ARTICLE IN BRIEF
Investigators found that ibrutinib, a drug approved for lymphoma and leukemia, inhibited a key protein in glioma stem cells and slowed the growth of gliomas in an animal model of glioblastoma. Clinical trials of ibrutinib will begin later this month.
Ibrutinib, a medication federally-approved for leukemia and lymphoma, slowed the course of glioblastoma multiforme (GBM) in a mouse model of the disease, according to the results of a study published May 30 in Science Translational Medicine.
Ibrutinib was able to target and inactivate a protein — the bone marrow and X-linked (BMX) kinase — in glioma stem cells. BMX kinase has a direct hand in activating signal transducer and is an activator of transcription 3 (STAT3) — both key players in glioma stem cells, which self-populate, drive malignant growth, and establish an immunosuppressive microenvironment that contributes to therapeutic resistance.
In the animal studies, the researchers found that ibrutinib not only infiltrated the brain tumor and inactivated BMX, it also seemed to boost the effects of radiation. The effects were so impressive that clinicians at the Cleveland Clinic, which conducted the preclinical studies, have designed a clinical trial set to begin this month.
Shideng Bao, PhD, professor of stem cell biology & regenerative medicine at the Lerner Research Institute at the Cleveland Clinic, and his colleagues found that BMX kinase is highly expressed in glioma stem cells and not in healthy neural stem cells. In addition, they observed that the protein controls activation of STAT3, which is important for maintaining cell renewal and tumorigenic potential of glioma stem cells.
These glioma stem cells are highly resistant to radiation, Dr. Bao pointed out. The glioma stem cells can differentiate into vascular pericytes that help maintain the stability of the blood-tumor-barrier. The more pericytes, the harder it is for drugs to make their way through this barrier. These pericytes also express BMX, which means that inhibiting this protein could disrupt the blood-tumor-barrier and allow chemotherapy treatments a more open road into the tumor.
STUDY METHODS, FINDINGS
The scientists conducted animal studies and human cell-based studies to test the effectiveness of ibrutinib on glioma stem cells and determine if it would slow or stop tumor growth in glioblastoma. They used patient-derived glioma stem cells, mice with glioma stem cell-derived xenografts, and human primary GBM tumor tissues for the various studies.
In one of the experiments. the animals were randomized to receive either the treatment or a control inert substance. The investigators found that ibrutinib significantly inhibited glioma stem cell-driven tumor growth. This disruption in the cancer stem cells also made the tumors more sensitive to radiation. They compared tumor growth and survival to animals treated with the standard of care for GBM, temozolomide (TMZ). The survival extension of animals treated with ibrutinib was ten times longer than that of the TMZ-treated animals.
The scientists also tested whether radiation was more effective with ibrutinib. Glioma stem cells are highly resistant to radiation and chemotherapies. Ibrutinib worked, and radiation worked to some extent, but the two modalities together showed the strongest effect: more tumor death and even longer survival of the animals. The average survival extension from the combinational treatment (from 33 to 91.8 days) is much longer than that from the radiation (from 33 to 47 days) or ibrutinib (from 33 to 52.8 days) treatment alone in this set of animal experiments.
In similar work with patient-derived glioma stem cells and mouse brain tissue, the researchers found that the drug was able to reduce tumor growth and burden.
Ibrutinib only seems to target glioma stem cells and has minimal effect on neural stem cells lacking BMX expression, said Dr. Bao.
“This drug could be used with standard treatments such as radiation and could have significantly more benefit for patients,” Dr. Bao said. “We may also be able to overcome the therapeutic resistance mediated by glioma stem cells and allow chemotherapeutic and other agents better access to tumor tissue by disrupting the blood-tumor barrier.”
Experts not involved in the study said the findings show a potential new targeted approach for GBM, which is worth exploring in clinical trials.
“It's a very exciting paper,” said Patrick Wen, MD, PhD, FAAN, professor and director of the Center for Neuro-Oncology at the Dana-Farber Cancer Institute. “This builds on their early work that BMX is a good target. The findings give strong support for going into [clinical trials] for patients.”
