Most of the drugs currently in clinical use are based on small molecules and use an "occupancy-driven" mechanism of action to inhibit the function of proteins so as to treat diseases. Unlike traditional small molecule inhibitors and antagonists, targeted protein degradation (TPD) has been rapidly developed in recent years due to its ability to induce the degradation of disease-causing target proteins, i.e., PROteolysis TArgeting Chimeras (PROTACs), providing a new way of thought for new drug development.
What Is PROTAC?
The concept of PROTAC (Proteolysis Targeting Chimeras) was first proposed by Crews et al. in 2001, which is able to lower the protein level instead of inhibiting the function of protein by using the naturally existing protein cleaning system in the body for the purpose of treating diseases.
Figure 1. Protac Structure, source: reference 
PROTACs are heterofunctional small molecules consisting of two ligands linked by an appropriate linker: one ligand recruits and binds the protein of interest (POI), while the other recruits and binds the E3 ubiquitin ligase. The mechanism of PROTACs is to use the ubiquitin-proteasome system (UPS) to ubiquitinate and degrade the target protein. Once the PROTAC molecule binds the target protein to the E3 ligase, a ternary complex is formed, which induces the E3 ligase to ubiquitinate the target protein and initiate the degradation process. The ubiquitinated target protein is recognized and degraded by the 26S proteasome, which is part of the UPS eukaryotic cell
Elements of PROTAC Design
Although PROTAC has been studied a lot and involves many targets, the conformational relationships are still not very clear. From the structural characteristics of PROTAC, the design considerations are mainly as follows.
(1) Selection of POI ligands
POI ligands are generally selected from listed or literature-reported inhibitors with certain activity, which can be divided into reversible inhibitors, covalent irreversible inhibitors and covalent reversible inhibitors according to whether the inhibitors can form covalent bonds. Depending on the binding pocket, they can be further classified into ATP-competitive inhibitors and metastable inhibitors. In order to obtain PROTAC with intellectual property, certain structural derivatization of the inhibitor is often done first, and the optimized inhibitor is used as a ligand for POI.
(2) Selection of E3 ligases and their ligands
The main E3 ligases reported in the literature for application to PROTAC are CRBN, VHL, cIAP and MDM2, among which the more effective and most frequently used are CRBN and VHL. Among them, the ligands of CRBN are mainly lenalidomide, pomalidomide and their analogues, and the ligands of VHL are mainly VHL-L.
Figure 8. E3 ligases and their ligands
(3) Selection of Linker
Alkyl, PEG, and extended glycol chains are by far the most common linker motifs appearing in published degrader structures. It has been reported in the literature that the length of the linker also affects the PROTAC degradation activity, and the commonly used linker lengths are generally in the range of 4-15 carbon atoms (or heteroatoms). Depending on the target site, the length of the linker has different effects on the degradation activity. In addition, Click chemistry is often applied among the linkers of PROTAC molecules for linking the two ends of the molecules due to milder reaction conditions and higher efficiency.
PEG linkers are the most common motifs incorporated into PROTAC linker structures. According to statistics, 54% of the reported PROTAC molecules used PEG as a linker. Huateng Pharma can provide high purity PEG linkers with various reactive groups to continuously assist your project development.
Figure 9. PROTAC Linker
(4) Selection of linker sites and composition of linker sites
PROTAC is structurally linked to two ligands by a linker. in addition to the composition and length of the linker, the site of linker connection also affects the degradation activity and even the selectivity. the linking site of POI ligand and E3 ligase ligand is generally in the region where the ligand is exposed to the solvent. The linkage sites are generally linked by amide bonds, carbon atoms or heteroatoms (e.g. O, N, etc.), by condensation reactions or nucleophilic substitution reactions, etc. The role of junction design in PROTAC was recently described in an article published in J Med Chem by Michael D. Burkart et al.
PROTAC vs. Traditional Small-Molecule Drugs: Advantages
(1) Wider range of action, higher activity, and the ability to target undruggable targets
Traditional small molecules and antibodies inhibit the function of target proteins through an "occupancy-driven" pharmacology model, which requires high concentrations of the inhibitor or monoclonal antibody to occupy the active site of the target and block downstream signaling pathways. In contrast, PROTAC is "event-driven", not affecting the function of the protein, but mediating the degradation of the disease-causing target protein. As long as PROTAC mediates the formation of the ternary complex and ubiquitinates target protein, it is theoretically recyclable and therefore can be used repeatedly in catalytic amounts. Moreover, PROTAC can induce the degradation of proteins without active sites, such as scaffold proteins, as long as they can produce binding effects, which can greatly increase the range of targets.
