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Cited by 252Protein Degradation and InhibitorsUbiquitin and proteasome pathwaysChemical Synthesis and Analysis

PROteolysis TArgeting Chimeras (PROTACs) as emerging anticancer therapeutics

Open access
Khan, Sajid, He, Yonghan, Zhang, Xuan, Yuan, Yaxia, Pu, Shaoyan, Kong, Qingpeng, Zheng, Guangrong, Zhou, Daohong·2020·Oncogene
DOI 10.1038/s41388-020-1336-yPMC PMC7319888
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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.

AI snapshot

Written from the paper’s full text.

TL;DR

This review argues that PROTACs are emerging anticancer therapeutics because they catalytically degrade cancer-relevant proteins via the ubiquitin-proteasome system, with potential advantages over small-molecule inhibitors. It emphasizes that tumor-specific/selective PROTACs using tissue- or tumor-selective E3 ligases could reduce on-target toxicities, and surveys E3 ligase ligands, design considerations, safety concerns, and preliminary bioinformatic identification of tumor-selective E3 ligases.

Key findings

  • PROTACs are heterobifunctional molecules linking a protein-of-interest ligand to an E3 ligase ligand; they induce proximity-driven ubiquitination and degradation, can act catalytically at lower concentrations than small-molecule inhibitors, can target undruggable proteins such as transcription factors, and can overcome resistance from target mutation or upregulation.
  • Commonly exploited E3 ligases include CRBN, VHL, MDM2, IAPs, DCAF15, DCAF16, RNF4, and RNF114; more than 600 E3 ligases exist in the human genome, but only a few have been exploited for PROTAC design. VHL-based PROTACs may offer target specificity and tumor selectivity but VHL is mutated in cancers such as clear cell renal cell carcinoma; CRBN-based PROTACs may degrade a broader range of targets and have lower molecular weight than VHL-based PROTACs.
  • In GTEx RNA-seq data from more than 50 non-diseased tissues across nearly 1000 individuals, 611 E3 ligases were analyzed: 3% were not detected, 9% were lowly expressed, 4% were highly expressed in almost all normal tissues, 9% were tissue-specific, and 12% were group-enriched in 2–7 tissue types.
  • In TCGA RNA-seq data from 12 cancer types, using Logfold change >1.5 and p < 0.05, 114 E3 ligases were highly expressed in at least one cancer type; overlapping these with E3 ligases not detected or lowly expressed in normal tissues yielded 69 candidate tumor-specific/selective E3 ligases, such as Birc7 in melanoma.
  • ARV-110 (AR) and ARV-471 (ER) are noted as the first PROTACs tested in clinic (NCT03888612; NCT04072952), and the review suggests PROTACs may be more efficient against hematological malignancies than solid tumors because of size, tissue penetration, and cell permeability constraints.

Methods

  • Narrative review of PROTACs as anticancer therapeutics, covering E3 ligase ligands, PROTAC design and optimization, safety concerns, tumor-specific/selective strategies, and relative efficacy in hematological versus solid cancers.
  • Bioinformatics analysis: extracted GTEx RNA-seq data from more than 50 non-diseased tissues across nearly 1000 individuals and analyzed 611 E3 ligases; extracted TCGA RNA-seq data from 12 cancer types and identified E3 ligases highly expressed in tumors using Logfold change >1.5 and p < 0.05.
  • Main analysis was descriptive and bioinformatic; no systematic review methodology, meta-analysis, or new primary wet-lab experiments are reported in the review itself.

Limitations

  • The paper states that using tissue- and/or tumor-selective E3 ligases faces challenges: only a fraction of the more than 600 E3 ligases have reported E3 ligase activity, protein structures for most E3 ligases have not been solved, and developing small-molecule E3 ligase ligands is challenging and time-consuming.
  • The paper states that PROTAC safety concerns remain, including on-target and off-target toxicities, degradation of neosubstrates, the Hook effect, and potential dysregulation of proteostasis by saturating the UPS; clinical translation is still early.
  • Evident from the text: as a review, it does not provide a systematic search strategy or inclusion criteria, and the bioinformatic identification of tumor-selective E3 ligases is described as preliminary and is not experimentally validated within this paper.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1038/s41388-020-1336-y

Cites

Statistics

Nothing to recompute.

Claims the paper makes

0 of 105 cited sources checked · 0 supported · 0 flagged

  • Findingcitation

    A1874, an MDM2-based PROTAC targeting BRD4, shows superior efficacy versus the corresponding VHL-based PROTAC utilizing the same ligand for the protein of interest in the wild-type p53-harboring colon cancer HCT116 cell line.

    “a MDM2-based PROTAC targeting bromodomain-containing protein 4 (BRD4), named A1874, shows superior efficacy versus the corresponding VHL-based PROTAC utilizing the same ligand for the POI in the wild-type p53-harboring colon cancer HCT116 cell line.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [38] Hines J, Lartigue S, Dong H, Qian Y, Crews CM. MDM2-recruiting PROTAC offers superior, synergistic antiproliferative activity via simultaneous degradation of BRD4 and stabilization of p53. Cancer Res. 2019;79:251–62. source

      Not checked in the free run.

