This is a multicenter, open, single arm clinical study to evaluate the safety, tolerability, and antitumor activity of FCN-437c in combination with letrozole in postmenopausal women with ER + / HER2 - advanced breast cancer, and to evaluate the PK characteristics of FCN-437c monotherapy and combined therapy.~The single drug administration period (7 days) . The continuous administration period made up of 21 days of continuous administration, followed by 7 days of withdrawal, which made up of 28 days as a treatment cycle. The evaluation was conducted every 8 weeks until one of the following happened, disease progression, intolerable toxicity, death, the researcher's decision or the patients' voluntary withdrawal from the study. The follow-up visit was conducted 30 days after the last administration. The telephone follow-up was conducted once every 3 months until the end of the study to record the survival period.~In the expansion period, FCN-437c was continuous administration per day for 21 days, followed by 7 days of withdrawal, making a treatment cycle of 28 days during which letrozole was continuously administrated 2.5 mg QD. Evaluation was conducted every 8 weeks until one of the following occurred, disease progression, intolerable toxicity, death, decision of the researcher or patients' voluntary withdrawal of the study. Follow up visit was conducted 30 days after the last administration, followed by the survival period telephone follow-up every 3 months until the end of the study.~End of of the study was defined as the last patient in the dose expansion stage took the treatment for more than one year, or terminated the treatment (depending on which occurred earlier.~At the end of the study, patients with no disease progression were determined to continue taking FCN-437c according to the clinical benefits.
This is a multicenter, open, single arm dose escalation and dose expansion clinical study to evaluate the safety, tolerability, pharmacokinetics, and antitumor activity of FCN-437c alone or in combination with letrozole in women with ER +/ HER2 - advanced breast cancer.