Abstract P3-14-05: Interim analysis of a phase 1 study of the antibody-drug conjugate SGN-LIV1A in patients with metastatic breast cancer

Abstract

Abstract Background LIV-1, a multispan transmembrane protein and downstream target of STAT3, is highly expressed in breast cancer cells (Sussman 2014). It has been associated with lymph node involvement and linked with malignant progression to metastasis (Manning 1994). SGN-LIV1A is an anti-LIV-1 antibody conjugated via a protease-cleavable linker to monomethyl auristatin E (MMAE). Upon binding to cell-surface LIV-1, SGN-LIV1A is internalized and releases MMAE, which binds to tubulin and induces G2/M arrest and apoptosis. Methods A phase 1, open-label, dose-escalation study is ongoing to evaluate the safety, tolerability, antitumor activity, pharmacokinetics (PK), and the maximum tolerated dose (MTD) of SGN-LIV1A monotherapy (q3 wks IV) in women with LIV-1-positive, metastatic breast cancer (MBC) (NCT01969643). Patients (pts) with either hormone receptor-positive/HER2-negative (HR+/HER2–) or triple-negative (TN) disease were eligible if they had measurable disease and received ≥2 prior cytotoxic regimens in the metastatic setting. Pts with ≥ Grade 2 neuropathy were excluded. Response was assessed per RECIST v1.1, and pts with stable disease (SD) or better were eligible to continue treatment until disease progression or intolerable toxicity. Fresh tumor biopsies were obtained pre- and post-treatment to evaluate the LIV-1 expression-response relationship, mechanism of action, and tumor sensitivity to SGN-LIV1A. Results To date, 21 pts (17 HR+/HER2–, 4 TN) have received a median of 3 cycles (range, 1–7) of SGN-LIV1A at doses of 0.5–2.8 mg/kg. Median age was 58 yrs (range, 34–73), and pts had a median of 8 prior systemic metastatic therapies (range, 2–15). At baseline, 19 pts had visceral disease and 15 had bone involvement. No dose-limiting toxicities (DLTs) in Cycle 1 were observed in the 18 DLT-evaluable pts; MTD has not yet been identified. Treatment-emergent adverse events (AEs) reported in ≥30% of pts were primarily Grade 1/2 in severity and were: nausea (57%), fatigue and peripheral neuropathy (43% each), alopecia (38%), and vomiting (33%). Two pts discontinued treatment due to AEs (1 nausea, 1 tachycardia). Preliminary PK data suggest a linear increase in antibody-drug conjugate (ADC) exposure with increasing dose. In the 19 efficacy-evaluable pts, the overall response rate (ORR) was 11% (2 partial responses [PRs]) with clinical benefit of SD or better achieved in 63% (2 PR, 10 SD) of pts. Of note, all 4 pts with TN MBC achieved clinical benefit: 2 PR and 2 SD. Currently, the median duration of clinical benefit is 12.7 wks (range, 6.1–26.3). Four pts remain on treatment. To date, of the 179 MBC tumor specimens evaluated for LIV-1, 156 (87%) were LIV-1-positive; moderate-to-high LIV-1 expression (H-score ≥100) was present in 91% (94 of 103) of HR+/HER2– and 81% (47 of 58) of TN samples. Conclusions LIV-1 is expressed in the majority of metastatic breast cancer tissue samples, with moderate-to-high expression in both HR+/HER2- and TN disease. To date, SGN-LIV1A monotherapy has been generally well tolerated and resulted in a clinical benefit of SD or better in 63% of these heavily pre-treated pts, including 2 PRs in the 4 TN MBC pts. Response duration data continue to evolve. Clinical subtype-specific expansion cohorts are planned. Citation Format: Forero A, Burris III H, Mita M, Specht J, Weise A, Liu MC, Modi S, Pusztai L, Kostic A, Yang J, Li M, Hengel S, Miller K. Interim analysis of a phase 1 study of the antibody-drug conjugate SGN-LIV1A in patients with metastatic breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-14-05.