The acetate salt of megestrol, a synthetic derivative of the naturally occurring female sex hormone progesterone, with progestogenic, antiestrogenic, and antineoplastic activities. Mimicking the action of progesterone, megestrol binds to and activates nuclear progesterone receptors (PRs) in the reproductive system and pituitary; ligand-receptor complexes are translocated to the nucleus where they bind to progesterone response elements (PREs) located on target genes. Megestrol’s antineoplastic activity against estrogen-responsive tumors may be due, in part, to the suppression of pituitary gonadotrophin production and the resultant decrease in ovarian estrogen secretion; interference with the estrogen receptor complex in its interaction with genes and; as part of the progesterone receptor complex, direct interaction with the genome and downregulation of specific estrogen-responsive genes. This agent may also directly kill tumor cells.