A routine blood test can make a kidney look like it is failing when it is not. A team from Kanazawa University and Shinshu University has shown that this happens often in patients with breast cancer who take abemaciclib, and that a simple baseline measurement can help doctors tell the difference.
Abemaciclib is widely used for hormone receptor–positive breast cancer. It is known to raise serum creatinine, the standard marker used to estimate kidney function. Because creatinine is also pushed out of the body by transporter proteins in the kidney, the rise may reflect blocked transport rather than damaged kidneys. In practice, however, a higher creatinine can still lead to dose reductions, treatment interruptions or extra tests.
The researchers followed 63 patients (62 women and one man) at Shinshu University Hospital. Creatinine rose in 62 of the 63 patients, from a median of 0.63 to 0.85 mg/dL, and the rise stayed stable through 24 weeks of treatment. It fell back after treatment was interrupted. The study is published in the journal Clinical Pharmacology & Therapeutics.
In the 29 patients whose blood had been tested for cystatin C both before and during treatment, cystatin C showed a different pattern. In 28 of them, creatinine and cystatin C moved in opposite directions. Cystatin C is a protein that is not secreted by the same transporters, so it is less affected by the drug.
Another finding was that, in 15 of the 29 patients (52%), the two markers already disagreed before treatment began. Without a baseline cystatin C, such a gap can be mistaken for a drug effect or for kidney damage.
The team also looked at a gene for the transporter MATE1 (SLC47A1). Patients with the GG type of the rs2289669 variant, which is thought to keep the transporter fully active, tended to show a larger creatinine rise. This result is exploratory and needs confirmation in larger studies.
“When creatinine rises after starting abemaciclib, it does not necessarily mean the kidneys are failing. What surprised us was that, in about half of the patients we could examine, creatinine and cystatin C already disagreed before treatment began. We therefore recommend measuring cystatin C at baseline, so that each patient has a personal reference point and can stay on an effective therapy without unnecessary dose reductions or interruptions,” said Hiroaki Watanabe, assistant professor, Faculty of Pharmacy, Kanazawa University.
More information
Hiroaki Watanabe et al., Creatinine–Cystatin C Discrepancy and Renal Transporter Polymorphisms in Japanese Patients with Breast Cancer Receiving Abemaciclib, Clinical Pharmacology & Therapeutics (2026). DOI: 10.1002/cpt.70490
Citation:
Breast cancer drug raises creatinine without necessarily signaling kidney damage, study finds (2026, October 30)
retrieved 1 November 2026
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