Inclusion Criteria

DISEASE CHARACTERISTICS:

- Histologically confirmed diagnosis of early-stage breast cancer

- HER2 positive by immunohistochemistry (IHC) (3+) or fluorescent in situ
hybridization (FISH)

- Ductal carcinoma in situ (DCIS) components should not be counted in the
determination of degree of IHC staining or FISH amplification

- No locally advanced tumors (i.e., T4) at diagnosis, including the following:

- Tumors fixed to chest wall

- Peau d'orange

- Skin ulcerations or nodules

- Clinical inflammatory changes (e.g., diffuse brawny cutaneous induration with an
erysipeloid edge)

- Has undergone mastectomy or lumpectomy with axillary node or sentinel node dissection
within the past 84 days

- Patients who have undergone a mastectomy must meet the following criteria:

- No evidence of gross or microscopic tumor (i.e., invasive DCIS) at the
surgical resection margins noted in final surgery or pathology reports

- Patients with close margins are eligible

- Radiation therapy is required for 4 or more positive lymph nodes and must
be started after completion of chemotherapy

- Patients who have undergone a lumpectomy with axillary node or sentinel node
dissection must meet the following criteria:

- No evidence of invasive cancer or DCIS at the surgical resection margins

- No gross residual adenopathy

- Planning to undergo radiation therapy to the breast with or without
regional lymphatics after completion of chemotherapy

- No active hepatic or biliary disease

- Patients with liver metastases, stable chronic liver disease, Gilbert's
syndrome, or asymptomatic gallstones are eligible

- Hormone receptor status:

- Estrogen receptor and progesterone receptor status known

PATIENT CHARACTERISTICS:

- Male or female

- Menopausal status not specified

- ECOG performance status 0-2

- Absolute neutrophil count ≥ 1,500/mm³

- Platelet count ≥ 100,000/mm³

- Hemoglobin ≥ 10.0 g/dL

- Bilirubin ≤ 1.5 times upper limit of normal (ULN)

- AST and ALT ≤ 2.5 times ULN

- Alkaline phosphatase ≤ 2.5 times ULN

- Creatinine normal OR creatinine clearance ≥ 60 mL/min

- LVEF ≥ 50% by MUGA scan or echocardiogram

- Able to complete questionnaire(s) by themselves or with assistance

- Able and willing to provide blood and tissue samples

- No known sensitivity to benzyl alcohol

- No sensory neuropathy ≥ grade 2

- No active cardiac disease, including any of the following:

- Myocardial infarction within the past 6 months

- Prior or concurrent congestive heart failure

- Prior or concurrent arrhythmia or cardiac valvular disease requiring medications
or that is clinically significant

- Uncontrolled hypertension, defined as diastolic blood pressure (BP) >100 mm Hg
or systolic BP > 200 mm Hg on 2 separate occasions ≥ 14 days apart

- Clinically significant pericardial effusion

- Prior or concurrent uncontrolled or symptomatic angina

- Other cardiac condition that, in the opinion of the treating physician, would
put the patient at hazardous risk

- No history of allergic reactions attributed to compounds of similar chemical or
biologic composition as lapatinib ditosylate

- No uncontrolled intercurrent illness including, but not limited to, the following:

- Ongoing or active infection

- Psychiatric illness or social situations that would preclude study compliance

- Able to swallow and retain oral medication

- No history of gastrointestinal (GI) disease resulting in an inability to take
oral medication, including any of the following:

- Malabsorption syndrome

- Requirement for IV alimentation

- Prior surgical procedures affecting absorption

- Uncontrolled inflammatory GI disease (e.g., Crohn's disease or ulcerative
colitis)

- Not pregnant or nursing

- Negative pregnancy test

- Fertile patients must use effective contraception during and for 6 months after
completion of study treatment

PRIOR CONCURRENT THERAPY:

- See Disease Characteristics

- No prior chemotherapy, radiation therapy, immunotherapy, or biotherapy for breast
cancer

- No primary breast radiation therapy as part of breast-conserving treatment

- No prior anthracycline or taxane therapy for any malignancy

- No prior epidermal growth factor receptor-targeting therapies (e.g., gefitinib,
cetuximab, erlotinib hydrochloride, rituximab, trastuzumab [Herceptin®], lapatinib
ditosylate, panitumumab, or nimotuzumab)

- At least 14 days since prior and no concurrent CYP3A4 inducers, including the
following:

- Rifamycin-class antibiotics (e.g., rifampin, rifabutin, or rifapentine)

- Anticonvulsants (e.g., phenytoin, carbamazepine, or barbiturates [e.g.,
phenobarbital])

- Antiretrovirals (e.g., efavirenz or nevirapine)

- Glucocorticoids (e.g., oral cortisone, hydrocortisone, prednisone,
methylprednisolone, or dexamethasone)

- Daily oral dexamethasone ≤ 1.5 mg (or equivalent) allowed

- Modafinil

- Hypericum perforatum (St. John's wort)

- At least 7 days since prior and no concurrent CYP3A4 inhibitors, including the
following:

- Antibiotics (e.g., clarithromycin, erythromycin, or troleandomycin)

- Antifungals (e.g., itraconazole, ketoconazole, fluconazole [> 150 mg daily], or
voriconazole)

- Antiretrovirals and protease inhibitors (e.g., delaviridine, nelfinavir,
amprenavir, ritonavir, indinavir, saquinavir, or lopinavir)

- Calcium channel blockers (e.g., verapamil or diltiazem)

- Antidepressants (e.g., nefazodone or fluvoxamine)

- Gastrointestinal agents (e.g., cimetidine or aprepitant)

- Grapefruit and grapefruit juice

- At least 6 months since prior and no concurrent amiodarone

- No herbal or alternative medicines or supplements ≥ 14 days before, during, and for
30 days after completion of study treatment

- No concurrent hormonal agents (e.g., birth control pills, ovarian hormonal
replacement therapy, or raloxifene)

- Adjuvant hormonal agents (e.g., tamoxifen, aromatase inhibitors) allowed after
completion of chemotherapy as part of treatment for breast cancer

- No concurrent antiretroviral therapy for HIV-positive patients

- No concurrent digitalis or beta-blockers for congestive heart failure

- No concurrent arrhythmia or angina pectoris medication

- No other concurrent investigational agents or anticancer therapies, including
cytotoxic agents or immunotherapy