Oncolytic Viral Therapies Offer One-Two Punch To Fight Glioblastoma

Sept. 18, 2018

On August 25, 2018, Senator John McCain died after several months of battling glioblastoma, a particularly aggressive form of brain cancer, also known as glioblastoma multiforme (GBM). GBM is one of the deadliest and most common forms of brain cancer in adults, affecting 2 or 3 out of every 100,000 people. GBM has claimed the lives of several other well-known public figures, including Senator Ted Kennedy, Joe Biden’s son Beau Biden, composer George Gershwin and NASA astronaut Deke Slayton. Most patients survive less than 20 months following diagnosis.

Development of effective GBM therapies is complicated by the blood-brain barrier, a specialized membrane that evolved to prevent toxins and pathogens in the blood from reaching the brain. The protective function of this barrier also limits the ability of chemotherapy agents to reach GBM tumors. The alternative, radiation therapy, bypasses the blood-brain barrier, but poses its own challenges because it damages healthy brain cells in addition to tumor cells. Surgeries to resect the tumors can extend a patient’s lifespan by several months but aren’t curative, since glioma tumor cells migrate and embed deep within brain tissue in the majority of cases.

To address the lack of effective GBM treatment strategies, there are over 200 interventional clinical trials ongoing in the US focused on innovating and/or optimizing therapies for managing GBM and improving survival. The complexity of the disease and its recalcitrance to traditional therapies have caused cancer researchers to develop approaches designed to specifically target tumors. The arsenal has broadened to include creative engineering of infectious viruses to specifically attack tumor cells.  

Oncolytic virotherapy: New designer viruses fight glioblastomas from the inside out

One of the key tenets for advancing and improving cancer therapies is the pursuit of a therapy that kills or inhibits the growth of cancer cells, without affecting healthy cells. The development of such targeted chemo- and immunotherapies has been successful for numerous types of cancer, which brings hope for the potential discovery of a targeted therapy for GBM.

Thus far, all attempts at developing immunotherapies for GBM have failed by not significantly improving patient survival compared to existing treatments. Many of these potential treatments rely on the patient’s immune system to recognize cancer cells, but these therapies are often rendered ineffective by steroid treatments (used to reduce symptoms, but as a consequence  suppresses the immune system) and the tumor’s ability to evade immune detection.

Figure 1: A. Different families of viruses that can have oncolytic effects upon cancer cells. B. Varying mechanisms of action that viruses can employ to kill cancer cells.
Figure 1: A. Different families of viruses that can have oncolytic effects upon cancer cells. B. Varying mechanisms of action that viruses can employ to kill cancer cells.

Scientists looking for GBM therapies have thus turned to a type of microbe that can differentially affect different cell types: viruses. Viruses are an attractive method for targeting cancer cells because they have a variety of natural properties that can target cancer cells, such as a preference for replicating inside of cells with tumor-specific receptors, causing spontaneous lysis of cancer cells, and/or increasing tumor cell susceptibility to chemotherapy and radiation (Figure 1). Reoviruses, in particular, innately infect and kill cancer cells by preferentially replicating inside of cells containing activated cell growth signals known as Ras proteins, which are often present in rapidly-dividing cancer cells. Reoviruses more efficiently infect cells with activated Ras proteins because the virus can uncoat and disassemble more efficiently. When the virus uncoats, the immune system recognizes the viral nucleic acids that are released and launches an attack against the infected tumor cell, killing the cancer cells as a byproduct of eliminating the virus.  

The concept of using viruses to fight cancer—also known as oncolytic virotherapy—is not new, and has been around since 1910, when a case of cervical cancer was successfully treated using the Pasteur-Rox live attenuated rabies vaccine. However, the advent of genetic engineering has put a new spin on an old trick, as scientists today have a powerful new tool for manipulating viral characteristics, such as receptors, target cells, replication, and lytic cycles. While not all viruses have an innate preference for tumor cells, genetic engineering can help those viruses that are less able to discriminate between cancerous and healthy cells to target cancer cells.

In 2015, a modified herpesvirus was approved for inoperable melanoma, demonstrating the power of oncolytic virotherapy combined with genetic engineering. 2018 has been an exciting year for emerging GBM-specific oncolytic viral therapies, as two promising, early-stage clinical trials and an animal study have been published this year for treating GBM.

