Unveiling the Power of PROTAC Valency: Navigating Cell Type-Specific Hook Effects

Targeted protein degradation (TPD) using bivalent proteolysis-targeting chimeras (PROTAC) technology has shown potential in expanding the "druggable" proteome. In their publication in Nature Chemical Biology, Imaide et al.1 posited that augmenting PROTAC valency could potentially lead to the formation of long-lived ternary complexes between PROTAC, the protein of interest (POI), and E3 ligase, thereby constraining the formation of potent binary complexes, as evidenced by a pronounced hook effect. The authors introduced SIM1, a trivalent von Hippel-Lindau (VHL)-based PROTAC, which exhibited a superior degradation profile in comparison to its parent molecule MZ1, towards bromo and extra terminal (BET) proteins, with a predilection for BRD2. The authors attributed this heightened degradation capability of SIM1 over bivalent MZ1 as supportive evidence for their hypothesis. While we concur with the notion that increasing valency and avidity could enhance the efficacy of a PROTAC, the claim that trivalent PROTACs unequivocally eliminate the hook effect is not entirely accurate. We propose that the presence or absence of a hook effect is influenced by numerous factors beyond PROTAC valency.