Abstract B043: Integrated organoid screening approaches enable preclinical assessment of protein degrader efficacy and target engagement
Proteolysis-targeting chimeras (PROTACs) and molecular glues (MGs) are emerging therapeutics that enable selective protein degradation and offer new strategies for targeted cancer treatment. To determine whether MGs/PROTACs function as designed, functional screening in clinically and physiologically relevant preclinical models is essential. This study establishes a preclinical functional screening platform for MGs/PROTACs using patient-derived (xenograft) tumor organoids (PD(X)O) and matched in vivo patient-derived xenograft (PDX) counterparts. The platform integrates in vitro efficacy, mode-of-action, CRISPR/Cas9-mediated resistance and off-target toxicity modeling, and in vivo translatability. The cereblon (CRBN)-mediated protein degrader of GSPT1, CC-885, was tested across 98 next-generation sequencing-characterized organoids spanning six cancer types (colorectal, cervical, gastric, head and neck, lung, and pancreatic), including non-diseased controls, using the OrganoidXplore™ CellTiter-Glo (CTG) assay. Sensitivity was confirmed in selected models by high-content imaging (HCI), with GSPT1 degradation assessed simultaneously by immunofluorescence. To model CC-885 resistance and off-target toxicity, CRBN-knockout and GSPT1-mutant organoids were engineered using CRISPR/Cas9. In vivo translatability of in vitro responses was evaluated in matched PDX models. Organoid screening revealed heterogeneous responses to CC-885 (IC50 ∼0.1–50 nM) that did not correlate with GSPT1 or CRBN mRNA levels. HCI validated OrganoidXplore™ responses and demonstrated target engagement and time-dependent GSPT1 degradation, linking efficacy to the designed mechanism-of-action of CC-885. CRBN knockout and GSPT1 mutation each abolished drug sensitivity, confirming target specificity without off-target toxicity. In vitro model sensitivity was reflected in vivo in patient-matched PDXs. PD(X)O/PDX models provide physiologically and clinically relevant patient-derived systems that capture patient diversity and response heterogeneity. The platform presented here facilitates, for the first time, functional preclinical screening of efficacy, mode-of-action, resistance, and toxicity of MGs/PROTACs in matched PD(X)O/PDX models. These results establish organoid-based screening platforms such as OrganoidXplore™, combined with CTG and HCI methods, as robust tools for preclinical protein degrader evaluation. By identifying responsive tumor subtypes and capturing both efficacy and mechanistic effects, this approach can strengthen biomarker discovery; alignment with PDX models further highlights its value for early oncology drug development and target selection. Marten Hornsveld, Dennis van der Grinten, Dorrith Verstegen, Shannon Kouters, Esther Kingma, Tomas Veenendaal, Mariusz Madej, Aaron Hua, Jinxi Wang, Lenno Krenning, Jessie Wang, Marrit Putker, Ludovic Bourre. Integrated organoid screening approaches enable preclinical assessment of protein degrader efficacy and target engagement [abstract]. In: Proceedings of AACR Drug Discovery and Development (AACR D3) Conference; 2026 Jul 21-24; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(14_Suppl):Abstract nr B043.