How important is reversible binding for efficient degradation using PROTACs?

PROteolysis TArgeting Chimeras (PROTACs) are chimeric small molecules that through binding to a specific E3 ligase on the one hand, and to a protein target on the other, induce ternary complex formation between the target, PROTAC and E3 ligase, leading to ubiquitination and subsequent degradation of the protein target. The potential of PROTACs to precisely and efficiently degrade target proteins may lead to their use as novel versatile therapeutic tools (1).
One quoted advantages of PROTACs compared to traditional small molecule inhibitors, is that after target degradation, the PROTAC dissociates and can lead to degradation of additional protein copies, acting ‘catalytically’ in sub-stoichometric concentrations. Thus, most reported PROTACs to date, were designed based on non-covalent reversible recognition, limiting the target space only to proteins with known high-affinity, reversible ligands.
Using PROTAC based on irreversible inhibitors can significantly increase protein target coverage, however, irreversible binding will likely negate the catalytic nature of the PROTAC activity, rendering it less efficient. While several irreversible PROTACs were developed (2), evidence suggests that the introduction of covalent binding may reduce the potency of PROTACs (3).
Here we designed a reversible covalent binding PROTAC targeting BTK, taking advantage of the covalent binding potency, and the catalytic efficiency stemming from reversibility. We show that while a reversible covalent PROTAC can work, optimizing target engagement, might be more important than reversibility. The general approach can guide the design of reversible covalent PROTACs for various targets, and the insights on affinity/reversibility trade-off can inform future PROTAC design.

(1)https://www.ncbi.nlm.nih.gov/pubmed/11438690

(2)https://pubs.acs.org/doi/10.1021/acscentsci.6b00280

(3)https://pubs.acs.org/doi/10.1021/acschembio.8b01094