Inclusion Criteria

Inclusion criteria:

- Histologically confirmed infiltrating primary breast cancer of 2.0 cm or more in
largest clinical diameter

- ER and/or PgR positive cancer (> 10% of positive cancer cell assessed by IHC)

- Postmenopausal status, defined by at least one of the following:

≥ 60 years of age < 60 years of age and amenorrheic for ≥ 12 months prior to day 1 <
60 years of age and amenorrheic for < 12 months prior to day, or without a uterus:
luteinizing hormone (LH) and follicle stimulating hormone (FSH) values within
postmenopausal range Prior bilateral oophorectomy Prior radiation castration with
amenorrhea for at least 6 months

- HER2 negative tumors (IHC 0-2+, or FISH negative)

- Availability of tumor tissue suitable for biological and molecular examination before
starting primary treatment

- Age over 18 years

- ECOG PS 0-1

- Normal organ and marrow function as defined below:

leukocytes > 3000/mL absolute neutrophil count > 1,500/mL platelets > 100,000/mL total
bilirubin within normal institutional limits AST (SGOT)/ALT(SGPT)< 2.5 X institutional
upper limit of normal Creatinine within normal institutional limits

- Cardiac ejection fraction within the institutional range of normal as measured by
echocardiogram or MUGA scan.

- Eligibility of patients receiving medications or substances known to affect, or with
the potential to affect the activity or pharmacokinetics of lapatinib will be
determined following review of their use by the Principal Investigator.

A list of medications and substances known or with the potential to interact with CYP450
isoenzymes is provided

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

- Ability to swallow and retain oral medication.

Exclusion criteria:

- Stage IIIB, IIIC, and inflammatory breast cancer

- Stage IV breast cancer

- Contraindication to the treatment with letrozole

- Prior treatment with chemotherapy, endocrine therapy or radiotherapy. Prior
treatment with EGFR targeting therapies

- Treatment with any other investigational agents, or with all herbal (alternative)
medicines

- History of allergic reactions attributed to compounds of similar chemical or biologic
composition to lapatinib

- Uncontrolled inter-current illness including, but not limited to, ongoing or active
infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
arrhythmia, or psychiatric illness/social situations that would limit compliance with
study requirements

- HIV-positive patients receiving combination anti-retroviral therapy

- GI tract disease resulting in an inability to take oral medication, malabsorption
syndrome, a requirement for IV alimentation, prior surgical procedures affecting
absorption, uncontrolled inflammatory GI disease (e.g., Crohn's, ulcerative colitis)

- Concomitant requirement for medication classified as CYP3A4 inducers or inhibitors
(See section 3.7.4.2 Other concomitant treatments)