CDK4/6 inhibition with endocrine therapy is now a standard of care for advanced estrogen receptor positive breast cancer. Mechanisms of CDK4/6 inhibitor resistance have been described pre-clinically, with limited evidence from clinical samples. We conducted paired baseline and end of treatment circulating tumor DNA sequencing from 195 patients in the PALOMA-3 randomized phase III trial of palbociclib plus fulvestrant versus placebo plus fulvestrant. We show that clonal evolution occurs frequently during treatment, reflecting substantial sub-clonal complexity in breast cancer that has progressed after prior endocrine therapy. RB1 mutations emerged only in the palbociclib plus fulvestrant arm and in a minority of patients (6/127, 4.7%, p=0.041). New driver mutations emerged in PIK3CA (p=0.00069) and ESR1 after treatment in both arms, in particular ESR1 Y537S (p=0.0037). Evolution of driver gene mutations was uncommon in patients progressing early on palbociclib plus fulvestrant but common in patients progressing later on treatment. These findings inform future treatment strategies to address resistance to palbociclib plus fulvestrant.
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