PROTACs: Emerging Targeted Protein Degradation Approaches for Advanced Druggable Strategies

Abstract

A potential therapeutic strategy to treat conditions brought on by the aberrant production of a disease-causing protein is emerging for targeted protein breakdown using the PROTACs technology. Few medications now in use are tiny, component-based and utilize occupancy-driven pharmacology (MOA), which inhibits protein function for a short period of time to temporarily alter it. By utilizing an event-driven MOA, the proteolysis-targeting chimeras (PROTACs) technology introduces a revolutionary tactic. Small-molecule-based heterobifunctional PROTACs hijack the ubiquitin-proteasome system to trigger the degradation of the target protein. The main challenge PROTAC's development facing now is to find potent, tissue- and cell-specific PROTAC compounds with favorable drug-likeness and standard safety measures. The ways to increase the efficacy and selectivity of PROTACs are the main focus of this review. In this review, we have highlighted the most important discoveries related to the degradation of proteins by PROTACs, new targeted approaches to boost proteolysis' effectiveness and development, and promising future directions in medicine.

Keywords: PROTACs; degradation; druggable technologies; emerging medicine; proteolysis.

Figure 1

Proteolysis-targeting chimeras (PROTACs) are shown schematically. The PROTAC molecule is a heterobifunctional chemical that includes an E3 ligase recruiter and a ligand for the protein of interest (POI) (in blue). To initiate ubiquitination (seen in light blue) and subsequent proteasomal destruction, PROTACs force the E3 ligase and POI to be close to one another. PROTACs, which assist the transfer of ubiquitin, connect the POI and E3 ligase. Proteasomes identify the POI associated with ubiquitin and break it down into peptides. E3 ligase and the target protein can both be bound by the PROTAC molecule at the same time. Under the control of E1, E2, and E3 ligases, ubiquitins can be constantly transported to the target protein, leading to the polyubiquitination of the target protein. The target protein that has been polyubiquitinated is then added.

Figure 2

The components of PROTACs are tiny compounds that consist of a ligand that binds to the target protein, a ligand that binds to an E3 ubiquitin ligase, and a linker that joins the two ligands. The cellular ubiquitin–proteasome system can destroy particular proteins using the proteolysis-targeting chimera (PROTAC) technology, a chemical method for protein knockdown. The E3 ubiquitin ligase is brought into close proximity to the target protein by the PROTAC, which encourages the transfer of ubiquitin molecules onto the protein, marking it for degradation by the proteasome. Through the targeted degradation of proteins important in tumor growth, this strategy has demonstrated potential for cancer therapy. Nevertheless, the structure of PROTACs can affect their activity, and recent studies have looked at the use of enzyme-catalyzed activation to regulate PROTAC activity and improve its selectivity for the target protein.

Figure 3

A photo-switchable BET PROTAC’s design philosophy. By substituting o-F4-azobenzene for the oligoether linker in ARV-771, an isomeric photo–PROTAC pair is produced, with the cis-isomer being much shorter than the trans-isomer.

Figure 4

(A) A schematic illustration of the activation of RT-PRO in mice carrying tumors. In the absence of radiation, RT-PRO is inactive and has no harmful effects on healthy tissues. The prodrug is activated to release the PROTAC molecule PRO, while the tumor location is exposed to X-ray radiation. The target proteins are specifically destroyed by the released PRO at the tumor site under the influence of UPS. (B) The response of RT-PRO to PRO’s release. The radioactive PROTAC controls protein breakdown spatiotemporally and has synergistic anticancer effectiveness.

Figure 5

The hypoxia-activated proteolysis-targeting chimera (ha-PROTAC) is a novel group of small molecules designed to degrade proteins selectively in hypoxic conditions. It comprises three components: a ligand that binds to the target protein and a hypoxia-activated leaving group.

Figure 6

Schematic presentation of the design strategies for folate-caged PROTACs, antibody–PROTAC conjugates (Ab-PROTACs), and aptamer–PROTAC conjugates (APCs). Upon special recognition by the cell membrane receptor (e.g., FOLR1, HER2, and nucleolin), these PROTAC-based conjugates are taken up by cells via endocytosis. Then, the linker is cleaved by hydrolases to release the active PROTAC molecule, leading to target protein degradation.

Figure 7

Timetable describing the evolution of PROTACs (2001–2016) and publication numbers during several years [56].

Figure 8

The Future of Drug Discovery with PROTAC Technology.

Figure 9

Due to the encouraging the ubiquitination and subsequent proteasomal destruction of the tumor suppressor protein p53, MDM2 has been identified as a possible target for anti-cancer therapy. This procedure is interfered with by nutlins and other MDM2/p53 interaction inhibitors, which prevent p53 from being degraded and instead cause it to accumulate. In turn, this strengthens p53′s anti-tumor properties and offers a viable plan for the creation of novel cancer treatments. The PROTAC molecule known as dBET1 is made up of two parts: thalidomide, which attracts the E3 ubiquitin ligase CRL4 CRBN, and JQ1, which binds to the oncoprotein BRD4. Using the cellular apparatus of the cell, dBET1 is designed to target BRD4 for breakdown by the proteasome. 7.2. PROTACs in neurodegenerative diseases.

Figure 10

A tiny molecule or antibody attached to a ligand, known as a LYTAC (lysosomal-targeting chimera), interacts with lysosome-targeting receptors (LTRs) such as the CI-MPR and ASGPR to cause protein degradation. All human tissues include CI-MPR, whereas ASGPR is exclusively found in the liver. As a result, ASGPR-based LYTAC could target particular protein breakdown in the liver, whereas CI-MPR is present in all human tissues. Endocytosis is used to pick up the proteins of interest (POI) and LYTAC molecules after CI-MPR or ASGPR binds to them. In contrast, CI-MPR or ASGPR is recycled to the plasma membrane for use in the future, while the POI is later broken down by lysosomes.

Figure 11

Specifically targeted proteins can be degraded by autophagy using a sort of synthetic chemical called AUTAC. Three components make up these molecules: a warhead that targets a particular protein of interest (POI), a linker, and a degradation tag based on cyclic guanosine monophosphate (cGMP). The POI is degraded by this tag by enlisting autophagosomes, cellular organelles, and structures that digest cytoplasmic proteins. The development of aberrant or undesirable proteins in the body can lead to a variety of disorders, and AUTACs have shown promise as potential treatments.