The field of immunotherapy, centered on the idea that a person’s own immune system can be used to fight cancer, is rife with potential breakthroughs. One such example is ipilimumab (YervoyTM), which in clinical trials has produced practice-changing results for people with melanoma. The drug works by boosting the body’s natural immune defense against tumors. For some patients, it shrinks tumors and significantly prolongs lives.
But immunotherapy doesn’t help everyone. In fact, about 80 percent of people with melanoma get little or no benefit from ipilimumab. And thus far, doctors have had no way of predicting which patients are more likely to respond to the drug.
Now Memorial Sloan Kettering scientists have gained new knowledge about how ipilimumab works and why it controls melanoma in only some people. In a report published today in the New England Journal of Medicine, the researchers show that in patients who respond to the drug, their cancer cells carry a high number of gene mutations — and some of these mutations make tumors more conspicuous to the immune system.
“For the first time, it might be feasible to develop a reliable diagnostic test to help guide treatment decisions by predicting who will respond,” says physician-scientist Timothy Chan, who led the research. The findings could also inspire new research that potentially may lead to more-powerful immunotherapies for melanoma as well as for other cancers.
T Cell Emancipation
MSK physicians and scientists played a major role in the development of ipilimumab, which works by blocking a protein called CTLA-4. Normally, CTLA-4 keeps the tumor-fighting activity of the immune system’s T cells in check. In the presence of the drug, T cells are unleashed and their inherent ability to recognize and destroy cancer cells is enhanced.
Recent studies have shown that about one in five patients with metastatic melanoma who are treated with the drug live for more than three years after starting treatment. Before the drug became available, the median life expectancy for the disease was seven to eight months.
Mining Clinical Samples for Clues
In the study, the researchers collected tumor samples from 64 melanoma patients who had been treated with ipilimumab or tremelimumab, an experimental drug that works in a similar way. The tumors were analyzed by whole-exome sequencing, a method of deciphering DNA changes across all parts of the genome that code for protein. About half of the tumors analyzed came from patients for whom the treatment had been successful and the other half from people who derived little or no benefit from it.
“We found that tumors that had responded to the drug had a higher mutational burden,” or overall number of DNA changes, explains fellow Alexandra Snyder Charen, the study’s first author. “But the correlation isn’t perfect. Not all patients with a high mutational burden in their tumors responded to the drug.”
“This made us ask, ‘What is the immune system seeing?’” adds Jedd Wolchok, a melanoma expert and immunologist who played a major role in the development of ipilimumab and co-led the NEJM study with Dr. Chan. “What is it about the mutational landscape of a tumor that helps the immune system recognize and attack it?”
Using sophisticated computational tools, the researchers were able to explore their data through the lens of immunity science. They found that drug-responsive tumors share a certain type of mutation that makes cancer cells express new antigens — substances that T cells can detect and recognize as foreign to the body.
“We identified a specific subset of tumor antigens found only in patients who respond well to CTLA-4 blockade therapy,” says Dr. Chan. The researchers believe these mutations make tumors express the new antigens that the immune system then reacts to — and ipilimumab works by enhancing that reactivity.
Making Immunotherapies More Personalized and Effective
In the future, these findings could translate into a diagnostic test to detect the mutations in melanoma patients’ tumors. Results from such testing could help doctors and patients make more-informed treatment choices, taking into account the potential benefit of ipilimumab as well as the risk of side effects — which may include potentially dangerous inflammation of the colon or other organs.
In addition, the MSK team plans to investigate whether specific tumor mutations influence the effectiveness of other immunotherapy drugs.
“By mapping the mutations that make different types of tumors sensitive to different drugs, we might ultimately be able to offer more patients successful immunotherapy,” Dr. Chan says. “If we know a patient won’t respond to ipilimumab, we may be able to identify other drugs that are more likely to be effective against this person’s tumor.”
“The case of ipilimumab is unique in that we’ve been using a drug that empirically was found to be quite effective, and yet we didn’t have a detailed understanding of how it was working in people,” adds Dr. Wolchok. “It’s quite possible that in the future, we might learn how to introduce new antigens in people’s cancer cells to make their tumors more vulnerable to immunotherapy.”
“These advances would not have been made without the generosity of patients who consented to having their tumor tissue collected and analyzed,” Dr. Snyder Charen concludes. “Jedd Wolchok and his lab members have spent many years banking samples, and it’s an invaluable resource for research.”