Although CAR T-cell therapy has not yet proved effective against solid tumors, a novel CAR that targets mesothelin proteins expressed in pancreatic cancer is showing early signs of activity.
Gregory L. Beatty, MD, PhD
Although chimeric antigen receptor (CAR) T-cell therapy has not yet proved effective against solid tumors, a novel CAR that targets mesothelin proteins expressed in pancreatic cancer is showing early signs of activity, according to Gregory L. Beatty, MD, PhD.
Mesothelin-specific CAR T cells (CARTmeso) are being explored at the Pancreatic Cancer Research Center at the University of Pennsylvania (Penn) in Philadelphia, where Beatty is director of translational research. He discussed these efforts— along with the challenges of developing CARs for solid tumors—during a presentation at the 2019 American Society of Clinical Oncology—Society for Immunotherapy of Cancer Clinical Immuno- Oncology Symposium.1
In terms of overall CAR T-cell therapy development, Beatty noted that multiple CAR targets are being tested in clinical trials, with CD19 being the most attractive one for therapy. CD19 is largely expressed by normal tissue, which is of importance with the recognition of on-target but off-tumor toxicities.
In solid malignancies, multiple clinical trials of CAR T-cell therapy have been conducted with a range of targets, with minimal dose-limiting toxicities observed. “[CAR T-cell therapy] has been largely tolerable,” said Beatty. “Nonetheless, there has been limited intratumoral activity that has been appreciated thus far.”
Unlike with hematologic malignancies, in which CAR T-cell trafficking and infiltration occur readily in the blood, lymph nodes, and bone marrow, getting CAR T cells into peripheral tissues and working in the microenvironment of solid malignancies is challenging, according to Beatty. “This is particularly true in pancreatic cancer, a disease that is currently the third leading cause of cancer-related deaths in the United States, with an incidence of about 50,000 per year,” he said.
The overall survival rate of patients with advanced pancreatic cancer in response to cytotoxic chemotherapy is <20% at 2 years and <5% at 5 years. Attempts to introduce immunotherapy have not generated improvements in response, with the exception of a small subset of patients with mismatch repair deficiency. The response rate is 0% to anti—PD-1/ PD-L1 and anti–CTLA-4 therapy and <10% in second and later lines with novel immuno-oncology combinations.2
“The tumor microenvironment shows a desmoplastic reaction [and] is poorly vascularized; there’s a large infiltration of myeloid cells and often a scarcity of T cells,” Beatty said, explaining some of the challenges.
Several targets for CAR therapy are being investigated for pancreatic cancer in clinical studies in the United States and China, according to Beatty (Table).At Penn, Beatty’s laboratory developed a CAR specific for mesothelin, a protein that is overexpressed by pancreatic ductal adenocarcinoma cells. It incorporates the mouse monoclonal antibody SS1 and contains the costimulatory CD137 domain to enhance proliferation and survival and a CD3ζ signaling domain to induce T-cell activation.3 In vitro, CAR T cells engineered to express mesothelin “will react and kill tumor cells that are mesothelin positive but not those that are negative,” he said. Even in autologous T cells, the CAR T cells were effective in recognizing mesothelin-positive tumor cells. One strategy to engineer CAR T cells uses an RNA platform and electroporation to transiently express the CAR in T cells. An alternative strategy involves using a lentiviral platform and transduction to permanently express the CAR. “Although permanent transduction leads to superior activity,” Beatty said, both exhibited antitumor activity in a mesothelioma human tumor model; however, the transient expression of CAR elicited through the RNA platform might appeal to safety.The safety, feasibility, and activity of RNA mesothelin-specific CARTmeso therapy was explored in a phase I study in 6 patients with chemotherapy- refractory metastatic pancreatic ductal adenocarcinoma.4 All patients had received 2 or more prior lines of therapy. The RNA CAR T cells were infused 3 times weekly for a total of 9 doses. “We observed no dose-limiting toxicities, and we found no evidence of cytokine release syndrome,” Beatty said. The most common adverse events included fatigue and abdominal pain, possibly a result of the pancreatic cancer itself. The best overall response was stable disease in 2 patients who experienced a progression-free survival time of 3.8 months and 5.4 months.
18F-2-fluoro-2-deoxy-D-glucose (FDG)—positron emission tomography/computed tomography (PET/CT) imaging was used to monitor the metabolic active volume of individual tumors. The total metabolic activity remained stable (for 1 month) in 3 patients and decreased by 69.2% after 1 month in 1 patient with biopsy-proven mesothelin expression who had significant liver tumor burden at baseline. All liver lesions had a complete reduction in FDG uptake at 1 month compared with baseline, although the primary lesion was not affected.
The study demonstrated that CAR T cells can have biologic activity in patients with pancreatic cancer and that significantly different immune escape mechanisms may operate within the distinct anatomic location where the disease is present, Beatty said.
A lentiviral CAR T-cell trial enrolled 15 patients with either mesothelioma (n = 5), ovarian cancer (n = 5), or pancreatic cancer (n = 5). Two doses (1-3 x 107/m2 and 1-3 x 108/m2) were infused (with or without prior cyclophosphamide) with a lymphodepleting regimen. The mean number of prior therapies was 5 (range, 1-11). At both doses, lymphodepletion improved CAR T-cell expansion in the peripheral blood. Lymphodepletion did not have an impact on CAR T-cell persistence; CAR T cells became undetectable in the peripheral blood by 2 months in nearly all patients. The best overall response was stable disease in 11 of the patients.Potential resistance mechanisms that could affect CAR T-cell therapy in solid malignancies are cellular fitness and trafficking, tumor microenvironment CAR target expression, and cellular fate. Patients with pancreatic cancer screened for the phase I study showed little T-cell proliferation in vitro. Further analysis showed a decreased proliferative expansion of CD8 T cells in vitro, along with a decrease in the number of naïve T cells and an increase in the number of effector cells in the peripheral blood in patients with pancreatic cancer, as well as altered cytokine secretion capacity.
“We also found that CAR T cells were capable of trafficking to tumors in some patients. Levels of CAR T cells, though, that were detected within the tumors were actually low,” Beatty said. “CARTmeso cells were detectable in some but not all tumor biopsies, so trafficking was a challenge.”
A large degree of heterogeneity was found in the tumor microenvironment, with some regions of the tumor lacking T cells and other regions having a strong influx of T cells. PD-L1 is not expressed in pancreatic cancer, and most T cells are not actively proliferating. The same biology exists within metastatic tumors, he said.
Multiple patterns of T-cell infiltration were identified—ones in which T cells are not present, those in which T cells are present but fail to engage tumor cells, and those that engage but fail to kill tumor cells.
Mesothelin is expressed in most pancreatic tumor specimens. “The expression profile is quite important. Many of our patients have intracellular expression, and that won’t work with CAR T cells,” Beatty said. “We did notice that the patient who had an FDG-PET/CT response had mesothelin expressed on the surface of the tumor cells.” Mesothelin expression is now an inclusion criterion for clinical trial enrollment in 1 such ongoing study (NCT03323944).