Phase I Trial of a Novel Anti-HER2 Antibody–Drug Conjugate, ARX788, for the Treatment of HER2-Positive Metastatic Breast Cancer

ARX788 is a novel antibody-drug conjugate (ADC) comprised of an anti-HER2 monoclonal antibody and a potent tubulin inhibitor payload AS269 that is site-specifically conjugated to the antibody via a non-natural amino acid incorporated into the antibody. Herein, we present the results of a phase I study of the safety, pharmacokinetics, and antitumor activity of ARX788 in HER2-positive metastatic breast cancer (MBC) patients.Patients with HER2-positive MBC received ARX788 at doses of 0.33, 0.66, 0.88, 1.1, 1.3, or 1.5 mg/kg Q3W, or 0.88, 1.1, or 1.3 mg/kg Q4W. The dose-limiting toxicity (DLT) was assessed for 84 days for pulmonary toxicity and at a duration of one cycle (21 or 28 days) for other toxicities.In total, 69 patients were enrolled. No DLT or drug-related deaths occurred. Most patients (67/69; 97.1%) experienced at least one treatment-related adverse event (TRAE). Common ({greater than or equal to} 30%) TRAEs included an increase in aspartate aminotransferase, an increase in alanine aminotransferase, corneal epitheliopathy, alopecia, hypokalemia, interstitial lung disease (ILD)/pneumonitis, and an increase in aldosterone. While 34.8% of participants experienced ILD/pneumonitis, only two had a severity of grade 3. At 1.5 mg/kg Q3W, the recommended phase II dose, the objective response rate was 65.5% (19/29, 95% confidence interval [CI], 45.7-82.1), the disease control rate was 100% (95% CI, 81.2-100), and the median progression-free survival was 17.02 months (95% CI, 10.09 to not reached).ARX788 demonstrated a manageable safety profile with promising preliminary signs of activity in HER2-positive MBC patients who progressed on prior anti-HER2 therapies.