Researchers at the São Paulo State Cancer Institute (ICESP) in Brazil have used a genetically manipulated virus to destroy tumor cells upon injection into mice with prostate cancer.
The virus also made tumor cells more sensitive to chemotherapy drugs, halting tumor progression and almost eliminating tumors in some cases.
The results were obtained by a team led by Bryan Eric Strauss, head of the Viral Vector Laboratory at ICESP’s Center for Translational Research in Oncology (CTO), and are described in an article in Gene Therapy.
“We used a combination of gene therapy and chemotherapy to combat prostate cancer in mice,” said Strauss.
“We chose the weapon we considered most likely to work as a tumor suppressant,” he said, referring to p53, a gene that controls important aspects of cell death and is present in both rodents and humans.
In the laboratory, the gene was inserted into the genetic code of an adenovirus.
The modified virus was then injected directly into tumors in mice.
“First, we implanted human prostate cancer cells in the mice and waited for tumors to grow.
We then injected the virus directly into the tumors.
We repeated this procedure several times.
On two of these occasions, we also systemically administered cabazitaxel, a drug commonly used in chemotherapy.
After that, we observed the mice to see if the tumors developed,” Strauss said.
The experiments used several groups of mice, all of which were inoculated with prostate tumor cells.
To verify the efficacy of the gene therapy, the researchers administered an unrelated virus to one of the groups as a control.
The second group received only the virus with p53.
The third group received only cabazitaxel.
The fourth group, corresponding to 25% of the mice, received a combination of the drug and the virus.
When the tumor cells were infected by the modified virus, it penetrated the cell nucleus – where genes act – and triggered cell death.
The p53 gene was particularly effective at inducing cell death in prostate cancer.
“Individual treatments with p53 or cabazitaxel alone had an intermediate effect in terms of controlling tumor growth, but the combination had the most striking result, totally inhibiting tumors,” Strauss said.
The experiments proved that the modified virus caused the death of the tumor cells it infected.
“The association of the drug with gene therapy resulted in full control of tumor growth.
In other words, we observed an additive or even synergistic effect. It can also be assumed that the virus with p53 made tumor cells more sensitive to the action of the chemotherapy drug,” he said.
According to Strauss, the virus cannot be injected into the bloodstream.
“For the therapy to work, we need to inject the virus directly into tumor cells,” he said.
Tumors can evidently be controlled using chemotherapy drugs alone, he recalled, but the high doses required can have significant side effects.
One is leukopenia, or loss of white blood cells, a constraint for this type of chemotherapy because it impairs the immune system.
“In our study, we used a subtherapeutic dose, which was not sufficient to control the tumor. This was done to avoid leukopenia,” Strauss said.
Immune system
Destroying tumor cells with p53 does not guarantee that all cancer cells will be eliminated, including metastases.
Stimulation of the organism’s immune response was the answer found by the researchers.
According to Strauss, if the combination of p53 and cabazitaxel is not sufficient to activate the immune system, the use of a second gene in addition to p53 can be considered.
The interferon-beta gene was chosen for its key role in the immune system.
Interferons are proteins produced by leukocytes (white blood cells) and fibroblasts that interfere with the replication of fungi, viruses, bacteria and tumor cells while also stimulating the defense activities of other cells.
“Both p53 and interferon-beta can kill tumor cells. We wanted to combine them for cell death to wake up the immune system.
This is known as immunogenic cell death,” Strauss said.
Previous studies by the group served as a basis for the idea. When a combination of ARF (a functional partner of p53) and interferon-beta was inserted into the tumor cell nucleus, the mouse’s immune system ceased recognizing the tumor cell as part of its organism and identified it as an external agent to be combated.
“When this happens, the immune system combats tumor cells both at the treatment site and in tumors located elsewhere,” Strauss said.
“Our goal now is to refine these approaches. We’re engaged in experiments to find out whether they deserve to advance to the stage of clinical trials in human patients.”
