Myxoma virus and oncolytic virotherapy: a new biologic weapon in the war against cancer

Oncolytic virotherapy is an innovative alternative to more conventional cancer therapies. The ability of some viruses to specifically target and kill malignant cancerous cells while leaving normal tissue unscathed has opened a large repertoire of new and selective cancer killing therapeutic candidates. Poxviruses, such as vaccinia virus, have a long history of use in humans as live vaccines and have more recently been studied as potential platforms for delivery of immunotherapeutics and attenuated variants of vaccinia have been explored as oncolytic candidates. In contrast, the poxvirus myxoma virus is a novel oncolytic candidate that has no history of use in humans directly, as it has a distinct and absolute host species tropism to lagomorphs (rabbits). Myxoma virus has been recently shown to be able to also selectively infect and kill human tumor cells, a unique tropism that is linked to dysregulated intracellular signalling pathways found in the majority of human cancers. This review outlines the existing knowledge on the tropism of myxoma virus for human cancer cells, as well as preclinical data exhibiting its ability to infect and clear tumors in animal models of cancer. This is an exciting new therapeutic option for treating cancer, and myxoma virus joins a growing group of oncolytic virus candidates that are being developed as a new class of cancer therapies in man.     

Myxoma virus and oncolytic virotherapy: a new biologic weapon in the war against cancer

Oncolytic virotherapy is an innovative alternative to more conventional cancer therapies. The ability of some viruses to specifically target and kill malignant cancerous cells while leaving normal tissue unscathed has opened a large repertoire of new and selective cancer killing therapeutic candidates. Poxviruses, such as vaccinia virus, have a long history of use in humans as live vaccines and have more recently been studied as potential platforms for delivery of immunotherapeutics and attenuated variants of vaccinia have been explored as oncolytic candidates. In contrast, the poxvirus myxoma virus is a novel oncolytic candidate that has no history of use in humans directly, as it has a distinct and absolute host species tropism to lagomorphs (rabbits). Myxoma virus has been recently shown to be able to also selectively infect and kill human tumor cells, a unique tropism that is linked to dysregulated intracellular signalling pathways found in the majority of human cancers. This review outlines the existing knowledge on the tropism of myxoma virus for human cancer cells, as well as preclinical data exhibiting its ability to infect and clear tumors in animal models of cancer. This is an exciting new therapeutic option for treating cancer, and myxoma virus joins a growing group of oncolytic virus candidates that are being developed as a new class of cancer therapies in man.     

Virus combinations and chemotherapy for the treatment of human cancers

Oncolytic viruses possess an inherent trophism for tumor cells or have been engineered in a variety of ways to selectively replicate in and destroy cancer cells. Because of the unique mode of tumor destruction, oncolytic virotherapy has the potential to augment the antineoplastic activity of chemotherapy and radiation therapy. Many oncolytic viruses, such as adenovirus, HSV, vaccinia, measles, reovirus, Newcastle disease virus and coxsackie virus, have entered into clinical trials and their efficacy and safety have been demonstrated with few, minor, side effects. Data obtained from several clinical trials of the oncolytic adenovirus, ONYX-015, in patients with cancer have been described in detail. Some preclinical studies of oncolytic viruses have demonstrated promising results, mainly when administered in combination with chemotherapeutic drugs. In this review, the clinical use of oncolytic viruses in combination with chemotherapy and radiation therapy, and future directions to enhance the efficacy of oncolytic virotherapy, are discussed.     

Luciferase imaging for evaluation of oncolytic adenovirus replication in vivo

Oncolytic viruses kill cancer cells by tumor-selective replication. Clinical data have established the safety of the approach but also the need of improvements in potency. Efficacy of oncolysis is linked to effective infection of target cells and subsequent productive replication. Other variables include intratumoral barriers, access to target cells, uptake by non-target organs and immune response. Each of these aspects relates to the location and degree of virus replication. Unfortunately, detection of in vivo replication has been difficult, labor intensive and costly and therefore not much studied. We hypothesized that by coinfection of a luciferase expressing E1-deleted virus with an oncolytic virus, both viruses would replicate when present in the same cell. Photon emission due to conversion of D-Luciferin is sensitive and penetrates tissues well. Importantly, killing of animals is not required and each animal can be imaged repeatedly. Two different murine xenograft models were used and intratumoral coinjections of luciferase encoding virus were performed with eight different oncolytic adenoviruses. In both models, we found significant correlation between photon emission and infectious virus production. This suggests that the system can be used for non-invasive quantitation of the amplitude, persistence and dynamics of oncolytic virus replication in vivo, which could be helpful for the development of more effective and safe agents.     

