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Review

Inhibitors, PROTACs and Molecular Glues as Diverse Therapeutic Modalities to Target Cyclin-Dependent Kinase

by
Sandeep Rana
1,
Jayapal Reddy Mallareddy
2,
Sarbjit Singh
2,
Lidia Boghean
2 and
Amarnath Natarajan
2,3,4,5,*
1
Division of Preclinical Innovation, National Center for Advancing Translational Sciences, National Institutes of Health, Rockville, MD 20850, USA
2
Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, Omaha, NE 68198, USA
3
Pharmaceutical Sciences and University of Nebraska Medical Center, Omaha, NE 68198, USA
4
Genetics Cell Biology and Anatomy, University of Nebraska Medical Center, Omaha, NE 68198, USA
5
Fred and Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, NE 68198, USA
*
Author to whom correspondence should be addressed.
Cancers 2021, 13(21), 5506; https://doi.org/10.3390/cancers13215506
Submission received: 28 September 2021 / Revised: 27 October 2021 / Accepted: 29 October 2021 / Published: 2 November 2021
(This article belongs to the Special Issue The Renaissance of CDK Inhibition in Cancer Therapy)
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Abstract

:

Simple Summary

Cyclin-dependent kinases (CDKs) are rich and viable therapeutic targets for various cancers. The emergence of event-driven pharmacology as an alternative to occupancy-driven pharmacology has begun to address the challenges associated with selectively targeting CDKs. In this review article, we summarize the CDK inhibitors that are currently in clinical trials. In addition, we provide an overview of PROTAC- and molecular glue-based strategies to modulate CDK function.

Abstract

The cyclin-dependent kinase (CDK) family of proteins play prominent roles in transcription, mRNA processing, and cell cycle regulation, making them attractive cancer targets. Palbociclib was the first FDA-approved CDK inhibitor that non-selectively targets the ATP binding sites of CDK4 and CDK6. In this review, we will briefly inventory CDK inhibitors that are either part of over 30 active clinical trials or recruiting patients. The lack of selectivity among CDKs and dose-limiting toxicities are major challenges associated with the development of CDK inhibitors. Proteolysis Targeting Chimeras (PROTACs) and Molecular Glues have emerged as alternative therapeutic modalities to target proteins. PROTACs and Molecular glues utilize the cellular protein degradation machinery to destroy the target protein. PROTACs are heterobifunctional molecules that form a ternary complex with the target protein and E3-ligase by making two distinct small molecule–protein interactions. On the other hand, Molecular glues function by converting the target protein into a “neo-substrate” for an E3 ligase. Unlike small molecule inhibitors, preclinical studies with CDK targeted PROTACs have exhibited improved CDK selectivity. Moreover, the efficacy of PROTACs and molecular glues are not tied to the dose of these molecular entities but to the formation of the ternary complex. Here, we provide an overview of PROTACs and molecular glues that modulate CDK function as emerging therapeutic modalities.

Graphical Abstract

1. Introduction

Protein kinases are a family of essential enzymes that make up ~2.6% of all human genes (based on estimated 20,000 human genes). They are clustered in different subgroups such as serine-threonine (S/T) kinases, tyrosine kinases, and tyrosine-like kinases, based on their sequence similarity and biochemical function [1]. Mechanistically, a kinase forms a ternary complex with ATP and its substrate to catalyze the transfer of the γ-phosphate group from an ATP to its substrate proteins. The resulting phosphorylation of the substrate serves as the on/off switch for downstream signal transduction pathways. Unlike the classical kinases, histidine kinases are two-component signal transduction systems, i.e., the transfer of the γ-phosphate group from an ATP to the histidine residue of the kinase, followed by the transfer of phosphate to the aspartate residue of the downstream substrate [2,3].
Changes to the phosphoproteome regulate several aspects of cellular functions, such as growth, migration, and survival, etc. Therefore, dysregulation of the kinase function has been associated with the pathogenesis of several human diseases, including cancer and neurodegenerative diseases [4,5,6]. Therapeutically, a small, well-defined ATP binding pocket makes kinases an attractive target for drug discovery. Twenty years ago, Imatinib (Gleevec), a BCR-Abl kinase inhibitor, was the first Food and Drug Administration (FDA)-approved kinase inhibitor to treat Philadelphia chromosome-positive chronic myelogenous leukemia. As of 30 March 2021, 65 kinase inhibitors have been approved by the FDA, and 85% of them are prescribed to treat cancer [7].
A protein kinase core is composed of two lobes; a smaller N-terminal lobe that mainly consists of five β-sheets and a Cα-helix, and a larger C-terminal lobe that primarily consists of six α-helices. These two lobes are connected by a flexible hinge region that interacts with the ATP [8]. On the other side of the hinge region is a flexible activation loop that begins with the conserved DFG motif and extends up to the APE motif. In the active conformation, the Asp (DFG in) residue orients inwards towards the ATP binding site and binds to the magnesium ion that interacts with the ATP phosphate. In the inactive conformation, the Asp (DFG out) residue points away from the ATP binding site. Kinase inhibitors that bind to the active kinase conformation are called type I inhibitors while those that bind to the inactive conformation are called type II inhibitors. Representative examples of type I and type II inhibitors are Gefitinib, Baricitinib, Ribociclib and Imatinib, Ponatinib, Sorafenib, respectively. Type III kinase inhibitors, such as Cobimetinib, Trametinib, and Binimetinib bind to an allosteric site of the kinase. Covalent kinase inhibitors bind irreversibly to the kinase by forming a carbon–sulfur bond through the sulfhydryl group of the surface-exposed cysteine residue proximal to the ATP binding pocket site [4].
To date, ~89% of approved kinase inhibitors are reversible and follow an occupancy-driven pharmacology model wherein the efficacy of the inhibitor correlates with its off constant [9]. Since it is impossible to accurately model the amount of drug required to maximally inhibit kinase function in vivo, efficacy studies are conducted at concentrations as close to the maximum tolerated dose as possible. Irreversible or covalent inhibitors that follow a two-step binding model overcome the above issue. First, the reversible component of the covalent inhibitor binds to the kinase (Ki), followed by the attack of the proximal nucleophile with an appropriate trajectory to the electrophilic component of the covalent inhibitor (Kinact) [10,11].
Cyclin-dependent kinases (CDKs) are members of a S/T kinase subfamily that comprises 21 CDK enzymes. CDK1, 2, 3, 4, and 6 play a key role in cell cycle regulation, whereas CDK7, 8, 9, and 11 are involved in the regulation of transcription. The biological functions of CDK10, 11, 14–18, and 20 are yet to be fully elucidated [12,13]. Cyclins bind to CDKs to regulate their activity, except CDK5, which binds to and is regulated by p35/p39 and their cleaved products p25/p29 [14]. Multiple cyclins bind to the same CDK to regulate different phases of the cell cycle. For instance, the CDK2-cyclin E complex regulates cell cycle reentry, G1 progression, and S phase entry, whereas CDK2-cyclin A regulates the later part, S and G2/M progression, of the cell cycle [15]. CDKs bind to different cyclins, resulting in the phosphorylation of an array of substrates [16]. For example, CDK9 can complex with either cyclin T1, T2a, T2b, or K to form the positive transcription elongation factor b (P-TEFb), which phosphorylates the C-terminal domain of the large subunit of RNA polymerase II (RNA pol II) and facilitates mRNA synthesis [13,14]. However, the CDK9–cyclin K complex has additional functions in genome maintenance, [15] and induces the phosphorylation of Ser33 within the transactivation domain (TAD) of p53 [16].
CDKs play important roles in a variety of functions including cell proliferation, transcriptional activity, and neuronal activity, and dysregulation of these proteins is often observed in cancer and neurological diseases [17]. Over the past two decades, several CDK inhibitors (1–13) entered clinical trials to treat cancer (Figure 1). In 2015, Palbociclib was the first CDK 4/6 inhibitor (6) to be approved by the FDA, [18] and shortly after, the CDK4/6 inhibitors ribociclib (7) [19] and abemaciclib (8) [20] also received FDA approval for the treatment of breast cancer. The above successes reinvigorated CDK inhibitor development, leading to several CDK inhibitors currently being evaluated alone or in combination with other drugs against several cancers (Table 1) [21].
Novel approaches to target dysregulated proteins are routinely explored. One such approach that has gained considerable attention is proteolysis targeting chimeras (PROTACs). PROTACs artificially induce the degradation of a targeted protein by hijacking the cellular quality-control machinery [22]. PROTACs are heterobifunctional molecules with two different ligands connected by a linker. One ligand attaches to the protein of interest (POI), whereas the other ligand binds to an E3 ligase, resulting in a ternary complex between POI, the PROTAC, and the E3 ligase. This enables proximity-driven polyubiquitination of the POI by the E3 ligase and the subsequent non-natural degradation of the target protein (Figure 2). A traditional small molecule inhibitor (SMI) primarily blocks the catalytic activity of the protein to inhibit the enzymatic function of the target protein via an occupancy-driven pharmacology. On the other hand, a PROTAC operates by degrading the target protein via an event-driven pharmacology model. Early studies showed superior effects via the PROTAC approach, likely due to the elimination of both catalytic and non-catalytic/scaffolding function of the target protein [23,24,25,26,27,28,29,30].
The developmental arc of PROTACs went from a proof-of-concept study to clinical trials in ~20 years (Figure 3). A typical PROTAC consists of three parts: a POI binding ligand, an E3 ubiquitin-binding ligand, and a linker connecting two ligands. Despite the presence of >600 putative E3 ligases in the human proteome, only a handful of E3 ligase ligands (CRBN [31,32], VHL [33,34,35], MDM [36,37,38], IAP [39], RNF114 [40,41], DCAF15 [42], DCAF 16 [43], and FEM1B [44]) are readily available for the design of a PROTAC (Figure 4). The PROTAC approach has been successfully applied to degrade kinases, G protein-coupled receptors, nuclear receptors, membrane proteins, transcription factors, and neurodegenerative proteins aggregates [23,26,27,28,29,30,36,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61]. Recent studies showed good potency and selectivity of PROTACs in animal models due to POI degradation [27,28,29,30]. Currently, two PROTAC candidates, ARV-471 (14, NCT04072952) and ARV-110 (15, NCT03888612), targeting the estrogen receptor and androgen receptor, respectively, are in Phase II clinical trials. Two additional PROTACs, viz. KT-474 (undisclosed structure) (NCT04772885) that degrades IRAK4 for the treatment of interleukin-1 receptor (IL-1R)/toll-like receptor (TLR)-driven immune-inflammatory diseases, and BTK degrader NX-2127 (NCT04830137) for the treatment of relapsed and refractory B-cell malignancies, are being evaluated in Phase I studies.
A typical flowchart for the successful development of a PROTAC is summarized in Figure 5. The PROTAC strategy has been effective in converting promiscuous binders to selective degraders [62,63,64]. Due to the catalytic nature of PROTAC technology, a sub-stoichiometric amount of PROTAC is adequate to induce robust POI degradation, thereby mitigating the off-target inhibitory effects observed with promiscuous SMI. Since PROTACs degrade the POI, unlike SMI’s, PROTAC treatment does not result in feedback induction of the POI. Future studies will focus on developing disease-specific PROTACs either by exploring novel disease-specific POI such as oncogenic KRASG12C, or by examining tissue-specific E3 ligases [65,66]. Alternative strategies to achieve tissue specificity could include the development of antibody PROTAC conjugates [67].

