Oncolytic virus

An oncolytic virus is a virus that preferentially infects and kills cancer cells. As the infected cancer cells are destroyed by oncolysis, they release new infectious virus particles or virions to help destroy the remaining tumour. Oncolytic viruses are thought not only to cause direct destruction of the tumour cells, but also to stimulate host anti-tumour immune system responses. An oncolytic virus is a virus that preferentially infects and kills cancer cells. As the infected cancer cells are destroyed by oncolysis, they release new infectious virus particles or virions to help destroy the remaining tumour. Oncolytic viruses are thought not only to cause direct destruction of the tumour cells, but also to stimulate host anti-tumour immune system responses. The potential of viruses as anti-cancer agents was first realised in the early twentieth century, although coordinated research efforts did not begin until the 1960s.A number of viruses including adenovirus, reovirus, measles, herpes simplex, Newcastle disease virus, and vaccinia have been clinically tested as oncolytic agents.Most current oncolytic viruses are engineered for tumour selectivity, although there are naturally occurring examples such as reovirus and the senecavirus, resulting in clinical trials. The first oncolytic virus to be approved by a national regulatory agency was genetically unmodified ECHO-7 strain enterovirus RIGVIR, approved in Latvia in 2004 for the treatment of skin melanoma. It was subsequently approved in Georgia and Armenia. An oncolytic adenovirus, a genetically modified adenovirus named H101, was approved in China in 2005 for the treatment of head and neck cancer. In 2015, talimogene laherparepvec (OncoVex, T-VEC), an oncolytic herpes virus which is a modified herpes simplex virus, became the first oncolytic virus to be approved for use in the U.S. and European Union, for the treatment of advanced inoperable melanoma. A connection between cancer regression and viruses has long been theorised, and case reports of regression noted in cervical cancer, Burkitt lymphoma, and Hodgkin lymphoma, after immunisation or infection with an unrelated virus appeared at the beginning of the 20th century. Efforts to treat cancer through immunisation or virotherapy (deliberate infection with a virus), began in the mid-20th century. As the technology for creating a custom virus did not exist, all early efforts focused on finding natural oncolytic viruses. During the 1960s, promising research involved using poliovirus, adenovirus, Coxsackie virus, ECHO enterovirus RIGVIR, and others. The early complications were occasional cases of uncontrolled infection, resulting in significant morbidity and mortality; the very frequent development of an immune response, while harmless to the patient, destroyed the virus and thus prevented it from destroying the cancer. Only certain cancers could be treated through virotherapy was also recognised very early. Even when a response was seen, these responses were neither complete nor durable. The field of virotherapy was nearly abandoned for a time, as the technology required to modify viruses didn't exist and chemotherapy and radiotherapy technology enjoyed early success. However, now these technologies have been thoroughly developed, cancer is still a major cause of mortality and there is still a need for novel cancer therapies, this sidelined therapy has now gained renewed interest. Herpes simplex virus (HSV) was one of the first viruses to be adapted to attack cancer cells selectively, because it was well understood, easy to manipulate and relatively harmless in its natural state (merely causing cold sores) so likely to pose fewer risks. The herpes simplex virus type 1 (HSV-1) mutant 1716 lacks both copies of the ICP34.5 gene, and as a result is no longer able to replicate in terminally differentiated and non-dividing cells but will infect and cause lysis very efficiently in cancer cells, and this has proved to be an effective tumour-targeting strategy. In a wide range of in vivo cancer models, the HSV1716 virus has induced tumour regression and increased survival times. In 1996, the first approval was given in Europe for a clinical trial using the oncolytic virus HSV1716. From 1997 to 2003, strain HSV1716 was injected into tumours of patients with glioblastoma multiforme, a highly malignant brain tumour, with no evidence of toxicity or side effects, and some long-term survivors. Other safety trials have used HSV1716 to treat patients with melanoma and squamous-cell carcinoma of head and neck. Since then other studies have shown that the outer coating of HSV1716 variants can be targeted to specific types of cancer cells, and can be used to deliver a variety of additional genes into cancer cells, such as genes to split a harmless prodrug inside cancer cells to release toxic chemotherapy, or genes which command infected cancer cells to concentrate protein tagged with radioactive iodine, so that individual cancer cells are killed by micro-dose radiation as well as by virus-induced cell lysis. Other oncolytic viruses based on HSV have also been developed and are in clinical trials. One that has been approved by the FDA for advanced melanoma is Amgen's talimogene laherparepvec. The first oncolytic virus to be approved by a regulatory agency was a genetically modified adenovirus named H101 by Shanghai Sunway Biotech. It gained regulatory approval in 2005 from China's State Food and Drug Administration (SFDA) for the treatment of head and neck cancer. Sunway's H101 and the very similar Onyx-15 (dl1520) have been engineered to remove a viral defense mechanism that interacts with a normal human gene p53, which is very frequently dysregulated in cancer cells. Despite the promises of early in vivo lab work, these viruses do not specifically infect cancer cells, but they still kill cancer cells preferentially. While overall survival rates are not known, short-term response rates are approximately doubled for H101 plus chemotherapy when compared to chemotherapy alone. It appears to work best when injected directly into a tumour, and when any resulting fever is not suppressed. Systemic therapy (such as through infusion through an intravenous line) is desirable for treating metastatic disease. It is now marketed under the brand name Oncorine. With advances in cancer immunotherapy such as immune checkpoint inhibitors, increased attention has been given to using oncolytic viruses to increase antitumor immunity. There are two main considerations of the interaction between oncolytic viruses and the immune system.