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BT8009 is a Bicycle® Toxin Conjugate (BTC™), a novel class of chemically synthesized molecules, comprising a bicyclic peptide targeting Nectin-4 tumor antigen, linked to cytotoxin (monomethyl auristatin E [MMAE]) via a valine-citrulline (val-cit) cleavable linker. Due to their small size, BTCs represent a unique therapeutic class, combining the pharmacology normally associated with a biologic with the pharmacokinetic properties of a small molecule. Here, we describe the preliminary monotherapy dose escalation results of the ongoing multicenter Phase I/II clinical trial (NCT04561362) that assesses the safety and tolerability of BT8009 administration in patients with advanced solid tumors associated with Nectin-4 expression, not exposed to enfortumab vedotin. Patients were treated until disease progression or intolerable toxicity, with tumor assessments for response per RECIST taken every two months. Nectin-4 expression levels were measured in tumor tissue. Thirty-four patients were initiated on weekly BT8009 (7 patients at 2.5 mg/m2, 20 patients at 5.0 mg/m2, and 4 patients at 7.5 mg/m2) or biweekly BT8009 (3 patient at 7.5 mg/m2). Baseline demographics were 62% patients were male, 44% and 56% were ECOG 0 and ECOG 1, respectively, median age was 65.5 years. BT8009 exhibits linear pharmacokinetics. The most common treatment-related adverse events were nausea (14 patients, 41%; ≥ G3 1 patient, 3%), fatigue (13 patients, 38%; ≥ G3 2 patients, 6%), anorexia (11 patients, 32%; ≥ G3 0 patients), constipation (10 patients, 29%; ≥ G3 1 patient, 3%) and anemia (10 patients, 29%; ≥ G3 3 patients, 9%). To date, BT8009 had low incidences of ocular disorders (3%), hyperglycemia (6%), neuropathy (6%), and rash (18%). There was 1 dose-limiting toxicity (G3 asthenia) reported at the 7.5 mg/m2 weekly dose level. Six patients (18%) experienced an SAE, with only 1 related SAE (vomiting). Treatment-emergent AEs resulting in treatment discontinuation, dose interruption, or reductions were observed in 2 patients (6%), 11 patients (32%), and 5 patients (15%), respectively. Preliminary confirmed responses observed in patients with urothelial carcinoma (UC) enrolled in the 5 mg/m2 cohort show 4/8 patients with a complete response (CR) or partial response (PR), including 1/8 patients with a CR and 3/8 patients with a PR, and 2/8 patients (25%) with stable disease (SD), reflecting a 50% overall response rate and a 75% disease control rate in UC patients for this cohort. BT8009 exhibits a promising preliminary tolerability profile and preliminary antitumor activity. The molecule will continue to be explored in the current dose-escalation/dose-expansion study of BT8009 monotherapy and in combination with nivolumab. Citation Format: Meredith McKean, Capucine Baldini, Loic Verlingue, Bernard Doger, Gerald Falchook, Antoine Italiano, Julia Lostes, Oscar Reig, Roshawn Watson, Sebastien Hazard, Dominic Smethurst, Giulia Fulci, Hongmei Xu, Phil Jeffery, Kevin Lee, Irene Braña, Sophie Cousin, Elisa Fontana, Hendrik-Tobias Arkenau. BT8009-100 phase I/II study of novel bi-cyclic peptide and MMAE conjugate BT8009 in patients with advanced malignancies associated with nectin-4 expression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr CT025.