2. CDK Degradation Using PROTAC

In the following sections, we summarize PROTACs developed against various CDKs and, where available, compare their effects with the corresponding inhibitors. In general, the identification of a successful CDK PROTAC requires a set of generic experiments that are common such as: (i) binding studies with the newly synthesized PROTACs via in vitro kinase assays; (ii) dose–response studies to determine their cellular DC50; (iii) time-course studies to demonstrate the robust degradation of a CDK kinase at different time points; (iv) competition studies with an inhibitor, an E3 ligand, and a proteasome inhibitor to show the formation of a ternary complex and subsequent ubiquitin-mediated degradation of the CDK; and (v) whole cell proteomics studies to show selective degradation of the target CDK. In the subsequent sections, we provide the structures of selective CDK PROTACs along with an overview of experiments conducted to establish their MOA. Moreover, we highlight the CDK inhibitor component of a PROTAC with a yellow box and an E3 ligand component with a green box.

2.1. CDK2

Cyclin-dependent kinase 2 (CDK2) plays a crucial role in cell cycle regulation and other biological processes such as DNA damage, intracellular transport, and signal transduction [68]. CDK2 binds with cyclin E to phosphorylate numerous proteins required for the cell cycle progression, DNA replication, and centrosome duplication. During the S phase, cyclin E is degraded, and CDK2 then forms complexes with cyclin A [69,70,71]. Although CDK2 mutations are rarely observed in human cancers, the CDK2/cyclin E and CDK2/cyclin A pathways are often altered. For instance, in ovarian and breast cancers, cyclin E1 gene amplification results in increased activity of CDK2/cyclin E complexes [72,73]. Similarly, cyclin A is often overexpressed in colorectal [74], breast [75], and high-grade serous ovarian cancers (HGSOC) [76]. Recent studies also suggest that inhibiting CDK2 could be a promising therapeutic strategy to treat acute myeloid leukemia (AML) [77,78]. Despite extensive efforts put forth by both pharma and academic labs, the discovery of a highly selective CDK2 inhibitor remains elusive, primarily due to the high homology of the ATP binding sites among the CDKs [68,79].
The PROTAC strategy relies on the overall three-dimensional architecture of CDK and involves structural complementarity with the E3 ligase complex. This approach has been explored to develop a selective CDK2 degrader. Three different labs reported the development of CDK2 PROTACs using pomalidomide (16) and three different CDK targeting ligands (27, 29 and 31, Figure 6). OVCAR8 cells treated with PROTAC 27 (TMX-2172) selectively degraded CDK2 and CDK5 over other CDKs [80]. The study also reported a control compound 28 (TMX-bump) in which the glutarimide nitrogen was methylated, thus blocking its ability to bind CRBN (E3 ligase). Ribociclib (7) does not inhibit CDK2; however, replacing the 1-(pyridyl-2-yl)piperazine with a methylbenzoate in ribociclib yielded an inhibitor that was selective of CDK2 (>50-fold) and bound CDK2 with improved affinity. PROTAC 29 derived from the modified ribociclib analog degraded CDK2 in addition to CDK4 and CDK6. PROTAC 29, degraded CDK2/4/6 in melanoma cells (B16F10 and A375), and inhibited retinoblastoma (Rb) protein phosphorylation. Moreover, PROTAC 29 inhibited colony formation and induced apoptosis in A375 and B16F10 cells [81]. A prodrug 30 of PROTAC 29, was synthesized by attaching a labile group on thalidomide which resulted in improvement of oral bioavailability from 1 to 68%. In a B16F10 xenograft model, oral administration (200 mg/kg) of 30 showed significant reduction in tumor growth. The third reported CDK2 PROTAC (31, Figure 6) was built using a CDK2 inhibitor with a diaminotriazole (J2) [82]. PROTAC 31 induced robust degradation of CDK2 in NB4 cells and Ramos cells at nanomolar concentrations. Accumulation of mutations within hematopoietic stem cells (HSCs) leads to AML [83]. In a cellular differentiation study, PROTAC 31 treatment induced the expression of CD11b, a marker for cell differentiation. Morphological changes indicated that PROTAC 31 could induce significant differentiation of myeloid cells [82]. Furthermore, PROTAC 31 induced granulocytic differentiation of HSCs, suggesting that PROTAC 31 is a promising lead for AML differentiation therapy.

2.2. CDK4

CDK4 and its close homolog CDK6 are a family of serine-threonine kinases that interact with D-type cyclins (D1, D2, and D3) to phosphorylate Rb to regulate the G1/S transition of the cell cycle [84]. Phosphorylation of Rb results in the release of transcription factors E2F to activate transcription of numerous genes that are responsible for cell cycle progression through the S phase as well as those involved in DNA replication and chromosome segregation [1]. Activation of the CDK4-Rb pathway is often observed in human cancers; for example, CDK4 amplification is observed in ~50% of glioblastomas [85]. Although CDK4 and CDK6 are highly homologous, they elicit different biological functions in cells. For instance, knockdown of CDK4 in the mouse model of NSCLC results in the CDK6-induced expression of mutant K-Ras [86] and CDK4/6 inhibitor treatment in breast cancer cells results in the amplification of CDK6 through a feedback mechanism [87]. These findings indicate the need to develop CDK4- and CDK6-selective inhibitors. The high homology (>90%) in the ATP binding pocket of CDK4 and CDK6 makes the development of CDK4- and CDK6-selective inhibitors a challenging task.
The synthesis and screening of a focused set of PROTACs derived from the CDK4/6 inhibitors palbociclib, ribociclib and abemaciclib identified selective and non-selective CDK4 and CDK6 degraders. The ribociclib-pomalidomide PROTAC 32 (Figure 6) with a 7-atom linker selectively degraded CDK4 in a CRBN-dependent manner in Jurkat cells [88]. Another study that evaluated a palbociclib and a ribociclib PROTAC linked through a triazole showed that the palbociclib-based PROTAC pal-pom (33) degraded CDK4 with three-fold selectivity over CDK6 in MDA-MB-231 cells [60]. Despite available CDK4/6 inhibitors and E3-ligase binding ligands, the development of fully characterized CDK4-selective degraders are yet to be achieved.

2.3. CDK6

The Gray lab, our lab and Rao lab independently discovered CDK6-selective PROTACs (34, 36 and 37, Figure 7) by conjugating the linker through the solvent exposed piperazine on Palbociclib and functionalizing the 4-position of phthalimide. The three PROTACs had different linker lengths and linker compositions [46]. PROTAC 34 exhibited potent and selective CDK6 degradation, while the designed N-methylated thalidomide containing compound BSJ-bump (35) did not degrade CDK4 or CDK6. Both PROTAC 34 and parent inhibitor 6 induced a similar transcriptional response in AML cells, suggesting that the transcriptional role of CDK6 in AML is mainly limited to its kinase-dependent function. PROTAC 36 exhibited similar in vitro potency for CDK4 and CDK6. In MiaPaCa2 cells, PROTAC 36 with a linker length of 17 atoms selectively degraded CDK6 while sparing its close homolog CDK4. We also observed Hook effects [89] at 5 and 10 μM concentrations in both HPNE and MiaPaCa2 cell lines [90,91]. PROTAC 37 (CP-10) showed robust CDK6 degradation in human glioblastoma U251 cells [92]. Since CDK6 overexpression could result in resistance to CDK6 inhibitors, additional copies of CDK6 were introduced in A-673, an Ewing’s sarcoma cell line. In the above cell line, treatment with degrader 37 reduced CDK6 levels, suggesting that CDK6 PROTACs could be used to reverse CDK6 overexpression-induced drug resistance [92].
Multiple studies explored selectivity for CDK4 or CDK6 degradation by building focused sets of analogs with palbociclib, ribociclib and abemaciclib as the POI ligand, varying linker compositions, and E3-ligase ligands [60,93]. For the most part, PROTACs with shorter hydrophilic linker lengths and ribociclib as the POI ligand favored CDK4 degradation while those with longer hydrophobic linkers and palbociclib as the POI ligand resulted in selectivity for CDK6.
Steinebach and co-workers also explored the effect of linkers’ lengths and the type of E3 ligand on the activity and selectivity of PROTACs derived from CDK4/6 dual inhibitors [94]. Several PROTACs with varied linker lengths and E3 ligands i.e., CRL4CRBN, von Hippel Lindau (CRL2VHL), a cellular inhibitor of apoptosis protein 1 (cIAP1) and mouse double minute 2 homolog (MDM2) were synthesized [94]. The pomalidomide-based PROTAC 38 (Figure 7) reduced CDK4 and CDK6 levels in multiple myeloma (MM) cell lines at 100 nM concentration, but also degraded the Ikaros (IKZF1) and Aiolos (IKZF3) transcription factors. Interestingly, the effect of the linker length was not significant but the lipophilicity of PROTACs was crucial in determining its selectivity. In general, PROTACs with log D < 4, showed superior CDK6 degradation. Additionally, the PROTACs derived from different VHL ligands also showed pronounced degradation of CDK4 and CDK6 with some selectivity for CDK6. PROTACs synthesized using VHL ligands linked through the phenyl ring showed excellent selectivity toward CDK6 compared to those derived from the terminal amine group (39 vs. 40, Figure 7). Both PROTACs 39 and 40 were effective in degrading CDK6 in multiple cell lines. The IAP-based PROTAC 41 (Analog 35, Figure 7) non-selectively degraded CDK4 and CDK6. However, the MDM2-based PROTAC was unable to degrade CDK4 and CDK6, probably due to their poor cellular permeability.

