Abstract 2115: Assessing BCL2 protein complex level in peripheral blood mononuclear cells as a surrogate pharmacodynamic biomarker of BCL2 inhibitors in hematological cancers

Abstract In the last decade, cancer therapeutics have adhered to the practice of administering drugs at the maximum tolerated dose (MTD) selected from limited patient cohorts. The emergence of targeted therapies has disrupted this dose selection paradigm, as optimal efficacy may be achievable at lower dose than MTD. Recently, a regulatory agency initiated "OPTIMUS" to redefine the dose selection process, recommending quantitative assessments of drug efficacy in relation to the target protein throughout development and clinical trials. ABT-199 is a protein interaction inhibitor that specifically disrupts the interaction of BCL2 protein. Consequently, the assessment of changes in the level of BCL2 protein complex is necessary to evaluate the ABT-199 efficacy for optimal dose selection. In this study, we adapted the Single-molecule Protein Interaction Detection platform for evaluating pharmacodynamics of BCL2 inhibitors in clinical samples. We assessed the dissociation of two BCL2-related biomarkers, BCL2-BAX and BCL2-BIM, from a model system under various ABT-199 doses. Following one hour treatment with 300 nM of ABT-199, the BCL2-BAX complex dissociated almost by 40%, while 80% of the BCL2-BIM complex remained. The reduction of the BCL2-BIM complex was not changed when extending the treatment for 3 hours, indicating that the BCL2-BAX complex may serve as a more sensitive biomarker for assessing the target engagement of ABT-199. To determine the assay sensitivity, we evaluated the ABT-199 response across different cell numbers and found that 100, 000 cells are sufficient for performing the BCL2-BAX assay. Subsequently, we validated the utility of our method in assessing clinical samples, specifically BMMCs and PBMCs. We collected 4 PBMCs and the corresponding BMMCs from hematological cancer patients and treated them with various doses of ABT-199 ex vivo. With 100, 000 BMMCs and 1, 000, 000 PBMCs, we can generate dose-response curves for the BCL2-BAX complex at doses of ABT-199 ranging from 1 to 1, 000 nM. To explore the feasibility of employing the BCL2-BAX complex in PBMCs as a surrogate biomarker for assessing ABT-199 engagement with BCL2 in BMMCs, we compared the averaged dose-response curves from 4 PBMCs with that of BMMCs. Remarkably, the two curves showed a coherent dissociation trend, indicating that the biochemical effect of ABT-199 on the BCL2-BAX complex may be conserved across different sample types. Collectively, we have developed a target engagement assay for BCL2 inhibitors by measuring the BCL2-BAX complex. Notably, we observed a consistent dissociation pattern of the BCL2-BAX complex by ABT-199 treatment in both PBMCs and BMMCs. This molecular information offers an alternative way to assess the pharmacodynamics of BCL2 inhibitors, particularly in the case where collecting a series of BMMCs with sufficient amounts may not be feasible. Citation Format: Hongwon Lee, Byungsan Choi, Saem Hong, Yunseo Lee, Tae-Young Yoon, Janghee Woo. Assessing BCL2 protein complex level in peripheral blood mononuclear cells as a surrogate pharmacodynamic biomarker of BCL2 inhibitors in hematological cancers 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 (6Suppl): Abstract nr 2115.