Abstract P4-01-20: Circulating tumor DNA as a predictive biomarker of response to palbociclib-fulvestrant in patients with estrogen receptor-positive, HER2-negative metastatic breast cancer

Abstract

Abstract Background: Following the PALOMA-3 study results, the combination of palbociclib, a cdk4/6 inhibitor, with fulvestrant has become a standard therapy in women with estrogen receptor-positive (ER+) HER2-negative (HER2-) metastatic breast cancer (MBC) whose disease progressed under hormone therapy. Palbociclib increased the progression-free survival (PFS) in all patient subgroups, and no predictive biomarker of palbociclib efficacy has been validated so far. In that context, we evaluated whether early changes of circulating tumor DNA (ctDNA) levels are associated with the efficacy of palbociclib plus fulvestrant. Methods: After providing written informed consent, ER+ HER2- MBC patients were included in a prospective observational cohort (“ALCINA”, NCT02866149) prior to the initiation of palbociclib plus fulvestrant. Tumor response was assessed by a radiological evaluation (RECIST v1.1) every 3 months. Fresh plasma samples were collected at baseline, before the initiation of treatment, at day 15, at day 30 and at disease progression. For each patient, DNA from archived tumor tissue was subjected to targeted NGS to identify a driver mutation. In patients with identified driver mutation, circulating cell-free tumor DNA extracted from plasma was subjected to digital droplet PCR (ddPCR). Ratios of ctDNA levels (J15/baseline and J30/baseline) were correlated with prospectively registered patient characteristics and outcomes. For the current analysis, “responders” were defined as patients who experienced a PFS longer than 3 months. Results: 61 MBC patients have been included and 26 were eligible for ctDNA mutation detection, after the characterization of single nucleotide variations in PIK3CA (N=21 patients), TP53 (N=3) and AKT (N=2). At baseline, 21 patients (81%) had detectable ctDNA levels, with a median level of ctDNA of 381 copies/ml of plasma; There was no significant correlation between ctDNA levels at baseline and PFS (p=0,06). ctDNA levels decreased under therapy, with 82% and 68% of patients displaying detectable levels of ctDNA at day 15 and day 30. Patients with undetectable ctDNA levels at day 15 and, principally, at 30 were more likely to experience a PFS > 3 month (HR=5,8, 95% IC=1,5-22,5, p=0.004). We explored ctDNA level ratios (day 15/baseline and day 30/baseline) and found that all patients whom ctDNA increases at day 30 (31%) will not respond to treatment. Moreover, among all patients, ctDNA detection at day 30 is correlated with shorter PFS [HR=2,634 IC95 (1,1 to 6,5)]. At time of tumor progression, all patients presented increased ctDNA levels. In addition to tumor tissue sequencing, activating ESR1 mutations were found at baseline in 5 patients (19%). Conclusion: Our study suggests that the efficacy of palbociclib and fulvestrant may be monitored by serial analyses of ctDNA levels, before radiological evaluation and that early ctDNA levels and dynamics are prognostics factors for PFS. A large randomized trial, PADA-1 (NCT03079011), is currently testing the utility of real time resistant subclones detection in ctDNA from ER+ HER2- MBC treated with palbociclib and aromatase inhibitor. Citation Format: Darrigues L, Pierga J-Y, Bernard A, Bièche I, Silveira A, Michel M, Sablin M-P, Cottu P, Dubot C, Geiss R, Ricci F, Proudhon C, Bidard F-C. Circulating tumor DNA as a predictive biomarker of response to palbociclib-fulvestrant in patients with estrogen receptor-positive, HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-01-20.