Development of Regulated Induced Proximity Targeting Chimeras (RIPTACs) for Cancer Specific Proteins

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Crews, Craig

Abstract

RIPTACs are heterobifunctional small molecules that induce the formation of a ternary complex between a cancer-specific target protein and a pan-essential effector protein to elicit cell-type selective cytotoxicity. Herein, we describe the development of RIPTACs designed to target the cancer-specific oncogenic driver ALK while inhibiting CDKs. Despite achieving ternary complex formation in vitro, ALK-CDK RIPTACs were found to induce off-target ternary complexes that compromised selectivity between ALK-expressing cells and the negative control cell line. To elucidate the nuances of the RIPTAC modality, we studied a RIPTAC, HLDA-6623, that induced formation of a ternary complex between two previously validated protein partners: the oncogenic driver BCL6 and the pan-essential effector protein BRD4. We hypothesized that a RIPTAC mechanism is also operative in the cytotoxic phenotype caused by BCL6-BRD4 TCIPs (transcriptional chemical inducers of proximity), a platform capable of inducing transcriptional rewiring to induce cell death. Transcriptional chemical inducers of proximity recruit BCL6 and BRD4 to differentially express pro-apoptotic genes. Using a structurally distinct BCL6-BRD4 RIPTAC, HLDA-6623, we observed that transcriptional rewiring occurred exclusively in cells expressing BCL6WT but was absent in cells expressing a DNA-binding mutant BCL6DBM. However, HLDA-6623 enhanced cytotoxicity in both the BCL6WT and BCL6DBM cell lines, suggesting that the RIPTAC mechanism, and not transcriptional rewiring, drives the antiproliferative phenotype of these heterobifunctional small molecules.

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