Christine Brown, PhD, entered the chimeric antigen receptor (CAR) T-cell therapy field right after completing her postdoctural work in 2002.
“It was in the very early stages of CAR T-cell therapy, and I don't think anyone believed it would become the therapy it is today with two drugs approved by the FDA and many more to come,” noted Brown, the Heritage Provider Network Professor in Immunotherapy and Deputy Director of T Cell Therapeutics Research Laboratories at City of Hope in Duarte, Calif. “There were a few key institutions looking at CAR T-cell therapy, but we were one of the first to apply it to brain tumors. In fact, we ran some very early trials in 2002 and 2004 with first-generation CARs.”
Brown has pioneered the use of CAR T-cell therapy for brain tumors ever since leading a team of research scientists and graduate students in an independent lab she has run since 2010 and supported by deep partnerships with colleagues.
In 2015, Brown along with her clinical partners became the first in the world to inject T cells into the cerebrospinal fluid (CSF) of a patient with glioblastoma (GBM). A case report on the remarkable response of one patient was published the following year in the New England Journal of Medicine (2016; 375:2561-2569 10.1056/NEJMoa1610497).
And now, Brown and her team are once again pioneering CAR T-cell therapy with an entirely new CAR design and targeting strategy using CARs with a novel recognition structure and “expanding the repertoire of tumor-selective CAR T cells with the potential to address GBM tumor heterogeneity and reduce antigen escape,” she told Oncology Times.
The new CAR is built upon the chlorotoxin peptide identified in the venom of the deathstalker scorpion (Leiurus quinquestriatus), which has been shown to bind glioma cells as far back as 1998. Chlorotoxin has been used to deliver radioisotopes and other therapeutics to GBM tumors, and also as an imaging agent to guide resection.
Brown and her team are fusing the peptide to the CAR—the antigen receptor—to develop a chlorotoxin-based CAR (CLTX-CAR) to redirect the specificity of the engineered T cells to recognize and destroy GBM tumors.
The CLTX-CAR T cells have shown exceptional promise in a preclinical study, the results of which were published earlier this year in Science Translational Medicine (2020;12(533):eaaw2672). The preclinical study “provides proof-of-principle data that confirm the chlorotoxin receptor as a useful immunotherapeutic target in [GBM] and establish that CAR T cells can mediate potent antitumor activity against a difficult-to-treat solid tumor,” the researchers concluded.
The impressive results of the preclinical study led to the first-ever FDA-approved clinical trial in humans of a peptide-based CAR-T therapy for GBM tumors, which recently got underway. The first patient has been enrolled and T cells are being manufactured, Brown said.
The results of the preclinical study and anticipation surrounding the clinical trial captivated the oncology community, including Oncology Times, which has named Brown and her team first runner-up for the 2020 Excellence in Oncology award.
“We started this project really inspired by overcoming the challenge of the tumor heterogeneity and supported by the strong literature suggesting that chlorotoxin would bind to GBM cells, as well as data in our lab showing it to be accurate,” Brown said. “And it indeed did target GBM cells and was highly effective in our preclinical models, which has gotten us very excited.”
Brown has spent her entire career at the not-for-profit City of Hope, which includes a hospital and a graduate school in addition to its vaunted cancer research programs that form a unique translational infrastructure. City of Hope currently has 10 open clinical trials evaluating CAR T cells for blood and solid tumors, she said, adding “that diversity and seeing all of the different areas in which we have and continue to make an impact is very inspiring.”
Brown has developed deep partnerships with colleagues, including, among others, lead neurosurgeon Behnam Badie, MD, who has led the clinical trials in GBM since the early 2000s; Stephen J. Forman, MD, the Francis & Kathleen McNamara Distinguished Chair in Hematology and Hematopoietic Cell Transplantation, her mentor and Co-Director of the T Cell Therapeutics Research Laboratories; and Michael Barish, PhD, Professor and Chair of the Department of Developmental and Stem Cell Biology, who initiated the development of the CLTX-CAR.
“It would be hard for me to imagine how many other institutions would have enabled me to bring to patients so many novel therapies along with my clinical partners,” she said. “We have been able to build a large and diverse program, mentor other investigators, and direct trials not only on brain tumors but a number of different cancers.”
A Personalized Approach
GBM tumors are the most common type of brain cancer, according to the American Cancer Society. They are also one of the most difficult to treat of all solid tumors, which are almost universally fatal within 2 years from diagnosis and 6-8 months from recurrence following aggressive standard-of-care treatment.
“From the beginning of our research, I knew there were big challenges,” Brown said. “One challenge is the heterogeneity of these cancers. Another was getting these cells into the CNS and across the blood-brain barrier and seeing how they traffic. And then a third challenge was figuring out how to overcome the suppressive microenvironment of these tumors.”
In the study published in NEJM, Brown and her team demonstrated CAR T-cell therapy to be safe and highly effective against GBM, providing the first-ever example of the cells mediating complete tumor remission in a human with recurrent GBM.
The patient, a 55-year-old male with multifocal disease in the spine, had a remarkable response to multiple infusions of CAR T cells targeting tumor-associated antigen interleukin-13 receptor alpha 2 (IL13Rα2) through two intracranial delivery routes: infusions into the resected tumor cavity, followed by infusions into the ventricular system.
