Abstract 334: proPBD-ADCs: optimization of therapeutic potential of ADCs based on capped PBD payload with attenuated potency

Abstract Antibody-drug conjugates (ADCs) with pyrrolobenzodiazepine (PBD) dimer payloads have been explored preclinically and several PBD-based ADCs entered clinical trials. PBD dimers produce highly cytotoxic DNA crosslinks and thus have a mode-of-action differentiated from microtubule inhibitors, TOPO1 inhibitors and other DNA targeting warheads. In April 2021, the FDA approved the first PBD-based ADC, the CD19 targeting loncastuximab tesirine, for treatment of adult patients with relapsed or refractory large B-cell lymphoma, including DLBCL, after 2 or more lines of therapy. However, loncastuximab tesirine carries safety warnings - for example for effusion and edema, myelosuppression, and cutaneous reactions. While PBD-based ADCs show efficacy in the clinic, they belong to a highly potent payload class with IC50 values on human cancer cell lines in the picomolar range. In fact, the ultrapotent character of the vast majority of PBD-based ADCs has highly limited the administration at clinically suitable dosing regimens. Therefore, increasing the therapeutic dose and at the same time improving the therapeutic windows of PBD-based ADCs will be key for the next generation of PBD-ADCs. Although conjugation of linker-payload to the antibody is typically based on random conjugation to cysteine or lysine, we have shown that the native glycan of monoclonal antibodies is a privileged site for attachment of cytotoxic payloads1 (GlycoConnect® technology), while the therapeutic index is further elevated by a highly polar spacer technology (HydraSpace®)2. Complemented by a broad choice of payloads with various mode-of-action (toxSYN® platform), these joint technologies have been applied in multiple clinical programs up to phase 3. Our strategy to optimize the therapeutic potential of PBD-ADCs that incorporate GlycoConnect® and HydraSpace® technology further, is two-fold: (a) apply PBD dimer payload with reduced potency versus common clinical structures and (b) incorporate a pro-drug concept (proPBD-ADC) by shielding the PBD payload by a capping structure that is stable in circulation but at the same time conditionally released by tumor-associated proteases or under reducing conditions. Data from efficacy studies with HER2-proPBD-ADCs in CDX tumor bearing mouse models will be shared, as well as tolerability studies performed both in rats and in the chicken egg chorioallantoic membrane (CAM) model. The data suggest that different cap structures of proPBD-ADCs, as well as the use of a PBD structure with attenuated potency, have a differential impact on efficacy and tolerability. 1 (a) van Geel et al. Bioconj. Chem. 2015, 26, 2233-2242. (b) Wijdeven et al. MAbs 2022, DOI: 10.1080/19420862.2022.2078466. 2 Verkade et al. Antibodies 2018, 7, 12, doi:10.3390/antib7010012. Citation Format: Anette Sommer, Remon van Geel, Sander van Berkel, Elias Post, Jorin Hoogenboom, Floris van Delft. proPBD-ADCs: optimization of therapeutic potential of ADCs based on capped PBD payload with attenuated potency [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 334.