T-DM1 vs Paclitaxel/Trastuzumab for Breast (ATEMPT Trial)

If a participant agrees to participate in this study she will be asked to undergo some
screening tests or procedures to confirm that she is eligible. Many of these tests and
procedures are likely to be part of regular cancer care and may be done even if turns out
that she does not take part in this research study. If she has had some of these tests or
procedures recently, they may or may not have to be repeated. These tests and procedures will
include: a medical history, performance status, assessment of your tumor, blood tests,
cardiac tests, pregnancy test and a collection of tumor tissue. If these tests show that she
is eligible to participate in the research study, she will begin the study treatment. If she
does not meet the eligibility criteria, she will not be able to participate in this research
study.

Because no one knows which of the study options is best, the participant will be "randomized"
into one of the study groups after she has had her breast surgery: Group 1 or Group 2.
Randomization means that she is put into a group by chance. Neither the participant nor the
research doctor will choose what group she will be in. The participant will have a one in
three chance of being placed in any group. Approximately 375 study participants will receive
the study drug, while 125 study participants will receive the standard therapy of trastuzumab
and paclitaxel.

Group 1 participants will receive the study drug T-DM1 every three weeks by IV (intravenous
injection) for 17 treatments (total of 51 weeks).

Group 2 participants will receive the FDA-approved drugs Paclitaxel and Trastuzumab once per
week by IV for 12 weeks. Then beginning week 13, participants will receive Trastuzumab only
by IV injection every three weeks for the next 13 treatments.

During all cycles the participant will have a physical exam and tumor assessment.

The investigators would like to keep track of the participant's medical condition for the
next five years after the final dose of study drug. The investigators would like to do this
by regular visits every 6 months for 3 years after completion of study treatment, and then
once a year for the next two years. The investigators may ask for additional follow-up by
phone after completion of these visits.

Participants who undergo lumpectomy (breast conserving surgery) need to receive breast
radiation therapy to participate in this study. Participants who have undergone a mastectomy
may receive chest wall and lymph node radiation (as determined by discussion with their
physician). Radiation Therapy will begin after the conclusion of all study paclitaxel doses,
and after 12 weeks fo the study drug T-DM1.

Inclusion Criteria:

- HER2-positive Stage I histologically confirmed invasive carcinoma of the breast

- ER/PR determination is required

- HER2 positive, confirmed by central testing: IHC 3+, FISH HER2/CEP17 <2.0 with an
average HER2 copy number >/=6.0, or FISH HER2/CEP17 >/= 2.0

- Bilateral breast cancers that individually meet eligibility criteria are allowed

- Subjects with multifocal or multicentric disease are eligible as long as each tumor
individually meets eligibility criteria

- Subjects with a history of ipsilateral DCIS are eligible if they were treated with
wide-excision alone, without radiation therapy; Patients with a history of
contralateral DCIS are not eligible.

- Should have tumor tissue available and a tissue block of sufficient size to make 15
slides, which must be sent to a DFCI site for testing

- Less than or equal to 90 days since most recent breast surgery for this breast cancer

- All tumor should be removed by either a modified radical mastectomy or a segmental
mastectomy (lumpectomy) with either a sentinel node biopsy or axillary dissection

- All margins should be clear of invasive cancer or DCIS

- May have received up to 4 weeks of tamoxifen therapy or other hormonal therapy, for
adjuvant therapy for this cancer

- Prior oophorectomy for cancer prevention is allowed

- Subjects who have undergone partial breast radiation (duration registration are eligible. Partial breast radiation must be completed prior to 2 weeks
before starting protocol therapy.

- Must have discontinued any investigational drug at least 2 weeks prior to
participation

- Willing to use one highly effective from of nonhormonal contraception or two effective
forms of nonhormonal contraception while on study and for 7 months after end of study
treatment

- Subjects undergoing lumpectomy must have no contraindications to radiation therapy

Exclusion Criteria:

- Pregnant or breastfeeding

- Use of potent CYP3A4 inhibitors during the study treatment period

- Excessive alcohol intake (more than 3 alcoholic beverages per day)

- Locally advanced tumors at diagnosis

- History of previous invasive breast cancer

- History of prior chemotherapy in the past 5 years

- History of prior trastuzumab or prior paclitaxel therapy

- Active, unresolved infection

- Active liver disease

- History of a different malignancy except for the following: disease free for at least
5 years and at low risk for recurrence; cervical cancer in situ, basal or squamous
cell carcinoma of the skin

- Active cardiac disease