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 tumour growth can be detected on imaging.
The early switch approach in women with hormone receptor-positive, HER2-negative breast cancer resulted in a 56 per cent 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 said she believes the data will likely result in a new treatment paradigm.
The trial involved 3256 patients with advanced hormone receptor-positive, HER2-negative breast cancer, the most common type in which hormones such as oestrogen 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 Eli Lilly's Verzenio, which block an enzyme that fuels cancer growth.
About 40 per cent of patients treated with aromatase inhibitors develop mutations in the oestrogen receptor 1 gene called ESR1 mutations, a sign of early drug resistance.
Camizestrant and similar drugs called selective oestrogen receptor degraders, or SERDS, block oestrogen receptor signaling in cancer cells.
In the trial, researchers used blood tests to identify 315 patients with signs of early drug resistance and then randomly assigned them to either switch to camizestrant plus the CDK4/6 inhibitor or continue with standard treatment plus a placebo.
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 a measure known as 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.
