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A selective CDK4 inhibitor shows promise after standard therapy fails. The phase 2 FOURLIGHT-1 trial found that atirmociclib plus fulvestrant cut the risk of disease progression or death by 40% compared to standard options in patients with HR+/HER2– advanced breast cancer who had already progressed on a CDK4/6 inhibitor. Median PFS improved from 6.6 to 9.4 months, offering a meaningful new option in a tough-to-treat setting.
A new CDK4 inhibitor combo shows real promise for patients who've run out of standard options. The phase 2 FOURLIGHT-1 trial tested atirmociclib — a highly selective CDK4 inhibitor — paired with fulvestrant in patients with hormone receptor–positive, HER2-negative advanced breast cancer whose disease had already progressed on a CDK4/6 inhibitor. Results, presented at the 2026 ESMO Congress, showed a meaningful improvement in progression-free survival (PFS) over investigator's choice of therapy.
What makes atirmociclib stand out is its up to 33-fold greater selectivity for CDK4 over CDK6 compared to currently approved agents. Since CDK4 drives tumor activity in estrogen receptor–positive disease while CDK6 inhibition is linked to neutropenia, this selectivity could mean better efficacy with a more manageable side effect profile — though neutropenia was still the most common adverse event reported.
By the Numbers:
Why it matters: After progression on CDK4/6 inhibitors, treatment options are limited and outcomes are poor. Atirmociclib's results in this second-line setting are encouraging, and a phase 3 trial (FOURLIGHT-3) is already fully enrolled to test it in the first-line setting.