Others commented on the paper's insights into GBM mechanisms. “The paper demonstrates the nuanced complexity of signaling pathways that can be co-opted to drive GBM pathogenesis,” said Paul S. Mischel, MD, head of the laboratory of molecular pathology at the Ludwig Institute for Cancer Research and professor at the University of California, San Diego. “By dissecting the signaling, the study suggests new and potentially actionable vulnerabilities that are generated by pathway rewiring in brain cancer.”
Jann Sarkaria, MD, professor of radiation oncology at Mayo Clinic in Rochester, MN, agreed. It is very important that the drug only targets glioma stem cells and not neural stem cells, he said, and that the combination treatment in the animals “had even more additive benefits. This is encouraging.”
Harley Kornblum, MD, PhD, professor in the departments of psychiatry, pharmacology and pediatrics at the David Geffen School of Medicine at the University of California, Los Angeles (UCLA) and associate director of the UCLA Intellectual and Developmental Disabilities Research Center at the Semel Institute for Neuroscience and Human Behavior, added that the research was encouraging, but he offered some caveats about the findings.
“The paper takes an approved drug, ibrutinib, and demonstrates that it can inhibit tumor growth, acting on the putative cancer stem cell in glioblastoma,” he said.
“The drug is acting on a tyrosine kinase, BMX, that is much more highly expressed in glioma cells than it is in normal brain cells. This receptor is linked to activation of a critical transcription factor in brain tumor growth, STAT3. An interesting and important element to this study is that while STAT3 is important for the function of normal brain cells, it is not regulated by the BMX tyrosine kinase, setting up the ideal therapeutic situation, where the cancer cells are utilizing a pathway that normal cells are not. The fact that ibrutinib is currently available and has been shown to cross the blood-brain barrier is also appealing.”
“However,” he added, as “with most drugs that target specific molecules, it remains to be seen whether the drug will reach high enough levels within human tumors to effectively inhibit its target. Furthermore, there may be more difficulty in the eradication of potentially very slow growing and/or deeply situated glioma stem cells in human tumors, compared to smaller, more rapidly growing mouse tumors.”
“Another issue to deal with is the highly heterogenous nature of GBM — both amongst different tumors and within a single tumor,” Dr. Kornblum continued. “It remains to be seen whether this pathway will be intact or important across this spectrum. Another challenge will be whether or how soon resistance mechanisms develop, as is generally the case when one attempts to target a single pathway. All in all, however, it seems as if there is a straightforward path from this study to implementation of clinical trials.”
CLINICAL TRIAL TO FOLLOW
Manmeet Ahluwalia, MD, the Dean and Diane Miller family endowed chair in neuro-oncology in the Rose Ella Burkhardt Brain Tumor and Neuro-Oncology Center at the Cleveland Clinic, has been following Dr. Bao's work for several years. He said the strong findings led him to initiate a trial in GBM patients testing the drug with radiation and TMZ in GBM patients. The trial is set to begin enrolling patients later this month.
The researchers will enroll 36 GBM patients, half of whom will receive the drug ibrutinib plus radiation and the others, TMZ and radiation. The patients' tumors will be genotyped to see which patients derive the most benefit from the treatment.
Dr. Ahluwalia noted that research has shown that certain tumors have an innate repair mechanism, a gene called O(6)-methylguanine-DNA methyltransferase (MGMT) that encodes a DNA repair enzyme. If the gene is methylated or inactivated, the gene's repair mechanisms are stalled, and patients have a better response to chemotherapy and/or radiation. MGMT-methylated tumors can live on average to 17 months.
Separating out patient tumors based on this biomarker will help the investigators determine whether some patients will respond better than others to the combination treatment.
“We are very excited about Dr. Bao's findings,” said Dr. Ahluwalia. “The primary endpoint in this clinical trial is safety of the combination treatments. We hope the study allows us to build a new standard of care.”