Results from the phase 1/2 study of patritumab deruxtecan, a HER3-directed antibody-drug conjugate (ADC), in patients with HER3-expressing metastatic breast cancer (MBC).

Abstract

1002Background: Patritumab deruxtecan (HER3-DXd) is a novel, investigational ADC composed of a human anti-HER3 monoclonal antibody covalently bound to a topoisomerase I inhibitor payload via a stable tetrapeptide-based cleavable linker. Here we report updated safety and efficacy data from this ongoing study (U31402-A-J101; NCT02980341; JapicCTI-163401) of HER3-DXd in pts with previously treated MBC. Methods: U31402-A-J101 is a phase 1/2, multicenter, open-label, first-in-human study of HER3-DXd in pts with HER3-expressing MBC (N = 182). The study enrolled pts in dose-escalation (3.2-8.0 mg/kg IV Q3W) and dose-finding portions across molecular subtypes (n = 66; including HER2+ MBC, n = 14) followed by dose expansion in the following subtypes: HER3 high (4.8 mg/kg [n = 33] or 6.4 mg/kg [n = 31]), HER3 low (6.4 mg/kg [n = 21]) HR+/HER2− MBC or HER3-high TNBC (6.4 mg/kg [n = 31]). HER3-high and -low were defined as ≥75% and 25% ‒ < 75% membrane positivity. The primary objective was to assess safety and efficacy; secondary objectives included determining the relationship between efficacy and HER3 expression. Results: At data cutoff (16 Aug 2021), median study duration was 31.9 mo (range, 15-56). Median age was 57 y (range, 30-83); 132 (72.5%) and 50 (27.5%) pts had an ECOG PS of 0 or 1. Pts had a median of 5 (range, 1-13) prior lines of therapy for locally advanced/metastatic disease. Median treatment duration with HER3-DXd was 5.9 mo (range, 0.7-30.6). In a pooled evaluation of dose escalation/finding and expansion, efficacy is shown in pts with HR+/HER2− MBC, TNBC, and HER2+ MBC in the Table. Overall, 130 pts (71.4%) had grade ≥3 TEAEs; the most common (≥15%) were decreased neutrophil count (39.6%), decreased platelet count (30.8%), anemia (18.7%), and decreased white blood cell count (18.1%). 12 pts (6.6%) experienced treatment-related interstitial lung disease according to central adjudication, including 1 grade 5 event. Conclusions: A pooled analysis in this heavily pretreated population showed promising efficacy in pts with HR+/HER2− and HER2+ MBC as well as TNBC. The safety profile with longer follow-up is consistent with previous reports and showed adequate safety and tolerability. Studies are ongoing in MBC tumor types, with a focus on biomarkers associated with efficacy. Clinical trial information: NCT02980341. [Table: see text]