CHICAGO (Reuters) -Treating breast cancer patients with AstraZeneca's ( AZN ) experimental pill camizestrant at the first sign of resistance to standard treatments cut the risk of disease progression or death by half, a finding that could change the way such cancers are treated, cancer experts said on Sunday.

The results, presented at the American Society of Clinical Oncology meeting in Chicago, mark the first use of a blood test called a liquid biopsy to indicate the need for a change in treatment in women with a common form of breast cancer, even before tumor growth can be detected on imaging.

The early switch approach in women with hormone receptor-positive, HER2-negative breast cancer resulted in a 56% reduction in the risk of disease progression or death, said Dr. Eleonora Teplinsky, an oncologist at Valley-Mount Sinai Comprehensive Cancer Care and an ASCO breast cancer expert.

"When patients progress on scans, we're already behind," Teplinsky said at a media briefing. She said an early switch approach, before disease progression, allows doctors "to essentially stay ahead of the curve."

Camizestrant is not yet FDA-approved, but Teplinsky believes the data will likely result in a new treatment paradigm.

The trial involved 3,256 patients with advanced hormone receptor-positive, HER2-negative breast cancer, the most common type in which hormones such as estrogen fuel cancer growth. These cancers lack high levels of HER2, another cancer driver.

Women in the trial had at least six months of treatment with aromatase inhibitors that block hormones fueling the cancer, as well as targeted drugs called CDK4/6 inhibitors such as Novartis' Kisqali, Pfizer's Ibrance or Lilly's Verzenio, which block an enzyme that fuels cancer growth.

About 40% of patients treated with aromatase inhibitors develop mutations in the estrogen receptor 1 gene called ESR1 mutations, a sign of early drug resistance.

Camizestrant and similar drugs called Selective Estrogen Receptor Degraders (SERDS) block estrogen receptor signaling in cancer cells.

In the trial, researchers used blood tests to look for ESR1 mutations until 315 patients were identified. They were randomly assigned to either switch to camizestrant plus the CDK4/6 inhibitor (157 patients) or continue with standard treatment plus a placebo (158 patients).

The researchers found that it took 16 months for the disease to progress in women who got camizestrant, compared with 9.2 months in those who continued on standard therapy, a statistically significant difference in progression-free survival.

No new side effects were reported and few patients from either group dropped out due to side effects.

"This is going to be very impactful for our patients," said Dr. Hope Rugo, head of breast medical oncology at City of Hope in Duarte, California. The question, she said, is how do doctors incorporate the testing into clinical practice.

Separately, adding AstraZeneca's ( AZN ) immunotherapy durvalumab to standard treatment before and after surgery in patients with early-stage stomach and esophageal cancers helped extend the time patients had without cancer progression or recurrence compared to chemotherapy alone.

The global study of nearly 950 patients tested durvalumab, sold under the brand Imfinzi, in combination with a chemotherapy regimen called FLOT given around the time of initial cancer surgery.

Patients in the durvalumab plus FLOT arm experienced a 29% better event-free survival than those who received the chemotherapy regimen.

"We demonstrate that immunotherapy works in early-stage disease, which is great," lead study author Dr. Yelena Jarnigan of Memorial Sloan Kettering Cancer Center in New York told reporters at the meeting.

"We did not see any new safety signals, so this will change practice for our patients, which is exciting to see."

Both studies were published on Sunday in the New England Journal of Medicine.