Potential acridinedione derivatives for the development of a heterobifunctional PROTAC for targeted degradation of PCSK9 protein

Proprotein convertase substilisin-like/kexin type 9 (PCSK9) interacts with the low-density lipoprotein (LDL) receptor by protein-protein interaction (PPI). The inhibition of this PPI lowers the LDL cholesterol levels, thereby reducing the risks of cardiovascular diseases. We identified in-house synthesized acridinedione scaffolds as binders of PCSK9 through microsecond timescale conventional and biased molecular dynamics (MD) simulations. The free energy of binding was calculated by the Molecular Mechanics Poisson-Boltzmann Surface Area and umbrella sampling methods for acridinedione molecules and compared with reference molecules (co-crystallized inhibitors). Three acridinedione molecules (DSPD-2, DSPD-4, and DSPD-6) were identified as potential binders of the PCSK9 allosteric binding site from preliminary docking and steered MD simulations. The stability of molecules inside the allosteric pocket was established by the low values of root mean square deviations of backbone C alpha atoms. The acridinedione derivatives also showed a shorter distance of hydrogen bonds as compared to the standard molecules. The molecule DSPD-6 showed the lowest binding free energy as compared to other selected and standard molecules. The binding of the selected molecules at the allosteric binding pocket had no effect on structural parameters of LDL receptor-binding site of PCSK9 protein. Finally, a proteolysis targeting chimera (PROTAC) was developed by attaching a proteasome recruiting tag to DSPD-6 (lowest binding affinity) to achieve ligand-induced protein degradation. These results qualify the development of the DSPD-6 molecule as a heterobifunctional PROTAC for targeted degradation of PCSK9 protein. Moreover, our computational strategy provides a framework for the identification of molecules against conventionally undruggable PPI targets.