  • Findingcitation

    The first IAP-based protein degraders (SNIPERs) using MeBS as an IAP ligand targeting CRABP-1/2 also induced auto-ubiquitination and subsequent degradation of cIAP1.

    “Hashimoto’s and Naito’s groups discovered the first IAP-based protein degraders or SNIPERs using methyl bestatin (MeBS) as an IAP ligand targeting CRABP-1/2. However, in addition to degrading its target protein, these SNIPERs also induced auto-ubiquitination and subsequent degradation of cIAP1 [ref 12].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [ref 12]

      Not checked in the free run.

  • Findingcitation

    The authors reported the first use of PROTAC technology to selectively target anti-apoptotic BCL-XL protein in tumor cells with reduced platelet toxicity by employing VHL or CRBN E3 ligases that are minimally expressed in platelets.

    “Recently, we reported the first use of PROTAC technology to selectively target anti-apoptotic BCL-XL protein in tumor cells with reduced platelet toxicity by employing VHL or CRBN E3 ligases that are minimally expressed in platelets [34, 35, 36].”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites [34] Zhang X, Thummuri D, He Y, Liu X, Zhang P, Zhou D, et al. Utilizing PROTAC technology to address the on-target platelet toxicity associated with inhibition of BCL-XL. Chem Commun (Camb). 2019;55:14765–8. source

      Not checked in the free run.

    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [36] Zhang X, Thummuri D, Liu X, Hu W, Zhang P, Khan S, et al. Discovery of PROTAC BCL-XL degraders as potent anticancer agents with low on-target platelet toxicity. Eur J Med Chem. 2020;192:112186. source

      Not checked in the free run.

  • Findingcitation

    Another MDM2-targeted PROTAC based on a niraparib derivative and a nutlin-3 derivative induces PARP1 degradation leading to apoptosis in MDA-MB-231 human breast cancer cells.

    “Another MDM2-targeted PROTAC based on a niraparib derivative (PARP1 ligand) and a nutlin-3 derivative has been reported, which induces poly (ADP) ribose polymerase 1 (PARP1) degradation leading to apoptosis induction in MDA-MB-231 human breast cancer cells [39].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [39] Zhao Q, Lan T, Su S, Rao Y. Induction of apoptosis in MDA-MB-231 breast cancer cells by a PARP1-targeting PROTAC small molecule. Chem Commun (Camb). 2019;55:369–72. source

      Not checked in the free run.

  • Findingcitation

    Replacing the ester group with an amide group at the linker attachment site of MeBS resulted in SNIPERs that degraded CRABP-2 only without any effect on cIAP1.

    “they replaced the ester group with amide group at the linker attachment site of MeBS, resulting in the generation of SNIPERs that degraded CRABP-2 only without any effect on cIAP1 [41].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [41] Itoh Y, Ishikawa M, Kitaguchi R, Sato S, Naito M, Hashimoto Y. Development of target protein-selective degradation inducer for protein knockdown. Bioorg Med Chem. 2011;19:3229–41. source

      Not checked in the free run.

  • May combine two claims. 33
  • Classified as probably not a substantial claim. 12
  • The quote only partly states this claim. 1

  • May combine two claims.

    Findingcitation

    PROTACs targeting MDM2 for degradation are highly efficient in degrading MDM2, leading to potent growth inhibition of human B-ALL RS4;11 cells both in culture and in mice.

    “These PROTACs have been shown to be highly efficient in degrading MDM2, leading to potent growth inhibition of human B-cell acute lymphoblastic leukemia (B-ALL) RS4;11 cells both in culture and in mice [40].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [40] Li Y, Yang J, Aguilar A, McEachern D, Przybranowski S, Liu L, et al. Discovery of MD-224 as a first-in-class, highly potent, and efficacious proteolysis targeting chimera murine double minute 2 degrader capable of achieving complete and durable tumor regression. J Med Chem. 2019;62:448–66. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Covalent ligands for RNF4 and RNF114 have been identified and used to generate PROTACs targeting BRD4.

    “covalent ligands for RNF4 and RNF114 have been identified recently and have been used to generate PROTACs targeting BRD4 [refs 71, 72].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [refs 71, 72]

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Testa et al. analyzed the crystal structure of the BRD4: MZ1: VHL ternary complex and rationally designed a macrocyclic-containing PROTAC with decent BRD4 degradation activity.

    “Testa et al. analyzed the crystal structure of the BRD4: MZ1: VHL ternary complex and rationally designed a macrocyclic-containing PROTAC with a decent BRD4 degradation activity [37].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [37] Testa A, Hughes SJ, Lucas X, Wright JE, Ciulli A. Structure-based design of a macrocyclic PROTAC. Angew Chem Int Ed Engl. 2020;59:1727–34. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingcitation

    A PROTAC that targets nuclear FKBP12 for degradation by engaging DCAF16 has been reported.

