Supplementary Material for: Circulating glypican-4 is a predictor of 24-month overall survival in metastatic breast cancer

Background & objective: Endocrine treatment combined with CDK4/6 inhibitors is the preferred treatment strategy in patients presenting with ER-positive/HER2-negative breast cancer, but the clinical course remains highly variable among individual patients. There is an unmet need for prognostic or predictive biomarkers in this important group of patients. Recently, we have identified circulating glypican-4 (GPC4) as a new biomarker of inferior outcomes in patients with metastatic colorectal cancer. The impact of plasma GPC4 levels on the survival of breast cancer patients is unknown and has been addressed in the present study. Methods: Our study included 47 patients with ER-positive/HER2-negative metastatic breast cancer prior to treatment with CDK4/6 inhibitors combined with endocrine therapy. The endpoint was overall survival (OS) at 24 months. GPC4 levels were measured in plasma using an enzyme-linked immunosorbent assay. Results: Increased circulating GPC4 levels were significantly linked to advanced age, postmenopausal state, visceral metastases, and invasive lobular carcinoma. During the two-year observational follow-up period, 25.5 % of patients died. The area under the receiver operating characteristic curve (ROC-AUC) analysis revealed an AUC of 0.713 [0.555-0.871]; P=0.029 for OS and an optimal cut-off value of GPC4 for predicting OS of 4.77 ng/mL. No patient showing GPC4 values below this cut-off died during the observational period. Cox regression analysis showed a hazard ratio of 2.14 [95% confidence interval: 1.24-3.67]; P=0.006 for one standard deviation change of plasma GPC4. Conclusions: In conclusion, our study suggests circulating GPC4 as a significant predictor of poor survival in metastatic breast cancer patients.