Given the infeasibility to achieve the study population together with the loss of originality in the scientific rationale, it was considered to cancel the study

This is a national, multicenter, retrospective, observational post-authorization study (EPA-OD) study. The study will be conducted by reviewing the medical records of patients up to the start of the study. In each case, only data from before the start of the study will be obtained in order to ensure they are retrospective in nature, thus reflecting the regular use of nab-paclitaxel in clinical practice and avoiding interference with the physician's clinical practice.

To ensure the observational nature of this study, these data will be collected whenever they are available in the patient's medical record, and so no diagnostic or therapeutic intervention outside regular clinical practice will be used.

This is a national, multicenter, retrospective, observational post-authorization study (EPA-OD) study. Investigators will include all consecutive adult patients diagnosed with HER2-negative MBC who have started treatment with nab-paclitaxel monotherapy no further than the third line of chemotherapy for metastatic disease during the past 3 years (2012-2014). These patients must meet all the inclusion criteria and none of the exclusion criteria established in this protocol.

The primary objective is to describe the effectiveness of nab-paclitaxel in terms of response in early lines of chemotherapy for metastatic breast cancer in routine clinical practice.

This study plans to collect data retrospectively, provided they are available in the patient's medical record and according to routine clinical practice.

Inclusion Criteria:

• Women ≥18 years of age.
• Confirmed diagnosis of MBC (stage IV).
• Breast adenocarcinoma confirmed histologically.
• HER2-negative according to the American Society of Clinical Oncology (ASCO) and Anatomical Pathology (CAP) criteria for the detection of HER2 in breast cancer.
• Patients who have started treatment with nab-paclitaxel monotherapy no further than the third line of chemotherapy for metastatic disease in HER2-negative breast cancer during the period 2012-2014 (3 years) and who have received at least one cycle of treatment.
• Ability to give informed consent, preferably in writing or orally in front of a witness, before the start of data collection (if it is able to be given).

Exclusion Criteria:

• Patients with any medical or psychological disorder which in the investigator's opinion might compromise the ability of the patient to give their informed consent.
• Patients who have received treatment with nab-paclitaxel combined with other chemotherapy agents or anti-angiogenic drugs or tumor-targeting drugs with anti-tumor activity.
• Patients who have taken part in any clinical trial (interventional) during the study period.

Adult patients diagnosed with HER2-negative MBC who have started treatment with nab-paclitaxel monotherapy no further than third-line chemotherapy for metastatic disease between 2012 and 2014 will be enrolled consecutively. In addition, patients must meet the selection criteria established in this protocol.