Sub-stoichiometric Degradation is Dispensable for Potent PROTACs: A Case Study for Irreversible Covalent BTK Degraders

Proteolysis-targeting chimeras (PROTACs) represent a transformative therapeutic modality, yet the viability of covalent PROTACs remains debated, as irreversible binding seemingly contradicts the catalytic mechanism central to their function. Here, we develop and characterize PSIRC3, a highly potent covalent PROTAC for Bruton's tyrosine kinase (BTK) that addresses this ambiguity. PSIRC3 induces potent and selective BTK degradation with a sub-nanomolar DC₅₀ of 0.75 nM and a D_max greater than 85%, while its non-covalent counterpart is completely inactive. This degradation activity is strictly dependent on covalent bond formation with the Cys481 residue, as evidenced by a total loss of efficacy against the C481S BTK mutant. PSIRC3 acts with remarkable speed, achieving maximum BTK degradation within 30 minutes, a kinetic profile linked to rapid cell permeation and efficient ternary complex formation. <i>In vivo</i>, a single administration of PSIRC3 leads to substantial BTK degradation in both PBMCs (>80%) and splenocytes (>50%). Computational modeling, parameterized with experimental data, reveals that degradation efficacy is governed by a delicate balance between E3 ligase and target protein affinities. Specifically, excessively high E3 affinity is detrimental by inducing a hook effect, while higher target affinity is generally beneficial. Our findings provide strong evidence that covalent engagement can drive potent and selective protein degradation, challenging the prevailing notion that catalytic turnover is indispensable for PROTAC efficacy. This work establishes a new benchmark for covalent degraders and opens new avenues for targeting previously intractable proteins.