Measuring Residual Estrogen Receptor Availability during Fulvestrant Therapy in Patients with Metastatic Breast Cancer

van Kruchten M, de Vries EG, Glaudemans AW, van Lanschot MC, van Faassen M, Kema IP, Brown M, Schroder CP, de Vries EF, Hospers GA

Cancer Discov 2014 Nov;

PMID: 25380844

Abstract

It is unknown whether the current dose of fulvestrant, an estrogen receptor (ER) downregulator, is sufficient for maximal ER downregulation in patients with metastatic breast cancer. We performed a feasibility study to assess ER availability before and during fulvestrant. Sixteen patients with ER-positive metastatic breast cancer underwent positron emission tomography/computed tomography (PET/CT) at baseline (Scan 1), day 28 (Scan 2) and day 84 (Scan 3) to monitor tumor [(18)F]fluoroestradiol (FES) uptake. Incomplete reduction in ER availability was predefined as < 75% decrease in median tumor FES uptake and a residual standardized uptake value (SUVmax) ≥ 1.5. In total 131 FES-positive lesions were identified (median SUVmax of 2.9, range 1.7-6.5). The median change in patients during fulvestrant treatment was -85% at Scan 2, but varied widely (-99% to +60%). Fulvestrant reduced tumor FES uptake incompletely at Scan 2 in 6 (38%) of the 16 patients, which was associated with early progression.