Changing treatment before scans show progression: a new relationship between breast cancer drugs and blood tests

Cancer treatment often changes only after there is evidence that a drug has stopped working. What if a blood test could detect emerging resistance before a change appeared on a scan? Would switching treatment at that point benefit patients?

On October 8, Guardant Health announced that its blood test, Guardant360 CDx, had received certification under the European In Vitro Diagnostic Regulation as a companion diagnostic for selecting patients for camizestrant. The drug itself was approved in Europe in July; this announcement adds certification on the testing side. Guardant Health announcement ↗

The approach is intended for certain patients with advanced breast cancer receiving endocrine therapy. Aromatase inhibitors reduce the supply of oestrogen to suppress cancer growth. But certain mutations in ESR1, the gene encoding the oestrogen receptor, can allow the receptor to remain active even when oestrogen levels are low, contributing to treatment resistance.

The blood test looks for tumour-derived DNA carrying these mutations. It is not a genetic test for an inherited predisposition; it tracks how the cancer changes during treatment. Camizestrant acts on the oestrogen receptor and promotes its degradation. EMA medicine overview ↗

SERENA-6 tested this approach by regularly checking patients' blood during treatment and identifying those who had developed an ESR1 mutation but whose scans did not yet show progression. These patients were assigned either to switch the endocrine component of their treatment to camizestrant or to continue their existing regimen. Their accompanying CDK4/6 inhibitor was continued. Original paper ↗

In an updated analysis released in June 2026, the median time to disease progression or death was 16.8 months in the switch group and 9.2 months in the continuation group. The sequence of detecting a change in the blood and changing treatment accordingly produced a clinical benefit. These numbers do not, however, directly establish that patients lived longer. Updated analysis ↗

An interesting point is that using the same drug from the outset does not necessarily produce a similar advantage for everyone. In the separate SERENA-4 trial, first-line camizestrant plus palbociclib did not demonstrate superiority on the primary endpoint over the comparator. The populations and conditions differed, so the two trials alone cannot establish why their results diverged. Even so, they highlight the importance of asking not only which drug to use, but in whom, after which change, and at what point. SERENA-4 results ↗

There is also a precise distinction between showing that switching early is better than continuing the current treatment and showing that using the same drug early is better than waiting until progression appears on imaging. As part of its accelerated approval, the US FDA required a confirmatory trial directly comparing these treatment timings. FDA approval letter ↗

From a business perspective, testing is moving beyond a one-off selection before a drug is started towards supporting the next decision during treatment. For a pharmaceutical company, it can identify not just the patients who may benefit from its drug, but the time at which they may benefit. For a diagnostics company, it creates demand for continued monitoring.

More frequent measurement is not itself the source of value. A test becomes necessary when an observed change leads to a defined action and that action benefits patients. This combination of testing and treatment points towards developing the whole sequence together.