Background: CDK4/6 inhibitors (CDKIs) are now standard of care for hormone receptor positive (HR+) metastatic breast cancer (MBC). However, there is no consensus on the effective subsequent treatment for patients refractory to CDKIs. This study aimed to describe the treatment outcomes of patients with HR+ MBC who received subsequent treatment after CDKIs.
Patients and methods: We retrospectively identified 51 patients with HR+ MBC who received CDKIs between December 1, 2017 and January 31, 2020 in Keio University Hospital, and focused on the population who received another line of therapy after CDKIs.
Results: Twenty-eight patients, with a median age of 68 years (range 37-82) and a follow-up of 18.6 months, were available for outcome analysis. The median number of prior treatment regimens for metastatic disease before CDKIs was 2 (range 0-12). Twenty-five and 3 patients received palbociclib and abemaciclib, respectively. The median TTF for CDKIs was 5.6 months and the median TTF for the subsequent therapy after CDKIs was 9.9 months. Eribulin was the most common treatment following CDKIs (n=9), S-1 in 5 patients, paclitaxel and bevacizumab (PTX+BEV) in 4 patients, and ET, including concomitant use of molecularly targeted agents, in 10 patients. The TTF for CDKIs per following treatment regimen was 10.6 months in the S-1 group, 6.6 months in the PTX+BEV group, 4.8 months in the eribulin group, and 3 months in the ET group. Comparing median TTFs for each of the following treatment regimens, excluding censored patients, was 9.9 months in the S-1 group (n=3), 9.4 months in the PTX+BEV group (n=4), 3.7 months in the eribulin group (n=7), and 4.0 months in the ET group (n=3).
Discussion: S-1 tended to be used as the subsequent therapy in cases with long TTFs on CDKIs, whereas ET was chosen as the subsequent treatment for patients with short TTFs on CDKIs, reflecting the impact of early discontinuation due to adverse events on CDKIs.