    “a PROTAC that targets nuclear FKBP12 for degradation by engaging DCAF16 has been reported [70].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [70] Zhang X, Crowley VM, Wucherpfennig TG, Dix MM, Cravatt BF. Electrophilic PROTACs that degrade nuclear proteins by engaging DCAF16. Nat Chem Biol. 2019;15:737–46. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Farnaby et al. replaced the PEG unit of a lead PROTAC targeting SMARCA2 with a phenyl ring, which recapitulated the geometry of the more flexible linker and formed an additional T-stack interaction to Y98 of VHL.

    “they replaced the PEG unit of a lead PROTAC targeting SMARCA2 with a phenyl ring, which recapitulated the geometry of the more flexible linker and formed an additional T-stack interaction to Y98 of VHL [78].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [78] Farnaby W, Koegl M, Roy MJ, Whitworth C, Diers E, Trainor N, et al. BAF complex vulnerabilities in cancer demonstrated via structure-based PROTAC design. Nat Chem Biol. 2019;15:672–80. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    VHL is differentially expressed between tumor cells and platelets, which helped design platelet-sparing BCL-XL PROTACs.

    “we have shown that VHL is differentially expressed between tumor cells and platelets, which helped us to design platelet-sparing BCL-XL PROTACs [35, 36].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [36] Zhang X, Thummuri D, Liu X, Hu W, Zhang P, Khan S, et al. Discovery of PROTAC BCL-XL degraders as potent anticancer agents with low on-target platelet toxicity. Eur J Med Chem. 2020;192:112186. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    The authors converted a dual inhibitor of BCL-XL and BCL-2, ABT263, into selective VHL-based BCL-XL PROTACs, providing evidence that promiscuous inhibitors can be converted into target-specific PROTACs.

    “Recently, we converted a dual inhibitor of BCL-XL and BCL-2, named ABT263 (or navitoclax), into selective VHL-based BCL-XL PROTACs, which provides further evidence that promiscuous inhibitors can be converted into target-specific PROTACs [35, 36].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [36] Zhang X, Thummuri D, Liu X, Hu W, Zhang P, Khan S, et al. Discovery of PROTAC BCL-XL degraders as potent anticancer agents with low on-target platelet toxicity. Eur J Med Chem. 2020;192:112186. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Using the same BCR-ABL ligands, CRBN-based PROTACs degraded both BCR-ABL and c-ABL, whereas VHL-based PROTACs could only degrade c-ABL.

    “using the same BCR-ABL ligands, the CRBN-based PROTACs degraded both BCR-ABL and c-ABL, whereas VHL-based PROTACs could only degrade c-ABL [44].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [44] Lai AC, Toure M, Hellerschmied D, Salami J, Jaime-Figueroa S, Ko E, et al. Modular PROTAC design for the degradation of oncogenic BCR-ABL. Angew Chem Int Ed Engl. 2016;55:807–10. source

      Not checked in the free run.

  • May combine two claims.

    Findingown data

    DT2216 induced BCL-XL degradation in a lysine 87-dependent manner, and since BCL-2 lacks the corresponding lysine, this provides further justification for the inability of DT2216 to degrade BCL-2.

    “DT2216 induced BCL-XL degradation in a lysine 87-dependent manner, and since BCL-2 lacks the corresponding lysine, this provides further justification for the inability of DT2216 to not degrade BCL-2.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    DT2216 could form a ternary complex with either BCL-XL and BCL-2 in a cell free system as analyzed by AlphaLISA assay, but could only form a stable ternary complex with BCL-XL in cells as analyzed by nanoBRET assay.

    “although DT2216 could form a ternary complex with either BCL-XL and BCL-2 in a cell free system as analyzed by AlphaLISA assay, it could only form a stable ternary complex with BCL-XL in cells as analyzed by a recently developed nanoBRET assay.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    The authors' VHL-based BCL-XL PROTACs with ABT263 as a warhead potently degraded BCL-XL in tumor cells both in vitro and in vivo, but not in platelets.

    “Our VHL-based BCL-XL PROTACs with ABT263 as a warhead potently degraded BCL-XL in tumor cells both in vitro and in vivo, but not in platelets.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    DT2216 was found to be a BCL-XL selective PROTAC without any degradation of BCL-2.

    “DT2216 was found to be a BCL-XL selective PROTAC without any degradation of BCL-2.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data

    DT2216 potently inhibited tumor growth in BCL-XL-dependent mouse and patient-derived xenografts and synergistically worked with other BCL-2 family inhibitors and standard-of-care chemotherapy.

