183PDNeoadjuvant endocrine therapy with palbociclib in patients with high-risk breast cancer

Abstract Background Optimal neoadjuvant therapy for luminal A breast cancer remains a topic of controversy. Methods In the phase 2 PETREMAC trial (NCT02624973), patients with large T2 (>4cm) or locally advanced breast cancers and luminal A characteristics (ER > 50%, HER2- and TP53 WT) received neoadjuvant endocrine therapy (NET) and CDK4/6 inhibition in concert. NET consisted of letrozole (postmenopausals) or tamoxifen + goserelin (premenopausals). Palbociclib (P) was added if Ki67 had decreased 50% on NET alone (CDH1 mutated: 15/20 vs CDH1 WT: 32/66; p = 0.042). Conclusions NET +/- P was effective at reducing cell proliferation and yielded an ORR of 77% in these ER+, HER2 negative breast cancers. NAC was required only among 33% of the patients. CDH1 mutations seem predictive of response to NET in this setting. Clinical trial identification NCT02624973; 2015-002816-34. Legal entity responsible for the study Haukeland University Hospital, Bergen, Norway. Funding Det Regionale Samarbeidsorganet/Helse Vest, Pfizer. Disclosure P.E. Lønning: Honoraria (self), Advisory / Consultanc