The Treatment of Melanoma: Oncolytic Virus Therapies

Cancer is the name that is given to a group of related diseases where the body’s cells begin to divide uncontrollably and have the capacity of invasion and prolonged survival. Despite of several advances made in treating cancer, there is always a need for a novel approach to tackle this condition due to the lack of effectiveness of the conventional therapies. The introduction of oncolytic viruses for this can be considered a promising approach. These are the genetically modified viruses which have the ability to kill the tumour cells selectively by combining tumour lysis and immune system activation. The efficiency of OVs as an anti-tumour candidate has been studied in many preclinical studies and also recently in humans. US Food and Drug Administration has also approved the oncolytic herpes virus Talimogene laherparepvec (T-VEC) in 2015 for treatment of advanced melanoma. This review mainly focuses on the introduction to oncolytic viruses, their mechanisms and the current status of these viruses particularly in the treatment of breast cancer.

Introduction

The greatest advantage proposed by the tumour cell is to escape the immune system called as immune-editing. This happens in three stages: elimination where the immune system identifies the tumour antigens and tries to attack these cells. Equilibrium where those cells which managed to survive the attack modify their antigens to become unrecognizable by the immune system and continue to grow in size. Escape where the immune system is no longer capable of controlling the growth of these malignant cells.

Immunotherapy is a method of treating cancers by encouraging the body’s own immune system to identify and kill the cancerous cells. It makes use of the substances made in a laboratory or by the body to re-establish the immune function. There are several types of immunotherapy which include:

• Oncolytic virus therapy
• Monoclonal antibodies and tumour agnostic therapies
• Cancer vaccines
• Non-specific immunotherapies
• T-cell therapy

Oncolytic virus therapies make use of genetically engineered viruses to kill selectively the tumour cells without affecting the host cells. Oncolytic virus therapy makes use of the virus itself as an active drug component in contrast to gene therapy where the virus is used as only a carrier for the delivery of transgene.

In the year 1949 Moore addressed regarding the use of OVs in treating cancer. His first preclinical study was based on capability of the Russian Far East Virus to inhibit the growth of mouse tumours. In the same year, 22 patients with Hodgkin’s disease were been treated with tissue extracts that contained hepatitis virus. Many attempts were made to use naturally attenuated or wild type viruses, such as hepatitis virus, yellow fever West Nile fever, adenoviruses and dengue fever for the treatment of cancer in 1950-1980s. However, as there no available technique to retain viral multiplication in cancer cells and attenuate their virulence at the same time, these were considered to be less useful. The advent of genetic engineering has regained the importance of using OVs as a treatment strategy.

Oncolytic viruses can be of 2 types: natural and genetically modified viruses. The natural virus can include wild type (ex: Reovirus) and weak virus variant strains. Genetic modification of viruses allows us to obtain strains with lower pathogenicity and increased immunogenicity. By inserting an exogenous gene of therapeutic importance it is possible to increase its selectivity and lethality towards malignant cells.

Mechanism of Action

OV tend to demonstrate anti-tumour activities by a combination of 2 mechanism: a) tumour lysis b) immune stimulation. Selectivity of OV to tumour cells can be due to the presence of virus-specific receptors on tumour cells. High metabolic activity in the malignant cells encourages the viral replication in them rather than the normal cells. Also, the tumour-driver mutations are responsible for increasing the selectivity of virus multiplication in malignant cells.

Protection mechanisms like interferon signalling is found to be absent in majority of tumour cells as a result of which most of the viruses can replicate to a much greater extent in cancer cells than in normal cells. It is necessary to see that the viruses don’t replicate in normal cells at the same time retain its ability to replicate in cancer cells.

OV are capable of causing immunogenic cell death (ICD) and hence triggers an inflammatory reaction. This is a type of apoptosis in which the death of malignant cells is capable of inducing an anti-tumour response through recruitment and activation of dendritic cells and also by stimulation of specific T lymphocytes. The process of ICD triggers the release of dangerous metabolites known as damage-associated molecular patterns (DAMPs) from the endoplasmic reticulum and tumour-associated antigens into the microenvironment.