2.4. CDK8

CDK8, a serine/threonine protein kinase, is overexpressed in colorectal, breast, and hematological malignancies, and in colon cancer, high CDK8 levels are associated with poor clinical outcomes [95]. CDK8 knockdown by short hairpin RNA (shRNA) suppressed proliferation in both in vitro and in vivo models [96]. CDK8 plays a key role in oncogenic signaling pathways, including the Wnt/β-catenin pathway [96], the p53 pathway [97], the NOTCH signaling pathway [98], and the TGF-β signaling pathway [99]. CDK8 regulates transcription through both its kinase activity and its scaffolding function [100], therefore CDK8 PROTACs are an attractive therapeutic option [101].
CDK8 has a high sequence similarity (>90%) with CDK19 [102]. Cortistatin A is a potent ATP competitive CDK8/19 selective inhibitor [103]. The complex total synthesis of Cortistatin (16–30 steps), along with the poor overall yield (0.012 and 2%) [104,105,106], limited the development of Cortistatin A-based PROTACs. Based on the X-ray crystal structure and mutagenesis studies [107], Hatcher et al. designed an analog by replacing the complex core in Cortistatin A with a simple steroid scaffold such as dehydroepiandrosterone (DHEA). Further optimization of this core resulted in the development of a potent CDK8 inhibitor with IC50 of 17 nM. A PEG linker that conjugated CDK8 inhibitor to pomalidomide yielded PROTAC JH-XI-10-02 (42, Figure 7) that induced partial degradation of CDK8 in Jurkat cells at 1 µM [103]. Studies with a negative control PROTAC 43 (Analog 31, Figure 7), that lacked the ability to bind CRBN, validated 42 as a CDK8-selective PROTAC.

2.5. CDK9

CDK9, a member of the positive transcription elongation factor b (P-TEFb) complex, associates with cyclin T and cyclin K to regulate transcriptional elongation by interacting with RNA polymerase II (RNAP II) [95]. Inhibition of CDK9 leads to reduced phosphorylation of RPB1, which results in a reduction in Mcl-1 levels. Mcl-1 is an anti-apoptotic protein that plays a crucial role in cancer cell survival [108,109,110]. Inhibition of CDK9 kinase activity resulted in the induction of apoptosis in several cancer cell lines [111,112,113,114,115]. These indicate that CDK9 is a promising cancer target, especially for cancers that are driven by transcriptional dysregulation [111,116].
We reported a PROTAC derived from a CDK2/5 aminopyrazole-based inhibitor that selectively degraded CDK9 without affecting the levels of CDK2, CDK5, AKT, FAK, and IKKβ [62,117]. In cellular studies, compound 3 reduced RPB1 phosphorylation, a direct substrate for CDK9, and reduced Mcl-1 levels. This was the first report that showed that the differential surface-exposed lysine residues among the CDKs could be exploited to convert a non-selective CDK inhibitor into a selective CDK9 degrader. In a follow-up study, we optimized linker length and linker composition to identify PROTAC 2 (44, Figure 8), which selectively degraded CDK9 with a DC50 value of 158 ± 6 nM [118]. Despite the remarkable selectivity for CDK9 degradation, as with compound 3, PROTAC 44 non-selectively inhibited CDK2, 5 and 9 in cell-free studies. Unlike the CDK9 inhibitor flavopiridol that reduced cyclin K levels, PROTAC 44 induced complete CDK9 degradation at 1 μM while sparing cyclin K. Recent studies demonstrated that concurrent inactivation of Mcl-1 and Bcl-xL resulted in robust induction of apoptosis [14,62,118,119,120] and CDK9 regulates the levels of pro-survival protein Mcl-1 [121,122]. Synergism studies showed PROTAC 44 sensitizes pancreatic cancer cells to Bcl2 inhibitors. PROTACs generated using two other aminopyrazole-based pan CDK inhibitors AT-7519, and FN-1501 resulted in CDK2/9 selective degraders. The FN-1501-based PROTAC induced G2/M arrest and inhibited the growth of PC3 cells with nanomolar potency [123].
Olson et al. developed a degrader, THAL-SNS-032 (45, Figure 8) by conjugating an N-acyl-2-aminothiazole CDK7/9 inhibitor 12 [124] to pomalidomide via a PEG linker. PROTAC 45 induced potent and selective degradation of CDK9 in a CRBN-dependent manner [111]. In a cell-free assay, PROTAC 45 non-selectivity inhibited CDK1/2/7/9. PROTAC 45 selectively induced CDK9 degradation and did not alter the levels of CDKs inhibited by 12 even at 5μM. At higher concentrations, PROTAC 45 exhibited a hook effect as well [125]. Consistent with CDK9′s role in promoting transcriptional elongation, PROTAC 45 inhibited phosphorylation of Ser2 C-terminal domain (CTD) of RNA Polymerase II in dose- and time-dependent studies, without effecting Ser5 and Ser7 phosphorylation [111].
Multiple independent PROTAC development studies with different potent and selective CDK9 ligands yielded mixed results. Replacing the N-acyl-2-aminothiazole kinase binding ligand in 45 with NVP-2, a highly selective aminopyrimidine-derived CDK9 inhibitor with sub-nM potency for CDK9, resulted in weaker CDK9 degradation [111,126]. Wogonin is a selective CDK9 inhibitor with an IC50 of 0.19 µM [127]. A docking guided PROTAC library design identified 11c that showed sub-micromolar biochemical inhibition of CDK9/CycT1 [128], but degraded CDK9 with a DC50 ~ 10 μM and reduced Mcl-1 levels in a dose-dependent study [129,130]. Conjugating the CDK9-selective inhibitor BAY-1143572 through the solvent-exposed group with pomalidomide led to the identification of PROTAC B03 (46, Figure 8) [131]. In cell-free studies, PROTAC 46 inhibited CDK9 with IC50 ~ 8 nM, which was comparable to the parent inhibitor (IC50 ~ 13 nM). PROTAC 46 reduced the levels of CDK9 (DC50 ~ 7 nM) and Mcl-1 levels in a time-dependent study. Moreover, PROTAC 46 exhibited improved anti-proliferative activity when compared to its parent inhibitor [131]. Together, these results suggest that the efficacy of CDK9 degradation by various PROTACs does not correlate with the potency and selectivity for CDK9 inhibition by the parent inhibitor.

2.6. CDK12

CDK12 phosphorylates RNA polymerase II to regulate transcription elongation [132]. In addition, CDK12 has also been reported to play a critical role in RNA splicing, DNA damage response (DDR), and the maintenance of genomic stability [133,134]. Loss of CDK12 results in reduced transcription of BRCA1 and increases sensitivity to PARP inhibitors and platinum chemotherapy. Thus, CDK12 inhibitors could be used in combination with DNA-damaging agents in HR-deficient cancers [12]. CDK12 is overexpressed in a variety of cancers, including breast cancer, ovarian cancer, prostate cancer, and gastric cancer, making it a viable cancer target [132]. The development of CDK12 inhibitors is particularly challenging due to high sequence similarity with its close homolog CDK13. The Gray lab developed a CDK12-selective degrader, BSJ-4-116 (47, Figure 8), by attaching 3-aminopiperidine moiety, which was important for the selectivity for CDK12 over its close homolog CDK13, to a promiscuous multi kinase degrader TL12-186 [64,135]. In cellular assays, PROTAC 47 selectively degraded CDK12 in a dose- and a time-dependent study, while sparing CDK13. A robust ternary complex was observed between CDK12 and CRBN in the presence of 47, which was abolished with inactive PROTAC BSJ-4-116-NC (48). On the other hand, no such complex was induced between CDK13 and CRBN. The control 48 was ten-fold less potent than 47 in inhibiting cell growth, indicating CDK12 degradation as the primary mode of action of 47. Since CDK12 is implicated in the regulation of DDR genes, degrader 47 treatment resulted in the downregulation of transcription genes involved in DDR pathways. In Jurkat and MOLT4 cells, PROTAC 47 and PARP inhibitors showed strong synergism. Interestingly, chronic exposure of MOLT-4 and Jurkat cells to degrader 47 resulted in the development of CDK12 degradation resistance via mutations of Ile733Val and Gly739Ser on the CDK12 G-loop, which resulted in reduced degrader 47 binding.

