Genomic markers of early progression on fulvestrant with or without palbociclib for ER+ advanced breast cancer in the PALOMA-3 trial.

1010 Background: Markers of early progression on endocrine therapy in combination with CDK4/6 inhibitors remain limited despite the key role of these combinations in treating of ER+ advanced breast cancer (ABC). We investigated genomic aberrations in patients treated with fulvestrant, with and without palbociclib, with a circulating tumor DNA analysis of baseline plasma in the PALOMA-3 trial. Methods: PALOMA-3 was a phase III trial that randomized 521 patients with ER+/HER2- ABC 2:1 to palbociclib plus fulvestrant (P+F) or placebo plus fulvestrant (F). Using baseline plasma samples, somatic mutations were assessed with a 17 gene panel. Copy number aberrations (CNA) were characterized with a 14 gene panel including ~800 SNPs in 8 commonly altered regions for estimation of tumor purity, the percentage of plasma DNA that was derived from tumor cells. Results for mutations and CNAs were available in 310 pts (203 P+F, 107 F) and were associated with clinical characteristics and progression free survival (PFS) using univariable and multivariable Cox proportional hazards models, including tumor purity and treatment as variables. Results: In the multivariable analysis of the whole cohort, higher baseline tumor purity in plasma was associated with worse PFS (HR 1.20, 95% CI 1.09 – 1.32, p = 0.0001, HR per 10% increase in purity). Baseline TP53 mutation was also associated with shorter PFS (HR 1.84, 95%CI 1.27 – 2.65, p = 0.0011), as was baseline FGFR1 amplification (HR 2.91, 95%CI 1.61 – 5.25, p = 0.0004). PIK3CA and ESR1 mutations had no significant association with PFS in the multivariable model. Palbociclib treatment effect was comparable with the overall trial result (HR 0.43, 95%CI 0.32 - 0.57, p < 0.0001). TP53 mutations were significantly associated with visceral (q = 0.046) and soft tissue/LN metastases (q = 0.042) and the number of disease sites (q = 0.0086) after correction for multiple testing. Conclusions: TP53 mutation, FGFR1 amplification , and tumor purity in plasma identified patients at risk of early progression In PALOMA-3. If validated these results could inform future clinical trials of CDK4/6 inhibitors combinations.