PET FES measures in vivo pharmacodynamics of endocrine therapy

Abstract

14110 Background: Many clinical options are available for management of hormone sensitive breast cancer, including agents which lower estrogen levels such as aromatase inhibitors (AIs) and agents with block ligand binding to receptor such as tamoxifen (TAM) or fulvestrant (FUL). Estrogen receptor (ER) function is essential for sensitivity to hormonal manipulation in breast cancer treatment. We and others have previously shown that functional ER imaging using PET FES predicts response to hormonal therapy using a quantitative threshold of SUV >1.5. Herein we report that FES PET provides a unique insight into in vivo pharmacodynamics of ER therapy. We hypothesized that the impact of therapy on estradiol binding to ER, measured by FES PET, differs between AIs and ER antagonists, and that early changes in receptor expression/occupancy show efficacy of drug at the tumor target. Methods: Patients undergoing treatment with AI, TAM, or FUL underwent baseline PET FDG and FES, and follow-up PET FES imaging at 2–8 weeks post initiation of therapy. Results: We observed the following changes in FES uptake on hormonal therapy: Mean percent change in FES SUV were 54% decline for TAM and FUL vs. 14% decline for AI treated patients (p<.001). Patients on TAM showed complete blockade of tumor FES uptake on therapy (5/5 with residual SUV <1.5), whereas patients on FUL had variable uptake and incomplete blockade at tumor sites in most patients (4/11 with residual SUV <1.5) (p < .05 FUL vs. TAM), despite consistent blockade of uterine FES uptake in patients where the uterus was visualized pre-FUL. Patients on AI therapy (n=14) had variable tumor uptake following treatment initiation. Conclusions: PET FES effectively monitors the in vivo activity of therapy. Estrogen blocking therapies result in a greater change in tumoral estradiol binding than in ligand depletion. TAM effectively blocks uptake of FES as would be predicted by the mechanism of action of this agent. However, FUL (while blocking uterine uptake) incompletely blocks tumor uptake, providing a mechanism to explain reduced activity of this agent in some patients. Ongoing analysis is designed to assess whether early changes in FES predict response or clinical benefit. No significant financial relationships to disclose.