A phase Ib study of xentuzumab plus abemaciclib and fulvestrant in patients (pts) with advanced hormone receptor-positive (HR+), HER2-negative breast cancer (BC) with visceral or non-visceral disease.

Abstract

1057 Background: Cyclin-dependent kinase (CDK) 4 & 6 inhibitors plus endocrine therapy (ET) are standard of care for advanced HR+ BC. Combining xentuzumab, an insulin-like growth factor (IGF) ligand-neutralizing antibody, with ET and everolimus, suggested progression-free survival (PFS) benefit in pts with advanced HR+ BC and non-visceral disease. Activation of the IGF pathway leads to an increase in cyclin D1, providing a rationale for combining IGF and CDK4 & 6 inhibition. This prospective, open-label study is investigating xentuzumab plus abemaciclib, a CDK4 & 6 inhibitor, with fulvestrant. In dose-finding cohorts, the recommended phase II dose (RP2D) was determined as xentuzumab 1000 mg weekly intravenously plus abemaciclib 150 mg every 12h orally (Q12h). Here, we report preliminary data on disease control rate (DCR) from two expansion cohorts in pts with advanced HR+ BC with visceral (D1) or non-visceral disease (D2). Methods: Postmenopausal women with advanced/metastatic HR+ BC that had progressed on or after ET (including adjuvant ET) were enrolled. Pts could not have received >1 line of ET or any chemotherapy for metastatic disease. No prior CDK4 & 6 inhibitor therapy was permitted. Pts had to have ≥1 documented visceral metastasis in D1 and no visceral metastases in D2. Pts received xentuzumab weekly plus abemaciclib Q12h (at RP2D) plus fulvestrant 500 mg per label. Protocol primary endpoint was PFS rate at 18 months (mos). Secondary endpoints included DCR (complete response [CR], partial response [PR] and non-CR/non-progressive disease [PD] or stable disease [SD] lasting ≥24 weeks [wks]). Results: In D1/D2, 33/31 pts were treated: median age 60/53 years. 19 pts in D2 had bone-only, non-measurable disease. At data cut-off (Jan 2021), median treatment duration was 7.5/9.2 mos in D1/D2; 40 pts remain on treatment. In D1, DCR was 64%: 17 (52%) pts had PR and 4 (12%) had SD lasting ≥24 wks. In D2, DCR was 55%: 5 (16%) pts had PR, 10 (32%) had non-CR/non-PD lasting ≥24 wks and 2 (7%) had SD lasting ≥24 wks. Median duration of disease control was 10.9 mos in each cohort. Some pts had not reached 24 wks; full DCR data will be presented. Most common AEs are shown in the table (≥33% all-grade in either cohort). No hypo/hyperglycemia was reported. Conclusions: Xentuzumab plus abemaciclib and fulvestrant demonstrated encouraging disease control in pts with advanced HR+ BC with visceral and non-visceral disease. The safety profile was manageable, and consistent with the known profiles of the three agents. Clinical trial information: NCT03099174 .[Table: see text]