In response to these metabolites, APCs in the microenvironment generate an immune response, thus disrupting the immuno-editing process. There are chances that the immune system can clear the virus prematurely along with its lytic effect which can result in loss of anti-tumour immunity. Hence its important to design OVs which can multiply and spread within malignant cells quickly to induce the maximal anti-tumour effect before its clearance.

Also, genetic engineering has given to rise to OV lacking thymidine kinases hence forcing them to replicate in cells where there is high Ras activity like in tumour cells. The best example is given by Talimogene Laherparepvec, known as T-Vec. Which is a modified herpex simplex virus (HSV). It has two viral gene deletions and contains the human GM-CSF gene. In phase II study it showed an increase in the number of tumour-specific CD8+ T cells and reduced the number of suppressor and regulatory T cells. When tested in phase III trial in patients with melanoma resulted, in its FDA approval in the year 2015 for the treatment of patients with melanoma.

Oncolytic Viruses in the Treatment of Breast Cancer

Breast cancer remains to be the leading cause of death in women. The currently available treatment modalities include surgery, radiation, chemotherapy, and hormonal and targeted therapies. But all of these have limitations when it comes to the treatment of advanced diseases. The use of novel oncolytic viruses in the treatment appears to be promising. Several viruses have been tried as a candidate here.

Herpes Simplex Virus

Herpes simplex virus type 1 (HSV-1) is an enveloped dsDNA. Several mutations have been incorporated in viral genes in the oncolytic HSV vectors to make them more target-specific. One example of such modification was the deletion of the γ134.5 gene which resulted in its inability to replicate in neurons. Also modifications to the entry mediator glycoprotein present on the HSV-1 envelope which allowed for retargeting to specific receptors overexpressed in breast cancer, such as the HER-2 receptors. R-LM249 is an example for this where it was engineered to contain anti-HER-2 single-chain antibody trastuzumab in the gD domain. This was successfully used to retarget the HER-2 receptor in breast cancer.

In one study, tumour size reduction and increase in CD8 + T cell in tumour cells where seen when patients with breast cancer recurrences received single or repeated doses of HF10 into tumour nodules.

Reoviruses

Reovirus is a dsRNA virus is generally non-pathogenic in humans. It depends on Ras overexpression in tumour cells. Pelareorep (Reolysin) combined with paclitaxel was investigated in patients with metastatic breast cancer in a phase II clinical trial. 74 women have randomised in which 36 patients received Pelareorep combined with paclitaxel and the remaining received alone paclitaxel.

Pelareorep was well tolerated and the median survival in first group patients who received the combination was longer than those who received paclitaxel alone (17.4 and 10.4 months, respectively). This trial suggested that the combination of both is more effective than paclitaxel alone. Some studies also showed that a panel of breast cancer cell lines were all susceptible to reovirus infection regardless of hormone receptor status, whereas normal epithelial cells of the breast were not affected. This can be attributed to overexpression of the Ras pathway or the presence of mutated Ras protein in malignant cells.

Vaccinia virus (VVDD)

The vaccinia virus (VV) is dsDNA viruses whose replication occurs entirely in the cytoplasm, and not in the nucleus of the cell and has natural tropism to tumours. This feature can be considered as an additional benefit in terms of safety for an oncolytic virus as the genome integration risk can be eliminated. Vaccinia virus is genetically engineered by deleting two of the essential genes namely thymidine kinase and vaccinia growth factor in order to obtain a tumour selective replication capacity. A trial of VVDD included 4 patients with breast cancer who were injected with 3×107 pfu intratumorally. The therapy was well tolerated and showed a high selective multiplication in tumour cells.

Conclusion

FDA approval of T-VEC for the treatment of melanoma in 2015 as gained a lot of interest in the field of developing additional OV for the treatment of other types of cancer. But there are multiple obstacles faced in terms of delivery of these viruses, suppression of viral replication, development of antibodies etc. A combinatorial approach using OV with conventional chemotherapy, or use of 2 different OV can be beneficial. Balancing between the immune anti-tumour and anti-viral response is important to enhance anti-tumour activity. It is further necessary to justify the efficiency and safety of OVs from large cohort studies and multiple clinical situations and to bring it into clinical practice.