Regression was observed in all of the patient's intracranial and spinal tumors along with corresponding increases in cytokine levels and immune cells in the CSF of the patient, Brown, et al, wrote in the case report, noting also that the clinical response occurred despite non-uniform tumor expression of IL13Rα2 without previous chemotherapy designed for the depletion of lymphocytes.
The patient, a physician with whom the team developed a strong kinship, was part of the third trial Brown and her collaborators had run on GBM. He was the first person in the world to be treated with CAR T cells injected directly into the CSF. His remarkable response and dramatically improved quality of life continued for about 7.5 months after the initiation of CAR T-cell therapy.
“When his tumor came back, it was heartbreaking because it was lacking the antigen we were targeting,” Brown said, “That patient was a unique responder. We are not seeing that dramatic of a response in other patients. But what that patient tells us is that it is possible. And so now we have to understand how to optimize the therapy further so we can achieve that sort of response in more patients.”
From Brown's perspective, there is no overstating the dramatically improved clinical outcomes CAR T cells can offer patients. “Seeing this individual respond to the therapy, return to normal work and life activities, and feel good as a person is what motivates our entire program at City of Hope,” she said. “We know these cells can be highly potent. Now, we just have to make it a reality for more patients.”
Brown has witnessed the benefits of a personalized medical approach to cancer care from both sides of the chart since being diagnosed with stage II breast cancer in 2012. Then-new data suggested an onco-score could rule out invasive therapies for some patients, enabling her to make more informed decisions along with her oncologist and avoid chemotherapy ultimately.
“A personalized approach to cancer care is extremely important. All cancers aren't the same,” she emphasized. “And that's why I think trying to understand which subset of patients with brain tumors and GBM respond to CAR therapies or combinations of therapies is an important goal.”
The CLTX-CAR clinical trial is among three recently launched by Brown and her team targeting diverse patient populations. Others include a trial combining CAR T cells with checkpoint inhibitors and another targeting HER2 breast cancer.
“What we hope to achieve is an understanding of the safety and bioactivity of these different CAR-T platforms in patients, so that we can combine the most effective therapies based on the patient population and using a personalized medicine approach,” Brown said.
Assembling a New CAR
In the preclinical study of the CLTX-CAR, Brown's team used tumor cells in resection samples from a cohort of patients with GBM to compare CLTX binding with the expression of antigens currently under investigation as CAR T-cell targets, including IL13Rα2, HER2, and EGFR.
In orthotopic GBM xenograft models, the study team found CLTX bound “broadly and specifically” to a greater proportion of GBM tumors and to stem-like cells thought to seed recurrence, but left alone non-tumor cells in the brain and other organs after adoptive transfer into mice.
The CLTX-CAR T cells not only elicited potent cytotoxic responses against GBM tumor cells with little or no expression of the other targetable antigens, but also appeared to address the heterogeneity of GBM tumors and reduction of antigen escape, two major stumbling blocks to developing effective immunotherapy.
The study team also discovered that effective targeting by CLTX-CAR T cells requires cell surface expression of matrix metalloproteinase–2 (MMP2). Dongrui Wang, a doctoral candidate in City of Hope's Irell & Manella Graduate School of Biological Sciences, was the lead scientist in developing the CLTX-CAR T-cell platform and determining that MMP2 is required for CLTX-CAR T-cell activation, Brown noted.
“The preclinical study that we just published really came out of our understanding that these tumors are quite heterogeneous and that we needed new targets to think about developing CARs so that they can be more effective against this disease. And so my colleagues and I drew on our knowledge and a long history characterization of this peptide that was discovered in the venom of a scorpion and shown to have selective binding properties to glioblastoma cells,” she said.
Neurological studies of chlorotoxin have been building upon each other for decades, Brown noted. She pointed to Jim Olson, MD, PhD, of Seattle Children's Hospital and Regional Medical and Fred Hutchinson Cancer Research Center, who discovered how to combine the protein with a non-toxic chemical to fluorescently label or “paint” malignant tumors, making them easier for surgeons to spot using imaging technologies.
Still, there were a lot of risks as no one had ever tried to build a CAR with a toxin-based peptide. “The attractive thing about using chlorotoxin in the CAR design was that chlorotoxin seemed to bind quite broadly to GBM cells in a greater number of patients, and a greater number of cells within the tumors of patients,” Brown said. “And so we thought that, if we could develop a CAR that functioned well, it might have more targeting ability than other CARs where you see high rates of antigen escape.”
The phase I clinical trial of the CLTX-CARs is designed to determine if they are right. The study had enrolled the first patient as of early May and Brown's team had begun manufacturing their T cells. Over the next 2 or 3 years, the team plans to enroll about 18 patients in the dose-escalation safety trial and they are hoping to show evidence of anti-tumor effectiveness, she said.
The results of the preclinical study, meanwhile, have challenged Brown and her team's assumptions on the best CAR design. “Based on our knowledge of what has worked in blood cancer CARs, there has been assumptions about the best signaling components of the CAR, which we found didn't apply to our chlorotoxin CAR design, emphasizing that there may be important differences in the best CAR design for all other platforms as well,” Brown noted.
”I am excited that we were able to, based on the preclinical work, rapidly open this clinical trial. And I think it's going to be very important, not only to the field, but hopefully helping the patients that we are trying to treat,” she said. “There are no effective therapies for recurrent GBM, which are pretty much uniformly lethal when they recur. We want to change that.”
Chuck Holt is a contributing writer.
2020 Excellence in Oncology: First Runner-Up