Interim analysis (IA) of the giredestrant (G) + everolimus (EVERO) arm in MORPHEUS Breast Cancer (BC): A phase I/II study of G treatment (tx) combinations in patients (pts) with estrogen receptor-positive (ER+), HER2-negative, locally advanced/metastatic BC (LA/mBC).

1059 Background: Endocrine therapy (ET) + a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) is a therapeutic mainstay for first-line tx of ER+ mBC. Finding effective ET combinations after progression remains a challenge; however, targeting the Akt/mTOR pathway is a promising approach. G is a highly potent, non-steroidal, oral (PO), selective ER antagonist and degrader that is well tolerated and achieves robust ER occupancy. EVERO is an approved mTOR inhibitor. Here, we present a 16-week IA of the G + EVERO arm in MORPHEUS BC (NCT04802759). Methods: Eligible pts with disease progression on 1–2 lines of ET (including a CDK4/6i) for LA/mBC were randomized 1:6 to receive G (30 mg PO daily QD) or G + EVERO (10 mg PO QD) until disease progression (PD)/unacceptable toxicity. The study was not designed to make explicit power and type I error considerations for a hypothesis test. Primary endpoints were safety and objective response rate (ORR); other endpoints included progression-free survival (PFS), overall survival, clinical benefit rate, disease control rate (DCR), duration of response, and pharmacokinetics. Steroid mouthwash was recommended for prophylaxis or treatment. Genetic alterations were defined using baseline circulating tumor DNA. Results: Results for the G arm have been presented previously and are not included here. As of July 24, 2023, 15 pts were enrolled in the G + EVERO arm: 66.7% and 33.3% received 1–2 prior lines of therapy for LA/mBC, respectively; 26.7% had prior fulvestrant, and 73.3% had liver metastasis at enrollment. One pt had prior capecitabine. Safety data are in the table. Confirmed ORR in the G + EVERO arm was 26.7% (n = 4). Partial responses were reported in 26.7% of pts (n = 4; all had ESR1 mutations, 1 also had an Akt pathway mutation), 46.7% of pts (n = 7) had stable disease, and the DCR was 53.3% (n = 8). Median PFS was not reached at data cutoff. No clinically relevant drug–drug interactions were observed. Conclusions: Safety of G + EVERO was aligned with the individual safety profile of each drug with no overlapping toxicities or new safety signals seen. An encouraging efficacy signal was observed with G + EVERO in pts with PD on 1–2 lines of ET (including a CDK4/6i). G + EVERO is being further investigated in the Phase III evERA BC trial (NCT05306340). Clinical trial information: NCT04802759 . Table: see text