    “The first lead VHL-based BCL-XL PROTAC, named DT2216, potently inhibited the tumor growth in BCL-XL-dependent mouse and patient-derived xenografts and synergistically worked with other BCL-2 family inhibitors and standard-of-care chemotherapy.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation

    Birc7 is highly and specifically expressed in melanoma and many other tumors.

    “For example, Birc7, a member of the IAP family, is highly and specifically expressed in melanoma [96] and many other tumors [97], which however is lowly or almost not expressed in normal tissues (Fig. 5b).”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [96] Vucic D, Stennicke HR, Pisabarro MT, Salvesen GS, Dixit VM. ML-IAP, a novel inhibitor of apoptosis that is preferentially expressed in human melanomas. Curr Biol. 2000;10:1359–66. source

      Not checked in the free run.

    • cites [97] Liu B, Han M, Wen JK, Wang L. Livin/ML-IAP as a new target for cancer treatment. Cancer Lett. 2007;250:168–76. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Several tissue-specific/selective E3 ligases have been identified by Crews' group, including the pancreas-selective E3 ligase ASB9, which can be potentially used to generate pancreatic cancer selective PROTACs if the targeted protein of interest is not essential for normal pancreatic functions.

    “several tissue-specific/selective E3 ligases have been identified by Crews’ group, including the pancreas-selective E3 ligase ASB9, which can be potentially used to generate pancreatic cancer selective PROTACs [82], if the targeted POI is not essential for normal pancreatic functions.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [82] Schapira M, Calabrese MF, Bullock AN, Crews CM. Targeted protein degradation: expanding the toolbox. Nat Rev Drug Disco. 2019;18:949–63. source

      Not checked in the free run.

  • May combine two claims.

    Findingcitation

    Several PROTACs targeting BTK to CRBN have been generated and exhibit the potential to treat ibrutinib-resistant non-Hodgkin's lymphomas.

    “several PROTACs targeting BTK, a protein that is important for normal B cell development and development of B cell lymphoma, to CRBN have been generated and exhibit the potential to treat ibrutinib-resistant non-Hodgkin’s lymphomas [6, 92].”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [6] Buhimschi AD, Armstrong HA, Toure M, Jaime-Figueroa S, Chen TL, Lehman AM, et al. Targeting the C481S ibrutinib-resistance mutation in Bruton’s tyrosine kinase using PROTAC-mediated degradation. Biochemistry. 2018;57:3564–75. source

      Not checked in the free run.

    • cites [92] Sun Y, Ding N, Song Y, Yang Z, Liu W, Zhu J, et al. Degradation of Bruton’s tyrosine kinase mutants by PROTACs for potential treatment of ibrutinib-resistant non-Hodgkin lymphomas. Leukemia. 2019;33:2105–10. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based). May combine two claims.

    Premisenone given

    More than 600 E3 ligases have been discovered in the human genome, of which only few have been exploited for PROTAC design.

    “More than 600 E3 ligases have been discovered in the human genome of which only few have been exploited for the PROTAC design.”
    Quote in the paperNot a substantial claim

  • The quote only partly states this claim.

    Interpretationreasoning

    PROTACs that recruit E3 ligases ubiquitously expressed in tumor and normal tissues can cause on-target toxicities if the protein of interest is not tumor-specific.

    “These PROTACs can cause on-target toxicities if the POIs are not tumor-specific.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationcitation

    A1874's superior efficacy is because it simultaneously stabilizes the p53 protein and targets BRD4 for degradation.

    “This is because A1874 simultaneously stabilizes the p53 protein and targets BRD4 for degradation [38].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [38] Hines J, Lartigue S, Dong H, Qian Y, Crews CM. MDM2-recruiting PROTAC offers superior, synergistic antiproliferative activity via simultaneous degradation of BRD4 and stabilization of p53. Cancer Res. 2019;79:251–62. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given

    PROTACs have been developed to degrade a variety of cancer targets with unprecedented efficacy against a multitude of tumor types.

    “PROTACs have been developed to degrade a variety of cancer targets with unprecedented efficacy against a multitude of tumor types.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Binding of a PROTAC to MDM2 can stabilize the p53 protein in p53-wild type tumors while inducing degradation of its target.

    “The binding of a PROTAC to MDM2 can stabilize the p53 protein in p53-wild type tumors, at the same time inducing the degradation of its target.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: MDM2 acts as an oncogene by suppressing the activity of the tumor suppressor p53.

  • May combine two claims.

    Interpretationcitation

    In the case of the authors' BCL-XL PROTACs, prolonged degradation of BCL-XL may be advantageous because BCL-XL exhibits functional redundancy in normal tissues except platelets, and therefore its prolonged depletion accounts for better therapeutic efficacy.

    “in the case of our BCL-XL PROTACs the prolonged degradation of BCL-XL may be advantageous because BCL-XL exhibits functional redundancy in normal tissues except platelets, and therefore its prolonged depletion accounts for better therapeutic efficacy [35, 80, 81].”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [80] Eichhorn JM, Alford SE, Sakurikar N, Chambers TC. Molecular analysis of functional redundancy among anti-apoptotic Bcl-2 proteins and its role in cancer cell survival. Exp Cell Res. 2014;322:415–24. source

      Not checked in the free run.

