Cystatin C Testing Helps Distinguish Abemaciclib Effect from Kidney Injury

Kidney function monitoring in breast cancer care often relies on serum creatinine, a routine blood marker used to estimate renal function. However, creatinine increases in patients receiving abemaciclib for hormone receptor-positive breast cancer can be difficult to interpret and may prompt dose reductions, treatment interruptions, or additional testing. To help clarify these findings, a new study shows that baseline cystatin C measurement can distinguish abemaciclib-related creatinine changes from possible kidney damage.

Kanazawa University (Kanazawa, Japan) researchers, working with Shinshu University (Matsumoto, Japan), evaluated baseline cystatin C measurement to help interpret renal marker changes in patients receiving abemaciclib. The drug is known to raise serum creatinine, which is eliminated in part through transporter proteins in the kidney. The study suggests that this increase may reflect blocked transport rather than kidney injury.

The researchers followed 63 patients, including 62 women and one man, at Shinshu University Hospital. Creatinine increased in 62 patients, rising from a median of 0.63 to 0.85 mg/dL. Levels remained stable through 24 weeks of treatment and declined after treatment was interrupted. Among 29 patients who underwent cystatin C testing before and during treatment, the two markers moved in opposite directions in 28 patients.

This difference is relevant because cystatin C is not secreted by the same transporters as creatinine and is therefore less affected by abemaciclib. However, the markers already disagreed before treatment in 15 of the 29 patients, or 52%. The team also examined SLC47A1, the gene encoding the MATE1 transporter, and found that patients with the GG type of the rs2289669 variant tended to have a larger creatinine increase. This exploratory finding requires confirmation in larger studies. The study was published in Clinical Pharmacology & Therapeutics.

“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.

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