CDK4 and breast cancer: <h4>Background</h4>This study evaluates the prognostic impact of the co-administration of proton pump inhibitors (PPI) on treatment outcomes in patients with hormone receptor-positive (HR+) metastatic and recurrent breast cancer who were receiving CDK4/6 inhibitor(CDK4/6i) treatment, providing real-world evidence to guide treatment strategies.<h4>Materials and methods</h4>This retrospective analysis included 359 patients with HR+ metastasis and relapse breast cancer treated between 2018 and 2023.