Pretreatment Liver Tests Often Overlooked in CDK4/6 Therapy

Study finds most breast cancer patients prescribed CDK4/6 inhibitors do not receive recommended liver function tests before starting treatment.

Published on Mar. 9, 2026

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A cohort study of nearly 16,000 US patients with breast cancer who were prescribed a cyclin-dependent kinase (CDK) 4/6 inhibitor found that only 46% underwent liver function testing in the 2 weeks before starting treatment, despite FDA package insert recommendations. However, most patients (80.9%) did receive liver function testing during the first 2 months of treatment.

Why it matters

Liver function tests are important for patients starting CDK4/6 inhibitor therapy due to the risk of hepatotoxicity. The lack of pretreatment testing raises concerns about potential liver injury in some patients, as 0.3% experienced acute liver injury in the 2 months after starting treatment.

The details

The study used a US claims database to analyze liver function testing patterns in 15,958 breast cancer patients prescribed CDK4/6 inhibitors between 2015-2025. Package inserts for two of the three approved CDK4/6 inhibitors recommend liver function testing before and during treatment, but the researchers found similar low rates of pretreatment testing regardless of which medication was prescribed.

  • The study included patients who began CDK4/6 inhibitor therapy between February 2015 and August 2025.
  • Liver function testing was analyzed during three time periods: 2 weeks prior to treatment initiation, 1 day before or on the day of treatment initiation, and 60 days following treatment initiation.

The players

Carelon Research

A subsidiary of health insurer Elevance Health that conducted the study.

Daniel C. Beachler

The lead author of the study, who is an employee of Carelon Research.

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What they’re saying

“Although liver function tests are commonly performed during treatment, this study found that a majority of patients initiating CDK4/6 therapies in the US did not receive liver function tests prior to treatment initiation as recommended in FDA-approved package inserts.”

— Study authors (JAMA Network Open)

What’s next

The study authors suggest that additional strategies, such as electronic health record prompts, may be needed to encourage timely pretreatment liver function testing for patients prescribed CDK4/6 inhibitors.

The takeaway

This study highlights the need for improved adherence to FDA recommendations for liver function testing before starting CDK4/6 inhibitor therapy for breast cancer patients, in order to monitor for potential hepatotoxicity and ensure patient safety.