Recent clinical progress in virus-based therapies for cancer

As our knowledge of the molecular basis of cancer expands, viral vectors have been increasingly studied as potential antitumour therapeutic agents. With their ability to invade and replicate within target cells, viruses have been utilised as oncolytic agents to directly lyse tumour cells. Viruses can also deliver their genetic payload into infected cells, allowing for the repair of defective tumour suppressor genes, disruption of oncogenic pathways, and production of cytokines that activate the immune system. Finally, viruses encoding tumour-associated antigens can infect dendritic cells, triggering the development of a tumour-specific immune response. The ability to engineer viruses with high levels of tumour specificity and efficient rates of infection has enhanced the safety profile of these agents, allowing for the development of viable therapeutic options that have been examined in the clinic, either alone or in conjunction with more conventional therapies. This review highlights the principles underlying virus-based therapies for cancer, with an emphasis on recent developments from the clinic.     

Oncolytic vaccinia virus: from bedside to benchtop and back

The field of oncolytic viral therapy has undergone a major shift in focus in the last few years. Less research has been directed at making incremental improvements in original vectors based mainly on strains of adenovirus and HSV; instead a variety of different viral strains have been suggested as potential backbones for future oncolytic viruses (including Newcastle disease virus, reovirus, vesicular stomatitis virus, polio virus, retrovirus, Sindbis virus, picornavirus, mumps and measles virus), with many of these progressing to clinical trials. Of these, vaccinia virus represents a particularly promising candidate. It possesses a variety of intrinsic molecular properties suitable for an oncolytic virus (such as rapid life cycle and lysis of infected cells, and an ability to infect various cell types), in addition to undergoing extensive study both in the laboratory and in the clinic. Although not a natural human pathogen, there are extensive data on the effects of vaccinia infection in humans. Preclinical models incorporating new oncolytic vaccinia strains, as well as data from the first clinical trials that have utilized the next-generation oncolytic vaccinia strains for the potential treatment of cancer have been described.     

Oncolysis of pancreatic tumour cells by a gamma34.5-deleted HSV-1 does not rely upon Ras-activation, but on the PI 3-kinase pathway

The ability of viruses to selectively target, replicate within, and destroy tumour cells without deleterious effects in normal cells (oncolysis), makes the use of viruses as an attractive tool for cancer treatment. Pancreatic adenocarcinoma, being insensitive to traditional therapy and having a rather poor prognosis, represents a suitable target to evaluate viral oncolysis as a novel therapeutic approach. Herpes simplex virus (HSV) has been reported to produce an oncolytic effect in cells overexpressing Ras. As Ras signalling is frequently aberrant in pancreatic cancer, we compared four pancreatic cell lines (which differ in the presence of mutated or wild-type ras) for their ability to support growth of gamma34.5-replication attenuated HSV-1 (R3616). Our data show that permissiveness to viral replication is neither associated with enhanced Ras signalling nor with defective PKR activity. By contrast, we provide evidence that disregulation of the PI 3-kinase signalling pathway allows conditionally replication-defective R3616 virus to overcome the cellular antiviral activity.     

Oncolytic adenoviruses for the treatment of brain tumors

In recent years, oncolytic viruses have been genetically engineered to target cancer cells selectively. Adenovirus is one such oncolytic virus that is being tested in clinical trials for the treatment of cancer. The observation that cells infected with replication-competent adenoviruses undergo autophagy has provided new options for investigating the mechanism of adenovirus-induced cell death. It has been suggested that the use of autophagy inducers, such as rapamycin, can enhance the oncolytic potency of recombinant adenoviruses. Additionally, several research groups have established that inserting microRNA (miRNA)-targeted sequences into the adenoviral genome can modulate adenoviral protein expression to confer tissue and tumor selectivity. Furthermore, the capability of adenoviruses to inhibit the expression of the DNA repair enzyme MGMT and to chemosensitize glioma cells to temozolomide has been demonstrated. This review discusses three aspects of the use of oncolytic adenoviruses to treat cancer: (i) the induction of autophagy and autophagic cell death during adenoviral replication; (ii) the opportunities and strategies involved in the exploitation of miRNA specificity to generate tissue- and tumor-selective oncolytic viruses; and (iii) the rationale for combining oncolytic adenoviruses with chemotherapeutic agents.     

