A type of inherited and incurable breast cancer that tends to affect younger women could be targeted by a new therapy, researchers have found.

A small study presented at the world’s largest cancer conference found treating patients with the drug olaparib could slow cancer growth by three months and be less toxic for patients with inherited BRCA-related breast cancer.

Researchers said there was not enough data to say whether patients survived longer as a result of the treatment.

“We are in our infancy,” said Dr Daniel Hayes, president of the American Society of Clinical Oncology and professor of breast cancer research at the University of Michigan. “This is clearly an advance; this is clearly proof of concept these can work with breast cancer.”

“Does it look like it’s going to extend life? We don’t know yet,” he said.

The drug is part of the developing field of “precision medicine”, which targets patients’ genes to tailor treatment.

“It is a perfect example of how understanding a patient’s genetics and the biology of their tumor can be used to target its weaknesses and personalize treatment,” said Andrew Tutt, director of the Breast Cancer Now Research Centre at The Institute of Cancer Research.

“Olaparib is already available for women with BRCA-mutant advanced ovarian cancer, and is the first drug to be approved that is directed against an inherited genetic mutation.” The study was the first to show olaparib can slow growth of inherited BRCA-related breast cancer. The drug is not yet approved for that use.

People with inherited mutations in the BRCA gene make up about 3% of all breast cancer patients, and tend to be younger. The median age of women in the olaparib trial was 44 years old.

BRCA genes are part of a pathway to keep cells reproducing normally. An inherited defect can fail to stop abnormal growth, thus increasing the risk of cancer. The study examined the effectiveness of olaparib against a class of BRCA-related cancers called triple negative. Olaparib is part of a class of four drugs called “PARP-inhibitors” that work by shutting down a pathway cancer cells use to reproduce.

The study from Memorial Sloan Kettering Cancer Center in New York randomly treated 300 women with advanced, BRCA-mutated cancer with olaparib or chemotherapy. All the participants had already received two rounds of chemotherapy.

About 60% of patients who received olaparib saw tumors shrink, compared with 29% of patients who received chemotherapy. That meant patients who received olaparib saw cancer advance in seven months, versus four months for only chemotherapy.

Researchers cautioned it is unclear whether olaparib extended life for these patients, and that more research was needed to find out which subset of patients benefit most from olaparib.