In order for estrogens and androgens to perform their physiologic functions, they must interact with and activate their hormone receptors. Hormone receptors are sites located in tissues where hormones bind. Once a hormone binds with its receptor, a series of cellular events is activated, resulting in estrogenic or androgenic tissue effects.

Pharmaceuticals that target hormone receptors for estrogens or androgens have been prescribed for over 50 years. The drugs that have been used to stimulate androgen receptors are natural or synthetic hormones known as steroids.

Steroids activate hormone receptors in all tissue types in a non-selective manner. The absence of selectivity may result in unwanted side effects, such as:

• Potential stimulation of latent prostate cancer
• Aggravation of existing BPH
• Acne
• Hair growth
• Gynecomastia

There are classes of small molecules that are not steroids, but which bind to hormone receptors. These small molecules may either stimulate or block hormone receptors depending on the type of tissue in which the receptor is found.

A drug that can either block or stimulate the same hormone receptor is called a receptor modulator. If it can block or stimulate a receptor in a tissue-selective manner, it may be able to mimic the beneficial and at the same time minimize the unwanted effects of the natural or synthetic hormones.