Abstract PD2-12: Molecular imaging for early identification of patients who benefit from palbociclib in addition to letrozole

Palbociclib plus letrozole has improved both progression free survival and overall response rate in metastatic breast cancer (MBC) patients. For response to palbociclib, the best biomarker is ER expression. 16α-[ 18 F]Fluoro-17β-estradiol (FES) -PET allows whole body ER level assessment, and provides insight in the heterogeneity of ER expression throughout the body. We hypothesized that lesions with low uptake on FES-PET are unlikely to respond to letrozole plus palbociclib. METHODS : Post-menopausal women with ER positive MBC were eligible for this pilot study. All patients were staged with fludeoxyglucose (FDG)-PET and CT scan, and in addition a FES-PET was performed at baseline. After 8 weeks treatment an FDG-PET/CT was used for response evaluation. The primary endpoint was the relation between standard uptake value (SUV) per lesion on FES-PET to response, as measured by RECIST 1.1 criteria in case of measurable disease. In case of non-measurable bone lesions, progression was defined as an increase in SUV on FDG-PET of more than 30% per lesion compared to baseline (based on PERCIST). RESULTS: 15 patients were included of which 14 were evaluable for primary endpoint. Mean age was 50 years (range 35-76). Median number of prior therapies was 1. A total of 280 lesions were detected on conventional imaging of which 50 showed low uptake (SUV DISCUSSION: this pilot trial indicates negative predictive value of low FES uptake for response to letrozol plus palbociclib in ER positive MBC. The FES-PET may therefore be a biomarker for response for this combination treatment. This will have to be explored further in future clinical trials. Citation Format: Venema CM, de Vries EFJ, Glaudemans AWJM, Hospers GAP, Schroder CP. Molecular imaging for early identification of patients who benefit from palbociclib in addition to letrozole [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD2-12.