Structure and biochemical characterization of BTK degraders. (A) Chemical structure and biochemical BTK IC₅₀ of BTK degraders. (B) Docking of lenalidomide-bound CRBN (Protein Data Bank [PDB]: 5FQD; shown as a gray surface) with apo BTK (PDB: 5P9J). BTK is overlaid with CGI1746-bound BTK (PDB: 3OCS; shown in blue). (C) TREEspot visualization of the kinome selectivity profile of DD-03-171 (1 μM). (D) Compound-induced ternary complex formation of recombinant BTK and CRBN-DDB1 (Lanthascreen duplicate mean ± standard error of the mean). (E) Assessment of cellular CRBN engagement by displacement of dBET6 by a degrader. Cells stably expressing BRD4_BD2-GFP with mCherry reporter were treated with 100 nM dBET6 and titration of compound. Percentage of GFP/RFP ratio of RFP-positive cells calculated from images using CellProfiler. Data are shown as means ± standard deviation of cell culture duplicates (n = 2). A.U., arbitrary units; IC₅₀, 50% inhibitory concentration.