AstraZeneca and Daiichi Sankyo’s Enhertu slows the progression of breast cancer by about five months in women whose disease worsened after endocrine therapy – even if they had very low levels of the mutant protein targeted by the drug, according to trial results announced on Sunday.

The findings, presented in Chicago at the annual meeting of the American Society of Clinical Oncology, significantly broaden the range of breast cancer patients that could benefit from Enhertu, an antibody-drug conjugate designed to deliver toxic chemotherapy directly to tumors.

The data showed an “unprecedented” improvement in progression-free survival, supporting the thesis that antibody-drug conjugates can deliver their payloads more specifically to cancer cells, ASCO President Dr. Lynn Schuchter said in a briefing with Reuters.

Study participants given Enhertu lived for a median of 13.2 months before their cancer worsened, compared with 8.1 months for those given chemotherapy. Results were similar for patients with low and “ultra-low” levels of HER2 – the protein targeted by the drug.

About 70% of breast cancer cases are hormone-receptor positive, and are initially treated with endocrine drugs that interfere with hormones like estrogen. If the cancer worsens, the only current option for those patients is chemotherapy. Another 20-25% of breast cancers are HER2 positive, or HER2 “high,” and can be treated with drugs like Roche’s Herceptin.

Enhertu is currently approved as a second-line treatment for HER2 positive and HER2 low breast cancer. Sales of the drug totaled about $2.6 billion last year.

If approved for HER2 low and ultra-low breast cancers after endocrine therapy, eight out of out 10 women with metastatic breast cancer could be treated with Enhertu, AstraZeneca oncology research chief Susan Galbraith told Reuters.

As tumor mutation testing improves, the number of patients with no HER2 could be so small that Enhertu becomes a preferred choice for almost all patients in approved settings, Jefferies analyst Peter Welford said in a research note on Sunday.

Galbraith said Astra is working with global regulatory agencies to submit the latest Enhertu breast cancer data.

A number of other trials are underway aimed at moving the drug into earlier lines of therapy.

Galbraith said Enhertu is effective at reaching tumors with low levels of HER2 due to the mechanism linking the antibody to the drug.

“Our linker is stable in the blood. When it gets to the tumor it gets cleaved and then it can go across the cell membrane,” Galbraith said.