Bivalent proteolysis-targeting chimeras (PROTACs) drive protein degradation by simultaneously binding a target protein and an E3 ligase and forming a productive ternary complex. We hypothesized that increasing binding valency within a PROTAC could enhance degradation. Here, we designed trivalent PROTACs consisting of a bivalent bromo and extra terminal (BET) inhibitor and an E3 ligand tethered via a branched linker. We identified von Hippel–Lindau (VHL)-based SIM1 as a low picomolar BET degrader with preference for bromodomain containing 2 (BRD2). Compared to bivalent PROTACs, SIM1 showed more sustained and higher degradation efficacy, which led to more potent anticancer activity. Mechanistically, SIM1 simultaneously engages with high avidity both BET bromodomains in a cis intramolecular fashion and forms a 1:1:1 ternary complex with VHL, exhibiting positive cooperativity and high cellular stability with prolonged residence time. Collectively, our data along with favorable in vivo pharmacokinetics demonstrate that augmenting the binding valency of proximity-induced modalities can be an enabling strategy for advancing functional outcomes.

二价蛋白水解靶向嵌合体 (PROTAC) 通过同时结合靶蛋白和 E3 连接酶并形成高效的三元复合物来驱动蛋白质降解。我们假设增加 PROTAC 内的结合价可以促进降解。在这里，我们设计了三价 PROTAC，由二价溴和额外末端 (BET) 抑制剂和通过分支接头连接的 E3 配体组成。我们将基于 von Hippel–Lindau (VHL) 的 SIM1 鉴定为低皮摩尔 BET 降解剂，优先选择含溴结构域 2 (BRD2)。与二价 PROTAC 相比，SIM1 显示出更持久和更高的降解功效，从而导致更有效的抗癌活性。从机制上讲，SIM1 同时以高亲和力与顺式中的两个 BET 溴域结合分子内时尚并与 VHL 形成 1:1:1 三元复合物，表现出正协同性和高细胞稳定性以及延长的停留时间。总的来说，我们的数据以及有利的体内药代动力学表明，增加接近诱导模式的结合效价可以成为推进功能结果的有利策略。