Abstract OT1-03-01: XMT-1660: A Phase 1b trial of a B7-H4 targeted Antibody Drug Conjugate (ADC) in Breast, Endometrial, and Ovarian Cancers

Abstract Background: Breast cancer (BC) is the most commonly diagnosed cancer and one of the leading causes of cancer death in women. Despite significant therapeutic advances, the majority of patients with unresectable or recurrent/metastatic disease eventually develop resistance to available standard of care (SOC) therapies. B7-H4 is a poor prognostic factor and is overexpressed in several cancers including endometrial, ovarian, and breast. As a member of the CD28/B7 family of cell surface proteins, it promotes tumorigenesis by suppressing anti-tumor immunity. XMT-1660 is a B7-H4-targeted Dolasynthen antibody drug conjugate with a precise, optimized drug-to-antibody ratio and a DolaLock microtubule inhibitor payload with controlled bystander effect. In the preclinical setting, XMT-1660 has demonstrated anti-tumor activity in TNBC and ER+/HER2- patient-derived xenograft mouse models, which included tumors from heavily pre-treated patients (Collins et al, AACR 2022). Increased anti-tumor activity tended to be more frequent in models with higher B7-H4 expression, providing rationale for a Ph1 clinical trial. Methods: The Ph1 trial includes a first-in-human open-label dose escalation (DES) portion followed by dose expansion (EXP) evaluating XMT-1660 in patients with BC, EC, and OC following progression on SOC as applicable (i.e., CDK4/6i + ET; platinum-based chemotherapy). In the DES, Bayesian Optimal Interval (BOIN) design will be used to determine the MTD. Patients will receive XMT-1660 IV Q3 weeks. Primary endpoints in DES are to assess safety and determine a recommended phase 2 dose (RP2D) and assessment of preliminary efficacy as a secondary endpoint. In the EXP portion, cohorts enrolling TNBC, ER+/HER2- BC, EC/OC are planned and additional patients may be enrolled based on emerging data. The primary endpoint of EXP is to assess safety and tolerability, overall response rate, disease control rate, and duration of response. Secondary endpoints include pharmacokinetic analysis and antidrug antibodies. Patients are not selected by B7-H4 status, but baseline tumors samples are collected for retrospective tumor tissue evaluation. The trial is currently enrolling patients. NCT05377996 Citation Format: Erika Hamilton, Arvind Chaudhry, Alexander I. Spira, Sylvia Adams, Nour Abuhadra, Antonio Giordano, Ritesh Parajuli, Hyo S. Han, Amy Weise, Aubri Marchesani, Kate Josephs, Chu Ri Shin, Kevin Kalinsky. XMT-1660: A Phase 1b trial of a B7-H4 targeted Antibody Drug Conjugate (ADC) in Breast, Endometrial, and Ovarian Cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-03-01.