The drug erlotinib works by blocking the activity of a protein called Epidermal Growth Factor Receptor (EGFR), which is located on the surface of many prostate cancer cells. Blockage of this protein has been shown to inhibit the growth of prostate tumor cells in a laboratory setting and in animal experiments. Erlotinib has been given to patients with other types of cancers. A recently completed study showed that erlotinib improved the survival of patients with advanced lung cancer who failed standard chemotherapy.