Four years ago, Karen Peterson received a life-altering diagnosis of stage I triple-negative breast cancer—the most aggressive type. The news brought a rough year of treatment that began with a lumpectomy and ended with a double mastectomy.

Hedging on the one-in-four chance that her cancer might return, Karen resolved to learn everything she could about it. A single mom with a nonverbal autistic adult son and a background in television production and social work, she began reading, googling, and speaking to nutritionists, yoga teachers, patient navigators, scientists—“anyone with a relationship to cancer,” she recalls.

So when she saw the scan showing metastatic tumors throughout her body 2 years later, Karen understood what she faced: a survival expectancy of 18 to 24 months. “I thought, ‘Okay, you’ve been preparing for this moment, even if you didn’t know it would come.’”

Her oncologist recommended sticking with chemotherapy, the only accepted treatment at the time for triple-negative breast cancer. But Karen’s awful experience with the side effects of chemotherapy made her vow never to undergo it again. What’s more, she knew from her extensive research that another round of chemotherapy could diminish her odds of qualifying for a clinical trial.

Scouring government and medical center websites for alternatives, she spotted an intriguing clinical trial for breast cancer on the website for Perlmutter Cancer Center at NYU Langone Health. It involved immunotherapy, an emerging class of drugs that trains the body’s own immune system to kill off cancer cells. Immunotherapy works only in a minority of cancer patients, and it can cause intense side effects, but in the past five years, it has emerged as one of the only therapies to rein in advanced cancers.

Karen thought she might qualify for the trial, so she mailed her test results to its principal investigator, oncologist Daniel C. Cho, MD, associate professor of medicine and director of Perlmutter Cancer Center’s Phase 1 Drug Development Program, which runs roughly 30 clinical trials to evaluate the safety and promise of new cancer therapies. “Dr. Cho called me himself, which surprised me,” Karen recalls. “It wasn’t the nurse or coordinator. He told me I couldn’t get into that trial, but there was another immunotherapy trial coming down the pipeline, and he asked if I would be interested. Wow! It felt like getting into Harvard. When it comes to clinical trials, there are only so many slots and many exclusions.”

“Dr. Cho called me himself, which surprised me. It wasn’t the nurse or coordinator. He told me I couldn’t get into that trial, but there was another immunotherapy trial coming down the pipeline, and he asked if I would be interested. Wow! It felt like getting into Harvard.”—Karen Peterson

With Dr. Cho’s help, Karen became the first triple-negative breast cancer participant, among 20 with other types of metastatic cancers, in the trial of two different types of immunotherapy drugs. The combination, the researchers hoped, would unleash the immune system’s T cells to find and destroy tumors and remember the cancer if it returned.

Karen’s odds of responding to the treatment stood between 4 percent and 15 percent, Dr. Cho cautioned. She spent much of the first week under observation for side effects. The second infusion, three weeks later, brought flulike symptoms—sometimes an encouraging sign that the immune system is gearing up. “I had faith,” says Karen.

Eight weeks later, a scan revealed heartening news: the lesions had shrunk. Within months, they were gone, except for a spot on Karen’s pelvis, which, Dr. Cho says, may be merely an unhealed hole in the bone where cancer once grew. In April, after a year of unchanged scans, Karen finished the trial as a “near complete responder.” She’s not alone: promising results have led to phase 2 trials for bladder and lung cancers, and phase 3 trials for kidney cancer and melanoma, edging this treatment closer to FDA approval. “The hope is that people like Karen will never need another treatment,” says Dr. Cho. “We can’t say they are actually cured until we are 5 to 10 years out, but that is the hope.”

Grateful and enthused, Karen has become a patient advocate, speaking and writing about her experience. “People say, ‘Karen, how did you find the trial?’” she says. “I tell them I did a lot of work and pushed forward on faith. Plus, I got lucky. I got to Dr. Cho, and he called me back.”