Figure 2. "occupancy-driven" pharmacology model of tranditional small molecule drugs, source: references 
Figure 3. "event-driven" pharmacology model of tranditional small molecule drugs, source: references 
According to incomplete statistics, more than 100 proteins have been successfully degraded. These targets include (1) kinase classes, such as RIPK2, BCR-ABL, EGFR, HER2, c-Met, TBK1, CDK2/4/6/9, ALK, Akt, CK2, ERK1/2, FLT3, PI3K, BTK, Fak, etc.; (2) BET proteins, such as BRD2/4/6/9; (3) nuclear receptors, such as AR, ER, etc. (4) other proteins, such as MetAp-2, Bcl-xL, Sirt2, HDAC6, Pirin, SMAD3, ARNT, PCAF/GCN5, Tau, FRS2, etc. These include " undruggable targets", such as the transcription factor regulatory protein pirin and the epigenetic-related protein PCAF/GCN5. In addition, according to a Nature report in Janurary, at least 21 protein degraders (including PROTAC and molecular glue) are in clinical trials.
When it comes to undruggable targets, it is imperative to mention RAS (KRAS, HRAS and NRAS). As the most common mutated gene in cancer, RAS is an important driver of lung, colorectal and pancreatic cancers. For more than 40 years of discovery and research, there have been no drugs available for this target. Finally, in May 2021, Amgen's KRAS-G12C irreversible inhibitor AMG510 (Sotorasib) was successfully launched, putting an end to the "undruggability" of this target. For KRAS-G12C mutation, Crews et al. designed and synthesized PROTAC based on KRAS inhibitors AMG510 and MRTX849, and the activity assay identified degraders with good degradation activity, which may provide a new solution for further overcoming RAS. It is foreseeable that not only G12C may be overcome by PROTAC technology, but also for other mutations.
Figure 4. PROTAC Based on AMG510 and MRTX849, source: references 
(2) Improved selectivity, activity and safety
Compared to traditional small molecule inhibitors, PROTAC can achieve selectivity at certain targets that is difficult to achieve with small molecules. For example, the multi-targeted tyrosine kinase inhibitor (TKI) Foretinib can bind more than 130 kinases. Crews et al. generated a VHL-recruiting PROTAC (compound 1) and a CRBN-recruiting PROTAC (compound 2) based on the foretinib warhead. The results showed that compound 1 and compound 2 could only degrade 36 and 62 proteins, respectively, and only 12 were degraded by both. Another study by Gray's group also demonstrated that PROTAC can achieve target selectivity.
Another study by Gray's group also demonstrated that PROTAC can achieve target selectivity. 2,4-diaminopyrimidine scaffold is a common scaffold for kinase inhibitors, and it is used as the parent nucleus for inhibitors of EGFR, ALK, CDK, Jak and other kinases. However, most of the drug molecules formed by this scaffold have poor target selectivity and often have strong inhibitory activity against many other kinases, and the off-target effect during the development process often becomes a major source of toxic side effects, which affects the success rate of new drug development. Although the PROTAC synthesized by Gray's group is capable of binding more than 190 kinases, it can only degrade 12 and 22 kinases in cellular experiments, which greatly improves the target selectivity. This shows that with rational drug design and iterative optimization, PROTAC molecules with higher selectivity, better activity and better safety are likely to be discovered.
PROTAC technology can not only achieve selectivity that is difficult to achieve with small molecule inhibitors, but also has very significant advantages in enhancing activity. Taking BET (BRD2/3/4), the earliest target applied by PROTAC technology, as an example, QCA570, discovered by Qin Chong et al, showed significantly better cellular anti-proliferative activity than inhibitors such as JQ1, whose activity was mostly at nanomolar level, while QCA570 increased its activity by three orders of magnitude, reaching an amazing picomolar level. Based on the excellent in vitro cellular anti-proliferative activity, the degraders also showed potent anti-tumor activity in vivo and exhibited low dose and low frequency of administration as well. This suggests that PROTAC has a clear advantage in targets like BET. However, no degradation agent for this target has been seen to enter the clinic for unknown reasons.