PVSRIPO: A GBM-targeting poliovirus that acts through CD155

The promising results of a Duke University-led phase I clinical trial (a trial with a small number of patients that tests the dosage, side effects, and toxicity of a treatment) with a genetically-modified poliovirus type 1 were recently published in the New England Journal of Medicine. While the virus is live and capable of replicating, it is nonvirulent and does not cause illness. Infectious poliovirus can infect neurons (causing paralytic poliomyelitis) but the modified virus, known as PVSRIPO, is a polio-rhinovirus chimera that is unable to infect healthy neurons due to replacement of a key poliovirus genomic sequence with a sequence from rhinovirus (best known for causing the common cold).

The PVSRIPO virus, like the unmodified poliovirus, targets GBM tumor cells through a cell surface receptor, CD155. CD155 is an immunoglobulin-like protein that is involved in immune-mediated killing and adhesion. It is expressed at low levels in healthy tissues, but is often overexpressed in solid tumors, including those present in GBM.  Once it recognizes CD155, PVSRIPO infects and lyses tumor cells to trigger the patient’s immune response.

Once the PVSRIPO virus detects CD155 on cancer cell surfaces, it initiates a two-pronged approach to eradicate the tumor cells. First, infection with PVSRIPO causes these cancer cells to lyse and die. Second, when the cells lyse, they release proteins that the immune system can recognize as a threat, causing production of interferons, which serve as “cancer alarms” for immune cells. Thus PVSRIPO both directly kills the cancer cells and alerts the immune system to the presence of tumor cells, which have many mechanisms to avoid immune detection (Figure 1). Conveniently, CD155 is also expressed on antigen-presenting cells (APCs), such as dendritic cells, which educate T cells on how to recognize potential pathogens and cancers. Unlike tumor cells, PVSRIPO does not cause APCs to lyse because the inserted rhinovirus genome sequence offers protection against cell lysis. PVSRIPO instead maintains a chronic infection inside the cell that keeps the immune system on the lookout for tumor cells.  

During the clinical trial, 61 GBM patients received PVSRIPO delivered directly into the brain using a catheter. At 36 months following treatment, 21% of patients in the PVSRIPO group were still alive, versus 4% in a historical control group of patients with GBM. Some patients that received PVSRIPO lived for more than 5 years—a very atypical outcome for GBM patients, and a positive sign for the continued success of PVSRIPO in future studies.

As Dr. Henry Friedman, Associate Professor of Medicine at Duke University, told WRAL Tech Wire, “The impressive results with PVSRIPO in this trial are the best we have seen to date in patients with recurrent glioblastoma and provide hope for these patients whose typical survival time is less than a year.” He also added that there is potential to use PVSRIPO for treatment of other cancers as well, due to the prevalence of CD155 in solid tumors across cancer types.   

DNX 2401: An adenovirus that seeks out cells with irregular growth patterns

Another phase I study of oncolytic virotherapy lead by Frederick F. Lang’s group at the University of Texas MD Anderson Cancer Center has demonstrated significant effects on GBM patient survival in a recent Journal of Clinical Oncology publication. The oncolytic viral treatment examined in this study is a modified adenovirus known as DNX 2401. DNX 2401 has been engineered to infect only cells that lack a functional retinoblastoma pathway, a pathway that controls cell growth and is disrupted in GBM. As a result, DNX 2401 cannot replicate in non-GBM tumor cells. The virus infects cells by binding integrins, proteins that help cells adhere to their environments and are highly expressed on tumor cells, including cancerous glioma stem cells.

Similar to PVSRIPO viral therapy, once DNX 2401 has infected its targeted cancer cells, it replicates until the cells lyse and burst open, releasing viral particles and cancer-identifying molecules for the immune system to recognize and attack. DNX 2401 utilizes the same two-pronged model as PVSRIPO, including direct cell killing through cell lysis and indirect killing through the activation of immune cells with cancer-killing capabilities.  

The clinical study of DNX2401 included 37 GBM patients, separated into 2 different treatment arms. All patients received intra-tumor injections of the viral therapy, but patients in Group A received single doses of different viral loads to determine which dose was most efficacious with the fewest side effects; patients in Group B received a fixed dose of DNX2401, followed by surgical intervention to remove GBM tumors, and a second dose of DNX2401.

Some patients experienced lengthy remission and long survival time, while other patients continued to progress, dying within 9.5 months. The median survival time in Group B was 13 months, with 2 patients surviving for 2 years. Findings from this study showed that not all patients were able to be infected. Infection inefficiency could account for the variabilities in patient outcomes.

