The primary objective of the open-label, dose-escalation study is to investigate the safety of single-dose monotherapy and repeated-dose of KHK2375 combined with exemestane in female subjects with advanced or recurrent breast cancer. The secondary objective is to investigate the pharmacokinetics and efficacy.

Inclusion Criteria:

• Postmenopausal women aged ≥ 20 and < 75 years at the time of consent;
• Estrogen receptor positive and/or progesterone receptors positive;
• HER2-negative
• Nonresectable advanced or recurrent breast cancer previously treated with nonsteroidal aromatase inhibitor (AI), and planning to be treated with exemestane

Exclusion Criteria:

• Radiation therapy or immuno therapy within 14 days before the first dose of investigational product;
• Chemotherapy, biological medicines, other pharmacotherapy or major surgery within 21 days before the first dose of the investigational product;
• Prior chemotherapies of ≥ 3 regimens for advanced or recurrent breast cancer;
• Ongoing treatment with other investigational product