (GI) cancers (especially pancreatic and oesophageal cancers) is dismal, despite the advance of conventional therapeutic strategies. Cancer gene therapy is a category of new therapeutics, among which conditionally replicative adenovirus (CRAd) is one promising strategy to overcome existing obstacles of cancer gene therapy. Various CRAds have been developed for GI cancer treatment by taking advantage of the replication biology of adenovirus. Some CRAds have already been tested in clinical trials, but have fallen short of initial expectations. Concerns for clinical applicability include therapeutic potency, replication selectivity and interval end points in clinical trials. In addition, improvement of experimental animal models is needed for a deeper understanding of CRAd biology. Despite these obstacles, CRAds continue to be an exciting area of investigation with great potential for clinical utility. Further virological and oncological research will eventually lead to full realisation of the therapeutic potential of CRAds in the field of GI cancers.

Selective gene transfer in vitro to tumor cells via recombinant Newcastle disease virus

We developed a novel strategy to target recombinant Newcastle disease virus (NDV) to tumor cells for gene therapy. Modifying the virus with a bispecific fusion protein allowed virus receptor-independent tumor cell binding and gene transfer. The targeting molecule (alpha)HN-IL-2 contains an scFv antibody cloned from a neutralizing hemagglutinin-neuraminidase (HN)-specific hybridoma linked to the human cytokine IL-2. A recombinant NDV expressing the enhanced green fluorescent protein (NDFL-EGFP) was applied to show the expression of foreign genes in virus-infected tumor cells. At 24 hours after infection with the modified virus (NDFL-EGFP/(alpha)HN-IL-2), FACS analysis and fluorescence microscopy revealed neutralization of natural infection in IL-2 receptor-negative Jurkat leukemia cells, but targeted expression of EGFP in IL-2 receptor-positive human leukemia-derived MT-2 cells. The targeted gene delivery of NDFL-EGFP/(alpha)HN-IL-2 in MT-2 cells was blocked by the target ligand human IL-2. Selective virus entry to IL-2 receptor bearing tumor cells was also observed in a mixture of Jurkat and MT-2 cell lines. These results demonstrate that a recombinant NDV carrying a foreign gene can be successfully targeted to a specific tumor through a bispecific protein, which thereby increases the selectivity of gene transfer.

Screening for cervical cancer in the developing world

Cervical cancer remains the most common malignancy amongst females in countries of low income, mainly due to a lack of screening. Responsible factors are centred around inadequacies of the Pap smear: high cost; low sensitivity; the need of a laboratory with high human expertise; and a demanding logistic system for mass screening. No alternative screening method seems to be clearly advantageous. Although combinations of tests have higher sensitivities, they are complex, costly and associated with low specificities. Adding the problem of effective treatment, it seems that mass screening with adequate coverage of the population is an unreachable goal for many developing countries. The most promising development in the control of cervical cancer seems to be vaccination against the human papillomavirus, either as a preventative measure or for stimulating immunity in infected women.

Oncolytic activities of approved mumps and measles vaccines for therapy of ovarian cancer

Oncolytic viruses are promising cytoreductive agents for cancer treatment but extensive human testing will be required before they are made commercially available. Here, we investigated the oncolytic potential of two commercially available live attenuated vaccines, Moraten measles and Jeryl-Lynn mumps, in a murine model of intraperitoneal human ovarian cancer and compared their efficacies against a recombinant oncolytic measles virus (MV-CEA) that is being tested in a phase I clinical trial. The common feature of these viruses is that they express hemagglutinin and fusion therapeutic proteins that can induce extensive fusion of the infected cell with its neighbors, resulting in death of the cell monolayer. In vitro, the three viruses caused intercellular fusion in human ovarian cancer cells but with marked differences in fusion kinetics. MV-CEA was the fastest followed by Jeryl-Lynn mumps virus while Moraten measles virus was the slowest, although all viruses eventually caused comparable cell death 6 days postinfection. Tumor-bearing mice treated with 10(6) or 10(7) pfu (one thousand times the vaccine dose) of each of the three viruses responded favorably to therapy with significant prolongations in survival. All three viruses demonstrated equivalent antitumor potency. Commercially available Moraten measles and Jeryl-Lynn mumps vaccines warrant further investigation as potential anticancer agents.

