Supplementary Material for: A Phase 1 Study of a CDH6-Targeting Antibody-Drug Conjugate in Patients with Advanced Solid Tumors with Evaluation of Inflammatory and Neurological Adverse Events

<b><i>Purpose:</i></b> This first-in-human study (NCT02947152) evaluated the safety, tolerability, pharmacokinetics, and preliminary efficacy of HKT288, a first-in-class CDH6-targeting antibody-drug conjugate (ADC). <b><i>Experimental Design:</i></b> HKT288 was administered intravenously (IV) every 3 weeks until patients experienced unacceptable toxicity or progressive disease (PD). The starting dose of 0.3 mg/kg was determined based on the highest nonseverely toxic dose in monkeys, which was 2 mg/kg IV weekly. Based on preclinical toxicology, skin, eyes, bone marrow, and liver were expected targets of toxicity. <b><i>Results:</i></b> Nine patients were enrolled: 5 with renal cell carcinoma and 4 with epithelial ovarian cancer. The best overall response on the 0.3 mg/kg cohort in patients with measurable disease was RECIST v1.1 stable disease in 3 patients and PD in 2 patients. The most frequent adverse events (AEs) regardless of causality were pyrexia (44.4%), constipation (44.4%), fatigue (33.3%), and vomiting (33.3%). Three suspected-related neurologic AEs (Grade 2) were reported on the 0.75 mg/kg cohort: seizure in 1 patient and another patient with aphasia and encephalopathy. Further studies were unable to identify the underlying mechanism of the neurologic AEs, and the study was terminated early. <b><i>Conclusions:</i></b> Preclinical toxicology did not predict the neurotoxicity observed with HKT288, and a comprehensive assessment performed post hoc did not identify the mechanism of toxicity. The development of further CDH6-targeting ADCs should be pursued with caution.