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Cancer cells proliferate using various mechanisms. One mechanism of preventing tumor cell growth is blockade of the cyclin-dependent kinase (CDK) 4/6 axis. Multiple CDK 4/6 inhibitors - ribociclib, palbociclib, and abemaciclib - have significantly improved progression-free survival rates. However, they can cause hepatotoxicity. We present a case of a 67-year-old female who was diagnosed with stage 1C invasive ductal carcinoma. She was treated with letrozole and ribociclib due to recurrence as metastatic disease, but within 10 days, she developed transaminitis. She then started palbociclib but experienced elevated transaminases within two weeks, needing discontinuation of palbociclib. Subsequent positron-emission tomography/computed tomography imaging showed disease progression, and she was started on fulvestrant. We considered adding abemaciclib, but the patient declined and has had stable disease for more than a year on fulvestrant. CDK 4/6 inhibitors are used to treat metastatic breast cancer and are generally well tolerated. The most common side effect is neutropenia; however, our patient developed transaminitis. The novelty of our case is the development of hepatotoxicity even after the introduction of another CDK 4/6 inhibitor, indicating at least some degree of class effect. In summary, CDK 4/6 inhibitors have significantly improved outcomes in hormone-positive metastatic breast cancers. However, a small percentage suffer from hepatic injury enough to warrant discontinuation of the drug, and we must continue to assess the risk versus benefit profile when offering them to our patients.

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