A Pilot Study Evaluating Megestrol Acetate Modulation in Advanced Breast Cancer With Positive Hormonal Receptor

Progesterone receptor (PR) expression has been considered a biomarker of oestrogen receptor-α (ERα) activity. This longstanding relationship has been recently challenged and instead of being merely an ERα-induced gene target, PR may be a critical determinant of ERα activity. The functional significance of this steroid receptor crosstalk is regulation of a gene expression program associated with low tumorigenicity; hence, better disease outcome. Genomic alterations in the PR genomic locus seem to be a relatively common mechanism for reduction of PR expression, which may consequently lead to altered ERα chromatin binding and target gene expression patterns that increase breast tumorigenicity and confers a poor clinical outcome. This ERα-PR crosstalk may be directly influenced by many variables, including the relative receptor levels and the hormonal milieu.

ER-positive advanced breast cancer is a heterogeneous group of diseases with considerable variability in outcome to a range of treatments. Prior response predicts the likelihood of subsequent benefit from another endocrine agent and this should be taken into account in the treatment decision process when assessing whether to prescribe a subsequent endocrine therapy. Despite the enormous progress made regarding the elucidation of breast cancer subgroups and their molecular drivers, most information comes from primary tumors. MA lacks cross-resistance and is active after acquired resistance to potent AI. This pilot trial evaluates in vivo MA modulation of steroidal receptors in advanced breast cancer.