Baseline vitamin D levels and progression-free survival in patients with HR+/HER2−metastatic breast cancer treated with ribociclib: a retrospective single-center analysis

BACKGROUND: Vitamin D has been implicated in cancer biology and immunity, yet its prognostic relevance in patients receiving CDK4/6 inhibitors remains unclear. METHODS: We retrospectively analyzed 89 patients with HR+/HER2 - metastatic breast cancer (MBC) treated with ribociclib plus endocrine therapy between January 2018 and December 2023 at a single institution. Baseline serum 25-hydroxyvitamin D3 [25(OH)D3] levels obtained prior to treatment initiation were categorized as deficient (≤ 20 ng/mL) or sufficient (> 20 ng/mL). The primary endpoint was progression-free survival (PFS). Survival outcomes were assessed using Kaplan-Meier estimates, log-rank tests, and Cox proportional hazards models. RESULTS: The median baseline vitamin D level was 19 ng/mL. The overall median PFS was 36 months. Patients with sufficient vitamin D had not reached median PFS, whereas those with deficiency had a median PFS of 14 months (95% confidence interval (CI), 12.6-15.3; p < 0.001). At 24 months, PFS was 100% in the sufficient group versus 23.1% in the deficient group. In multivariate Cox analysis, baseline vitamin D deficiency remained associated with inferior PFS (HR 32.33, 95% CI 6.90-151.5; p < 0.001), although the wide confidence intervals should be interpreted cautiously given the limited number of progression events in the vitamin D-sufficient subgroup. Other clinicopathological variables did not show statistically significant associations with PFS. CONCLUSION: Baseline vitamin D status was associated with progression-free survival outcomes in patients with HR+/HER2 - metastatic breast cancer receiving ribociclib-based therapy. Patients with sufficient vitamin D levels demonstrated longer progression-free survival compared with those with vitamin D deficiency. However, given the retrospective design, limited sample size, residual confounding, and substantial censoring, these findings should be interpreted cautiously and considered exploratory and hypothesis-generating. Larger prospective studies are required to validate these observations and to clarify the potential clinical relevance of baseline vitamin D status in patients treated with CDK4/6 inhibitors.