Abstract PO3-19-10: Phase 1b study of EZH1/2 inhibitor valemetostat in combination with trastuzumab deruxtecan in subjects with HER2 low/ultra-low/null metastatic breast cancer

Abstract Background Low HER2-expressing breast cancers have traditionally been classified as HER2-negative and treated as TNBC or hormone receptor (HR)-positive. Trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate, composed of an anti-HER2 antibody conjugated to topoisomerase I payload, is approved for clinical use in HER2-low (IHC 1+ or IHC 2+/ISH negative) metastatic breast cancer (MBC). In the DAISY study (NCT04132960), meaningful clinical response was observed with T-DXd in HER2 IHC 0 MBC. Valemetostat is an oral, selective dual inhibitor of enhancer of zeste homolog 1 and 2 (EZH1/2, methyltransferases that specifically methylate histone H3 lysine 27. It is currently approved for patients with relapsed or refractory adult T-cell leukemia/lymphoma in Japan. EZH2-mediated PP2A inactivation has been shown to confer resistance to HER2-targeted therapy. Additionally, valemetostat has been shown to upregulate Schlafen11 (SLFN11), a putative DNA/RNA helicase, that regulates the sensitivity to DNA damaging agents such as topoisomerase I inhibitors. Consequently, this study examines the safety and anti-tumor activity of valemetostat in combination with T-DXd in subjects with HER2 low/ultra-low/null MBC. Trial Design This is a single-arm, phase-1b study to evaluate the safety and clinical activity of T-DXd in combination with valemetostat in patients with HER2 low/ultra-low/null MBC. The dosing for T-DXd is 5.4 mg/kg Q3W administered intravenously as indicated for current clinical use. Valemetostat will be evaluated at three dose levels (100 mg, 150 mg, and 200 mg orally), with starting dose (level 1) at 100 mg QD. The dose-limiting toxicity (DLT) evaluation period will be the first 2 treatment cycles (42 days). Eligibility criteria Specific aims Statistical methods Approximately 12 evaluable patients will be enrolled for the dose-escalation portion based on the Bayesian optimal interval design with a target DLT rate of 25%. Patients enrolled in cohorts of 3. The expansion will be performed at the RDE using the 2-stage Bayesian optimal dose-expansion design. In the first stage, 13 evaluable patients (including those treated at the RDE in the dose-escalation part) will be enrolled. If 5 patients respond in the first stage, the study will be stopped for futility. If ≥ 5 responses are observed, 13 additional evaluable patients will be enrolled. If 11 or more responses are observed among the total of 26 patients the treatment will be regarded as promising. This two-stage design yields 78% power under the alternative hypothesis of ORR=50% (null ORR = 30%) while controlling the one-sided type I error at 10%. Contact information for people with a specific interest in the trial sdamodaran@mdanderson.org (NCT05633979) Citation Format: Senthil Damodaran, Toshiaki Iwase, Angela Marx, Jie Willey, Funda Meric-Bernstam, Debu Tripathy, Carlos Barcenas, Jangsoon lee, Naoto Ueno. Phase 1b study of EZH1/2 inhibitor valemetostat in combination with trastuzumab deruxtecan in subjects with HER2 low/ultra-low/null metastatic breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-19-10.