Mechanisms of action of progestational agents.

An in vitro tissue culture system utilizing human breast cancer cells in long-term culture was employed to examine mechanisms of action of progestational agents such as progesterone and megestrol acetate. Growth experiments were performed under various defined media conditions. Concentrations of both progesterone and megestrol acetate (10(-5) mol/L), which are attainable in patients, led to direct cytotoxicity of hormone-dependent cells and also to a modification of the known estrogen stimulation of growth in these cells.