Dr. Frederick Lang told Science Daily, “Of those five long-term survivors, three had durable complete responses, which is impressive for a phase I clinical trial in glioblastoma. Many phase I trials might have one patient who does well, so our result is unusual, but we're always cautious in assessing results with this very difficult disease.” The development of gliosarcoma in patients who survived for 3 or more years following DNX2401 administration may be of interest for future, extended studies involving DNX2401.

ZIKV-LAV: A modified Zika virus that destroys insidious GBM progenitor cells

Even after GBM-associated tumors are surgically removed from the brain, glioma stem cells can get left behind and embed deep within the brain. These stem cells can cause the cancer to grow back, worsening the patient’s prognosis for survival. Therapies that target glioma stem cells are needed to eradicate GBM at its root to prevent recurrence. A new study from ASM’s mBio showed that these GBM glioma stem cells can be targeted by a modified Zika virus, the virus that caused a global health crisis in 2016. The study, conducted by Pei-Yong Shi at University of Texas Medical Branch in Galveston, Jianghong Man of the National Center of Biomedical Analysis in Beijing, and Cheng-Feng Qin of the Chinese Academy of Military Medical Sciences, showed that Zika virus preferentially infects glioma stem cells, due to its natural proclivity to infect neural progenitor cells—a fact learned by scientists studying the mechanism by which Zika causes microcephaly in developing fetuses.

The investigators modified the live attenuated Zika virus (ZIKV-LAV) to reduce its virulence and increase its safety by deleting 10 amino acids in the 3’ untranslated region of the virus’ genome. This deletion slows viral RNA replication and makes the virus more susceptible to inhibition by the immune system, thereby preventing the development of an uncontrolled Zika virus infection. Previously published studies of ZIKV-LAV suggest that the virus eliminates cancer cells by inducing apoptotic cell death. Like PSVRIPO and DNA 2401, ZIKV-LAV is a potentially suitable as a GBM therapy because it preferentially selects for human glioma cells in vitro, unlike other Flaviviruses, such as West Nile virus, which kills all cell types indiscriminately.

In mice, ZIKV-LAV appears to be safe when intracerebrally injected, as no neurovirulence, disease, or behavioral disturbances were observed in ZIKV-LAV treated mice, in comparison to healthy mice. The investigators also found that the modified Zika virus was unable to be spread to its usual vector, the Aedes mosquito, due to slow viral RNA replication and inhibition by interferons, alleviating concerns for the spread of ZIKV-LAV in the general population.

In addition to its robust safety profile, ZIKV-LAV demonstrated effectiveness in controlling GBM in mice with implanted GBM tumors, as evidenced by increased survival, smaller tumor size, and, for some mice, complete elimination of visible tumors. As the previously discussed therapies do, ZIKV-LAV uses a two-pronged approach by causing direct oncolytic cell destruction and stimulating an immune response against infected tumor cells.

Looking Ahead: What is the future of tumor-targeting viruses?

While the results of these studies are promising, additional research is needed to understand and validate their efficacy and safety. The PVSRIPO study experienced several serious adverse events that may have been related to treatment, and both therapies involve invasive dosing procedures, which must be properly regulated and administered. As a spokesperson for the National Brain Tumor Society told CNET, regarding PVSRIPO, “The evaluation of this treatment – and clinical trial process to ultimately determine if the treatment is safe and effective – is still in its very early phases, and there is a lot more data that needs to be seen.”

The investigators who developed these viruses are now experimenting with combining viral therapies with chemotherapy agents and immunomodulators to enhance their efficacy. The researchers testing PVSRIPO have begun recruiting for their phase II clinical trial, which involves more patients and the addition of a chemotherapy drug, lomustine and DNAtrix Therapeutics, the company now developing DNX 2401, is conducting phase II clinical trials of DNX 2401 in combination with the immunotherapy pemrolizumab. Similarly, Chen et al, the ZIKA-LAV group, are working to further develop ZIKV-LAV for clinical testing in patients with GBM and have expressed interest in further modifying the virus to include an immunomodulatory component in the viral genome, which the virus would release to alert the immune system. If these current studies are successful, viruses may soon be accepted as integral components of GBM therapy.

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Author: Rita Algorri

Rita Algorri
Rita Algorri is a freelance writer, Ph.D. candidate in Clinical and Experimental Therapeutics, and Master's student in Regulatory Science at the University of Southern California.