Abstract 1667: Antibody encapsulated drug: Tr-ACT2 for targeted treatments of various cancers

Abstract Purpose: HER2 is overexpressed in 15-20% of breast cancers and some other cancers, and the development of HER2-targeted therapies has revolutionized cancer treatment in past 20 years. Trastuzumab (Tr) has demonstrated efficacy in treatment of HER2+ breast cancer and advanced gastric cancer. Recently, Tr-based antibody drug conjugates (ADCs), Kadecyla and Enhertu, have emerged as a new class of anti-HER2 therapies by combining targeted antibodies with cytotoxic agents via linkers. However, ADC approach faces great challenges, such as in vivo instability, manufacturing processes, limited availability of antitumor payloads and suboptimal payload release. Therefore, smarter and more efficient designs are urgently needed. Methods: Our patented single protein encapsulation (SPE) platform, allowing encapsulation of small-molecule drugs by a single protein (albumins or globulins) without artificial nanoparticles and chemical modifications to drugs and proteins, has achieved great success in development of 2 drug products, SPEDOX-6 under human clinical trial (NCT0764018) and SPESN38-8 (IND #: 164346) under IND-enabling study based on albumin, which have prompted us to utilize antibody, such as Tr to encapsulate cytotoxic payload, actinomycin D (ACT, RNA polymerase inhibitor), forming antibody encapsulated drugs (AEDs). We developed Tr-ACT2 as a first-in-class AED drug, featuring each Tr molecule to encapsulate two ACT molecules without linkers. Tr-ACT2 was well characterized by UV, fluorescence, membrane dialysis, particle size distribution and molecular docking. In vitro and in vivo anticancer efficacy of Tr-ACT2 against various HER2+ & HER2- cancers were evaluated. Results: Tr-ACT2 has been investigated in following. In vitro cytotoxicity of Tr-ACT2 against breast cancers, HER2+ (SKBR3, JIMT1) and HER2- (BT549, MDA-MB-231) and A549 (NSCLC, HER2-) was evaluated, leading to a time-dependent and significant reduction in cell viability, induction of apoptosis and reduction of Akt activation. Tr-ACT2's cytotoxic activity appeared to be independent of HER2 expression levels. The internalization study on Tr-ACT2 demonstrated that early-stage internalization of Tr-ACT2 is HER2 dependent, but the rate of late-stage internalization of Tr-ACT2 is not limited by HER2 expression level, confirming that Tr-ACT2 could be effectively internalized into both HER2+ and HER2- cancer cells. In vivo anticancer efficacy of Tr-ACT2 at 1 mg/kg vs irinotecan at 50 mg/kg using A549 mouse model has been evaluated, showing that Tr-ACT2 was significantly more effective in suppressing growth of A549 than irinotecan. Conclusions: We have developed the first-in-class AED nanocomplex, Tr-ACT2, as a potent anticancer agent, demonstrating that the SPE technology can be applied to antibody for encapsulating small-molecule drugs without covalent conjugation and marking a significant advancement in antibody-based therapeutics. Citation Format: C. J. Yu, Linrong Li, leslie Wang, Kinsley Wang, Mengmeng Liu, Faqing Huang, Warren A. Chow, Xiaojiang Cui. Antibody encapsulated drug: Tr-ACT2 for targeted treatments of various cancers [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 1667.