Evaluation of tumour heterogeneity by (18)F-fluoroestradiol PET as a predictive measure in breast cancer patients receiving palbociclib combined with endocrine treatment

BACKGROUND: Predictive biomarkers are needed to identify oestrogen receptor-positive, human epidermal growth factor receptor 2-negative (ER + /HER2-) metastatic breast cancer (MBC) patients who would likely benefit from cyclin-dependent kinase 4 and 6 inhibitors combined with endocrine therapy. Therefore, we performed an exploratory study to evaluate the tumour heterogeneity parameters based on 16α-(18)F-fluoro-17β-oestradiol ((18)F-FES)-PET imaging as a potential marker to predict progression-free survival (PFS) in MBC patients receiving palbociclib combined with endocrine therapy. METHODS: Fifty-six ER + MBC patients underwent (18)F-FES-PET/CT before the initiation of palbociclib. (18)F-FES uptake was quantified and expressed as the standardized uptake value (SUV). Interlesional heterogeneity was qualitatively identified according to the presence or absence of (18)F-FES-negative lesions. Intralesional heterogeneity was measured by the SUV-based heterogeneity index (HI = SUVmax/SUVmean). Association with survival was evaluated using the Cox proportional hazards model. RESULTS: A total of 551 metastatic lesions were found in 56 patients: 507 lesions were identified as (18)F-FES-positive, 38 lesions were distributed across 10 patients without (18)F-FES uptake, and the remaining 6 were liver lesions. Forty-three patients obtained a clinical benefit, and 13 developed progressive disease (PD) within 24 weeks. Nine out of 10 patients with an (18)F-FES-negative site developed PD, and the median PFS was only 2.4 months. Among 46 patients with only (18)F-FES-positive lesions, only four patients had PD, and the median PFS was 23.6 months. There were statistically significant differences between the two groups (P < 0.001). For the subgroup of patients with only (18)F-FES-positive lesions, low FES-HI patients experienced substantially longer PFS times than those with high FES-HI (26.5 months vs. 16.5 months, P = 0.004). CONCLUSIONS: (18)F-FES-PET may provide a promising method for identifying and selecting candidate ER + /HER2- MBC patients who would most likely benefit from palbociclib combined with endocrine treatment and could serve as a predictive marker for treatment response. Trial registration NCT04992156, Date of registration: August 5, 2021 (retrospectively registered).