    • cites [81] Mason KD, Carpinelli MR, Fletcher JI, Collinge JE, Hilton AA, Ellis S, et al. Programmed anuclear cell death delimits platelet life span. Cell. 2007;128:1173–86. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    The introduction of a hydrophilic PEG-containing linker can improve water solubility.

    “The introduction of a hydrophilic PEG-containing linker can improve water solubility.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    CRBN-based PROTACs have relatively lower molecular weight than VHL-based PROTACs, and thus it is more feasible to develop orally bioavailable PROTACs using a CRBN ligand.

    “The advantage with CRBN-based PROTACs is their relatively lower molecular weight than VHL-based PROTACs, and thus it is more feasible to develop orally bioavailable PROTACs using a CRBN ligand.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    Complete protein degradation with PROTACs can block both enzymatic activity and remove the scaffolding function of proteins, which may sometimes have undesirable consequences.

    “complete protein degradation with PROTACs can block both enzymatic activity as well as also remove the scaffolding function of such proteins and the latter may sometimes have undesirable consequences.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Some proteins have scaffold functions that are essential for normal cellular functions in certain cells.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationnone given

    The toxicities associated with PROTACs can be broadly categorized into on-target and off-target toxicities.

    “the toxicities associated with PROTACs can be broadly categorized into on-target and off-target toxicities.”
    Quote in the paperNot a substantial claim

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationcitation

    Hydrophilic motifs such as piperazine hold the potential to improve water solubility and balance lipophilicity.

    “Hydrophilic motifs such as piperazine hold the potential to improve water solubility and balance lipophilicity [50].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [50] Han X, Wang C, Qin C, Xiang W, Fernandez-Salas E, Yang CY, et al. Discovery of ARD-69 as a highly Potent Proteolysis Targeting Chimera (PROTAC) degrader of Androgen Receptor (AR) for the treatment of prostate cancer. J Med Chem. 2019;62:941–64. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S4 (based).

    Interpretationnone given

    It is possible to achieve tumor-specific/selective degradation of a target protein with PROTACs by using ligands for tissue-specific and/or tumor-selective E3 ligases.

    “it is possible to achieve tumor-specific/selective degradation of a target protein with PROTACs by using ligands for tissue-specific and/or tumor-selective E3 ligases.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationnone given

    PROTACs have several advantages over conventional small molecule inhibitors pertaining to their unique mechanism of action: catalytic mode of action, high selectivity, potential to target undruggable proteins, and capacity to overcome resistance from small molecule inhibitors by targeting mutated proteins.

    “PROTACs have several advantages over conventional SMIs pertaining to their unique mechanism of action. The main advantages are their catalytic mode of action, high selectivity, potential to target undruggable proteins, and the capacity to overcome resistance from SMIs by targeting mutated proteins.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    The broader target adaptation of CRBN compared with VHL may be because the protein-binding surface of CRBN is much larger than that of VHL, so CRBN is expected to contribute more in forming the ternary complex than is necessary for triggering ubiquitination.

    “This may be because the protein-binding surface of CRBN is much larger than that of VHL, so CRBN is expected to contribute more in forming the ternary complex than is necessary for triggering ubiquitination, which results in broader target adaptation of CRBN compared with VHL.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: The protein-binding surface of CRBN is much larger than that of VHL.

  • May combine two claims.

    Interpretationcitation

    The Hook effect results from binary bindings of a PROTAC with either the target protein or E3 ligase, and leads to lesser degradation of the target protein at higher concentrations.

    “This phenomenon is known as the “Hook effect” which results from the binary bindings of a PROTAC with either the target protein or E3 ligase, and lead to lesser degradation of the target protein at higher concentrations [75].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [75] Pettersson M, Crews CM. PROteolysis TArgeting Chimeras (PROTACs) - Past, present and future. Drug Disco Today Technol. 2019;31:15–27. source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    Since platelets depend on BCL-XL for survival, BCL-XL inhibition by ABT263 or monoselective BCL-XL inhibitors causes severe platelet toxicity, which limits the clinical development of these inhibitors as safer anticancer therapeutics.

    “Since platelets depend on BCL-XL for survival, BCL-XL inhibition by the dual BCL-XL/BCL-2 inhibitor ABT263 or monoselective BCL-XL inhibitors (A1155463 or A1331852) causes severe platelet toxicity [81, 90], which limits the clinical development of these inhibitors as safer anticancer therapeutics.”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [81] Mason KD, Carpinelli MR, Fletcher JI, Collinge JE, Hilton AA, Ellis S, et al. Programmed anuclear cell death delimits platelet life span. Cell. 2007;128:1173–86. source

      Not checked in the free run.

