Background: PALLET randomized 307 postmenopausal women with ER+ primary breast cancer to one of 4 treatment groups (3:2:2:2 ratio): A: L for 14wks; B: L for 2wks then L+P to 14wks; C: P for 2wks then L+P to 14wks; D: L+P for 14wks. This allowed a randomized 1:2 comparison of L (Group A) vs L+P (Groups B+C+D) at 14wks. P was given 125mg/d PO (21 days on, 7 days off). Adding P to L markedly enhanced Ki67 suppression and Complete Cell Cycle Arrest (CCCA, Ki67 < 2.7%) by 14wks but did not substantially increase clinical response. We now report exploratory analysis of the association of baseline expression of 6 pre-specified biomarkers involved in estrogen and CDK4/6 signaling with CCCA at 14wks and changes in their expression during therapy. Methods: Estrogen receptor (ER), progesterone receptor (PgR), RB and CCNE1 were measured by IHC and CCND1 by IHC and FISH (CCND1/CEP11 ratio≥2.0 amplified). Baseline biomarker values were available with 14wk Ki67 values in up to 64 patients for L alone and up to 124 patients for L+P. Of these 59% and 90%, respectively, achieved CCCA. Results: With L alone CCCA was significantly less frequent (indicating relative resistance) with low baseline PgR (odds ratio [OR] 0.22, 95%CI 0.05-0.96, p = 0.04) or high CCNE1 levels (OR 10.39, 95%CI 1.19-90.48, p = 0.03). With L+P CCCA was also significantly less frequent with high CCNE1 (OR 50.34 95%CI 5.12-495.34, p = 0.001) or with low baseline ER (OR 0.21 95%CI 0.08-0.60, p = 0.004). CCCA was not significantly different with either treatment according to CCND1 amplification status or expression overall. However, CCCA showed a tendency to being less frequent in non-amplified cases with low baseline cyclin-D1 expression when treated with L+P (p = 0.10). There were no significant changes in ER levels or CCND1 amplification over 14wks. By 14 wks PgR, RB, CCND1 and CCNE1 levels were significantly suppressed by L or L+P (geomeans PgR: -96.4% vs -94.9%; CCND1: -79.9% vs -70.7%; CCNE1: -68.2% vs -74.7%; RB: -23.5% vs 26.1%, respectively) and there was no significant difference between the treatments. Conclusions: These data support low ER, possibly indicating limited luminal status, and high CCNE1 as markers of poor Ki67 response to L+P in primary disease and are consistent with findings in studies in advanced disease. Clinical trial information: NCT02296801