Modified virus used to kill cancer cells
It is the first time that cancer-associated fibroblasts within solid tumours – healthy cells that are tricked into protecting the cancer from the immune system and supplying it with growth factors and nutrients – have been specifically targeted in this way.
The researchers, who were primarily funded by the MRC and Cancer Research UK, say that if further safety testing is successful, the dual-action virus – which they have tested in human cancer samples and in mice – could be tested in humans with carcinomas as early as next year.
Currently, any therapy that kills the ‘tricked’ fibroblast cells may also kill fibroblasts throughout the body – for example in the bone marrow and skin – causing toxicity.
In this study, published in the journal Cancer Research, the researchers used a virus called enadenotucirev, which is already in clinical trials for treating carcinomas.
It has been bred to infect only cancer cells, leaving healthy cells alone.
They added genetic instructions into the virus that caused infected cancer cells to produce a protein called a bispecific T-cell engager.
The protein was designed to bind to two types of cells and stick them together. In this case, one end was targeted to bind to fibroblasts.
The other end specifically stuck to T cells – a type of immune cell that is responsible for killing defective cells. This triggered the T cells to kill the attached fibroblasts.
Dr Joshua Freedman, from the Department of Oncology at the University of Oxford, who was first author on the study said:
‘We hijacked the virus’s machinery so the T-cell engager would be made only in infected cancer cells and nowhere else in the body. The T-cell engager molecule is so powerful that it can activate immune cells inside the tumour, which are being supressed by the cancer, to attack the fibroblasts.’
Dr Kerry Fisher, from the Department of Oncology at the University of Oxford, who led the research said: ‘Even when most of the cancer cells in a carcinoma are killed, fibroblasts can protect the residual cancer cells and help them to recover and flourish.
Until now, there has not been any way to kill both cancer cells and the fibroblasts protecting them at the same time, without harming the rest of the body.
‘Our new technique to simultaneously target the fibroblasts while killing cancer cells with the virus could be an important step towards reducing immune system suppression within carcinomas and should kick-start the normal immune process.
‘These viruses are already undergoing trials in people, so we hope our modified virus will be moving towards clinical trials as early as next year to find out if it is safe and effective in people with cancer.’
The scientists successfully tested the therapy on fresh human cancer samples collected from consenting patients, including solid prostate cancer tumours which reflect the complex make-up of real tumours.
They also tested the virus on samples of healthy human bone marrow and found it did not cause toxicity or inappropriate T cell activation.
Dr Nathan Richardson, head of molecular and cellular medicine at the MRC said: ‘Immunotherapy is emerging as an exciting new approach to treating cancers. This innovative viral delivery system, which targets both the cancer and surrounding protective tissue, could improve outcomes for patients whose cancers are resistant to current treatments.
Further clinical studies will be crucial to determine that the stimulation of the patient’s immune system does not produce unintended consequences.’
Dr Michelle Lockley, Cancer Research UK’s expert on immunotherapy, said: ‘Using the power of the body’s own immune system to tackle cancer is a growing area of research.
This work in human tumour samples is encouraging, but can be complicated – one of the biggest challenges of immunotherapies is predicting how well they will work with the patient’s immune system, and understanding what the side effects could be.
The next stage will be using clinical trials to test whether this is both a safe and effective way to treat the disease in people.’
The virus targets carcinomas, which are the most common type of cancer and start in cells in the skin or in tissues that line or cover internal organs, such as the pancreas, colon, lungs, breasts, ovaries and prostate.
The study was funded by Cancer Research UK, the Medical Research Council, the Kay Kendall Leukaemia Fund and the Oxford NIHR Biomedical Research Centre. Materials were provided by PsiOxus Therapeutics Ltd.
More information: Rodrigo Esaki Tamura et al, Combination of cabazitaxel and p53 gene therapy abolishes prostate carcinoma tumor growth, Gene Therapy (2019). DOI: 10.1038/s41434-019-0071-x
