Recently, Pfizer and Arvinas jointly announced that they have reached an agreement to co-develop and promote the PROTAC protein degrader ARV-471. Arvinas will receive $650 million in upfront payments and potentially up to $1.4 billion in milestone payments. What is exactly the PROTAC technology that fevers in recent years? What has been achieved during the development of the past 20 years? What are the targets and drugs being studied currently? Let’s discuss it together.
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Milestone of PROTAC
In 2001, the first PROTAC molecule PROTAC-1 was born, reported by PROTAC technology pioneer Crews and his colleague Deshaies. In 2008, the first small molecule PROTAC was born that is still designed and synthesized by the Crews research group, which is composed of small molecule inhibitors nutlin and AR small molecule ligands targeting MDM2 E3 ubiquitin ligase and intermediate PEG Linker. After that, researchers developed various small molecule PROTACs with CRL4CRBN, CRL2VHL, and cIAPE3 ubiquitin ligand ligands. Later, two PROTAC molecules, ARV-110 and ARV-471, developed by Arvinas, entered clinical trials to treat prostate cancer and breast cancer, respectively.
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Mechanism of PROTAC
Proteolysis-targeting chimera (PROTAC) functions through heterozygous bifunctional small molecular compounds to draw the target protein closer to the intracellular E3 ubiquitin ligase, and use the ubiquitin-proteasome protein degradation pathway to specifically degrade the target protein. The drug consists of three parts, that is, the specific E3 ubiquitin ligand, the specific ligand of the target protein, and the intermediate linker, thus forming the “target protein-PROTAC-E3 ubiquitin ligase” structure.
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Ubiquitin process
Eukaryotic cells have been trying to maintain protein balance through protein production and degradation pathways. A key to maintaining this balance is ubiquitin. After the protein is ubiquitinated, it will be transported to the proteasome for degradation.
Ubiquitin is mainly catalyzed by three enzymes, namely, E1 ubiquitin activating enzyme, E2 ubiquitin binding enzyme, and E3 ligase. At the same time, the process is reversible and ubiquitin can be removed from the ubiquitin chain by ubiquitin enzyme to form reverse regulation. The whole ubiquitin-proteasome system is called UPS.
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The main battlefield of PROTAC: Tumor
The initial PROTAC technology used peptides as ligands to successfully target MetAP-2, androgen receptor (AR), estrogen receptor α (Erα) and phosphatidylinositol 3-kinase (PI3K). The technology laid a good theoretical foundation for the development of small molecule PROTACs. Later, the discovery of small molecular ligands of E3 ubiquitin ligase, such as CRL4CRBN, CRL2VHL, and cIAP, provided a new direction for the development of PROTAC technology. Nowadays, PROTAC in the field of oncology mainly focuses on some nuclear receptors, epigenetic proteins, kinases, other types of proteins/enzymes, RNA, and so on:
- Nuclear receptors such as MDM2, AR, IAP;
- BET, HDACs in the epigenetic direction;
- BTK, FAK, EGFR, PARP, STAT3 in the direction of kinase
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Examples
Targeting AR: ARV-110 is the first oral bioavailable PRAOTC small molecule drug in clinical trials targeting AR, which was approved by the FDA fast track in May 2019. ARV-110 can significantly inhibit tumor growth in the Enzalutamide resistance model.
Targeting ER: ARV-471 can significantly reduce the level of ER expression in patients’ tumor tissues, reducing the level of ER by 62%, up to a maximum of nearly 90%. Moreover, ARV-471 showed degradation effect on wild-type ER and ER mutants.
Targeting BTK: MT-802, BTK, which can effectively degrade wild-type BTK and ibrutinib-resistant C481S mutation, has better selectivity than ibrutinib. It can weaken the BTK signal transduction of primary B cells isolated from patients with C481S mutation, and does not produce adverse reactions of ibrutinib.
Targeting EGFR: PROTAC-C1/3/4 are the PROTAC products of EGFR inhibitor gefitinib/afatinib/lapatinib separately. Preclinical studies have shown that C1 can penetrate the cell membrane of human ovarian cancer cell line OVCAR8 and induce EGFR protein degradation at low nanomole concentration. C3 can degrade the EGFR protein of L858R activation point mutation caused by the deletion of exon 19 of H3255 in human lung cancer cell line H3255. C4 can degrade the dual mutant (L858R/T790M) type EGFR protein of gefitinib resistance in non-small cell lung adenocarcinoma H1975 cells.
Targeting ERK: ERK-CLIPTAC, based on covalent inhibitor design, targets ERK1/2 degradation and inhibition of phosphorylated ERK1/2 signal pathway in malignant melanoma cell line A375 and human colon cancer cell line HCT116, and the binding with CRBN is proved to be the key to cause degradation.
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Envision
After a short period of 20 years of development, some varieties of PROTAC technology have entered the clinical stage. It is predicted that many varieties will enter the clinic in the next few years, and currently it has attracted the attention of a large number of pharmaceutical companies and capital. Of course, while gaining advantages in efficacy, this technology also has many insufficient medicinal properties, such as high molecular weight, strong molecular rigidity, poor water solubility, poor oral absorption, and poor membrane permeability. However, technical problems are often solved as time goes by, and once a breakthrough is made in clinic, it will certainly form a series of potential blockbuster.