3. Perturbing CDK Function by a Molecular Glue

In contrast to heterobifunctional PROTAC degraders (Figure 2), molecular glues are low molecular weight inducers (novel de novo contacts) or stabilizers of protein–protein interactions [136,137]. Upon binding to a protein, the small molecule induces a conformational change and makes the small molecule–protein complex a “neo-substrate” for E3 ligase (Figure 9). Following the formation of a ternary complex, the “neo-substrate” is ubiquitinated, which leads to UPS-mediated protein degradation. Although these glues were identified serendipitously, this is an emerging and exciting therapeutic strategy for the inactivation of intractable targets.
Thalidomide was prescribed to treat morning sickness in pregnant women in the late 1950s and was withdrawn in the early 1960s because of catastrophic teratogenicity. Studies conducted to understand the associated biology resulted in the characterization of Thalidomide as a molecular glue [138]. The mechanism of action of Thalidomide and its analogs, termed immune-modulatory imine drugs (IMiDs), involves the recruitment of the Ikaros zinc family transcription factor (IKZF1 and IKZF3), CK1α, GSPT1, and others to cullin-RING E3 ligase CUL4CRBN, resulting in their proteasomal degradation [31,32,139,140,141,142,143]. IMiDs, thalidomide, lenalidomide, and pomalidomide are first line therapeutics for multiple myeloma [144,145]. This discovery of IMiDs binding to CRBN led to the explosion of heterobifunctional PROTACs that recruit novel proteins of interest for proteasomal degradation [62,90,118,125]. Another class of molecular glue degrader is Indisulam, an aryl sulfonamides drug that recruits splicing protein RNF39 to DDB1-associated and CUL4-associated factor 15 (DCAF15 E3 ligase), which resulted in RNF39 ubiquitination, followed by its degradation [56,146,147,148]. As a result, Indisulam is being used as an E3 ligase recruiter for PROTAC development. Li et al. successfully demonstrated BRD4 degradation by recruiting DCAF15 in a PROTAC-dependent manner [42].
A rational design was used by Slabicki et al. to discover (R)-CR8 (49, Figure 8) as a cyclin K molecular glue degrader [149]. The glue 49 is an analog of roscovitine (2), which was previously reported as a CDK1/2/3/5/7/9 inhibitor [150]. A correlation of drug-sensitivity data of over 4500 drugs against 578 cancer cell lines and the expression of 499 E3 ligase components showed a positive correlation between the cytotoxicity of 49 and the mRNA levels of DDB1, a CUL4 adaptor protein. A quantitative proteome-wide mass spectrometry study showed a decrease in cyclin K in 49-treated cells, which was rescued by the pretreatment of cells with MLN7243 (an E1 ubiquitin-activating enzyme inhibitor), MLN4924, and MG132. Biophysical, point mutation, and X-ray crystallographic studies showed that in the presence of 49, CDK12 interacts with DDB1. Cyclin K is a binding partner of CDK12, and overlays showed that it did not make direct contact with DDB1. Interestingly, in the DDB1-CDK12-Cyclin K conformation, cyclin K was transformed into a “neo-substrate” for CUL4 and was degraded. Glue 49-induced cyclin K degradation was blocked by co-treatment with either the inhibitor of E1 ubiquitin-activating enzyme (MLN7243) or MLN4924. CRISPR-Cas9 resistance screens demonstrated the involvement of the CUL4 complex components in mediating 49-induced toxicity. In vitro ubiquitination assays showed that the CUL4 complex core that included RBX1-DDB1 was sufficient for 49 induced cyclin K degradation. Other CDKs (9 and 13) were also associated with DDB1 in the presence of 49. However, these complexes did not degrade cyclin K to the same degree as the CDK12-DDB1 complex. X-ray crystallographic studies showed that the purine core of analog 49 occupied the ATP-binding site of CDK12, whereas the hydrophobic phenylpyridine ring interacted with the BPC domain of DDB1. Structure–activity relationship (SAR) studies showed that any alteration to the phenylpyridine ring resulted in diminished activity, suggesting the importance of the terminal groups that interact with DDB1 for robust cyclin K degradation. This study, along with another recent report, showed that the minor modification of a surface-exposed region of a small molecule converted it from an inhibitor to a glue [151,152], which suggests that exploiting the surface-exposed regions of CDK inhibitors could yield CDK-selective molecular glues.

4. Conclusions and Outlook

Novel small molecules that modulate the function or levels of specific proteins are beginning to emerge as the primary modality for cancer therapy over other strategies such as surgery, chemotherapy, and radiation therapy. In recent years, kinase inhibitors have emerged as a successful class of targeted therapeutics. Several kinase inhibitors are in clinical use or under clinical/preclinical studies. The CDK family of proteins are an important class of kinases that play prominent roles in regulating cellular transcription, mRNA processing, and the cell cycle, and thus, have become attractive therapeutic targets for cancer. The similar ATP binding sites among CDKs make the development of selective CDK inhibitors a challenging task. Even FDA-approved drugs such as palbociclib, ribociclib and abemaciclib are dual CDK4/6 inhibitors. Moreover, most of the CDKs inhibitors developed to date are non-covalent; thus, accurately estimating the dosage required to inhibit the kinase continues to vex translational scientists. PROTACs and molecular glues have emerged as an alternative approach that relies on artificially inducing degradation of the targeted protein by hijacking the cellular quality-control machinery. PROTACs and molecular glues rely on the topology of surface-exposed lysine residues for their activity and, therefore, provide a different modality to develop CDK-selective modulators. Moreover, the “catalytic nature” and “event-driven” pharmacology of PROTACs/molecular glues allows sub-stoichiometric dosing to eliminate the targeted CDK.
Here, we summarized the reported CDK selective PROTACs. Several factors such as the differential distribution of lysine residues, stable ternary complex formation, structure complementarity of CDK, and the components of the E-ligase complex dictate selective degradation. Unlike kinase inhibitors, whose efficacy is closely associated with its binding affinity, the linker length, linker composition, and the choice of the E3 ligand play a greater role in dictating the potency and selectivity of PROTACs. Nonetheless, the design of any PROTAC is an iterative process, and extensive optimization is needed to develop a potent and selective degrader. Among others, hook effects and unknown off-target effects are likely to be the growing pains as the PROTAC-based strategies mature. Most of the PROTACs developed thus far have not been evaluated for in vivo efficacy and, therefore, would require further optimization to achieve the desired pharmacokinetic (PK) profile. The successful development of prodrug 33 (Figure 7), which improved the oral bioavailability of parent PROTAC from 1 to 68%, suggests that PROTACs can be optimized for PK [81]. Presently, the PROTACs ARV-471 and ARV-110 are in Phase II clinical trials, and the PROTACs KT-474 and NX-2127 are in phase I clinical trials. These early clinical candidates provide the scientific community with the necessary drive to continue to pursue PROTACs as an alternative therapeutic strategy. Future studies will focus on developing disease-specific PROTACs either by exploring novel disease-specific POI and/or using ligands that target tissue-specific E3 ligases. Alternative strategies to achieve tissue specificity could include developing antibody PROTAC conjugates for targeted delivery to the tumors [67]. The recent discovery of molecular glues, which are small molecule inhibitors with minor modification of surface-exposed regions, for targeted protein degradation [151,152], also indicates a bright future for these therapeutic modalities that exploit the cellular protein degradation machinery.

Author Contributions

A.N. and S.R. conceptualized the work. S.R., J.R.M., S.S., L.B. and A.N. wrote the manuscript. S.R. and A.N. designed the tables and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported in part by NIH grants GM121316 and CA036727. L.B. is supported by T32CA009476.

Conflicts of Interest

The authors declare no conflict of interest, financial or otherwise.

