A Study of Etirinotecan Pegol (NKTR-102) Versus Treatment of Physician's Choice (TPC) in Patients With Metastatic Breast Cancer Who Have Stable Brain Metastases and Have Been Previously Treated With an Anthracycline, a Taxane, and Capecitabine

Inclusion Criteria:

* Female or male, age ≥ 18 years.
* Histologically-confirmed carcinoma of the breast (either the primary or metastatic lesions) for whom single-agent cytotoxic chemotherapy is indicated. Patients may have either measurable or non-measurable disease according to RECIST version 1.1.
* Patients must have a history of brain metastases that are non-progressing.
* For triple-negative breast cancer, a minimum of 1 prior cytotoxic chemotherapy regimen must have been administered for the indication of metastatic disease.Depending on receptor status, 1 or 2 prior cytotoxic regimens are required prior to enrollment in this trial; hormonal and/or human epidermal growth factor receptor 2 (HER2) -targeted agents may be required.
* Have had prior therapy (administered in the neoadjuvant, adjuvant, and/or metastatic setting) with an anthracycline, a taxane, and capecitabine (prior anthracycline can be omitted if not medically appropriate or contraindicated for the patient).
* Last dose of anticancer therapy must have been administered within 6 months of the date of randomization into this study.
* All anticancer- and radiation therapy-related toxicities must be completely resolved or downgraded to Grade 1 or less (neuropathy may be Grade 2 or less).
* Have an Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
* Demonstrate adequate organ function obtained within 14 days prior to randomization and analyzed by the central laboratory.
* Women of childbearing potential (WCBP) must agree to use highly effective methods of birth control throughout the duration of the study until 6 months following the last dose of study drug.
* Males with female partners of child-bearing potential must agree to use a barrier contraception (e.g., condom with spermicidal foam/gel/film/cream/suppository) throughout the duration of the study until 6 months following the last dose of study drug; in addition to their female partner using either an intrauterine device or hormonal contraception and continuing until 6 months following the last dose of study drug. Male patients should not donate sperm until 6 months following the last dose of study drug.

Exclusion Criteria:

* Last dose of anticancer therapy (including HER2-targeted therapy) within 14 days prior to randomization.
* High-dose chemotherapy followed by stem cell transplantation (autologous or allogeneic).
* Major surgery within 28 days prior to randomization.
* Concomitant use of any anticancer therapy or use of any investigational agent(s).
* Received prior treatment for cancer with a camptothecin-derived agent.
* Lesions on imaging, by cerebrospinal fluid or with neurological findings that are consistent with leptomeningeal disease or meningeal carcinomatosis.
* Chronic or acute GI disorders resulting in diarrhea of any severity grade.
* Patients who are pregnant or lactating, plan to get pregnant, or have a positive serum pregnancy test prior to randomization.
* Enzyme-inducing anti-epileptic drugs (EIAEDs) within 14 days of randomization.
* Hepatitis B or C, tuberculosis, or HIV.
* Cirrhosis.
* Prior malignancy (other than breast cancer) unless diagnosed and definitively treated more than 5 years prior to randomization.
* Daily use of oxygen supplementation.
* Significant known cardiovascular impairment.
* Prior treatment with NKTR-102.
* Psychiatric illness, social situation, or geographical situation that preclude informed consent or limit compliance.
* Known intolerance or hypersensitivity to any of the products used in this study or their excipients.
* For patients selecting vinorelbine or gemcitabine as the TPC agent, patients may not receive yellow fever vaccine in the 28 days prior to randomization.