Abstract 4427: Development of CLIO-8221: A HER2-targeted multi-payload ADC to address ADC resistance

Abstract Antibody drug conjugates (ADCs) are transforming the treatment landscape for patients across tumor types, particularly HER2 expressing cancers, however they continue to be limited by narrow therapeutic indexes and rapidly emerging resistance. Studies have confirmed that most ADC patients retain target expression post progression, suggesting that treatment resistance emerges in part because of payload insensitivity. Patient samples have highlighted DNA damage response activity as one important cause of reduced efficacy and resistance to Topoisomerase 1 inhibitor (TOP1i) payloads, therefore, blockade of DNA damage repair pathways in conjunction with TOP1i may drive better and more durable efficacy. Dual payload ADCs are a novel modality with the potential for efficient delivery of combination therapies to the same tumor cell at the same time, balancing PK and exposure profiles. We have developed CLIO-8221, a novel anti-HER2 dual payload ADC conjugated to both TOP1i and a potent DNA damage response inhibitor (ATRi), designed for optimal efficacy and tolerability. CLIO-8221 is an Fc-engineered anti-HER2 antibody conjugated site-specifically via MTGase to a hydrophilic branched linker carrying both a Topo1 inhibitor and an ATR inhibitor, with a DAR of 4 for each payload. ELISA, SPR, flow cytometry and live cell imaging were used for ADC characterization. The pharmacologic activity of CLIO-8221 was assessed using proliferation assays, cell line-derived xenograft studies and non-human primate tolerability studies (NHP). CLIO-8221 is rapidly internalized into lysosomes in HER2 expressing tumor cells to release TOP1i and ATRi payloads with strong bystander effect. CLIO-8221 shows in vitro cell killing activity across cell lines with a range of HER2 expression levels and Trastuzumab-DXd (T-Dxd) sensitivities, and drives tumor regressions after a single dose in T-DXd-resistant and refractory xenograft models, with strong durable activity seen at 3 mg/kg. The optimized Fc and linker engineering reduces off-target uptake via abrogated FcγR binding and decreased macropinocytosis. Toxicokinetic studies in NHPs demonstrate high tolerability with minimal systemic free payload exposure and no significant CLIO-8221 changes in safety endpoints up to 70mg/kg. CLIO-8221 has dose dependent exposure with comparable PK profiles between the total antibody (mAb) and conjugated mAb and a half-life of 8-12 days. CLIO-8221 is a first-in-class anti-HER2 dual payload ADC that allows targeted delivery of TOP1i and ATRi payloads to HER2 expressing tumors to maximize anti-tumor efficacy, overcome TOP1i resistance, and reduce systemic toxicity. Our findings strongly support the development of CLIO-8221 as a potential therapeutic option for HER2-positive cancers and a phase 1/2 study of CLIO-8221 in patients with advanced solid tumors has been initiated. Citation Format: Alyson Smith, Ben Ayers, Shalini Paliwal, Bhushan Dharmadhikari, Jerome Boyd-Kirkup. Development of CLIO-8221: A HER2-targeted multi-payload ADC to address ADC resistance abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4427.