Longitudinal comparison of serum creatinine, estimated glomerular filtration rate, and blood urea nitrogen-to-creatinine ratio after initiation of palbociclib or abemaciclib: A single-center retrospective study

Introduction Abemaciclib inhibits renal tubular creatinine secretion and can increase serum creatinine without a true reduction in glomerular filtration rate. Direct real-world comparisons with palbociclib using routinely available laboratory markers are limited. Methods We conducted a single-center retrospective study of 56 women with advanced or recurrent hormone receptor-positive, human epidermal growth factor receptor 2-negative breast cancer who initiated palbociclib (n = 29) or abemaciclib (n = 27). Data from January 1, 2018, to June 30, 2026, were extracted. Serum creatinine, creatinine-based estimated glomerular filtration rate (eGFR), blood urea nitrogen (BUN), and the BUN/creatinine ratio were evaluated at baseline and Days 14, 28, 42, and 56 using adjusted generalized estimating equations. Results Drug-by-time interactions were significant for serum creatinine ( P = 0.004), eGFR ( P < 0.001), and the BUN/creatinine ratio ( P = 0.001), but not BUN ( P = 0.080). At Day 14, the adjusted between-group difference in change (abemaciclib minus palbociclib) was 0.180 mg/dL for serum creatinine and −11.4 mL/min/1.73 m 2 for eGFR. The BUN/creatinine ratio difference was not significant at Day 14 but was significant at Days 28 (−5.12), 42 (−4.11), and 56 (−3.83). Sensitivity analyses yielded consistent interaction results. Conclusions Abemaciclib showed greater and more persistent changes in serum creatinine, creatinine-based eGFR, and the BUN/creatinine ratio than palbociclib. Serial BUN and BUN/creatinine ratio measurements may provide supportive context for creatinine elevations but cannot exclude true kidney injury.