Potential of tumour cells for delivering oncolytic viruses

Autologous or allogenic tumour cells have long been used in the fight against cancer as vaccines to awaken the patient's immune system. On the other hand, oncolytic viruses have emerged in recent years as powerful therapeutic tools for selectively killing tumour cells. Yet despite recent improvements in virus production, administration and targeting, the latter strategy remains limited by poor access of oncolytic viruses to primary and metastatic tumour cells. The present review focuses on how to overcome these limitations on oncolytic virus delivery, at least in part, through the use of tumour-derived or in vitro transformed carrier cells. On the basis of existing evidence, novel strategies are proposed for using such cell vehicles, alone or in combination, both as virus factories and as anticancer vaccines.     

Naturally oncolytic viruses

Naturally oncolytic viruses are replication-competent viruses that have an innate ability to selectively infect and kill tumor cells. Despite being used in the original attempts to treat cancer with live viruses five decades ago, interest in naturally oncolytic viruses has lagged behind the support for engineered adenoviruses and herpesviruses as cancer therapeutics. Recently, however, there has been renewed interest in the high potency and selectivity of these naturally occurring agents. In this review, the mechanism of selectivity as well as the preclinical and clinical activity of naturally oncolytic viruses will be discussed.     

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.     

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.     

Integrating the biological characteristics of oncolytic viruses and immune cells can optimize therapeutic benefits of cell-based delivery

Despite significant advances in the development of tumor-selective agents, strategies for effective delivery of these agents across biological barriers to cells within the tumor microenvironment has been limiting. One tactical approach to overcoming biological barriers is to use cells as delivery vehicles, and a variety of different cell types have been investigated with a range of agents. In addition to transporting agents with targeted delivery, cells can also produce their own tumoricidal effect, conceal a payload from an immune response, amplify a selective agent at the target site and facilitate an antitumor immune response. We have reported a therapeutic combination consisting of cytokine induced killer cells and an oncolytic vaccinia virus with many of these features that led to therapeutic synergy in animal models of human cancer. The synergy was due to the interaction of the two agents to enhance the antitumor benefits of each individual component. As both of these agents display broad tumor-targeting potential and possess unique tumor killing mechanisms, together they were able to recognize and destroy a far greater number of malignant cells within the heterogeneous tumor than either agent alone. Effective cancer therapy will require recognition and elimination of the root of the disease, the cancer stem cell, and the combination of CIK cells and oncolytic vaccinia viruses has this potential. To create effective tumor-selective agents the viruses are modified to take advantage of the unique biology of the cancer cell. Similarly, if we are to develop targeted therapies that are sufficiently multifaceted to eliminate cancer cells at all stages of disease, we should integrate the virus into the unique biology of the cell delivery vehicle.     

Exploiting diversity: genetic approaches to creating highly potent and efficacious oncolytic viruses

Oncolytic viruses possess several key attributes that make them a highly attractive treatment for cancer. They exhibit clinically validated synergy with chemotherapy and an ability to selectively destroy tumor cells to the exclusion of normal cells. Oncolytic viruses can replicate and, therefore, amplify their dose in a tumor-dependent manner. In addition, they can be genetically manipulated to include additional therapeutic factors to create a multimodal anticancer agent. These characteristics lead to the expectation that oncolytic viruses will serve as an additional tool in the treatment repertoire of clinical oncologists. In their clinical development to date, these agents were safe and well tolerated, but lacked efficacy as monotherapies. In this review, three genetic-based methods to increase the potency and efficacy of oncolytic viruses, in which human adenovirus is utilized as an example of a prototype oncolytic virus, are discussed.     

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.     

Replicative oncolytic adenoviruses in multimodal cancer regimens

The use of replication-competent viruses that have a cytolytic cycle has emerged as a viable strategy (oncolytic virotherapy) to specifically kill tumor cells and the field has advanced to the point of clinical trials. A theoretical advantage of replicative oncolytic viruses is that their numbers should increase via viral replication within infected tumor cells and resulting viral progeny can then infect additional cells within the tumor mass. The life cycle of a virus involves multiple interactions between viral and cellular proteins/genes, which maximize the ability of the virus to infect and replicate within cells. Understanding such interactions has led to the design of numerous genetically engineered adenovirus (Ad) vectors that selectively kill tumor cells while sparing normal cells. These viruses have also been modified to function as therapeutic gene delivery vehicles, thus augmenting their anticancer capacity. In addition, the oncolytic mode of tumor killing differs from that of standard anticancer therapies, providing the possibility for synergistic interactions with other therapies in a multimodal antitumor approach. In this review, we describe the oncolytic Ad vectors tested in preclinical and clinical models and their use in combination with chemo-, radio-, and gene therapies.