Genetically modified “hunter” T cells successfully migrated to and penetrated a deadly type of brain tumor known as glioblastoma (GBM) in a clinical trial of the new therapy, but the cells triggered an immunosuppressive tumor microenvironment and faced a complex mutational landscape that will need to be overcome to better treat this aggressive cancer, Penn Medicine researchers report in a new study in the journal Science Translational Medicine.

A team led by principal investigator Donald M. O’Rourke, MD, an associate professor of Neurosurgery at Penn, and Marcela Maus, MD, PhD, showed that CART–EGFRvIII cells had an acceptable safety profile, crossed the blood–brain barrier, infiltrated the tumor, and prompted an immune response, resulting in reduction of the EGFRvIII tumor antigen in GBM cells. Maus – a former Penn faculty member who is now the Director of Cellular Immunotherapy at the Massachusetts General Hospital Cancer Center and an assistant professor of Medicine at Harvard Medical School – is the senior author on the study.

The new study includes full results from the first 10 patients treated. The paper identified two barriers: a wide variation in EGFRvIII expression in patients and a resistance in the tumor microenvironment, which researchers showed became even more immunosuppressive following CAR T cell infusion. Although the former may require targeting additional antigens, the authors said, the latter may be overcome with existing drugs that target immunosuppressive molecules, such as checkpoint inhibitors used to successfully treat other cancers.

While no clinical benefit could be definitively determined from the study, one patient did achieve stable disease at the 18–month follow up, a response that remains today, following infusion of CART–EGFRvIII. Two other patients are alive but their disease progressed, as revealed via MRI imaging criteria. The additional seven patients lived incrementally longer than one would have predicted based on the number of previous treatments they received and the multifocal nature of the GBM tumor recurrences.

“This trial showed that there is a need to target additional antigens in glioblastoma, as well as overcome the immunosuppressive environment that the CAR T cells encountered in the tumor,” Maus said.

Within the first two weeks of infusion, a detectable number of CART–EGFRvIII cells trafficked to the tumors, with signs of activation in the four patients who had “early surgery”, the researchers reported. All the infused patients also had detectable circulating CART–EGFRvIII cells in the blood in the first month after infusion. However, levels of the infused cells began to steadily decline after the two–week mark, and became undetectable after one month.

The immune activation from the CAR cells was also met with resistance mechanisms, including an upregulation of immunosuppressive pathways which may work against the patient and for the tumor, the researchers found.

“There may be a synergy between CAR T cells and inhibition of these pathways with small molecule drugs or checkpoint blocking antibodies,” O’Rourke added.

An evaluation of tumors removed from five of the patients who had surgery did reveal decreased levels of the target antigen EGFRvIII, a driver of growth found in about 30 percent of these tumors. However, an analysis of the samples showed a wide variation of EGFRvIII expression in patients and over time and in different areas of their tumors. With so many variants at play, the authors suggest that using a single target to address a heterogeneous antigen may not suffice to achieve a durable clinical benefit.

The therapy was found to have an acceptable safety profile in all patients, with no clinical or laboratory signs of systemic cytokine release syndrome. One patient experienced a seizure which was successfully treated.