    • cites [90] Schoenwaelder SM, Jarman KE, Gardiner EE, Hua M, Qiao J, White MJ, et al. Bcl-xL-inhibitory BH3 mimetics can induce a transient thrombocytopathy that undermines the hemostatic function of platelets. Blood. 2011;118:1663–74. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Interpretationcitation

    These findings are in agreement with Craig Crews' recent suggestion that it is advantageous to develop tumor-specific/selective PROTACs to reduce on-target toxicities where a protein of interest is important for normal tissue function.

    “These findings are in agreement with Craig Crews’ recent suggestion that it is advantageous to develop tumor-specific/selective PROTACs to reduce on-target toxicities where a POI is important for normal tissue function [82].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [82] Schapira M, Calabrese MF, Bullock AN, Crews CM. Targeted protein degradation: expanding the toolbox. Nat Rev Drug Disco. 2019;18:949–63. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given

    Most of these PROTAC on- and off-target toxicities can potentially be reduced by generating tumor-specific/selective PROTACs.

    “Most of these PROTAC on- and off-target toxicities can potentially be reduced by generating tumor-specific/selective PROTACs as discussed in the next section.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationown data

    Many of these E3 ligases are shared by multiple tumors and can be considered tumor-selective because they are also more or less expressed in normal tissues.

    “Many of these E3 ligases are shared by multiple tumors and can be considered tumor-selective because they are also more or less expressed in normal tissues.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning

    Photo-PROTACs require an external light source to cause tumor-specific degradation of target proteins, which can only be accomplished in a clinical setting by photodynamic therapy for limited types of cancer.

    “Photo-PROTACs require an external light source to cause tumor-specific degradation of target proteins. This can only be accomplished in a clinical setting by the use of photodynamic therapy (PDT) for limited types of cancer.”
    Quote in the paperNot a substantial claim

  • May combine two claims.

    Interpretationreasoning

    These phospho-PROTACs utilized peptide moieties as ligands for target proteins and thus are not considered tissue/tumor-specific.

    “However, these PROTACs utilized peptide moieties as ligands for the target proteins and thus are not considered tissue/tumor-specific.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    PROTACs have been developed against different POIs for solid tumors and hematological malignancies and exhibit high potency against some tumor cells in a target-dependent manner.

    “PROTACs have been developed against different POIs for solid tumors and hematological malignancies and exhibit a high potency against some tumor cells in a target-dependent manner.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Premisecitation

    PROTACs targeting BCL-XL and ALK have broad-spectrum antitumor activities, effectively killing leukemia and solid tumor cells in vitro and in xenograft models.

    “Some of the PROTACs, such as PROTACs targeting BCL-XL and ALK [35, 105], have broad-spectrum antitumor activities, which can effectively kill leukemia and solid tumor cells in vitro and in xenograft models.”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [105] Powell CE, Gao Y, Tan L, Donovan KA, Nowak RP, Loehr A, et al. Chemically induced degradation of anaplastic lymphoma kinase (ALK). J Med Chem. 2018;61:4249–55. source

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationcitation

    Exploiting E3 ligases that have a tumor suppressor natural substrate could stabilize the tumor suppressor protein and lead to enhanced antitumor activity, as in MDM2-based BRD4 PROTAC.

    “Another strategy could be exploiting E3 ligases that have a tumor suppressor natural substrate. This is because the engagement of the E3 ligase with a PROTAC could stabilize the tumor suppressor protein leading to enhanced antitumor activity, as in the case of MDM2-based BRD4 PROTAC [38].”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites [38] Hines J, Lartigue S, Dong H, Qian Y, Crews CM. MDM2-recruiting PROTAC offers superior, synergistic antiproliferative activity via simultaneous degradation of BRD4 and stabilization of p53. Cancer Res. 2019;79:251–62. source

      Not checked in the free run.

  • Classified as probably not a substantial claim.

    Interpretationown data

    The development of PROTACs based on tissue-specific and tumor-selective E3 ligases could be a potential strategy that can revolutionize the field of PROTACs as cancer therapeutics.

    “The development of PROTACs based on tissue-specific and tumor-selective E3 ligases could be one such potential strategy that can revolutionize the field of PROTACs as cancer therapeutics, as we recently demonstrated [35, 36].”
    Quote in the paperSubstantial claim0 of 2 sources checked
    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [36] Zhang X, Thummuri D, Liu X, Hu W, Zhang P, Khan S, et al. Discovery of PROTAC BCL-XL degraders as potent anticancer agents with low on-target platelet toxicity. Eur J Med Chem. 2020;192:112186. source

      Not checked in the free run.

  • May combine two claims.

    Premisecitation

    ARV-110 and ARV-471 are the first PROTACs against AR and ER to be tested in clinic.

