Treating prostate cancer with immunotherapy is currently difficult to do. But preliminary results from a first in-human phase 1 trial using a chimeric antigen receptor (CAR) T cell therapy developed by researchers from City of Hope® in patients with advanced prostate cancer showed promise, with minimal side effects. The study published today in Nature Medicine.
The trial used CAR T cell therapy to treat 14 prostate stem cell antigen (PSCA)-positive patients who had metastatic castration resistant prostate cancer (mCRPC) that had spread beyond the prostate and developed resistance to hormone treatment. More than 34,000 men with this type of prostate cancer die each year in the United States.
Saul Priceman, Ph.D., City of Hope associate professor in the Department of Hematology & Hematopoietic Cell Transplantation, and team developed CAR T cells that target prostate stem cell antigen (PSCA), which is highly expressed in prostate cancer. The treatment took a patient’s immune cells — known as T cells — from the bloodstream and reprogrammed the cells in a lab to recognize and attack the PSCA protein on the surface of cancer cells. CAR T cells were then infused back into the patient’s system to destroy the cancer cells.
“Prostate cancer has been called an immune desert — the tumor microenvironment is difficult to treat with immunotherapies because you don’t get a lot of T cells inside the tumor,” said Tanya Dorff, M.D., chief of the genitourinary disease program and professor in the Department of Medical Oncology & Therapeutics Research. “It takes something really powerful to overcome that. Our study showed that City of Hope’s CAR T cell therapy for prostate cancer could be a step closer to doing that.”
The trial’s preliminary major finding is that PSCA-directed CAR T cells may be effective against mCRPC, the research team said. This opens up the opportunity to continue to develop this type of cellular immunotherapy for these patients, who currently have no other effective treatment options.
The trial’s goals were to examine the therapy’s safety and dose-limiting toxicities, or side effects that limit the amount of treatment that can be administered, as well as gather preliminary data on the treatment’s efficacy.
Patients received a single infusion of 100 million CAR T cells without prior lymphodepletion chemotherapy, which is used routinely in blood cancers to improve the efficacy of CAR T cell treatment. Since this was a first-in-human CAR T cell trial, it was important to assess the safety of CAR T cells alone in patients.
At that same CAR T cell dose and with lymphodepletion, there was a side effect of cystitis, or irritation of the bladder. Dr. Dorff explained that PSCA is also found in the bladder, so the CAR T cells most likely attacked bladder cells, causing inflammation. Researchers then added a new cohort to the study using reduced lymphodepletion, which mitigated this toxicity.
The Study's Findings
Four out of 14 patients had declines in their PSA levels, which is a serum marker of disease progression in people with prostate cancer, including one patient with a significant decline. Imaging showed therapeutic responses in a subset of treated patients.
Five out of 14 patients had mild or moderate cytokine release syndrome, which can be caused by a large, rapid release of cytokines into the blood from immune cells and is a common side effect of CAR T cell therapy. CRS was a treatable side effect.
CAR T cells did not persist at high levels beyond the 28-day monitoring period, which limits the therapy’s effectiveness. This is a common challenge in the solid tumor CAR T cell field that researchers plan to address in a follow-up City of Hope trial using the therapy that is now open for enrollment.
One patient who had received several prior therapies responded well to the CAR T cell therapy. His PSA level decreased by 95% and cancer in his bones and soft tissue also declined. He experienced this positive response for approximately eight months.
“The patient’s results were very encouraging, and we are deeply grateful for his participation in our study, as well as other patients and their families,” Dr. Dorff said. “We want to continue with this therapy and increase the amount of CAR T cells, and continue to carefully monitor for any health problems, as we think this can improve the therapy’s effectiveness.”
The phase 1b trial using the PSCA-CAR T cell therapy in combination with radiation to enhance anti-tumor activity aims to enroll up to 24 patients.
City of Hope, a recognized leader in CAR T cell therapies, has treated nearly 1,500 patients since its CAR T program started in the late 1990s. The institution continues to have one of the most comprehensive CAR T cell clinical research programs in the world — it currently has about 70 ongoing CAR T clinical trials, which include 13 different solid tumor types. The trials use City of Hope-developed therapies and industry-sponsored products. A recent study published in Nature Medicine featured City of Hope’s CAR T cell therapy for brain tumors.
City of Hope manufactured the CAR T cells in its own facility, the Cell Therapy Production Center on its Los Angeles campus.
The Prostate Cancer Foundation helped fund the trial.
Main image: A CAR T cell attacks a tumor cell.