Abstract P2-13-33: Preclinical development of DHES0815A: A HER2-directed antibody-drug conjugate comprised of a reduced potency PBD dimer linked to a domain I binding HER2 antibody (2022)

Abstract Background: Many HER2-directed antibody-drug conjugates (ADCs) in early development employ trastuzumab linked to tubulin binding agents as the cytotoxic drug. DHES0815A is an ADC consisting of a THIOMABTM humanized IgG1 anti-HER2 monoclonal antibody (hu7C2), which binds domain 1 of the HER2 ECD, conjugated via a disulfide linker to PBD-monoamide (MA), a DNA mono-alkylating agent. The reduced potency of the PBD-MA payload compared to PBD dimers and the stability of the conjugation site and linker were designed to improve tolerability, whereas the binding of hu7C2 to a HER2 epitope distinct from trastuzumab and pertuzumab allows combination with existing HER2 therapies. . Methods: Modifications at one imine in the PBD (pyrrolobenzodiazepine) dimer produced mono-alkylating PBDs that were assessed for DNA binding, permeability and cell potency. Each modified PBD was conjugated to hu7C2 LC K149C via a disulfide linker and assessed in HER2-positive models in vitro and in vivo. Safety studies were performed in female and male cynomolgus monkey treated every 3 weeks X 5, with a 7-week follow-up. Assessments included hematology, clinical chemistry, anti-drug antibody, standard clinical, neurological and ophthalmic exams and histopathology. Results: Our goal was design of a reduced potency PBD dimer ADC to achieve dosing in the linear PK range for optimal exposure, efficacy and safety. PBD dimers contain 2 reactive imines that bis-alkylate and crosslink DNA. Mono-alkylating PBD dimers were assessed in DNA binding, potency and permeability assays to select the optimal mono-alkylator, PBD-MA, for further investigation. hu7C2-disulfide-PBD-MA, or DHES0815A, was potent across HER2-positive breast and gastric cancer models in vitro and in vivo, with little effect on HER2-negative tumors or normal primary cells in vitro. DHES0815A was more potent than T-DM1 in HER2-positive and HER2-low breast and gastric cancer PDX models and demonstrated combination activity with standard of care agents T-DM1 or docetaxel. Mechanistically, treatment of HER2-positive cells with DHES0815A resulted in late S-phase cell cycle arrest; induction of DNA damage markers γH2AX, phospho-p53, phospho-CHK2, induction of apoptosis, and bystander activity. Pharmacokinetic analysis showed dose-proportional PK, with linear biphasic PK &amp;gt;/= 4mg/kg in cynomolgus monkey. Safety studies in cynomolgus monkey treated with 4, 8 or 12 mg/kg demonstrated that findings were non-adverse, monitorable and manageable. All doses were tolerated with few clinical observations and minimal to mild clinical pathology changes. Target organs were skin, bone marrow, lung and eye. The HNSTD (highest non-severely toxic dose) was 12 mg/kg. This dose enabled a starting dose of 0.6 mg/kg for phase 1 testing. Conclusions: DHES0815A is a HER2-directed ADC comprised of a domain I binding antibody linked to a reduced potency PBD dimer, PBD-MA, via a reducible disulfide linker. The antibody does not interfere with trastuzumab or pertuzumab binding, allowing combination with existing HER2 therapies. The reduced potency payload allows dosing in the linear PK range, supporting our hypothesis. DHES0815A demonstrated dose-dependent anti-tumor efficacy in multiple HER2+ breast and gastric cancer models, and showed efficacy in HER2-low models, where T-DM1 was inactive. Enhanced anti-tumor activity was observed combining DHES0815A with T-DM1, trastuzumab/pertuzumab or docetaxel. Studies in cynomolgus monkey demonstrated a safety profile compatible with initiating a phase 1 clinical trial. The phase 1 data will be presented in a separate presentation. Citation Format: Gail Lewis Phillips, Guangmin Li, Jun Guo, Shang-Fan Yu, Genee Lee, Donglu Zhang, Peter Dragovich, Thomas Pillow, Binqing Wei, M. Violet Lee, Ola Saad, Shab Masih, Douglas Leipold, Lisa Crocker, Melissa Schutten. Preclinical development of DHES0815A: A HER2-directed antibody-drug conjugate comprised of a reduced potency PBD dimer linked to a domain I binding HER2 antibody [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-33.