Abstract 5891: An image-based phenotypic screen identified CBP/p300 as new cancer drug resistance target and enabled the development of the clinical candidate TT125-802
Authors: Gruber Dorothea , Fabritius Charles-Henry , Bohnacker Thomas , Schwill Martin , Laudato Sara , Herrador Raquel , Westritschnig Katrin , Pattupara Thushara , Flückiger-Mangual Stefanie 
Published: March 2024

Abstract
Resistance to targeted therapies is a major challenge in oncology. Genetic mechanisms of resistance such as gatekeeper mutations in the primary drug target can be readily identified. However, drug targets responsible for non-genetic resistance mechanisms such as transcriptional reprogramming remain elusive. Here we describe an innovative phenotypic screening approach that led to the development of the clinical candidate TT125-802 and identification of its target CBP/p300 which we discovered as a novel regulator of transcriptional resistance to a range of targeted therapies. Targeted cancer therapies have been reported to induce profound transcriptional reprogramming leading to drug resistance. For instance, inhibition of oncogenic MAPK signaling using a BRAF/MEK inhibitor combination in BRAF-V600E mutated melanoma, triggers expression of the pluripotency transcription factor SOX2 along with broader expression of associated stemness and EMT genes. We used this system as a model to screen for small molecules that prevent transcriptional escape mechanisms to targeted cancer therapies. For this, we developed an automated high-throughput immunofluorescence staining and analysis method of SOX2 as a surrogate marker for resistance-conferring transcriptional reprogramming. A hit-like high diversity library of 16’000 small molecules was screened using an image-based multi-parameter readout with an integrated counter-screen for general cell toxicity. A battery of downstream assessments including patentability, synthetic tractability, physico-chemical properties, and dose-dependent modulation of transcriptional resistance signatures was used to select the chemical scaffold TT125 for further hit to lead optimization. Hit to lead optimization was initially target agnostic. To de-orphan the scaffold, a chemical probe of TT125 was developed in house and used in a chemoproteomics assay. The transcriptional co-activators CBP/p300 were identified as the target of TT125. Biochemical assays revealed that the TT125 scaffold binds specifically to the bromodomain of CBP/p300 but not the histone acetyl transferase domain. Moreover, TT125 was confirmed to bind highly selectively to the bromodomain of CBP/p300 but not other bromodomain-containing proteins such as BRD2 or BRD4. Structure-based drug design along with in vivo PD models using transcriptional short-term readouts were used for lead optimization that resulted in the development of the potent and highly selective clinical candidate TT125-802. Validating our phenotypic approach to drug discovery and development, TT125-802 is currently being evaluated in a phase I oncology trial. The modular setup of our phenotypic screening system has the potential to power the discovery of additional novel modulators of transcriptional drug resistance.
Citation Format: Dorothea Gruber, Charles-Henry Fabritius, Thomas Bohnacker, Martin Schwill, Sara Laudato, Raquel Herrador, Katrin Westritschnig, Thushara Pattupara, Stefanie Flückiger-Mangual. An image-based phenotypic screen identified CBP/p300 as new cancer drug resistance target and enabled the development of the clinical candidate TT125-802 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5891.