Abstract P4-01-18: Real-world second-line treatment patterns and associated clinical outcomes for 2795 patients with advanced HR+ HER2- breast cancer treated with first-line CDK4/6 inhibitors
Authors: Clifton Katherine K. , Wander Seth A. , Ma Cynthia , Davis Andrew A. , Weipert Caroline , Zhang Nicole , Bucheit Leslie 
Published: March 2023

Abstract
Background: CDK4/6 inhibitors (CDK4/6i) are standard first-line (1L) regimens for HR+/HER2- advanced breast cancer (aBC). Recent data from the randomized phase II MAINTAIN trial reported a PFS benefit for patients (pts) who received a new endocrine therapy plus ribociclib (ribo) versus new endocrine therapy alone) following progression on CDK4/6i as compared to pts who received endocrine therapy alone. However, second-line (2L) treatment patterns and patient outcomes following 1L CDK4/6i are relatively undescribed. Here we describe real-world 2L treatment patterns following 1L CDK4/6i and associated clinical outcomes from a large clinical genomics database.
Methods: Real-world evidence (RWE) was sourced from the GuardantINFORM (Guardant Health) database which comprises aggregated commercial payer health claims and de-identified records from over 207,000 pts with comprehensive circulating tumor DNA (ctDNA) results via Guardant360 (G360) from 2014 to 2021. Pts with HR+/HER2- aBC with &gt;1 claim of CDK4/6i and &gt;1 claim for treatment after the index G360 test were included. Real-world time to treatment discontinuation (rwTTD) and real-world time to next treatment (rwTTNT) were assessed in months as proxies for progression-free survival. Real-world overall survival (rwOS) was also reported in months.
Results: 2,795 pts met criteria for inclusion; 2,361 (84.5%) were treated with 1L palbociclib (palbo), 271 (9.7%) with 1L abemaciclib (abema) and 163 (5.8%) with 1L ribo. Chemotherapy (chemo,35.5%) and endocrine-only therapy (32.8%) were the most common 2L therapy regardless of the 1L CDK4/6i agent (Table 1). Other 2L agents included endocrine backbone change (14.7%) or CDK4/6i change (7.7%). Endocrine backbone changes were observed more frequently (15.6%) in pts receiving 1L palbo while CDK4/6i changes were more frequent in pts receiving abema (14.0%) or ribo (22.0%). Pts treated with 2L CDK4/6i had improved rwTTNT, rwTTD and rwOS compared to 2L chemo regardless of 1L agent [rwTTNT: 10.2 (95% CI: 7.2-11.7) vs. 7.2 (6.5-8.1); rwTTD: 6.8 (95% CI: 5.8-8.5) vs. 4.3 (95% CI:3.9-4.7); rwOS: NR (95% CI: 40.0-NR) vs. 34.8 (95% CI: 31.3-37.2)]; improvement in rwTTNT, rwTTD and rwOS were also observed for pts with 2L endocrine backbone changes compared to chemo [rwTTNT: 8.5 (95% CI: 7.2-9.6) vs. 7.2 (6.5-8.1); rwTTD: 6.9 (95% CI: 5.8-7.9) vs. 4.3 (95% CI:3.9-4.7); rwOS: 63.4 (95% CI: 51.2-NR) vs. 34.8 (95% CI: 31.3-37.2)]. Pts treated with 2L alpelisib had the shortest rwOS regardless of 1L CDK4/6i agent used [any 1L: NR (95% CI: 23.6-NR)].
Conclusions: A variety of 2L regimens following 1L CDK4/6i were observed, with an improvement in rwTTNT, rwTTD and rwOS in pts receiving 2L CDK4/6i or 2L endocrine backbone change only relative to 2L chemo. These data are hypothesis generating, and the observed improvement may be secondary to therapy choice versus pts who received 2L chemo having more aggressive disease. Larger randomized trials are ongoing to study sequencing and efficacy of 2L treatments following 1L CDK4/6i.
Table 1. Distribution of 2L therapies by 1L CDK4/6i agent.
Citation Format: Katherine K. Clifton, Seth A. Wander, Cynthia Ma, Andrew A. Davis, Caroline Weipert, Nicole Zhang, Leslie Bucheit. Real-world second-line treatment patterns and associated clinical outcomes for 2795 patients with advanced HR+ HER2- breast cancer treated with first-line CDK4/6 inhibitors [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-01-18.