Longitudinal Changes in the Blood Urea Nitrogen-to-Creatinine Ratio After Initiation of Abemaciclib: A Single-Center Retrospective Observational Study

Abstract

Serum creatinine (Cr) elevation is frequently observed after the initiation of abemaciclib treatment. Because abemaciclib inhibits tubular creatinine secretion, Cr may increase without a true decline in glomerular filtration rate. In Japan, cystatin C is not routinely measured in many clinical settings, highlighting the need for practical markers based on standard laboratory tests. Therefore, we investigated longitudinal changes in the blood urea nitrogen-to-creatinine (BUN/Cr) ratio after abemaciclib initiation in a single-center retrospective observational study. A total of 51 female patients who received abemaciclib were included in this study. The BUN/Cr ratio was assessed at baseline (Day 0) and follow-up time points, and the early change (ΔBUN/Cr) was defined as the difference between Day 14 and baseline. Subgroup analyses were performed using two creatinine-elevation thresholds: ≥ 0.3 mg/dL (absolute increase) and ≥ 1.25-fold (relative increase) from baseline. The BUN/Cr ratio decreased significantly after the initiation of abemaciclib, with a median decline from 21.29 at baseline to 17.65 at Day 14 (P < 0.001), and the decrease persisted through Day 28 and Day 56. The median ΔBUN/Cr was −4.39, and 76.5％ of the patients showed a negative ΔBUN/Cr. Similar decreases were observed among patients with elevated creatinine levels, and the findings were consistent in sensitivity analyses excluding patients who received intravenous fluids or systemic corticosteroids. Longitudinal assessment of the BUN/Cr ratio may provide practical supportive information for interpreting Cr elevations that may not reflect true renal dysfunction in routine clinical practice, where cystatin C measurement is not readily available.