LAUSR

1024 Background: A166 is an antibody-drug conjugate composed of a novel cytotoxic drug (Duo-5, anti-microtubule agent) site-specifically conjugated to an anti-HER2 antibody (transtuzumab) via a stable protease-cleavable valine citrulline linker. In a phase I trial in US patients (pts) with relapsed or refractory advanced solid tumor, A166 had an acceptable toxicity profile and best objective response rate (ORR) of 36% at efficacious dose levels (Yongheng Liu et al. ASCO 2020). Here we report a phase I study of A166 in Chinese pts with locally advanced or metastatic solid tumors (CTR20181301). Methods: KL166-I-01-CTP is a single arm, open-label, dose-escalation and dose-expansion phase I study evaluating A166 in pts with HER2-expressing locally advanced or metastatic solid tumors. Pts received A166 at doses of 0.1, 0.3, 0.6, 1.2, 2.4, 3.6, 4.8, 6.0 mg/kg IV Q3W. Dose cohorts were expanded at 4.8 and 6.0 mg/kg Q3W. The objectives were to determine the safety and tolerability, pharmacokinetics and antitumor activity of A166. Results: 57 pts (median age 53 [range 26-74], 50 female, 7 male) enrolled from Aug 1, 2018 to Nov 30, 2020. HER2 expression was available for all 57 pts: 51 HER2-positive (3+ or 2+/ISH+), 6 HER2-low (1+ or 2+/ISH-). 61.4% (35/57) had received ≥5 prior lines of therapy. No DLTs were observed in all dose groups. Any grade treatment-related AEs (TRAEs) were documented in 96.5% (55/57) of pts, with 31.6% (18/57) being grade 3 or higher. Common TRAEs were corneal epitheliopathy (73.7%), vision blurred (59.6%), peripheral sensory neuropathy (26.3%), dry eye (21.1%), anemia (19.3%), hyponatremia (19.3%). Most common grade ≥3 TRAEs were corneal epitheliopathy (17.5%), hypophosphatemia (5.3%), and dry eye (5.3%). Four pts had serious AEs, two of which were possibly related to the study drug, including thrombosis and fatigue. TRAEs led to 5.3% (3/57) dose reduction and 5.3% (3/57) treatment discontinuation. One death occurred during the treatment due to progressive disease. At the doses of 0.3-6.0 mg/kg, the exposure of ADC in serum were dose dependent and the mean half-life was found to be 1.17-11.04 days. Serum free toxins was about 0.1% and 0.2% of total A166 (ADC) on a molar basis with the Cycle 1 Cmax and AUC, respectively. At efficacious dose, 36 HER2-positive breast cancer pts with measurable disease were assessed for efficacy, best ORR were 59.1% (13/22) and 71.4% (10/14) in 4.8 and 6.0 mg/kg cohort, respectively. Median progression-free survival (PFS) was not reached, and one patient in 4.8 mg/kg cohort has undergone the treatment for more than 19 months. Conclusions: A166 had a manageable safety profile and high stability in the circulation with much lower acute hematological and gastrointestinal toxicities in terms of incidence rate and grade. It demonstrated promising antitumor activity with clinically meaningful responses in heavily pretreated subjects with HER2-positive breast cancer. Clinical trial information: CTR20181301 .