Virotherapy represents a new, fast-growing field in oncology. Even though virotherapy medicines have entered the pharmaceutical market less than 15 years ago, the idea of virus a killing cancer cells can be traced back to the 19th century. It is hard to pinpoint the exact moment of discovery of virotherapy, since there were many important discoveries and observations, which helped virotherapy evolve to the level we know now.
Observations of tumour regression after infectious diseases
The first written reports where doctors had observed spontaneous disease remission of cancer patients after some infectious diseases date back to 19th century. Although at that time, they did not know that infectious diseases were caused by a particular virus or bacteria. Information about tumour exposure to viruses dates back to 1829, when Guillaume Dupuytren described a case of breast tumour regression after febrile illness. In 1868, W. Busch was the first to intentionally inoculate a cancer patient with erysipelas and noticed shrinkage of the malignancy. In 1896 Dock described a 42 years old woman with leukaemia, who went into remission after influenza infection. In 1904 a woman in Italy with cervical cancer was bitten by a dog. The doctors gave her a rabies vaccine, after which the tumour disappeared and the woman lived for another 8 years. Soon thereafter several other Italian patients with cervical cancer also received the vaccine – a live rabies virus that had been weakened. As reported by Nicola De Pace in 1910, tumours in some patients shrank. [1, 4, 5, 9]
Development of microbiology
The development of virotherapy went step by step together with the development of microbiology. The history of microorganisms’ as we know them now dates back to the 18th century, when the Dutch scientist Antonie Van Leeuwenhoek (1632-1723) discovered and studied microorganisms by help of a microscope that he had designed himself . Although people had enjoyed benefits of microorganisms since ancient times, it was not clear why grape juice turns into wine or why milk can be turned into cheese. Antonie Van Leeuwenhoek was the first who observed that there are some tiny, living beings that cannot be observed with the naked eye although he did not particularly connect them with fermentation or other processes involving microorganisms. After his discoveries, the field of microbiology flourished. The Italian scientist Lazzaro Spallanzani (1729-1799) proved that heat would kill microorganisms and new microorganisms could get into sterile broth only if it would be exposed to air, thus, no living being would appear de novo . Louis Pasteur (1822-1895), a scientist whose name is among the most popular, expanded Spallanzani’s experiments and proved once more that microorganisms did not develop in sterile broth but they came from outside.
Chamberland-Pasteur filter and it’s role in recognizing viruses
A connection between microorganisms and disease was made by the German scientist Robert Koch (1843-1910). He observed that animals taken ill with anthrax had large amounts of microorganisms in their blood. He also observed that if he transferred a small amount of blood of a sick animal to healthy, it would cause disease in that animal . In 1884 another French scientist Charles Chamberland (1851-1908) invented the Chamberland-Pasteur porcelain filter, which could be used to completely remove all bacteria or other cells known at the time from a liquid suspension. [1,6]
The filter was used by the Russian scientist Dmitri Ivanovsky (1864-1920) to try to isolate the bacteria that caused tobacco mosaic disease. However, after filtration, the crushed leaf extract still remained infectious properties. Then he thought that it was a toxin produced by the bacteria that caused the disease. The work was continued by Martinus Beijerinck (1851-1931), who transferred this filtered solution from plant to plant and saw that action did not get weaker and therefore, developed a theory that this infectious agent was not a toxin, but something that could replicate.  However, most active research began after Frederick Twort (1877-1950) and Felix d’Herelle (1873-1949) found out that some viruses – bacteriophages – can infect bacteria. Since it was easy to grow bacteria in cultures, it became relatively easy to investigate bacteriophages and plant viruses. However, this was difficult with animal and human viruses. The first success came with influenza virus in 1931, which could be grown in fertilized chicken eggs. This method is still used today to produce vaccines 
Oncotropism of viruses
In 1922 a Romanian physician, cytologist and immunologist Constantin Levaditi (1874-1953) noticed that tumours are more susceptible to viruses than normal cells and that viruses preferentially replicate in tumour cells. It was the beginning of the understanding of oncotropism of viruses. Also, the introduction of electron microscopes in 1939 was a huge development in science, since it made possible the visualization of viruses. [1, 3] It is also crucial to mention the work of three scientists – John F. Enders (1897-1985), Thomas Weller (1915-2008) and Frederick Robbins (1916-2003) who managed to grow poliovirus in cultured human embryonal cells. This was the beginning of cell cultures, which helped grow animal viruses outside the animal body 
