To verify the cell permeability of 30, 32 and 33, together with 1 (VH298) and cisVH298, a bioluminescence resonance energy transfer (NanoBRET) target engagement assay was conducted. This method relies on measuring the displacement of a fluorescent NanoBRET tracer by the test compound under live-cell and permeabilized-cell conditions. HEK293 cells expressing VHL fused to luciferase were treated with the unlabeled test compound and a VH298-BODIPY conjugate as the tracer. The outcome of this experiment is shown in Figure . While compounds 30, 32 and 33 showed tighter binding to VHL in both live and permeabilized mode compared to 1, with 30 being the most potent binder, the correlation of their IC50 values in live and permeabilized mode indicates overall comparable permeability of these compounds.