PROteolysis TArgeting Chimeras (PROTACs) as emerging anticancer therapeutics

Abstract

Using PROteolysis TArgeting Chimeras (PROTACs) to degrade proteins that are important for tumorigenesis has emerged as a potential therapeutic strategy for cancer. PROTACs are heterobifunctional molecules consisting of one ligand for binding to a protein of interest (POI) and another to an E3 ubiquitin (E3) ligase, connected via a linker. PROTACs recruit the E3 ligase to the POI and cause proximity-induced ubiquitination and degradation of the POI by the ubiquitin-proteasome system (UPS). PROTACs have been developed to degrade a variety of cancer targets with unprecedented efficacy against a multitude of tumor types. To date, most of the PROTACs developed have utilized ligands to recruit E3 ligases that are ubiquitously expressed in both tumor and normal tissues. These PROTACs can cause on-target toxicities if the POIs are not tumor-specific. Therefore, identifying and recruiting the E3 ligases that are enriched in tumors with minimal expression in normal tissues holds the potential to develop tumor-specific/selective PROTACs. In this review, we will discuss the potential of PROTACs to become anticancer therapeutics, chemical and bioinformatics approaches for PROTAC design, and safety concerns with a special focus on the development of tumor-specific/selective PROTACs. In addition, the identification of tumor types in terms of solid versus hematological malignancies that can be best targeted with PROTAC approach will be briefly discussed.

Fig. 1.. Mechanisms of PROTAC-mediated protein degradation.

a, Schematic representation of the general mechanism of PROTAC-induced degradation of POI. A PROTAC molecule recruits an E3 ligase to a POI followed by polyubiquitin of the latter by E2 conjugating enzyme. The polyubiquitinated POI is recognized by the proteasome for its degradation. The PROTAC molecule is recycled to induce next round of degradation. The three different structural components of a PROTAC are depicted in the box. b, Mechanism of the self-degradation of an E3 ligase with homo-PROTACs. A homo-PROTAC recruits an E3 ligase molecule (e.g., CRBN or VHL) to another E3 ligase molecule followed by bidirectional polyubiquitination of E3 ligase molecules and their subsequent degradation by the proteasome. c, Mechanism of POI degradation by Photo-PROTACs. In a photo-PROTAC, a photo-removable group is attached to the POI ligand. Upon irradiation with external light, the photo-removable group is detached from the photo-PROTAC converting it to an active PROTAC for the degradation of POI.

Fig. 2.. Milestones in PROTAC discovery.

Numbers in superscript indicate reference number.

Fig. 3.. Profile of E3 ligase expression in normal tissues.

The original RNA-sequencing (RNA-seq) data used for the analyses were obtained from the GTEx Portal (https://www.gtexportal.org). Heatmap was drawn based on their gene expression levels (RPKM). Red circle indicates selective E3 cluster in tissues.

Fig. 4.. Distribution of E3 ligases in normal tissues.

a, Percentage of E3 ligases in normal tissues according to the classification criteria shown in Table 3. b, Immunohistochemistry (IHC) staining of representative E3 ligases in 7 normal tissues. The figures were acquired from the Human Protein Atlas (THPA) (https://www.proteinatlas.org). Red rectangle indicates high expression of E3 ligases in the tissues.

Fig. 5.. Identification of tumor-specific/selective E3 ligases.

a, Tumor-specific/selective E3 ligases in 12 tumor tissues. BLCA, bladder urothelial carcinoma; BRCA, breast invasive carcinoma; COAD, colon adenocarcinoma; HNSC, head and neck squamous cell carcinoma; KICH, kidney chromophobe; KIRC, kidney renal clear cell carcinoma; KIRP, kidney renal papillary cell carcinoma; LIHC, liver hepatocellular carcinoma; LUAD, lung adenocarcinoma; LUSC, lung squamous cell carcinoma; PRAD, prostate adenocarcinoma; THCA, thyroid carcinoma. b, BIRC7 expression (RPKM) in 12 tumor and corresponding normal tissues. Comparisons were made by two-tailed Student’s t-test.