Inclusion Criteria

Inclusion Criteria

- Females aged ≥20 years and <75 years at the time of informed consent.

- Histologically or cytologically confirmed with breast cancer

- Score 3+ by immunohistochemistry (IHC) or HER2 positive by Fluorescence in Situ
Hybridization (FISH) method

- Subjects who meet any of the following criteria:

- Evidence of recurrence during adjuvant chemotherapy with trastuzumab and taxane

- Evidence of recurrence within 6 months after adjuvant chemotherapy with
trastuzumab and taxane

- Experienced prior chemotherapy including trastuzumab and taxane for advanced or
recurrent breast cancer

- Adequate organ function

- Eastern Cooperative Oncology Group (ECOG)-Performance Status (PS) is 0 or 1

- Subjects who have submitted written informed consent for study entry

Exclusion Criteria

- Subjects with known brain metastasis accompanied by clinical symptoms or requiring
active treatment

- Subjects with severe active infection requiring active treatment

- Subjects with large pleural effusions, ascites, or pericardial effusions requiring
drainage.

- Hypersensitivity to trastuzumab, halicondrin B or halicondrin B chemical derivatives

- Known positive for human immunodeficiency virus (HIV) test or positive for hepatitis
B surface (HBs antigen) or hepatitis C (HCV) by serum test.

- Subjects who are pregnant (positive β-hCG test) or breastfeeding

- Subjects judged to be ineligible for this study by the principal investigator or
sub-investigator.