Structure-Guided Design of ISOX-DUAL-Based Degraders Targeting BRD4 and CBP/EP300: A Case of Degrader Collapse.
Edmonds AK., Balourdas D-I., Marsh GP., Felix R., Brasher B., Cooper J., Graber-Feesl C., Kollareddy M., Malik K., Stewart H., Chevassut TJT., Lineham E., Morley S., Fedorov O., Bennett J., Rajasekaran MB., Ojeda S., Harrison DA., Ott CJ., Joerger AC., Maple HJ., Spencer J.
Degraders with dual activity against BRD4 and CBP/EP300 were designed. A structure-guided design approach was taken to assess and test potential exit vectors on the dual BRD4 and CBP/EP300 inhibitor, ISOX-DUAL. Candidate degrader panels revealed that VHL-recruiting moieties could mediate dose-responsive ubiquitination of BRD4. A panel of CRBN-recruiting thalidomide-based degraders was unable to induce ubiquitination or degradation of target proteins. High-resolution protein cocrystal structures revealed an unexpected interaction between the thalidomide moiety and Trp81 on the first bromodomain of BRD4. The inability to form a ternary complex provides a potential rationale for the lack of degrader activity with these compounds, some of which have remarkable affinities close to those of (+)-JQ1, as low as 65 nM in a biochemical assay, vs 1.5 μM for their POI ligand, ISOX-DUAL. Such a "degrader collapse" may represent an under-reported mechanism by which some putative degrader molecules are inactive with respect to target protein degradation.