Can estrogen receptor overexpression in normal tissues due to previous estrogen deprivation explain the fulvestrant efficacy in breast cancer therapy?

Abstract

Fulvestrant is a down-regulator of estrogen receptors (ERs) with still evolving optimal dosage for ER-positive breast cancer patients. The CONFIRM phase III trial in women with advanced breast cancer proved fulvestrant 500-mg to be associated with a longer time till progression (TTP) than the 250-mg schedule. Detailed results suggest that the fulvestrant in both schedules depended on the previous endocrine therapy. All complete responses and the only significant TTP difference between the two schedules was found among women previously treated with tamoxifen (TAM) and not in women after aromatase inhibitors (AIs).Noting that TAM competes with estrogen binding to ERs is important, so the optimal TAM dosage produces drug concentrations comparable to concentrations of available ER ligands. All AIs diminish production of the main ER ligand, so the optimal AI dosage depends on the overall pool of aromatase molecules in the body. Both treatments are not directly related to the pool of available ERs in the body.Here proposed interpretation is that estrogen deprivation due to years of endocrine breast cancer therapy increases ER expression in breast cancer cells and in other healthy estrogen target tissues. The breast cancer exposure to fulvestrant depends on the presence of all ERs in the body. Only when this overall pool is sufficiently saturated with fulvestrant, we can expect to achieve some breast cancer response due to down-regulation of ER in cancer tissue.The CONFIRM data suggest that among patients switching from TAM to fulvestrant, only the 500-mg schedule could down-regulate the moderately enlarged total body ER pool and thus induce breast cancer regression. In patients switching from previous AI treatments, both 250 and 500-mg schedules were unable to prolong the TTP, suggesting that in both doses, fulvestrant showed no efficacy since the overall ER pool was more enlarged after AIs.Fulvestrant might be more effective before TAM and AIs, in the first line endocrine therapy of metastatic breast cancer, since an unaltered ER pool in normal tissues is expected in this setting.