MO58-4 CDK4/6 inhibitors as maintenance therapy after initial chemotherapy: A retrospective single-institute study

The benefit of hormone therapy (ET) and/or capecitabine as maintenance therapy after chemotherapy (CTx) for metastatic breast cancer (MBC) was reported in the KBCSG-TR1214 study [1], similarly, the BOOSTER trial, a phase 2 study, demonstrated the efficacy of hormone therapy as switch maintenance therapy after response to initial chemotherapy [2]. However, the data on the efficacy and safety of ET plus CDK4/6 inhibitor (CDK4/6i) as maintenance therapy are insufficient [3][4][5]. To investigate the safety and efficacy of ET + CDK4/6i as maintenance therapy after initial CTx response in patients with MBC. Among patients who received CDK4/6i for MBC at our center from the approval of palbociclib (PLB) in December 2017 and abemaciclib (ABM) in November 2018 to the end of October 2021, patients who had received CDK4/6i as maintenance therapy after initial CTX response were selected to evaluate patient background, safety, and efficacy of CDK4/6i. Of the 179 patients who received CDK4/6i, 21 patients received ET plus CDK4/6i as maintenance therapy after achieving stable disease or better response to CTx (14 PLB and 7 ABM). The median TTF for ET+CDK4/6i was 6.8 months (PLB 2.0 months, ABM 9.4 months) for maintenance therapy after CTx. The safety profile was similar to that reported previously. ET plus CDK4/6i as maintenance therapy was able to control disease after initial CTx response. Further case series and prospective clinical trials are needed to verify the usefulness of ET+CDK4/6i as maintenance therapy.