Although cyclin-dependent kinase (CDK) 4/6-inhibitors, such as palbociclib and abemaciclib, have been shown to slow disease progression in advanced breast cancer, there are no known biomarkers to predict response to these therapies and women have to wait two to three months to find out if they are working using a scan.
Reporting in Nature Communications (online, 1 March 2018), researchers analysed plasma samples from 73 women with advanced breast cancer carrying a frequently mutated gene (PIK3CA) who took part in a randomised trial of palbociclib added to the endocrine therapy, fulvestrant
. They studied the levels of circulating tumour DNA (ctDNA), which is shed from the tumour and found in the bloodstream.
The team found that 15 days after treatment, the change in PIK3CA ctDNA levels strongly predicted progression-free survival for patients receiving palbociclib and fulvestrant (hazard ratio 3.94, 95% confidence interval 1.61–9.64).
They said the results represented the first biomarker for predicting response to palbociclib. In future, early ctDNA levels could be used to determine which patients should have their therapy switched or augmented with adjuvant therapies, the researchers concluded.
 O’Leary B, Hrebien S, Morden JP et al. Early circulating tumour DNA dynamics and clonal selection with palbociclib and fulvestrant for breast cancer. Nat Commun 2018;9:896. doi: 10.1038/s41467-018-03215-x