Figure 5. QCA570, source: references 
(3) Overcome drug resistance
Small molecule inhibitors or antagonists are inevitably subject to acquired drug resistance during clinical use, such as EGFR-T790M and C797S resistance. Although resistance can be addressed by developing new generations of inhibitors, such as third- and fourth-generation EGFR, new resistance will emerge with the use of new generations of drugs.PROTAC technology has shown some advantages in overcoming drug resistance. Groups such as Crews, Jian Jin, Sanqi Zhang and Gray have related EGFR-PROTACs studies aiming to overcome drug resistance mutations through protein degradation pathways or find breakthrough inhibitors for protein degradation therapies. It can be seen that the first, second, third and fourth generation inhibitors have been applied to the design of PROTACs as ligands for binding the target protein EGFR. Among them, Gray's group has also applied metamorphic inhibitors to PROTAC with good results, selectively degrading different EGFR mutants while circumventing the degradation of wild type. This selectivity was further validated by in vitro anti-proliferative activity.
Figure 6. EGFR-PROTACs.
C4 Therapeutics has announced its EGFR degrader CFT8919 to the public, leaving the fourth generation inhibitors in development inferior. CFT8919 has potent degradation activity against a wide range of EGFR mutants, including the currently clinically unavailable C797S resistance mutation, and is able to circumvent the degradation effect on EGFR wild type. The in vivo transplantation tumor model also validated the good in vivo tumor-suppressive activity of this degrader with a low impact on body weight, indicating a good safety profile. Although the specific structure of CFT8919 has not been disclosed, analysis of the relevant patents reveals that the ligands attached to bind EGFR are based on the mutagenesis inhibitor and the ligands attached to the E3 ligase are based on CRBN.
Figure 7. CFT8919, source: C4 Therapeutics
Leaders and Related Companies of PROTAC
The development of any emerging technology cannot be achieved without the tireless efforts of the individuals and groups involved, and PROTAC is certainly no exception; Crews, Bradner, Ciulli and Shaomeng Wang have all made significant contributions to the advancement of PROTAC technology. They have founded companies to develop protein degradation technologies. Some of the earlier and better companies that have done this include Arvinas, C4Therapeutics, Kymera Therapeutics, Vividion, Nurix, Oncopia Therapeutics, etc.
Figure 10. PROTAC Pioneers and Leaders
Arvinas, founded by Crews in 2013, was one of the first companies to lay out PROTAC, developing protein degradation technologies for the treatment of oncology and neurological diseases. The most advanced ARV-110 and ARV-471 are both in Phase II clinical trials for prostate cancer and breast cancer, respectively. The therapeutic effects of neurodegenerative diseases (e.g. AD, Parkinson's, etc.) and other central nervous system diseases are currently very limited, and Arvinas also has a layout for proteins related to these diseases (e.g. Tau proteins, etc.) and expects to make a breakthrough.
Figure 11. ARV-110 and ARV-471
On July 22, 2021, Pfizer and Arvinas entered into an agreement to jointly develop and commercialize the ER degrader ARV-471. Under the agreement, Arvinas will receive $650 million in upfront payments and up to $1.4 billion in milestone payments, and Pfizer will also make a $350 million equity investment in Arvinas. Pfizer entered into an $830 million partnership agreement with Arvinas back in January 2018. In addition to being favored by Pfizer, Arvinas has also established partnerships with pharmaceutical giants such as Merck Sharp & Dohme, Genentech, and Bayer. In April 2015, a $430 million partnership agreement was reached with Merck Sharp & Dohme. In November 2017, a $650 million partnership agreement was reached with Genentech.
Figure 12.Arvinas Pipeline, source Arvinas website
C4 Therapeutics, founded in 2015 by James Bradner (currently leading the development of Novartis PROTAC.) C4 Therapeutics currently lays out targets mainly related to tumors, such as IKZF1/3, BRD9, EGFR, BRAF-V600E and RET. It has a platform focused on protein degrader development, C4T TORPEDO, for PROTAC design, synthesis and activity evaluation, aiming to discover high-quality protein degraders. In January 2019, a $415 million and $900 million collaboration agreement was reached with Biogen and Roche, respectively.