References

  1. Duong-Ly, K.C.; Peterson, J.R. The human kinome and kinase inhibition. Curr. Protoc. Pharmacol. 2013, 60, 2–9. [Google Scholar] [CrossRef] [Green Version]
  2. Gao, R.; Stock, A.M. Biological insights from structures of two-component proteins. Annu. Rev. Microbiol. 2009, 63, 133–154. [Google Scholar] [CrossRef] [Green Version]
  3. Ferris, H.U.; Dunin-Horkawicz, S.; Hornig, N.; Hulko, M.; Martin, J.; Schultz, J.E.; Zeth, K.; Lupas, A.N.; Coles, M. Mechanism of regulation of receptor histidine kinases. Structure 2012, 20, 56–66. [Google Scholar] [CrossRef] [Green Version]
  4. Bhullar, K.S.; Lagaron, N.O.; McGowan, E.M.; Parmar, I.; Jha, A.; Hubbard, B.P.; Rupasinghe, H.P.V. Kinase-targeted cancer therapies: Progress, challenges and future directions. Mol. Cancer 2018, 17, 48. [Google Scholar] [CrossRef]
  5. Gross, S.; Rahal, R.; Stransky, N.; Lengauer, C.; Hoeflich, K.P. Targeting cancer with kinase inhibitors. J. Clin. Investig. 2015, 125, 1780–1789. [Google Scholar] [CrossRef]
  6. Benn, C.L.; Dawson, L.A. Clinically Precedented Protein Kinases: Rationale for Their Use in Neurodegenerative Disease. Front. Aging Neurosci. 2020, 12, 242. [Google Scholar] [CrossRef]
  7. Roskoski, R., Jr. Properties of FDA-approved small molecule protein kinase inhibitors: A 2021 update. Pharmacol. Res. 2021, 165, 105463. [Google Scholar] [CrossRef]
  8. Modi, V.; Dunbrack, R.L., Jr. Defining a new nomenclature for the structures of active and inactive kinases. Proc. Natl. Acad. Sci. USA 2019, 116, 6818–6827. [Google Scholar] [CrossRef] [Green Version]
  9. Bernetti, M.; Masetti, M.; Rocchia, W.; Cavalli, A. Kinetics of Drug Binding and Residence Time. Annu. Rev. Phys. Chem. 2019, 70, 143–171. [Google Scholar] [CrossRef] [Green Version]
  10. Potashman, M.H.; Duggan, M.E. Covalent modifiers: An orthogonal approach to drug design. J. Med. Chem. 2009, 52, 1231–1246. [Google Scholar] [CrossRef]
  11. Abdeldayem, A.; Raouf, Y.S.; Constantinescu, S.N.; Moriggl, R.; Gunning, P.T. Advances in covalent kinase inhibitors. Chem. Soc. Rev. 2020, 49, 2617–2687. [Google Scholar] [CrossRef]
  12. Malumbres, M.; Barbacid, M. Cell cycle, CDKs and cancer: A changing paradigm. Nat. Rev. Cancer 2009, 9, 153–166. [Google Scholar] [CrossRef]
  13. Sonawane, Y.A.; Taylor, M.A.; Napoleon, J.V.; Rana, S.; Contreras, J.I.; Natarajan, A. Cyclin Dependent Kinase 9 Inhibitors for Cancer Therapy. J. Med. Chem. 2016, 59, 8667–8684. [Google Scholar] [CrossRef]
  14. Robb, C.M.; Kour, S.; Contreras, J.I.; Agarwal, E.; Barger, C.J.; Rana, S.; Sonawane, Y.; Neilsen, B.K.; Taylor, M.; Kizhake, S.; et al. Characterization of CDK(5) inhibitor, 20-223 (aka CP668863) for colorectal cancer therapy. Oncotarget 2018, 9, 5216–5232. [Google Scholar] [CrossRef] [Green Version]
  15. Chi, Y.; Carter, J.H.; Swanger, J.; Mazin, A.V.; Moritz, R.L.; Clurman, B.E. A novel landscape of nuclear human CDK2 substrates revealed by in situ phosphorylation. Sci. Adv. 2020, 6, eaaz9899. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  16. Loyer, P.; Trembley, J.H.; Katona, R.; Kidd, V.J.; Lahti, J.M. Role of CDK/cyclin complexes in transcription and RNA splicing. Cell. Signal. 2005, 17, 1033–1051. [Google Scholar] [CrossRef]
  17. Kour, S.; Rana, S.; Contreras, J.I.; King, H.M.; Robb, C.M.; Sonawane, Y.A.; Bendjennat, M.; Crawford, A.J.; Barger, C.J.; Kizhake, S.; et al. CDK5 Inhibitor Downregulates Mcl-1 and Sensitizes Pancreatic Cancer Cell Lines to Navitoclax. Mol. Pharmacol. 2019, 96, 419–429. [Google Scholar] [CrossRef] [PubMed]
  18. Finn, R.S.; Dering, J.; Conklin, D.; Kalous, O.; Cohen, D.J.; Desai, A.J.; Ginther, C.; Atefi, M.; Chen, I.; Fowst, C.; et al. PD 0332991, a selective cyclin D kinase 4/6 inhibitor, preferentially inhibits proliferation of luminal estrogen receptor-positive human breast cancer cell lines in vitro. Breast Cancer Res. 2009, 11, R77. [Google Scholar] [CrossRef] [Green Version]
  19. Rader, J.; Russell, M.R.; Hart, L.S.; Nakazawa, M.S.; Belcastro, L.T.; Martinez, D.; Li, Y.; Carpenter, E.L.; Attiyeh, E.F.; Diskin, S.J.; et al. Dual CDK4/CDK6 inhibition induces cell-cycle arrest and senescence in neuroblastoma. Clin. Cancer Res. 2013, 19, 6173–6182. [Google Scholar] [CrossRef] [Green Version]
  20. Corona, S.P.; Generali, D. Abemaciclib: A CDK4/6 inhibitor for the treatment of HR+/HER2- advanced breast cancer. Drug Des. Dev. Ther. 2018, 12, 321–330. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  21. ClinicalTrials.gov. Available online: https://clinicaltrials.gov/ (accessed on 31 March 2021).
  22. Sakamoto, K.M.; Kim, K.B.; Kumagai, A.; Mercurio, F.; Crews, C.M.; Deshaies, R.J. Protacs: Chimeric molecules that target proteins to the Skp1-Cullin-F box complex for ubiquitination and degradation. Proc. Natl. Acad. Sci. USA 2001, 98, 8554–8559. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  23. Cromm, P.M.; Samarasinghe, K.T.G.; Hines, J.; Crews, C.M. Addressing Kinase-Independent Functions of Fak via PROTAC-Mediated Degradation. J. Am. Chem. Soc. 2018, 140, 17019–17026. [Google Scholar] [CrossRef]
  24. Burslem, G.M.; Smith, B.E.; Lai, A.C.; Jaime-Figueroa, S.; McQuaid, D.C.; Bondeson, D.P.; Toure, M.; Dong, H.; Qian, Y.; Wang, J.; et al. The Advantages of Targeted Protein Degradation Over Inhibition: An RTK Case Study. Cell Chem. Biol. 2018, 25, 67–77. [Google Scholar] [CrossRef]
  25. Wang, S.; Han, L.; Han, J.; Li, P.; Ding, Q.; Zhang, Q.-J.; Liu, Z.-P.; Chen, C.; Yu, Y. Uncoupling of PARP1 trapping and inhibition using selective PARP1 degradation. Nat. Chem. Biol. 2019, 15, 1223–1231. [Google Scholar] [CrossRef]
  26. Burslem, G.M.; Song, J.; Chen, X.; Hines, J.; Crews, C.M. Enhancing Antiproliferative Activity and Selectivity of a FLT-3 Inhibitor by Proteolysis Targeting Chimera Conversion. J. Am. Chem. Soc. 2018, 140, 16428–16432. [Google Scholar] [CrossRef]
  27. Bai, L.; Zhou, H.; Xu, R.; Zhao, Y.; Chinnaswamy, K.; McEachern, D.; Chen, J.; Yang, C.-Y.; Liu, Z.; Wang, M.; et al. A Potent and Selective Small-Molecule Degrader of STAT3 Achieves Complete Tumor Regression In Vivo. Cancer Cell 2019, 36, 498–511. [Google Scholar] [CrossRef] [PubMed]
  28. Han, X.; Wang, C.; Qin, C.; Xiang, W.; Fernandez-Salas, E.; Yang, C.-Y.; Wang, M.; Zhao, L.; Xu, T.; Chinnaswamy, K.; 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, 941–964. [Google Scholar] [CrossRef] [PubMed]
  29. He, Y.; Zhang, X.; Chang, J.; Kim, H.-N.; Zhang, P.; Wang, Y.; Khan, S.; Liu, X.; Zhang, X.; Lv, D.; et al. Using proteolysis-targeting chimera technology to reduce navitoclax platelet toxicity and improve its senolytic activity. Nat. Commun. 2020, 11, 1996. [Google Scholar] [CrossRef]
  30. Hu, J.; Hu, B.; Wang, M.; Xu, F.; Miao, B.; Yang, C.-Y.; Wang, M.; Liu, Z.; Hayes, D.F.; Chinnaswamy, K.; et al. Discovery of ERD-308 as a Highly Potent Proteolysis Targeting Chimera (PROTAC) Degrader of Estrogen Receptor (ER). J. Med. Chem. 2019, 62, 1420–1442. [Google Scholar] [CrossRef]
  31. Ito, T.; Ando, H.; Suzuki, T.; Ogura, T.; Hotta, K.; Imamura, Y.; Yamaguchi, Y.; Handa, H. Identification of a Primary Target of Thalidomide Teratogenicity. Science 2010, 327, 1345–1350. [Google Scholar] [CrossRef] [Green Version]
  32. Fischer, E.S.; Boehm, K.; Lydeard, J.R.; Yang, H.; Stadler, M.B.; Cavadini, S.; Nagel, J.; Serluca, F.; Acker, V.; Lingaraju, G.M.; et al. Structure of the DDBI-CRBN E3 ubiquitin ligase in complex with thalidomide. Nature 2014, 512, 49–53. [Google Scholar] [CrossRef] [Green Version]
  33. Buckley, D.L.; Gustafson, J.L.; Van Molle, I.; Roth, A.G.; Tae, H.S.; Gareiss, P.C.; Jorgensen, W.L.; Ciulli, A.; Crews, C.M. Small-Molecule Inhibitors of the Interaction between the E3 Ligase VHL and HIF1a. Angew. Chem. Int. Ed. 2012, 51, 11463–11467. [Google Scholar] [CrossRef] [Green Version]
  34. Buckley, D.L.; Van Molle, I.; Gareiss, P.C.; Tae, H.S.; Michel, J.; Noblin, D.J.; Jorgensen, W.L.; Ciulli, A.; Crews, C.M. Targeting the von Hippel-Lindau E3 Ubiquitin Ligase Using Small Molecules To Disrupt the VHL/HIF-1 alpha Interaction. J. Am. Chem. Soc. 2012, 134, 4465–4468. [Google Scholar] [CrossRef] [PubMed]
  35. Galdeano, C.; Gadd, M.S.; Soares, P.; Scaffidi, S.; Van Molle, I.; Birced, I.; Hewitt, S.; Dias, D.M.; Ciulli, A. Structure-guided design and optimization of small molecules targeting the protein-protein interaction between the von Hippel-Lindau (VHL) E3 ubiquitin ligase and the hypoxia inducible factor (HIF) alpha subunit with in vitro nanomolar affinities. J. Med. Chem. 2014, 57, 8657–8663. [Google Scholar] [CrossRef]
