N3-PEG3-CH2CH2COOH, a PEG-based PROTAC linker, is utilized in the synthesis of BI-3663, BI-4216, and BI-0319. Additionally, Azido-PEG3-acid, a non-cleavable 3-unit PEG ADC linker, is employed in the synthesis of antibody-drug conjugates (ADCs).

PROTACs contain two different ligands connected by a linker; one is a ligand for an E3 ubiquitin ligase and the other is for the target protein. PROTACs exploit the intracellular ubiquitin-proteasome system to selectively degrade target proteins. ADCs are comprised of an antibody to which is attached an ADC cytotoxin through an ADC linker.