Figure 13. C4 Therapeutics Pipeline, source: C4 Therapeutics website
Kymera Therapeutics, founded in 2016, is focused on treating cancer and immune inflammation with protein degradation technologies, laying out targets such as IRAK4 and STAT3. The most advanced program is KT-474, which is currently in Phase I clinic. Its partners are primarily Sanofi and GlaxoSmithKline, among others. In July 2020, Kymera Therapeutics entered into a strategic collaboration with Sanofi for multiple programs, receiving an upfront payment of $150 million and potentially over $2 billion in potential development, regulatory and sales milestones, as well as sizable royalties.
Figure 14. Kymera Pipeline, source: Kymera website
Nurix, founded in 2009, focuses on the development of orally available degraders, laying out targets such as BTK and CBL-B. The company has leveraged their deep expertise in E3 ligases and proprietary DNA-encoded compound library (DEL) to build a proprietary drug discovery platform, DELigase, for developing protein degradation drugs or inhibiting E3 ligases to increase levels of beneficial proteins. One of the company's most advanced protein degradation drugs is NX-2127, currently in clinical phase I for B-cell malignancies that have failed prior therapy. The other fast progressing program is NX-5948, which is currently in clinical phase I. Both molecules are orally effective BTK-PROTACs. Nurix is also one of the few companies with a presence in the antiviral field using PROTAC technology, which offers new hope for antiviral drug development, although the targets are not disclosed.
Figure 15. Nurix pipeline, source: Nurix website
In January 2020, Nurix announced a strategic global collaboration with Sanofi to develop innovative targeted protein degradation drugs for a variety of challenging diseases. Under the agreement, Nurix received an upfront payment of $55 million, with subsequent payments upon expansion of the number of targets in the collaboration. In addition, Nurix will be eligible for up to approximately $2.5 billion in total payments upon successful completion of preclinical, clinical, filing and sales milestones.In June 2019, Nurix entered into a strategic agreement with Gilead to develop novel anti-cancer drugs that break down disease-causing proteins using Nurix's DELigase platform. Under the agreement, Nurix received an upfront payment of $45 million. In addition, Nurix will have the opportunity to receive up to $2.3 billion in milestone payments and a share of future sales over the course of the collaboration with Gilead.
PROTAC has evolved over the past 20 years and has grown rapidly in the past 5 years in particular. The future is promising with the wide range of targets that can be laid out using this technology and the huge market. However, the disadvantages of PRORAC, such as large molecular weight, poor bioavailability, and difficulty in druggability, are also obvious, and we look forward to the continuous progress and improvement of this technology. It is believed that as the difficulty of poor druggability is overcome, PROTAC can become a successful therapy like small molecule inhibitors, monoclonal antibodies and immunotherapy, etc. Of course, we also expect PROTAC to be marketed and used to cure diseases, prolong patients' lives and improve their quality of life.
Huateng Pharma, as a leading PEG linker supplier, has over 3,000 high purity PEG linkers kept in stock to empower your development of PROTAC.
 Zhao HY, Yang XY, Lei H, et al. Discovery of potent small molecule PROTACs targeting mutant EGFR. Eur J Med Chem. 2020;208:112781. doi:10.1016/j.ejmech.2020.112781
 Salami J, Crews CM. Waste disposal-An attractive strategy for cancer therapy. Science. 2017;355(6330):1163-1167. doi:10.1126/science.aam7340
 Targeted Degradation of Oncogenic KRASG12C by VHL-Recruiting PROTACs, Michael J. Bond, Ling Chu, Dhanusha A. Nalawansha, Ke Li, and Craig M. Crews, ACS Central Science 2020 6 (8), 1367-1375, DOI: 10.1021/acscentsci.0c00411
 Bondeson DP, Smith BE, Burslem GM, et al. Lessons in PROTAC Design from Selective Degradation with a Promiscuous Warhead. Cell Chem Biol. 2018;25(1):78-87.e5. doi:10.1016/j.chembiol.2017.09.010
 Huang, Hai-Tsang et al. “A Chemoproteomic Approach to Query the Degradable Kinome Using a Multi-kinase Degrader.” Cell chemical biology vol. 25,1 (2018): 88-99.e6. doi:10.1016/j.chembiol.2017.10.005
 Qin C, Hu Y, Zhou B, et al. Discovery of QCA570 as an Exceptionally Potent and Efficacious Proteolysis Targeting Chimera (PROTAC) Degrader of the Bromodomain and Extra-Terminal (BET) Proteins Capable of Inducing Complete and Durable Tumor Regression. J Med Chem. 2018;61(15):6685-6704. doi:10.1021/acs.jmedchem.8b00506
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