Lapatinib in Treating Patients With Prostate Cancer That Did Not Respond to Hormone Therapy

OBJECTIVES: Primary * Determine the proportion of patients with hormone-refractory prostate cancer who experience \> 50% decline in prostate-specific antigen (PSA) after treatment with lapatinib ditosylate. Secondary * Determine the safety of this drug in these patients. * Determine the time to PSA progression in patients treated with this drug. * Determine the molecular correlates and predictive biomarkers of response in patients treated with this drug. OUTLINE: This is a multicenter, open-label study. Patients receive oral lapatinib ditosylate once daily. Treatment continues in the absence of disease progression or unacceptable toxicity. Serum samples are collected for biomarker analysis at baseline and every 4 weeks. After completion of study treatment, patients are followed at 4 weeks.