Switching breast-cancer drugs on a blood test, before the scan turns bad

In a trial of women with advanced hormone-driven breast cancer, doctors used a blood test to catch a resistance mutation as it emerged and switched drugs immediately — before any scan showed the cancer growing — and that pre-emptive switch to camizestrant lengthened the time to progression from a median of 9.2 months to 16.0 months [s1]. On the strength of that result, the SERENA-6 trial, the US Food and Drug Administration granted accelerated approval to camizestrant (Etcamah) on 4 September 2026 [s2][s3].

The idea being tested was not just a new drug but a new moment to act. In estrogen-receptor-positive, HER2-negative breast cancer, the standard first-line treatment is an aromatase inhibitor plus a CDK4/6 inhibitor. Its most common route to failure is a mutation in the ESR1 gene, which lets the tumour ignore the aromatase inhibitor. SERENA-6 asked whether catching that mutation in the blood, and swapping the aromatase inhibitor for camizestrant on the spot, beats waiting for the cancer to visibly advance [s1].

What they did

Investigators monitored circulating tumour DNA — genetic fragments the tumour sheds into the blood — every two to three months in patients who had been on first-line aromatase-inhibitor-plus-CDK4/6 therapy for at least six months [s1]. A total of 3,256 patients were screened this way [s1]. The 315 who developed an ESR1 mutation but whose scans still showed no progression were randomly assigned, 157 to switch to camizestrant (75 mg daily) and 158 to stay on their aromatase inhibitor, each continuing the same CDK4/6 inhibitor [s1]. The main outcome was investigator-assessed progression-free survival.

What it showed

At a median follow-up of 12.6 months, median progression-free survival was 16.0 months (95% confidence interval, 12.7 to 18.2) with camizestrant versus 9.2 months (95% confidence interval, 7.2 to 9.5) on the aromatase inhibitor — a hazard ratio of 0.44 (95% confidence interval, 0.31 to 0.60; P<0.0001) [s1]. That is a 56% lower risk of progression or death, and it is the number the approval turns on [s1].

Patients also held their quality of life longer: the median time to a meaningful deterioration in self-reported global health status was 21.0 months with camizestrant against 6.4 months without it (hazard ratio, 0.54; 95% confidence interval, 0.34 to 0.84) [s1]. Treatment discontinuation because of side effects was uncommon and similar in both groups, 1.3% versus 1.9% [s1].

What it does not show, yet

Two limits sit inside the headline. First, this is progression-free survival at an interim analysis, not overall survival: the trial has not shown that switching early helps patients live longer, only that it delays the cancer's documented growth [s1]. Delaying progression is worth having, but it is not the same outcome, and the two do not always move together. Second, the benefit was measured by the treating investigators rather than a blinded central review, which can flatter an open-label comparison [s1].

There is also the arithmetic of screening. To find the 315 patients who could benefit, the trial tested 3,256 [s1]. Serial ctDNA surveillance of everyone on first-line therapy — the infrastructure this strategy assumes — is not yet routine care, and the trial does not tell us how the approach performs outside a setting built to run it. The same promise and the same caveat attach to ctDNA-guided treatment decisions more broadly: the blood test can see resistance early, but seeing it early only helps if acting early changes the outcome that matters.

Where it fits

Camizestrant is a next-generation oral selective estrogen-receptor degrader, a class designed to work against ESR1-mutant tumours that no longer respond to aromatase inhibitors [s1]. The FDA approved it in combination with a CDK4/6 inhibitor for HR-positive, HER2-negative advanced breast cancer once an ESR1 mutation is detected during aromatase-inhibitor therapy by an authorised test [s3]. The clearance was granted under the accelerated pathway, on the basis of progression-free survival [s3].

What SERENA-6 genuinely establishes is narrower and more interesting than "a new drug works": it is evidence that the timing of a switch — driven by a blood test rather than a scan — can itself be the intervention. Whether that translates into longer lives, and whether health systems can run the endocrine-therapy monitoring it requires, are the open questions the confirmatory data will have to answer.