Study of Antibody Drug Conjugate in Patients With Advanced Breast Cancer Expressing HER2

The dose escalation segment of the study utilizes a 3+3 design. Initially, 3 patients will be
dosed at each dose level. The first 3-week cycle of treatment constitutes the dose limiting
toxicity (DLT) evaluation period. If none of the 3 patients experience a DLT during the
evaluation period and the Safety Review Committee agrees this was a reasonably well tolerated
dose, 3 patients will be enrolled at the next dose level. However, in the event of 1 DLT, 3
additional patients will be enrolled at the same dose level. Any dose level with 2 or more
DLTs will be considered to have exceeded the maximum tolerated dose and subsequent patients
will be enrolled at lower dose levels. After the first cycle, patients may continue to
receive XMT-1522 until disease progression as long as the drug is well-tolerated and patients
continue to derive clinical benefit in the opinion of the Investigator.

After completion of the dose escalation, the expansion segment will enroll the patients with
the following kinds of cancer:

- Cohort 1: Advanced breast cancer, HER2 IHC 1+, or HER2 IHC 2+ without HER2 gene
amplification

- Cohort 2: Advanced breast cancer, HER2-positive, who have received prior ado-trastuzumab
emtansine

- Cohort 3: Advanced gastric cancer, HER2-positive, who have received prior trastuzumab

- Cohort 4: Advanced non-small cell lung cancer, HER2 IHC 2+ or 3+, any HER2 gene
amplification or mutation status

Inclusion Criteria:

- Able and willing to give informed consent

- Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

- Measurable disease via RECIST

- Resolution of all toxic side effects from prior oncology treatments

- Adequate organ function as measured by various blood parameters

- Not pregnant or lactating, willing to prevent pregnancy while on study and for 6
months after the last dose of XMT-1522

- Histologically or cytologically confirmed adenocarcinoma of the breast with
unresectable locally advanced disease, or metastatic disease and HER2 IHC 1+ or 2+ OR

- Histologically or cytologically confirmed adenocarcinoma of the breast with
unresectable locally advanced disease, or metastatic disease and HER2 IHC 3+ or
positive for HER2 gene amplification

- Progressed following all standard of care therapies for advanced breast cancer. OR

- Histologically or cytologically confirmed locally advanced or metastatic gastric
cancer and HER2 IHC 3+ or positive for HER2 gene amplification OR

- Histologically or cytologically confirmed Stage IIIb or IV non-small cell lung cancer
HER2 IHC 2+ or 3+ by local laboratory assessment.

Exclusion Criteria:

- Major surgery, radiation therapy, or systemic anti-cancer therapy within 28 days of
starting study treatment.

- Some types of brain metastases

- Peripheral neuropathy of Grade 2 within 3 weeks prior to the first study therapy

- History of exposure to cumulative doxorubicin dose ≥ 360 mg/meter squared. If another
anthracycline or more than one anthracycline has been used, then the cumulative dose
must not exceed the equivalent of 360 mg/meter squared of doxorubicin

- History of clinically significant cardiac dysfunction

- Current known active infection with HIV, hepatitis B virus, or hepatitis C virus

- Current severe, uncontrolled systemic disease

- Severe dyspnea at rest, due to complications of advanced malignancy, or requiring
supplementary oxygen therapy.

- History of other malignancy within the last 5 years, except for appropriately treated
carcinoma in situ of the cervix, non-melanoma skin carcinoma, or other malignancy with
a similar expected curative outcome

Patients who participate in the dose escalation segment of the study cannot participate in
the expansion segment of the study.