On March 17th, 2011, the European Commission issued a marketing authorization valid throughout the European Union for Eribulin mesylate (Halaven; Eisai Limited), for the treatment of patients with locally advanced or metastatic breast cancer who have progressed after at least two chemotherapic regimens for advanced disease.

As the use of Eribulin will be widespread in this tumor setting, a better knowledge of its safety profile outside clinical trials is warranted.

Indeed the possibility to select patients at risk for developing Eribulin-induced neuropathy, will allow the exclusion from these treatment of those patients harbouring the specific single nucleotide polymorphism (SNP). Given that Eribulin toxicity often results in treatment discontinuation, the ability to anticipate which patients will experience severe toxicity could allow for either early intervention or even possibly for prophylactic therapy, or for selection of the patients to be treated.

This study is primarily aimed at surveying the tolerability profile of Eribulin in an unselected population of patients with metastatic breast cancer in relation to toxicities already described in clinical trials, and neurotoxicity in particular.

The secondary objectives of this trial include:

• To study the relationship between specific genetic polymorphism and incidence and severity of peripheral neuropathy
• To describe treatment efficacy in terms of duration of treatment and impact on survival.

All toxicities will be collected and classified according to National Cancer Institute Common Terminology criteria for Adverse Events (NCI CTCAE) version 4.0 and monitored during all the treatment period and up to 30 days after therapy discontinuation.

In particular, evaluation of incidence and outcome of any grade AEs already recorded in previous clinical trials will be collected, as follows:

• asthenia/fatigue,
• neutropenia,
• alopecia,
• nausea,
• peripheral neuropathy
• constipation

Any other unexpected AEs shall be evaluated likewise.

Patients must be followed for AEs until every ongoing Eribulin-related/unrelated toxicity and AE have been resolved, or the Investigator assesses them as "chronic" or "stable" or until the end of the trial, whichever comes first. For patients who will begin a new anticancer therapy after the last study drug administration, the AEs reporting period will end at the time the new treatment starts.

For the determination of polymorphisms, a routine blood collection of two tubes with 3-5 ml of blood be performed. The sample can be collected at any time during the participant's first two treatment cycles. Blood will be collected in a Vacutainer containing ethylendiaminetetraacetic acid (EDTA). Immediately after blood collection, tubes have to be inverted (at least five times) and then stored at - 20° C.

Inclusion Criteria:

• Diagnosis of metastatic breast cancer
• Previous treatment with anthracyclines and taxanes
• Patients who will start Eribulin or who have already received only the first dose (cycle 1, day 1) of Eribulin according to the approved indication
• Ability to comply with sample collection
• Patient has signed the study Informed Consent Form (ICF) and the specific Pharmacogenetic ICF.
• Absence of any contraindication to treatment

Exclusion Criteria:

• Previous treatment with Eribulin in a previous line of treatment
• Previous treatment with Eribulin off label