The similar biophysical profiles of the amide-ester matched pairs allow more robust assessment of the contribution of cell permeability to PROTAC degradation activity for these compounds. Analysis of these matched pairs makes it clear that increased protein degradation for these compounds must be driven by increased cell permeability rather than ternary complex formation. For example, the matched pair of MZ1 (cf. 25 and 21), 25, was found to be 1.5fold more potent than 21 at degrading Brd4 (Figure ), despite 25 having 3-fold weaker binary and ternary affinities to VHL ± Brd4 BD2 than 21 and also forming a less cooperative complex (α = 39 vs 54, respectively). Therefore, it is evident that the increased potency is derived from the 10fold increase in membrane permeability of 25 compared to 21 (Figures ). Similarly, the ester matched pair of ARV-771 (cf. 26 and 22), 26, displayed a 5.5-fold increase in Brd4 degradation potency relative to 22, despite having 2-to 3-fold weaker binary and ternary affinities to VHL ± Brd4 BD2 than 22, and also forming a less cooperative ternary complex (α = 8.5 vs 13, respectively). Finally, ester 28, not only has the highest PAMPA permeability (Pe = 0.6 × 10 -6 cm/s) of the entire PROTAC series and is 7.5-fold more permeable than its amide counterpart, 24, but 28 also displays the most potent degradation of Brd4 in cells when compared with the other VH032-based PROTACs within the series (21, 25, 23, 27 and 24). This is despite 28 having the lowest ternary affinity and cooperativity with Brd4 BD2 (Kd ternary = 21 nM, α = 7).