3551 Background: ALT-P7 is an antibody-drug conjugate, in which two molecules of monomethyl auristatin E (MMAE) are site-specifically conjugated to a cysteine-containing peptide motif of trastuzumab variant. This is the first-in-human study evaluating safety and pharmacokinetics of ALT-P7 in patients with HER2-positive advanced breast cancer. Methods: This was an open label, 3+3 dose-escalation phase 1 trial. Eligible patients were with HER2-positive advanced breast cancer progressive to at least two kinds of prior anti-HER2 treatment. ALT-P7 at doses from 0.3mg/kg to 4.8mg/kg were intravenously administered once every 3 weeks. Dose limiting toxicities were evaluated over the first cycle of 21 days. Primary objective was to define the maximum tolerated dose. Results: ALT-P7 were administered in 27 patients (n=4 at 0.3mg/kg, n=3 at each of 0.6, 1.2, 2.4, 3.6, 4.2, 4.5 mg/kg, n=5 at 4.8mg/kg) between Jan 2018 and Feb 2020. The median number of previous line of systemic therapy was six, including median four prior anti-HER2 agents. The most common grade (G) 3/4 adverse event (AE) was neutropenia (n=4). The other common drug-related AEs of all grade were myalgia (n=9), fatigue (n=7), sensory neuropathy (n=6), alopecia (n=6), pruritus (n=6), and neutropenia (n=6). Dose limiting toxicities(DLTs) were observed in three patients at 4.8mg/kg (G4 febrile neutropenia, G4 thrombocytopenia, G4 hyperbilirubinemia, G3 myalgia, G4 hyponatremia). No DLTs have been observed up to 4.2mg/kg, and safety evaluation at 4.5mg/kg is currently ongoing. Toxicokinetic analysis for total antibody, drug-conjugated antibody, and free payload suggested that there were no accumulation of ALT-P7 upon repeated injection. In 22 patients with response evaluation, disease control rate at 6 weeks of ALT-P7 treatment was 77.3%(17/22) and partial response was achieved in two out of fifteen patients with measurable lesion. The median PFS at doses from 2.4 to 4.8mg/kg was 6.2 months (95% CI 2.5-9.9 months). Conclusions: ALT-P7 was well tolerated to a dose of 4.2mg/kg in heavily pretreated HER2-positive advanced breast cancer. DLTs were observed at 4.8mg/kg, and 4.5mg/kg is under evaluation. The observed clinical activity warrants further evaluation in a phase 2 trial. Clinical trial information: NCT03281824. Clinical trial information: NCT03281824 .

Here, we demonstrate that a metal ion binding motif could serve as an efficient and robust tool for site-specific conjugation strategy. Cysteine-containing metal binding motifs were constructed as single repeat or tandem repeat peptides and their metal binding characteristics were investigated. The tandem repeats of the Cysteine-Glycine-Histidine (CGH) metal ion binding motif exhibited concerted binding to Co(II) ions, suggesting that conformational transition of peptide was triggered by the sequential metal ion binding. Evaluation of the free thiol content after reduction by reducing reagent showed that metal-ion binding elicited strong retardation of cysteine oxidation in the order of Zn(II)>Ni(II)>Co(II). The CGH metal ion binding motif was then introduced to the C-terminus of antibody heavy chain and the metal ion-dependent characteristics of oxidation kinetics were investigated. As in the case of peptides, CGH-motif-introduced antibody exhibited strong dependence on metal ion binding to protect against oxidation. Zn(II)-saturated antibody with tandem repeat of CGH motif retains the cysteine reactivity as long as 22 hour even with saturating O2 condition. Metal-ion dependent fluorophore labeling clearly indicated that metal binding motifs could be employed as an efficient tool for site-specific conjugation. Whereas Trastuzumab without a metal ion binding site exhibited site-nonspecific dye conjugation, Zn(II) ion binding to antibody with a tandem repeat of CGH motif showed that fluorophores were site-specifically conjugated to the heavy chain of antibody. We believe that this strong metal ion dependence on oxidation protection and the resulting site-selective conjugation could be exploited further to develop a highly site-specific conjugation strategy for proteins that contain multiple intrinsic cysteine residues, including monoclonal antibodies.