Inclusion Criteria:

- History of adenocarcinoma of the prostate

- Evidence of metastatic disease

- Recommendation by patient's radiation oncologist to receive palliative external beam
radiation for bone metastases secondary to metastatic prostate cancer

- Age 18 years or older

- Patients may be undergoing concurrent therapy with GNRH agonists or combined androgen
blockade, as this is standard care for all advanced prostate cancers

- Ability to understand and the willingness to sign a written informed consent document

Exclusion Criteria:

- Any contraindication to lapatinib treatment.

- Prior radiation therapy within 30 days of the start of the planned course of
treatment.

- Prior cytotoxic chemotherapy within 4 weeks of planned first dose of radiation.

- Prior administration of an ERBB1 or ERK1/2 inhibitor within 30 days of the start of
the planned course of treatment.

- Patients with a medical necessity to continue active therapy with CYP3A4 strong
inhibitors or inducers. The concomitant use of strong CYP3A4 inhibitors should be
avoided (e.g., ketoconazole, itraconazole, clarithromycin, atazanavir, indinavir,
nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, voriconazole,
aprepitant). Grapefruit may also increase plasma concentrations of lapatinib and
should be avoided. Once the strong inhibitor or inducer is discontinued, a washout
period of approximately 1 week should be allowed before the lapatinib administration.
If a patient is unable to discontinue these medications, they are not eligible for
enrollment.

- Concurrent administration of any other investigational agents.

- Uncontrolled concurrent illness including, but not limited to ongoing or active
infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
arrhythmia, active hepatic or biliary disease with a Child-Pugh class of B or C, or
psychiatric illness/social situations that would limit compliance with study
requirements or that would interfere with accomplishing the study objectives.