Further research continued and in 1951 probably the most used cell line in the world – HeLa cell line was derived. The HeLa cell line was derived from cervical cancer cells taken on February 8, 1951 from Henrietta Lacks, a patient who died of cancer on October 4, 1951. They were the first human cells grown in a lab that were «immortalized». The HeLa cell line has contributed to many medical breakthroughs, it was used in the development of polio vaccine in 1954, and they were also the first cells to be cloned. Cell culturing techniques became one of the cornerstones in further research of oncolytic viruses. [1, 2, 3, 8]
First clinical trials of oncolytic viruses
Meanwhile, first clinical trials of oncolytic viruses began in the 1940s, Hoster and his colleagues injected 22 patients suffering from Hodgkin’s disease with hepatitis B virus. One of the patients died, 13 developed hepatitis, but 7 showed some improvement. Scientists started actively testing various viruses in patients with a wide range of different cancers. [1, 5]
In 1952 many clinical trials were started involving West Nile virus, although scientists observed replication of the virus in the tumour, neurotoxicity was observed as well. Overall, tumour response also was observed, however, rarely. Screening of possible oncolytic viruses was carried out and several viruses – adenoviruses, herpes viruses, paramyxoviruses, picornaviruses were selected for future research. In 1956 scientists started trials of adenovirus in cervical cancer patients. In patients, who responded to adenovirus, tumour tissue was shed in large amounts, but survival wasn’t significantly prolonged and more than half of the patients died a few months after starting therapy. Adenovirus trials were followed by picornavirus trials, however, neither produced significant results. Therefore, research next shifted to mumps virus. Scientists tested mumps virus on 18 cancer types using different virus delivery methods. Results were very positive, in 37 of 90 patient’s the tumour disappeared completely or decreased at least by half. [1,5]
Adaptation of viruses
During these studies, the scientists realized that there is a need for the virus to be more specific. And here, scientist Alice Moore concluded that viruses can be more effective in the tumour tissues in which they were propagated. This is now considered targeted evolution. At that time, virus adaptation was the only way to make viruses more specific to a particular cell type. [1, 9]
Virus adaptation was widely used, and a success story is from Latvia. After the polio vaccine was developed, the research group that worked on this project in Latvia was left with many different viruses from the gastrointestinal tract of healthy children. They started to screen those viruses for oncolytic properties. ECHO group viruses were found to show oncolytic activity and an ECHO-7 virus was adapted to melanoma. In 1968 the first clinical trial of the oncolytic ECHO group viruses started. The ECHO-7 virus was tested in over 700 patients with late stage melanoma, stomach and gastrointestinal tract cancer, who had failed on standard treatment of that time. The ECHO-7 virus was also shown to be safe. [12, 16]
Genetic engineering of a viral genome
The field of genetic engineering has developed rapidly since 1950’s, and in the 1990s genetic engineering was a common laboratory technique, which allowed easy manipulation with DNA and RNA, and thus viral genomes. Now it was possible to modify a viral genome to improve its selectivity, decrease pathogenicity and insert different genes that would eventually produce proteins that help to kill cancer cells. ONYX-015 became the first virus to enter Phase I clinical trials in 1996, followed by many other viruses. [1, 5, 10]
Huge interest in oncolytic viruses and first approved drugs
1990’s also could be considered as a start of the new wave of clinical research in the field of virotherapy. Since then scientists work intensively and in 2015 there were more than 100 ongoing clinical studies of oncolytic viruses. Most of the studies are in Phase I and Phase II, however, some companies made it to the registration and there are currently 3 registered oncolytic viruses on the market. [10, 16, 17]
The first oncolytic virotherapy medicine was approved in 2004 in Latvia, after many decades of research the genetically not modified ECHO-7 virus strain Rigvir® was registered. The second was Oncorine-15® in 2005, a genetically modified virus registered by the Chinese company Shanghai Sunway Biotech. Ten years later, in 2015, Imlygic®, a genetically altered HSV-1 (herpes simplex virus) was approved in the USA by FDA and in Europe by EMA (manufacturer Amgen). [1, 10, 16, 17]
Scientists continue to work on oncolytic viruses, both genetically modified and non-genetically modified. It is just a matter of time when new virotherapy drugs will be approved.
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