 The drug Lapatinib has been shown to inhibit both epidermal growth factor receptor EGFR and erbB2 tyrosine kinases resulting in an effective slowing of disease progression in breast cancer. It has also been demonstrated that erbB is overexpressed in human pituitary adenomas. The investigators are therefore assessing tumor size stabilization and pituitary tumor secretory profiles during the course of a six month therapy of lapatinib. The purpose of this trial will be to estimate the activity of lapatinib in slowing the growth rate of pituitary tumors. Lapatinib is an FDA approved drug used to treat breast cancer. However, in this study, the drug will be used to treat pituitary cancer. 