  36. Hines, J.; Lartigue, S.; Dong, H.; Qian, Y.; Crews, C.M. MDM2-Recruiting PROTAC Offers Superior, Synergistic Antiproliferative Activity via Simultaneous Degradation of BRD4 and Stabilization of p53. Cancer Res. 2019, 79, 251–262. [Google Scholar] [CrossRef] [Green Version]
  37. Tovar, C.; Rosinski, J.; Filipovic, Z.; Higgins, B.; Kolinsky, K.; Hilton, H.; Zhao, X.L.; Vu, B.T.; Qing, W.G.; Packman, K.; et al. Small-molecule MDM2 antagonists reveal aberrant p53 signaling in cancer: Implications for therapy. Proc. Natl. Acad. Sci. USA 2006, 103, 1888–1893. [Google Scholar] [CrossRef] [Green Version]
  38. Wang, S.; Zhao, Y.; Aguilar, A.; Bernard, D.; Yang, C.-Y. Targeting the MDM2-p53 Protein-Protein Interaction for New Cancer Therapy: Progress and Challenges. Cold Spring Harb. Perspect. Med. 2017, 7, a026245. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  39. Shibata, N.; Nagai, K.; Morita, Y.; Ujikawa, O.; Ohoka, N.; Hattori, T.; Koyama, R.; Sano, O.; Imaeda, Y.; Nara, H.; et al. Development of Protein Degradation Inducers of Androgen Receptor by Conjugation of Androgen Receptor Ligands and Inhibitor of Apoptosis Protein Ligands. J. Med. Chem. 2018, 61, 543–575. [Google Scholar] [CrossRef]
  40. Tong, B.; Spradlin, J.N.; Novaes, L.F.T.; Zhang, E.; Hu, X.; Moeller, M.; Brittain, S.M.; McGregor, L.M.; McKenna, J.M.; Tallarico, J.A.; et al. A Nimbolide-Based Kinase Degrader Preferentially Degrades Oncogenic BCR-ABL. ACS Chem. Biol. 2020, 15, 1788–1794. [Google Scholar] [CrossRef] [PubMed]
  41. Luo, M.; Spradlin, J.N.; Boike, L.; Tong, B.; Brittain, S.M.; McKenna, J.M.; Tallarico, J.A.; Schirle, M.; Maimone, T.J.; Nomura, D.K. Chemoproteomics-enabled discovery of covalent RNF114-based degraders that mimic natural product function. Cell Chem. Biol. 2021, 28, 559–566. [Google Scholar] [CrossRef]
  42. Li, L.; Mi, D.; Pei, H.; Duan, Q.; Wang, X.; Zhou, W.; Jin, J.; Li, D.; Liu, M.; Chen, Y. In vivo target protein degradation induced by PROTACs based on E3 ligase DCAF15. Signal Transduct. Target. Ther. 2020, 5, 129. [Google Scholar] [CrossRef]
  43. Zhang, X.; Crowley, V.M.; Wucherpfennig, T.G.; Dix, M.M.; Cravatt, B.F. Electrophilic PROTACs that degrade nuclear proteins by engaging DCAF16. Nat. Chem. Biol. 2019, 15, 737–746. [Google Scholar] [CrossRef] [PubMed]
  44. Henning, N.J.; Manford, A.G.; Spradlin, J.N.; Brittain, S.M.; McKenna, J.M.; Tallarico, J.A.; Schirle, M.; Rape, M.; Nomura, D.K. Discovery of a Covalent FEM1B Recruiter for Targeted Protein Degradation Applications. bioRxiv 2021. [Google Scholar] [CrossRef]
  45. An, Z.; Lv, W.; Su, S.; Wu, W.; Rao, Y. Developing potent PROTACs tools for selective degradation of HDAC6 protein. Protein Cell 2019, 10, 606–609. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  46. Brand, M.; Jiang, B.; Bauer, S.; Donovan, K.A.; Liang, Y.; Wang, E.S.; Nowak, R.P.; Yuan, J.C.; Zhang, T.; Kwiatkowski, N.; et al. Homolog-Selective Degradation as a Strategy to Probe the Function of CDK6 in AML. Cell Chem. Biol. 2019, 26, 300–306. [Google Scholar] [CrossRef]
  47. Burslem, G.M.; Schultz, A.R.; Bondeson, D.P.; Eide, C.A.; Stevens, S.L.S.; Druker, B.J.; Crews, C.M. Targeting BCR-ABL1 in Chronic Myeloid Leukemia by PROTAC-Mediated Targeted Protein Degradation. Cancer Res. 2019, 79, 4744–4753. [Google Scholar] [CrossRef]
  48. Cance, W.G.; Kurenova, E.; Marlowe, T.; Golubovskaya, V. Disrupting the Scaffold to Improve Focal Adhesion Kinase-Targeted Cancer Therapeutics. Sci. Signal. 2013, 6, pe10. [Google Scholar] [CrossRef] [Green Version]
  49. Crew, A.P.; Raina, K.; Dong, H.; Qian, Y.; Wang, J.; Vigil, D.; Serebrenik, Y.V.; Hamman, B.D.; Morgan, A.; Ferraro, C.; et al. Identification and Characterization of Von Hippel-Lindau-Recruiting Proteolysis Targeting Chimeras (PROTACs) of TANK-Binding Kinase 1. J. Med. Chem. 2018, 61, 583–598. [Google Scholar] [CrossRef]
  50. Demizu, Y.; Okuhira, K.; Motoi, H.; Ohno, A.; Shoda, T.; Fukuhara, K.; Okuda, H.; Naito, M.; Kurihara, M. Design and synthesis of estrogen receptor degradation inducer based on a protein knockdown strategy. Bioorg. Med. Chem. Lett. 2012, 22, 1793–1796. [Google Scholar] [CrossRef]
  51. Demizu, Y.; Shibata, N.; Hattori, T.; Ohoka, N.; Motoi, H.; Misawa, T.; Shoda, T.; Naito, M.; Kurihara, M. Development of BCR-ABL degradation inducers via the conjugation of an imatinib derivative and a cIAP1 ligand. Bioorg. Med. Chem. Lett. 2016, 26, 4865–4869. [Google Scholar] [CrossRef] [PubMed]
  52. Farnaby, W.; Koegl, M.; Roy, M.J.; Whitworth, C.; Diers, E.; Trainor, N.; Zollman, D.; Steurer, S.; Karolyi-Oezguer, J.; Riedmueller, C.; et al. BAF complex vulnerabilities in cancer demonstrated via structure-based PROTAC design. Nat. Chem. Biol. 2019, 15, 672–680. [Google Scholar] [CrossRef]
  53. Fry, D.W.; Harvey, P.J.; Keller, P.R.; Elliott, W.L.; Meade, M.A.; Trachet, E.; Albassam, M.; Zheng, X.X.; Leopold, W.R.; Pryer, N.K.; et al. Specific inhibition of cyclin-dependent kinase 4/6 by PD 0332991 and associated antitumor activity in human tumor xenografts. Mol. Cancer Ther. 2004, 3, 1427–1437. [Google Scholar] [PubMed]
  54. Gabizon, R.; Shraga, A.; Gehrtz, P.; Livnah, E.; Shorer, Y.; Gurwicz, N.; Avram, L.; Unger, T.; Aharoni, H.; Albeck, S.; et al. Efficient Targeted Degradation via Reversible and Irreversible Covalent PROTACs. J. Am. Chem. Soc. 2020, 142, 11734–11742. [Google Scholar] [CrossRef]
  55. Gechijian, L.N.; Buckley, D.L.; Lawlor, M.A.; Reyes, J.M.; Paulk, J.; Ott, C.J.; Winter, G.E.; Erb, M.A.; Scott, T.G.; Xu, M.; et al. Functional TRIM24 degrader via conjugation of ineffectual bromodomain and VHL ligands. Nat. Chem. Biol. 2018, 14, 405–412. [Google Scholar] [CrossRef]
  56. Han, T.; Goralski, M.; Gaskill, N.; Capota, E.; Kim, J.; Ting, T.C.; Xie, Y.; Williams, N.S.; Nijhawan, D. Anticancer sulfonamides target splicing by inducing RBM39 degradation via recruitment to DCAF15. Science 2017, 356, eaal3755. [Google Scholar] [CrossRef] [PubMed]
  57. Han, X.; Zhao, L.; Xiang, W.; Qin, C.; Miao, B.; Xu, T.; Wang, M.; Yang, C.-Y.; Chinnaswamy, K.; Stuckey, J.; et al. Discovery of Highly Potent and Efficient PROTAC Degraders of Androgen Receptor (AR) by Employing Weak Binding Affinity VHL E3 Ligase Ligands. J. Med. Chem. 2019, 62, 11218–11231. [Google Scholar] [CrossRef] [PubMed]
  58. Hsu, J.H.-R.; Rasmusson, T.; Robinson, J.; Pachl, F.; Read, J.; Kawatkar, S.; O’Donovan, D.H.; Bagal, S.; Code, E.; Rawlins, P.; et al. EED-Targeted PROTACs Degrade EED, EZH2, and SUZ12 in the PRC2 Complex. Cell Chem. Biol. 2020, 27, 41–46. [Google Scholar] [CrossRef]
  59. Humphreys, P.G.; Bamborough, P.; Chung, C.-w.; Crags, P.D.; Gordon, L.; Grandi, P.; Hayhow, T.G.; Hussain, J.; Jones, K.L.; Lindon, M.; et al. Discovery of a Potent, Cell Penetrant, and Selective p300/CBP-Associated Factor (PCAF)/General Control Nonderepressible 5 (GCN5) Bromodomain Chemical Probe. J. Med. Chem. 2017, 60, 695–709. [Google Scholar] [CrossRef] [PubMed]
  60. Jiang, B.; Wang, E.S.; Donovan, K.A.; Liang, Y.; Fischer, E.S.; Zhang, T.; Gray, N.S. Development of Dual and Selective Degraders of Cyclin-Dependent Kinases 4 and 6. Angew. Chem. Int. Ed. 2019, 58, 6321–6326. [Google Scholar] [CrossRef]
  61. Khan, S.; Zhang, X.; Lv, D.; Zhang, Q.; He, Y.; Zhang, P.; Liu, X.; Thummuri, D.; Yuan, Y.; Wiegand, J.S.; et al. A selective BCL-X-L PROTAC degrader achieves safe and potent antitumor activity. Nat. Med. 2019, 25, 1938–1947. [Google Scholar] [CrossRef]
  62. Robb, C.M.; Contreras, J.I.; Kour, S.; Taylor, M.A.; Abid, M.; Sonawane, Y.A.; Zahid, M.; Murry, D.J.; Natarajan, A.; Rana, S. Chemically induced degradation of CDK9 by a proteolysis targeting chimera (PROTAC). Chem. Commun. 2017, 53, 7577–7580. [Google Scholar] [CrossRef] [PubMed]
  63. Bondeson, D.P.; Smith, B.E.; Burslem, G.M.; Buhimschi, A.D.; Hines, J.; Jaime-Figueroa, S.; Wang, J.; Hamman, B.D.; Ishchenko, A.; Crews, C.M. Lessons in PROTAC Design from Selective Degradation with a Promiscuous Warhead. Cell Chem. Biol. 2018, 25, 78–87. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  64. Huang, H.-T.; Dobrovolsky, D.; Paulk, J.; Yang, G.; Weisberg, E.L.; Doctor, Z.M.; Buckley, D.L.; Cho, J.-H.; Ko, E.; Jang, J.; et al. A Chemoproteomic Approach to Query the Degradable Kinome Using a Multi-kinase Degrader. Cell Chem. Biol. 2018, 25, 88–99. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  65. Bond, M.J.; Chu, L.; Nalawansha, D.A.; Li, K.; Crews, C.M. Targeted Degradation of Oncogenic KRAS(G12C) by VHL-Recruiting PROTACs. ACS Cent. Sci. 2020, 6, 1367–1375. [Google Scholar] [CrossRef]
