More than two-thirds of patients with hormone receptor–positive metastatic breast cancer respond to personalized immunotherapy with tumor-infiltrating lymphocytes (TILs) and more than half of those treated experience measurable tumor shrinkage, according to study findings published in the Journal of Clinical Oncology.

Researchers used whole-genome sequencing to identify biomarker variants in tumor samples from 42 patients with metastatic breast cancer whose cancers had progressed despite treatment. They isolated the tumors’ TILs—a type of T cell—and tested their reactivity against the genetic variants in vitro. In total, 28 of the 42 patients’ TILs produced an immune response (67%).

The researchers used TILs as treatment in six of the responding patients, who also received four doses of pembrolizumab prior to the TIL infusion to prevent the new T cells from becoming inactivated. Of those, three experienced objective tumor regression: one complete response (ongoing for more than 5.5 years) and two partial responses (6 and 10 months).

“The findings suggest that this form of immunotherapy can be used to treat some people with metastatic breast cancer who have exhausted all other treatment options,” the researchers concluded.

The new evidence is from a larger, ongoing phase II study of TIL treatment for metastatic epithelial cancers, including breast cancer. Hormone receptor–positive breast cancer does not typically respond well to immunotherapy, the researchers said in a press release.

“It’s fascinating that the Achilles’ heel of these cancers can potentially be the very genetic variants that caused the cancer,” they said. “We’re using a patient’s own lymphocytes as a drug to treat the cancer by targeting the unique variants in that cancer. This is a highly personalized treatment.”

Learn more about immuno-oncology and genomics and precision oncology in ONS’s learning libraries.