Birociclib

Primary objective: To compare the efficacy of pirocitabine tablets combined with letrozole/anastrozole and placebo combined with letrozole/anastrozole in the treatment of subjects with HR-positive, HER2-negative locally advanced, recurrent or metastatic breast cancer. Secondary objective: To compare the efficacy, safety and population pharmacokinetic characteristics of pirocitabine tablets combined with letrozole/anastrozole and placebo combined with letrozole/anastrozole in the treatment of subjects with HR-positive, HER2-negative locally advanced, recurrent or metastatic breast cancer. Exploratory objective: To evaluate patient-reported outcomes (PROs) of disease- or treatment-related symptoms in subjects treated with pirocitabine tablets combined with letrozole/anastrozole versus placebo combined with letrozole/anastrozole, including quality of life scores and health status scores.