  66. Zeng, M.; Xiong, Y.; Safaee, N.; Nowak, R.P.; Donovan, K.A.; Yuan, C.J.; Nabet, B.; Gero, T.W.; Feru, F.; Li, L.; et al. Exploring Targeted Degradation Strategy for Oncogenic KRAS(G12C). Cell Chem. Biol. 2020, 27, 19–31 e16. [Google Scholar] [CrossRef]
  67. Maneiro, M.; Forte, N.; Shchepinova, M.M.; Kounde, C.S.; Chudasama, V.; Baker, J.R.; Tate, E.W. Antibody-PROTAC Conjugates Enable HER2-Dependent Targeted Protein Degradation of BRD4. ACS Chem. Biol. 2020, 15, 1306–1312. [Google Scholar] [CrossRef]
  68. Tadesse, S.; Anshabo, A.T.; Portman, N.; Lim, E.; Tilley, W.; Caldon, C.E.; Wang, S. Targeting CDK2 in cancer: Challenges and opportunities for therapy. Drug Discov. Today 2020, 25, 406–413. [Google Scholar] [CrossRef]
  69. Otto, T.; Sicinski, P. Cell cycle proteins as promising targets in cancer therapy. Nat. Rev. Cancer 2017, 17, 93–115. [Google Scholar] [CrossRef] [Green Version]
  70. Ma, T.; Van Tine, B.A.; Wei, Y.; Garrett, M.D.; Nelson, D.; Adams, P.D.; Wang, J.; Qin, J.; Chow, L.T.; Harper, J.W. Cell cycle-regulated phosphorylation of p220(NPAT) by cyclin E/Cdk2 in Cajal bodies promotes histone gene transcription. Genes Dev. 2000, 14, 2298–2313. [Google Scholar] [CrossRef] [Green Version]
  71. Okuda, M.; Horn, H.F.; Tarapore, P.; Tokuyama, Y.; Smulian, A.G.; Chan, P.K.; Knudsen, E.S.; Hofmann, I.A.; Snyder, J.D.; Bove, K.E.; et al. Nucleophosmin/B23 is a target of CDK2/cyclin E in centrosome duplication. Cell 2000, 103, 127–140. [Google Scholar] [CrossRef] [Green Version]
  72. Scaltriti, M.; Eichhorn, P.J.; Cortes, J.; Prudkin, L.; Aura, C.; Jimenez, J.; Chandarlapaty, S.; Serra, V.; Prat, A.; Ibrahim, Y.H.; et al. Cyclin E amplification/overexpression is a mechanism of trastuzumab resistance in HER2+ breast cancer patients. Proc. Natl. Acad. Sci. USA 2011, 108, 3761–3766. [Google Scholar] [CrossRef] [Green Version]
  73. Etemadmoghadam, D.; Weir, B.A.; Au-Yeung, G.; Alsop, K.; Mitchell, G.; George, J.; Australian Ovarian Cancer Study Group; Davis, S.; D’Andrea, A.D.; Simpson, K.; et al. Synthetic lethality between CCNE1 amplification and loss of BRCA1. Proc. Natl. Acad. Sci. USA 2013, 110, 19489–19494. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  74. Handa, K.; Yamakawa, M.; Takeda, H.; Kimura, S.; Takahashi, T. Expression of cell cycle markers in colorectal carcinoma: Superiority of cyclin A as an indicator of poor prognosis. Int. J. Cancer 1999, 84, 225–233. [Google Scholar] [CrossRef]
  75. Michalides, R.; van Tinteren, H.; Balkenende, A.; Vermorken, J.B.; Benraadt, J.; Huldij, J.; van Diest, P. Cyclin A is a prognostic indicator in early stage breast cancer with and without tamoxifen treatment. Br. J. Cancer 2002, 86, 402–408. [Google Scholar] [CrossRef] [Green Version]
  76. Han, Y.; Wei, Y.; Yao, J.; Chu, Y.Y.; Li, C.W.; Hsu, J.L.; Nie, L.; Hung, M.C. Inhibition of CDK2 reduces EZH2 phosphorylation and reactivates ERα expression in high-grade serous ovarian carcinoma. Am. J. Cancer Res. 2020, 10, 1194–1206. [Google Scholar] [PubMed]
  77. Ying, M.; Shao, X.; Jing, H.; Liu, Y.; Qi, X.; Cao, J.; Chen, Y.; Xiang, S.; Song, H.; Hu, R.; et al. Ubiquitin-dependent degradation of CDK2 drives the therapeutic differentiation of AML by targeting PRDX2. Blood 2018, 131, 2698–2711. [Google Scholar] [CrossRef]
  78. Berthet, C.; Aleem, E.; Coppola, V.; Tessarollo, L.; Kaldis, P. Cdk2 knockout mice are viable. Curr. Biol. 2003, 13, 1775–1785. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  79. Tadesse, S.; Caldon, E.C.; Tilley, W.; Wang, S. Cyclin-Dependent Kinase 2 Inhibitors in Cancer Therapy: An Update. J. Med. Chem. 2019, 62, 4233–4251. [Google Scholar] [CrossRef] [PubMed]
  80. Teng, M.; Jiang, J.; He, Z.; Kwiatkowski, N.P.; Donovan, K.A.; Mills, C.E.; Victor, C.; Hatcher, J.M.; Fischer, E.S.; Sorger, P.K.; et al. Development of CDK2 and CDK5 Dual Degrader TMX-2172. Angew. Chem. Int. Ed. Engl. 2020, 59, 13865–13870. [Google Scholar] [CrossRef] [PubMed]
  81. Wei, M.; Zhao, R.; Cao, Y.; Wei, Y.; Li, M.; Dong, Z.; Liu, Y.; Ruan, H.; Li, Y.; Cao, S.; et al. First orally bioavailable prodrug of proteolysis targeting chimera (PROTAC) degrades cyclin-dependent kinases 2/4/6 in vivo. Eur. J. Med. Chem. 2021, 209, 112903. [Google Scholar] [CrossRef]
  82. Wang, L.; Shao, X.; Zhong, T.; Wu, Y.; Xu, A.; Sun, X.; Gao, H.; Liu, Y.; Lan, T.; Tong, Y.; et al. Discovery of a first-in-class CDK2 selective degrader for AML differentiation therapy. Nat. Chem. Biol. 2021, 17, 567–575. [Google Scholar] [CrossRef]
  83. Corces, M.R.; Chang, H.Y.; Majeti, R. Preleukemic Hematopoietic Stem Cells in Human Acute Myeloid Leukemia. Front. Oncol. 2017, 7, 263. [Google Scholar] [CrossRef] [Green Version]
  84. O’Leary, B.; Finn, R.S.; Turner, N.C. Treating cancer with selective CDK4/6 inhibitors. Nat. Rev. Clin. Oncol. 2016, 13, 417–430. [Google Scholar] [CrossRef]
  85. Schmidt, E.E.; Ichimura, K.; Reifenberger, G.; Collins, V.P. CDKN2 (p16/MTS1) gene deletion or CDK4 amplification occurs in the majority of glioblastomas. Cancer Res. 1994, 54, 6321–6324. [Google Scholar]
  86. Puyol, M.; Martín, A.; Dubus, P.; Mulero, F.; Pizcueta, P.; Khan, G.; Guerra, C.; Santamaría, D.; Barbacid, M. A synthetic lethal interaction between K-Ras oncogenes and Cdk4 unveils a therapeutic strategy for non-small cell lung carcinoma. Cancer Cell 2010, 18, 63–73. [Google Scholar] [CrossRef] [PubMed]
  87. Yang, C.; Li, Z.; Bhatt, T.; Dickler, M.; Giri, D.; Scaltriti, M.; Baselga, J.; Rosen, N.; Chandarlapaty, S. Acquired CDK6 amplification promotes breast cancer resistance to CDK4/6 inhibitors and loss of ER signaling and dependence. Oncogene 2017, 36, 2255–2264. [Google Scholar] [CrossRef] [Green Version]
  88. Zhao, B.; Burgess, K. PROTACs suppression of CDK4/6, crucial kinases for cell cycle regulation in cancer. Chem. Commun. 2019, 55, 2704–2707. [Google Scholar] [CrossRef] [PubMed]
  89. An, S.; Fu, L. Small-molecule PROTACs: An emerging and promising approach for the development of targeted therapy drugs. EBioMedicine 2018, 36, 553–562. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  90. Rana, S.; Bendjennat, M.; Kour, S.; King, H.M.; Kizhake, S.; Zahid, M.; Natarajan, A. Selective degradation of CDK6 by a palbociclib based PROTAC. Bioorg. Med. Chem. Lett. 2019, 29, 1375–1379. [Google Scholar] [CrossRef]
  91. Lu, H.; Schulze-Gahmen, U. Toward understanding the structural basis of cyclin-dependent kinase 6 specific inhibition. J. Med. Chem. 2006, 49, 3826–3831. [Google Scholar] [CrossRef]
  92. Su, S.; Yang, Z.; Gao, H.; Yang, H.; Zhu, S.; An, Z.; Wang, J.; Li, Q.; Chandarlapaty, S.; Deng, H.; et al. Potent and Preferential Degradation of CDK6 via Proteolysis Targeting Chimera Degraders. J. Med. Chem. 2019, 62, 7575–7582. [Google Scholar] [CrossRef] [PubMed]
  93. Anderson, N.A.; Cryan, J.; Ahmed, A.; Dai, H.; McGonagle, G.A.; Rozier, C.; Benowitz, A.B. Selective CDK6 degradation mediated by cereblon, VHL, and novel IAP-recruiting PROTACs. Bioorg. Med. Chem. Lett. 2020, 30, 127106. [Google Scholar] [CrossRef]
  94. Steinebach, C.; Ng, Y.L.D.; Sosic, I.; Lee, C.S.; Chen, S.; Lindner, S.; Vu, L.P.; Bricelj, A.; Haschemi, R.; Monschke, M.; et al. Systematic exploration of different E3 ubiquitin ligases: An approach towards potent and selective CDK6 degraders. Chem. Sci. 2020, 11, 3474–3486. [Google Scholar] [CrossRef] [Green Version]
  95. Firestein, R.; Shima, K.; Nosho, K.; Irahara, N.; Baba, Y.; Bojarski, E.; Giovannucci, E.L.; Hahn, W.C.; Fuchs, C.S.; Ogino, S. CDK8 expression in 470 colorectal cancers in relation to beta-catenin activation, other molecular alterations and patient survival. Int. J. Cancer 2010, 126, 2863–2873. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  96. Firestein, R.; Bass, A.J.; Kim, S.Y.; Dunn, I.F.; Silver, S.J.; Guney, I.; Freed, E.; Ligon, A.H.; Vena, N.; Ogino, S.; et al. CDK8 is a colorectal cancer oncogene that regulates beta-catenin activity. Nature 2008, 455, 547–551. [Google Scholar] [CrossRef] [PubMed]
  97. Donner, A.J.; Szostek, S.; Hoover, J.M.; Espinosa, J.M. CDK8 is a stimulus-specific positive coregulator of p53 target genes. Mol. Cell 2007, 27, 121–133. [Google Scholar] [CrossRef] [Green Version]