Oncolytic measles viruses for cancer therapy

New strategies using biological agents are being developed to treat cancer. Live viruses are among these new agents. Virotherapy uses replication-competent viral vectors with strong oncolytic properties. With the use of molecular virology techniques, viruses have been genetically engineered to replicate selectively in tumour cells and are under preclinical and clinical investigation at present. Measles virus (MV) is being used for this purpose. Replication-competent attenuated Edmonston B measles vaccine strain (MV-Edm) is non-pathogenic and has potent antitumour activity against several human tumours. The virus is selectively oncolytic in tumour cells, eliciting extensive cell-to-cell fusion and ultimately leading to cell death. Therefore, MV-Edm is a safe and efficient means to kill tumour cells. Further improvements in existing MV vectors may increase tumour selectivity and oncolytic activity. This review discusses the discovery and development of replication-competent oncolytic MV for cancer therapy.

Future directions for the field of oncolytic virotherapy: a perspective on the use of vaccinia virus

Oncolytic virotherapy is an emerging biotherapeutic platform based on genetic engineering of viruses capable of selectively infecting and replicating within cancer cells. Such viruses have been found to be both safe and to produce antitumour effects in a number of Phase I and II clinical trials. Early work in this field has been pioneered with strains of adenovirus which, although well suited to gene therapy approaches, have displayed certain limitations in their ability to directly destroy and spread through tumour tissues, particularly after systemic administration. Investigators have subsequently been examining the feasibility of using a variety of different viruses as oncolytic agents. Vaccinia virus is perhaps the most widely administered and successful medical product in history; it displays many of the qualities thought necessary for an effective antitumour agent and is particularly well characterised in people due to its role in the eradication of smallpox. Vaccinia has a short life cycle and rapid spread, strong lytic ability, inherent systemic tumour targeting, a large cloning capacity and well-defined molecular biology. In addition, the virus produces no known disease in humans, has been delivered safely to millions of people and has already demonstrated antitumoural efficacy in trials with vaccine strains. These qualities, along with strategies for further improving the safety and antitumour effectiveness of vaccinia, will be discussed in relation to the broad spectrum of clinical experience already achieved with this virus in cancer therapy.

Library-based selection of retroviruses selectively spreading through matrix metalloprotease-positive cells

Viruses conditionally replicating in cancer cells form an attractive novel class of antitumoral agents. To engineer such viruses infectivity can be coupled with proteolytic activity of the target cell by modifying the envelope (Env) protein of murine leukaemia virus (MLV) with blocking domains that prevent cell entry unless they are cleaved off by tumour-associated proteases like the matrix metalloproteases (MMP). Here we show that MLV variants selectively spreading through MMP-positive cells can be evolved from virus libraries, in which a standard MMP-2 substrate peptide connecting the blocking domain CD40L with the Env protein was diversified. Passaging the virus library on human fibrosarcoma or glioma cell lines resulted in the selection of about 10 virus clones, of which the three most frequent ones were shown to become activated by MMPs and to be replication competent on MMP-positive cells only. On these cells, the selected linker peptides improved the spreading by several orders of magnitude in vitro, as well as in tumour xenografts in vivo, approaching the kinetic of the unmodified wild-type virus. The data suggest that retroviral protease substrate libraries form a potent tool for the engineering of viruses conditionally replicating in a given cancer cell type of interest.

High CD46 receptor density determines preferential killing of tumor cells by oncolytic measles virus

Live attenuated Edmonston B strain of measles virus (MV-Edm) is a potent and specific oncolytic agent, but the mechanism underlying its tumor selectivity is unknown. The virus causes cytopathic effects (CPEs) of extensive syncytial formation in tumor cells but minimal damage or cell killing in normal cells. The CPE is dependent on expression of viral proteins and the presence of CD46, the major cellular receptor of MV-Edm. Using a virally encoded soluble marker peptide to provide a quantitative readout of the level of viral gene expression, we determined that tumor cells and normal cells expressed comparable levels of viral proteins. CD46 mediates virus attachment, entry, and virus-induced cell-to-cell fusion. Using engineered cells expressing a range of CD46 densities, we determined that whereas virus entry increased progressively with CD46 density, cell fusion was minimal at low receptor densities but increased dramatically above a threshold density of CD46 receptors. It is well established that tumor cells express abundant CD46 receptors on their surfaces compared with their normal counterparts. Thus, at low CD46 densities typical of normal cells, infection occurs, but intercellular fusion is negligible. At higher densities typical of tumor cells, infection leads to extensive cell fusion. Intercellular fusion also results in enhancement of viral gene expression through recruitment of neighboring uninfected cells into the syncytium, further amplifying the CPE. Discrimination between high and low CD46 receptor density provides a compelling basis for the oncolytic specificity of MV-Edm and establishes MV-Edm as a promising CD46-targeted cancer therapeutic agent.