307 patients were randomized to LET (n=103) or LET+PALBO (n=204) for 14 wks...At the individual gene level, genes significantly associated with non-CCCA after 14wks LET+PALBO included CCNE1, CDK2 and several E2F-related genes (p<0.05)….PALBO resistance was associated with higher baseline expression of cyclin-E1 (both IHC and RNA), CDK2, and genes related to E2F, MYC, interferon and MTOR signalling. These results suggest that multiple identifiable mechanisms of de novo resistance to PALBO are likely to exist in primary ER+ BC.

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).