Exploring Non-Covalent Pin1-PROTACs Insights Into Degradation Efficiency in Multiple Myeloma Cell Line

Pin1 is a peptidyl-prolyl isomerase involved in key signalling pathways and upregulated in different cancers, including Multiple Myeloma (MM), where it is linked to poor prognosis/drug resistance. Despite its therapeutic relevance, Pin1 remains challenging to target with conventional inhibitors thereby motivating interest in Targeted Protein Degradation (TPD) strategies. All reported Pin1-PROTACs rely on covalent Pin1 ligands, leaving non-covalent degraders largely unexplored. Since PROTACs (PROteolysis TArgeting Chimeras) act catalytically/reversibly, non-covalent designs are attractive. Here, we describe the design/synthesis/evaluation of 12 non-covalent potential Pin1-PROTACs (P1-P12) based on reversible peptidomimetic ligand and in a prodrug form to allow cell permeability. Despite rational design, stability and measurable target engagement, P1-P12 displayed none or very limited degradation activity, with the most active molecule, P3, achieving only 39% of Pin1 depletion in MM cells. To confirm Pin1 degradability in MM, MM1.R cells were treated with the covalent PROTAC P1D-34, which induced robust dose-dependent degradation (DC₅₀ = 3.16 µM, D_MAX = 90%). Overall, while demonstrating the feasibility of Pin1 depletion in MM, this work highlights the challenge of achieving effective Pin1 degradation with non-covalent micromolar ligands, underscores the current advantage of covalent engagement, and provides a foundation for the future development of non-covalent Pin1 degraders.