    “Two PROTACs against AR and ER, named ARV-110 and ARV-471, respectively, are the first to be tested in clinic (Identifier: NCT03888612; NCT04072952).”
    Quote in the paperNot a substantial claim0 of 2 sources checked
    • cites NCT03888612

      Not checked in the free run.

    • cites NCT04072952

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning

    Due to high molecular weight and tissue penetration constraints, some PROTACs can only be administered intravenously and may be more efficient against hematological malignancies than solid tumors, or require higher doses to kill solid tumor cells than leukemia/lymphoma cells in vivo.

    “Due to their high molecular weight and tissue penetration constraints, some PROTACs can only be administered by the intravenous route and thus may be more efficient against hematological malignancies compared to solid tumors, or they may be required at higher doses to efficiently kill solid tumor cells than leukemia/lymphoma cells in vivo.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: PROTACs are larger in size than SMIs, and tissue penetration and cell permeability remain major challenges.

  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Several other PROTAC drug candidates with promising preclinical data can potentially be tested in clinical trials in the near future.

    “In addition, there are several other PROTAC drug candidates with promising preclinical data that can potentially be tested in clinical trials in the near future [1, 35, 40].”
    Quote in the paperNot a substantial claim0 of 3 sources checked
    • cites [1] Bai L, Zhou H, Xu R, Zhao Y, Chinnaswamy K, McEachern D, et al. A potent and selective small-molecule degrader of STAT3 achieves complete tumor regression in vivo. Cancer Cell. 2019;36:498–511. e17. source

      Not checked in the free run.

    • cites [35] Khan S, Zhang X, Lv D, Zhang Q, He Y, Zhang P, et al. A selective BCL-XL PROTAC degrader achieves safe and potent antitumor activity. Nat Med. 2019;25:1938–47. source

      Not checked in the free run.

    • cites [40] Li Y, Yang J, Aguilar A, McEachern D, Przybranowski S, Liu L, et al. Discovery of MD-224 as a first-in-class, highly potent, and efficacious proteolysis targeting chimera murine double minute 2 degrader capable of achieving complete and durable tumor regression. J Med Chem. 2019;62:448–66. source

      Not checked in the free run.

44 of 44

  • The paper relies on this without support.

    PROTACs recruit an E3 ligase to a protein of interest and cause proximity-induced ubiquitination and degradation of the protein by the ubiquitin proteasome system.

    “PROTACs recruit the E3 ligase to the POI and cause proximity-induced ubiquitination and degradation of the POI by the ubiquitin proteasome system (UPS).”

  • The paper relies on this without support.

    MDM2 acts as an oncogene by suppressing the activity of the tumor suppressor p53.

    “MDM2 acts as an oncogene by suppressing the activity of the tumor suppressor p53.”

  • The paper relies on this without support.

    Some tumor cells upregulate IAPs to evade apoptosis.

    “some tumor cells upregulate IAPs to evade apoptosis.”

  • The paper relies on this without support.

    CRBN is ubiquitously expressed in tumor cells and normal cells.

    “Since CRBN is ubiquitously expressed in tumor cells and normal cells”

  • The paper relies on this without support.

    The protein-binding surface of CRBN is much larger than that of VHL.

    “the protein-binding surface of CRBN is much larger than that of VHL”

  • The paper relies on this without support.

    Some proteins have scaffold functions that are essential for normal cellular functions in certain cells.

    “some proteins such as kinases have not only enzymatic functions but also scaffold functions. The latter may be essential for normal cellular functions in certain cells.”

  • The paper relies on this without support.

    Small molecule inhibitors rarely completely inhibit the functions of their targets.

    “SMIs rarely completely inhibit the functions of their targets”

  • The paper relies on this without support.

    Incomplete inhibition of certain proteins with small molecule inhibitors may be tolerable.

    “An incomplete inhibition of certain proteins with SMIs may be tolerable”

  • The paper relies on this without support.

    Complete depletion of certain proteins with a PROTAC can be detrimental if the proteins carry vital functions.

    “complete depletion of the same proteins with a PROTAC can be detrimental if the proteins carry some vital functions.”

  • The paper relies on this without support.

    A small molecule inhibitor may cause transient inhibition of a protein function while a PROTAC causes prolonged depletion of the protein.

    “a SMI may cause the transient inhibition of a protein function while a PROTAC causes prolonged depletion of the protein”

  • The paper relies on this without support.

    Prolonged degradation of a target protein can be either beneficial or harmful depending on the target and cellular/tissue context.

    “this prolonged degradation of a target protein can be either beneficial or harmful depending on the target and cellular/tissue context.”

  • The paper relies on this without support.

    Off-target toxicities often result from degradation of proteins to which the binding affinity of a PROTAC is lower than that needed to inhibit their activity by inhibitors.

    “Off-target toxicities often result from the degradation of proteins to which the binding affinity of a PROTAC is lower to otherwise inhibit their activity by inhibitors.”

  • The paper relies on this without support.

