DISEASE CHARACTERISTICS:

• Histologically confirmed prostate cancer

• Biochemical disease progression after prior definitive surgery or radiotherapy, as evidenced by all of the following:

  • Three consecutive rising prostate-specific antigen (PSA) levels obtained ≥ 6 weeks apart within the past 6 months
  • Most recent PSA > 0.4 ng/mL after prostatectomy (1.5 ng/mL after radiotherapy)
  • PSA doubling time < 1 year
• Hormone-sensitive disease, as evidenced by total testosterone > 150 ng/dL within the past 4 weeks
• No evidence of metastatic disease by physical exam, CT scan, MRI, or bone scan

PATIENT CHARACTERISTICS:

Age

• 18 and over

Performance status

• ECOG 0-1

Life expectancy

• Not specified

Hematopoietic

• WBC ≥ 3,000/mm^3
• Granulocyte count ≥ 1,500/mm^3
• Platelet count ≥ 100,000/mm^3

Hepatic

• Bilirubin normal
• Alkaline phosphatase normal
• AST and ALT ≤ 2.5 times upper limit of normal
• PT/INR normal

Renal

• Creatinine normal OR
• Creatinine clearance ≥ 60 mL/min

Cardiovascular

• Cardiac ejection fraction normal by echocardiogram or MUGA within the past 4 weeks

• No unstable arrhythmias on ECG

  • Rate-controlled asymptomatic atrial fibrillation allowed
• No symptomatic congestive heart failure
• No unstable angina pectoris

Gastrointestinal

• Able to swallow and retain oral medication

• No GI tract disease resulting in either of the following:

  • Malabsorption syndrome
  • Requirement for IV alimentation
• No uncontrolled inflammatory GI disease (e.g., Crohn's disease or ulcerative colitis)

Other

• Fertile patients must use effective contraception
• No ongoing or active infection
• No psychiatric illness or social situation that would preclude study compliance
• No history of allergic reaction attributed to compounds of similar chemical or biologic composition to lapatinib
• No other uncontrolled illness
• No other malignancy within the past 5 years except curatively treated nonmelanoma skin cancer

PRIOR CONCURRENT THERAPY:

Biologic therapy

• No prior vaccine therapy for prostate cancer
• No prior immunotherapy for prostate cancer

Chemotherapy

• At least 1 year since prior neoadjuvant or adjuvant chemotherapy

Endocrine therapy

• At least 1 year since prior neoadjuvant or adjuvant hormonal therapy
• At least 1 year since prior therapy that modulates testosterone levels (e.g., luteinizing hormone-releasing hormone agonists/antagonists or antiandrogens)
• More than 6 months since prior 5α-reductase inhibitors
• More than 6 months since prior megestrol
• More than 6 months since prior systemic steroids
• More than 6 months since prior and no concurrent ketoconazole

Radiotherapy

• See Disease Characteristics
• Prior radiotherapy as curative salvage therapy allowed

Surgery

• See Disease Characteristics
• Prior surgery as curative salvage therapy allowed
• No prior surgery affecting gastrointestinal (GI) tract absorption

Other

• Prior local ablative procedures as curative salvage therapy allowed
• At least 1 year since prior investigational agents

• No concurrent CYP3A4 inducers or inhibitors

  • At least 7 days to 6 months since prior CYP3A4 inducers or inhibitors as determined by the treating physician
• More than 6 months since prior herbal supplements
• No concurrent antacids within 1 hour before or after study drug administration
• No other concurrent investigational agents
• No other concurrent anticancer therapy
• No concurrent antiretroviral therapy for HIV-positive patients