Immunotherapy combination stifles pancreatic cancer in mouse models

Combining two immunotherapies caused tumor rejection in mouse models of pancreatic cancer and provided long-term immune memory that prevented recurrence when tumors were reimplanted in responders months later, researchers MD Anderson report today in Gastroenterology.

The promising combination -- an anti-PD-1 immune checkpoint inhibitor and an antibody that activates the immune-stimulating molecule OX40 – merits further development for clinical trials after additional research, the researchers note.

“There remains work to be done in the preclinical stage before we can see whether this is translatable to the clinic,” says Cassian Yee, M.D., professor of Melanoma Medical Oncology and senior author of the paper.

Single-cell sequencing of gene expression on immune cells will help identify unique cell populations involved in this response and shed more light on the mechanisms involved, Yee says.

PD-1 inhibitor and OX40 stimulator eliminate tumors

PD-1 inhibitors, which block a brake on CD8-positive cytotoxic T cells, freeing those cells to attack tumors, have so far failed in human clinical trials of pancreatic cancer. Several OX40 activating antibodies have been tried against other cancer types, so far with disappointing results.

In this research, the combination boosted the presence of CD4-positive T cells penetrating the tumors, as well as memory cells for both CD4 and CD8-positive T cells. It reduced the presence of regulatory T cells that dampen immune response and the levels of exhausted CD8 cells.

Mice with orthotopic tumors – tumors implanted in the mouse pancreas – were treated with either control antibody, the OX40 agonist alone, PD-1 inhibitor alone or the combination.

Mice treated with control and anti-PD-1 had no response and all died within 50 days. Those treated with OX40 agonist alone had an initial 60% response rate and 43% of 90 treated survived for at least 225 days. All mice treated with the combination responded, and 51 of 55 (93%) survived for 225 days.

"The combination therapy eliminated orthotopic tumors and these mice survived as long as healthy mice would," said first author Ying Ma, M.D., Ph.D., instructor in Melanoma Medical Oncology.

The experiment was repeated in an even more aggressive model of pancreatic cancer that mimics the human pancreatic tumor microenvironment and is largely resistant to treatment. Mice treated with combination therapy had median survival of 88 days, compared with 50 days or less for all other treatment arms.

Long-term immunity blocks recurrence

Next, the team re-challenged mice that had responded to OX40 or combination therapy by implanting new tumors months after the initial treatment and response. Mice who had been treated with the combination rejected all tumors when reimplanted with the same tumor type as the original.

Mice treated with the combination also had an additional median survival of 70 days when the second tumor implanted was a more aggressive model of the disease than the original tumor, compared to 38.5 days for OX40 single-agent treatment.

To better understand the role of various immune cells in the response, the team treated mice and then depleted either CD4+ T cells, CD8+ T cells or natural killer cells.

No CD4 T cells, no response

Depleting CD4 T cells completely eliminated the therapeutic response of the combination or OX40 treatment.  Depleting CD8 cells had a weaker effect, and natural killer cell depletion had no effect.

Analysis of the immune cell signature left by the combination showed high levels of CD4+ cells and T cells positive for CD127, a protein marker of memory T cells that endure to combat reoccurrence of their antigen targets.

"CD4 memory, but not CD8 memory, is required for the protective effect of OX40 agonist and PD1 antagonist,” Ma says. “This finding may be insightful for developing potent immunotherapy in pancreatic cancer."

While FDA-approved anti-PD-1 therapies are readily available in the clinic, existing OX40 agonists are unlikely to have the same strong effect as the agonist used in mice, Yee says.

“We hope these findings resuscitate interest in looking at other OX40 agonists that can be evaluated in this combination strategy,” Yee says.

Yee has specialized in developing cellular immune therapies for cancer, mainly melanoma, but returned to pancreatic cancer with the encouragement of MD Anderson Pancreatic Cancer Moon Shot® leader and co-author Anirban Maitra, M.B.B.S., professor of Pathology, and the presence of Ma in his lab.

Maitra notes that OX40 has failed mainly in metastatic disease settings but thinks that the immune memory provided by the combination might be useful to prevent recurrence or progression of earlier stage disease after surgery.

Ma conducted the principal experiments, testing a variety of immunotherapies and combinations to identify the OX40/PD-1 combination. She plans to explore the potential mechanism of OX40 signaling on eradicating tumors and to improve OX40-based immunotherapy in pancreatic cancer patients.

This research was funded by the Pancreatic Cancer Moon Shot, a Hirshberg Foundation for Pancreatic Cancer Research grant, MD Anderson’s cancer center support grant from the National Cancer Institute (P30CA016672) and NCI grant (U24CA224020), and foundation grant from Parker Institute for Cancer Immunotherapy.

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