    Accumulation of off-target ubiquitinated proteins can dysregulate normal proteostasis by saturating the ubiquitin proteasome system.

    “the accumulation of off-target ubiquitinated proteins can dysregulate the normal proteostasis by saturating the UPS.”

  • The paper relies on this without support.

    Many tumor-associated POIs are widely expressed in both tumor and normal tissues and are important for normal cell functions, even though they are frequently upregulated to promote tumorigenesis.

    “However, many tumor-associated POIs are widely expressed in both tumor and normal tissues and are also important for normal cell functions, even though they are frequently upregulated to promote tumorigenesis.”

  • The paper relies on this without support.

    PROTACs are larger in size than SMIs, and tissue penetration and cell permeability remain major challenges.

    “This is important because unlike SMIs, PROTACs are larger in size, therefore tissue penetration and cell permeability remain major challenges.”

  • The paper relies on this without support.

    Only a fraction of E3 ligases among more than 600 have been reported to have E3 ligase activity to induce protein ubiquitination and degradation.

    “First, only a fraction of E3 ligases among more than 600 E3 ligases have been reported to have E3 ligase activity to induce protein ubiquitination and degradation.”

  • The paper relies on this without support.

    Protein structures for the majority of E3 ligases have yet to be solved.

    “In addition, protein structures for the majority of the E3 ligases have yet to be solved.”

  • The paper relies on this without support.

    Development of small molecule ligands of E3 ligases is challenging and time consuming.

    “Furthermore, development of small molecule ligands of E3 ligases is challenging and time consuming but is urgently needed to design PROTACs.”

  • The paper relies on this without support.

    Research has mainly focused on the antitumor potency rather than the safety of PROTACs.

    “So far, the research has mainly focused on the antitumor potency rather than the safety of such molecules.”
19 of 19

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Several recent reviews have analysed publicly available datasets for ligase expression in healthy and diseased states to highlight ligases with clear and distinct expression profiles.

    “several recent reviews have analysed and categorized publicly available datasets — such as GTEx, The Cancer Genome Atlas (TCGA), ProteinAtlas and others — for ligase expression in healthy and diseased states, to highlight ligases with clear and distinct expression profiles80,99,143,144.”

    From PROTAC targeted protein degraders: the past is prologue (2022) · cites this paper as [143]

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Conflicting statements about DT2216 and BCL-2 degradation

    Minor

    The paper says DT2216 does not degrade BCL-2, but the later phrase "inability ... to not degrade BCL-2" says the opposite.

1 of 1

The citation lists haven’t been fetched yet

References 111 · Cited by 252

Authors
Khan, S., He, Y., Zhang, X., Yuan, Y., Pu, S., Kong, Q., Zheng, G., & Zhou, D.
Affiliations
University of Florida (US)
Corresponding author
Zhou, Daohong
ORCID
Venue
Oncogene
Publisher
Springer Science and Business Media LLC
Published
2020-05-31
Volume
39
Issue
26
Pages
4909-4924
ISSN
  • 0950-9232 (ISSN-L)
  • 1476-5594
DOI
10.1038/s41388-020-1336-y
PMC
PMC7319888
OpenAlex
W3031903798
Semantic Scholar
a34280b8f40eeb15feb50a74636153d340c11080
Cited by
252
References
111
Citations by year
2026: 24 · 2025: 36 · 2024: 45 · 2023: 38 · 2022: 52 · 2021: 48 · 2020: 9
FWCI
3.64
Type
journal article
Language
en
Open access
Licence
Locations
  • Oncogene (publishedVersion)
  • PubMed (publishedVersion)
  • PubMed Central (acceptedVersion)
Funders
U.S. Department of Health & Human Services | NIH | National Cancer Institute (R01CA219836, R01CA242003, R01CA211963, R21CA223371)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Protein Degradation and Inhibitors
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Ubiquitin and proteasome pathways
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Chemical Synthesis and Analysis
Keywords
PROTACs, protein degradation, E3 ubiquitin ligase, ubiquitin-proteasome system, heterobifunctional molecules, PROTAC design, on-target toxicity, anticancer agents, solid tumors, hematologic malignancies
MeSH terms
Antineoplastic Agents, Humans, Neoplasms, Protein Binding, Proteins, Ubiquitin-Protein Ligases, Proteasome Endopeptidase Complex, Drug Discovery, Proteolysis, Carcinogenesis

About these checks

Registry, retraction, Cites and statistics results are computed, not judged, with one exception: a printed reference that no index lists is read once by a model before it counts as a citation error. Claims are extracted by a model and kept only when their quote appears verbatim in the text; each cited source is then checked, with a model’s help, for whether it says what the claim relies on. Those verdicts can be wrong — read the claim’s quote and the cited source before relying on one.

Claims read by deepseek-v4.1-flash under definition version 3 (extract-v3) on 2026-10-10.

Classified by jev-1.13-20260917 (gold-guideline-v0.2) on 2026-10-10.

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