Clinical efficacy and safety profile of palbociclib (P) in combination with letrozole (L) as first-line treatment in patients (pts) with ER+ and HER2- advanced breast cancer (ABC) who have not received any systemic treatment (ST): A subgroup analysis of PALOMA-1/TRIO-18.

575 Background: P is an orally active inhibitor of CDK4/6. In a randomized phase II study, P + L significantly prolonged progression-free survival (PFS) vs L alone (20 vs 10 mo; HR = 0.488, P =0.0004; Finn et al, Lancet Oncol, 2015) in ER+/HER2- ABC as first-line treatment. Given that pts who did not receive any systemic treatment (ST) may have different disease biology/course from those treated and relapsed from early stage disease and many pts may initiate endocrine therapy (ET) alone, we investigated the benefit of P + L in this subgroup in the PALOMA-1/TRIO-18 study. Methods: 165 ER+ and HER2- postmenopausal pts who are treatment naive for their ABC were randomized to receive L (N = 81) or P+L (N = 84). A subset of pts did not receive any ST in the adjuvant setting prior to randomization (P+L n = 44, L n = 37). The primary endpoint was investigator-assessed PFS. Tumor assessment was performed every 8 weeks. Tumor tissues were collected for correlative biomarkers. Results: Clinical characteristics at b...