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.