  98. Fryer, C.J.; White, J.B.; Jones, K.A. Mastermind recruits CycC:CDK8 to phosphorylate the Notch ICD and coordinate activation with turnover. Mol. Cell 2004, 16, 509–520. [Google Scholar] [CrossRef]
  99. Alarcón, C.; Zaromytidou, A.I.; Xi, Q.; Gao, S.; Yu, J.; Fujisawa, S.; Barlas, A.; Miller, A.N.; Manova-Todorova, K.; Macias, M.J.; et al. Nuclear CDKs drive Smad transcriptional activation and turnover in BMP and TGF-beta pathways. Cell 2009, 139, 757–769. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  100. Philip, S.; Kumarasiri, M.; Teo, T.; Yu, M.; Wang, S. Cyclin-Dependent Kinase 8: A New Hope in Targeted Cancer Therapy? J. Med. Chem. 2018, 61, 5073–5092. [Google Scholar] [CrossRef]
  101. Menzl, I.; Witalisz-Siepracka, A.; Sexl, V. CDK8-Novel Therapeutic Opportunities. Pharmaceuticals 2019, 12, 92. [Google Scholar] [CrossRef] [Green Version]
  102. Dannappel, M.V.; Sooraj, D.; Loh, J.J.; Firestein, R. Molecular and in vivo Functions of the CDK8 and CDK19 Kinase Modules. Front. Cell Dev. Biol. 2018, 6, 171. [Google Scholar] [CrossRef] [Green Version]
  103. Hatcher, J.M.; Wang, E.S.; Johannessen, L.; Kwiatkowski, N.; Sim, T.; Gray, N.S. Development of Highly Potent and Selective Steroidal Inhibitors and Degraders of CDK8. ACS Med. Chem. Lett. 2018, 9, 540–545. [Google Scholar] [CrossRef]
  104. Nicolaou, K.C.; Sun, Y.P.; Peng, X.S.; Polet, D.; Chen, D.Y. Total synthesis of (+)-cortistatin A. Angew. Chem. Int. Ed. Engl. 2008, 47, 7310–7313. [Google Scholar] [CrossRef]
  105. Shenvi, R.A.; Guerrero, C.A.; Shi, J.; Li, C.C.; Baran, P.S. Synthesis of (+)-cortistatin A. J. Am. Chem. Soc. 2008, 130, 7241–7243. [Google Scholar] [CrossRef] [Green Version]
  106. Simmons, E.M.; Hardin-Narayan, A.R.; Guo, X.; Sarpong, R. Formal total synthesis of (+/−)-cortistatin A. Tetrahedron 2010, 66, 4696–4700. [Google Scholar] [CrossRef] [Green Version]
  107. Pelish, H.E.; Liau, B.B.; Nitulescu, I.I.; Tangpeerachaikul, A.; Poss, Z.C.; Da Silva, D.H.; Caruso, B.T.; Arefolov, A.; Fadeyi, O.; Christie, A.L.; et al. Mediator kinase inhibition further activates super-enhancer-associated genes in AML. Nature 2015, 526, 273–276. [Google Scholar] [CrossRef]
  108. Albert, T.K.; Rigault, C.; Eickhoff, J.; Baumgart, K.; Antrecht, C.; Klebl, B.; Mittler, G.; Meisterernst, M. Characterization of molecular and cellular functions of the cyclin-dependent kinase CDK9 using a novel specific inhibitor. Br. J. Pharmacol. 2014, 171, 55–68. [Google Scholar] [CrossRef] [Green Version]
  109. Yin, T.; Lallena, M.J.; Kreklau, E.L.; Fales, K.R.; Carballares, S.; Torrres, R.; Wishart, G.N.; Ajamie, R.T.; Cronier, D.M.; Iversen, P.W.; et al. A novel CDK9 inhibitor shows potent antitumor efficacy in preclinical hematologic tumor models. Mol. Cancer Ther. 2014, 13, 1442–1456. [Google Scholar] [CrossRef] [Green Version]
  110. Contreras, J.I.; Robb, C.M.; King, H.M.; Baxter, J.; Crawford, A.J.; Kour, S.; Kizhake, S.; Sonawane, Y.A.; Rana, S.; Hollingsworth, M.A.; et al. Chemical Genetic Screens Identify Kinase Inhibitor Combinations that Target Anti-Apoptotic Proteins for Cancer Therapy. ACS Chem. Biol. 2018, 13, 1148–1152. [Google Scholar] [CrossRef]
  111. Olson, C.M.; Jiang, B.; Erb, M.A.; Liang, Y.; Doctor, Z.M.; Zhang, Z.; Zhang, T.; Kwiatkowski, N.; Boukhali, M.; Green, J.L.; et al. Pharmacological perturbation of CDK9 using selective CDK9 inhibition or degradation. Nat. Chem. Biol. 2018, 14, 163–170. [Google Scholar] [CrossRef]
  112. Bajrami, I.; Frankum, J.R.; Konde, A.; Miller, R.E.; Rehman, F.L.; Brough, R.; Campbell, J.; Sims, D.; Rafiq, R.; Hooper, S.; et al. Genome-wide profiling of genetic synthetic lethality identifies CDK12 as a novel determinant of PARP1/2 inhibitor sensitivity. Cancer Res. 2014, 74, 287–297. [Google Scholar] [CrossRef] [Green Version]
  113. Franco, L.C.; Morales, F.; Boffo, S.; Giordano, A. CDK9: A key player in cancer and other diseases. J. Cell Biochem. 2018, 119, 1273–1284. [Google Scholar] [CrossRef] [PubMed]
  114. Rahaman, M.H.; Lam, F.; Zhong, L.; Teo, T.; Adams, J.; Yu, M.; Milne, R.W.; Pepper, C.; Lokman, N.A.; Ricciardelli, C.; et al. Targeting CDK9 for treatment of colorectal cancer. Mol. Oncol. 2019, 13, 2178–2193. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  115. Wang, J.; Dean, D.C.; Hornicek, F.J.; Shi, H.; Duan, Z. Cyclin-dependent kinase 9 (CDK9) is a novel prognostic marker and therapeutic target in ovarian cancer. FASEB J. 2019, 33, 5990–6000. [Google Scholar] [CrossRef]
  116. Krystof, V.; Uldrijan, S. Cyclin-dependent kinase inhibitors as anticancer drugs. Curr. Drug Targets 2010, 11, 291–302. [Google Scholar] [CrossRef] [PubMed]
  117. Rana, S.; Sonawane, Y.A.; Taylor, M.A.; Kizhake, S.; Zahid, M.; Natarajan, A. Synthesis of aminopyrazole analogs and their evaluation as CDK inhibitors for cancer therapy. Bioorg. Med. Chem. Lett. 2018, 28, 3736–3740. [Google Scholar] [CrossRef]
  118. King, H.M.; Rana, S.; Kubica, S.P.; Mallareddy, J.R.; Kizhake, S.; Ezell, E.L.; Zahid, M.; Naldrett, M.J.; Alvarez, S.; Law, H.C.; et al. Aminopyrazole based CDK9 PROTAC sensitizes pancreatic cancer cells to venetoclax. Bioorg. Med. Chem. Lett. 2021, 43, 128061. [Google Scholar] [CrossRef]
  119. Lopez, H.; Zhang, L.; George, N.M.; Liu, X.; Pang, X.; Evans, J.J.D.; Targy, N.M.; Luo, X. Perturbation of the Bcl-2 network and an induced Noxa/Bcl-xL interaction trigger mitochondrial dysfunction after DNA damage. J. Biol. Chem. 2010, 285, 15016–15026. [Google Scholar] [CrossRef] [Green Version]
  120. Rana, S.; Kour, S.; Sonawane, Y.A.; Robb, C.M.; Contreras, J.I.; Kizhake, S.; Zahid, M.; Karpf, A.R.; Natarajan, A. Symbiotic prodrugs (SymProDs) dual targeting of NFkappaB and CDK. Chem. Biol. Drug Des. 2020, 96, 773–784. [Google Scholar] [CrossRef]
  121. MacCallum, D.E.; Melville, J.; Frame, S.; Watt, K.; Anderson, S.; Gianella-Borradori, A.; Lane, D.P.; Green, S.R. Seliciclib (CYC202, R-Roscovitine) induces cell death in multiple myeloma cells by inhibition of RNA polymerase II-dependent transcription and down-regulation of Mcl-1. Cancer Res. 2005, 65, 5399–5407. [Google Scholar] [CrossRef] [Green Version]
  122. Abid, M.; Sonawane, Y.A.; Contreras, J.I.; Rana, S.; Natarajan, A. Recent Advances in Cancer Drug Development: Targeting Induced Myeloid Cell Leukemia-1 (Mcl-1) Differentiation Protein. Curr. Med. Chem. 2017, 24, 4488–4514. [Google Scholar] [CrossRef] [PubMed]
  123. Zhou, F.; Chen, L.; Cao, C.; Yu, J.; Luo, X.; Zhou, P.; Zhao, L.; Du, W.; Cheng, J.; Xie, Y.; et al. Development of selective mono or dual PROTAC degrader probe of CDK isoforms. Eur. J. Med. Chem. 2020, 187, 111952. [Google Scholar] [CrossRef] [PubMed]
  124. Misra, R.N.; Xiao, H.Y.; Kim, K.S.; Lu, S.; Han, W.C.; Barbosa, S.A.; Hunt, J.T.; Rawlins, D.B.; Shan, W.; Ahmed, S.Z.; et al. N-(cycloalkylamino)acyl-2-aminothiazole inhibitors of cyclin-dependent kinase 2. N-[5-[[[5-(1,1-dimethylethyl)-2-oxazolyl]methyl]thio]-2-thiazolyl]-4- piperidinecarboxamide (BMS-387032), a highly efficacious and selective antitumor agent. J. Med. Chem. 2004, 47, 1719–1728. [Google Scholar] [CrossRef]
  125. Winter, G.E.; Buckley, D.L.; Paulk, J.; Roberts, J.M.; Souza, A.; Dhe-Paganon, S.; Bradner, J.E. Phthalimide conjugation as a strategy for in vivo target protein degradation. Science 2015, 348, 1376–1381. [Google Scholar] [CrossRef] [Green Version]
  126. Barsanti, P.A.; Hu, C.; Jeff, J.; Keyes, R.; Kucejko, R.; Xiaodong, L.; Yue, P.; Pfister Kb, S.M.; Sutton, J.; Lifeng, W. Pyridine and Pyrzaine Derivatives as Protein Kinase Modulators. International Patent PCT/JP2008/073864 (WO/2011/012661), 3 February 2011. [Google Scholar]
  127. Polier, G.; Ding, J.; Konkimalla, B.V.; Eick, D.; Ribeiro, N.; Kohler, R.; Giaisi, M.; Efferth, T.; Desaubry, L.; Krammer, P.H.; et al. Wogonin and related natural flavones are inhibitors of CDK9 that induce apoptosis in cancer cells by transcriptional suppression of Mcl-1. Cell Death Dis. 2011, 2, e182. [Google Scholar] [CrossRef] [Green Version]
  128. Bian, J.; Ren, J.; Li, Y.; Wang, J.; Xu, X.; Feng, Y.; Tang, H.; Wang, Y.; Li, Z. Discovery of Wogonin-based PROTACs against CDK9 and capable of achieving antitumor activity. Bioorg. Chem. 2018, 81, 373–381. [Google Scholar] [CrossRef]
  129. Chen, R.; Keating, M.J.; Gandhi, V.; Plunkett, W. Transcription inhibition by flavopiridol: Mechanism of chronic lymphocytic leukemia cell death. Blood 2005, 106, 2513–2519. [Google Scholar] [CrossRef]
  130. Chen, R.; Guo, L.; Chen, Y.; Jiang, Y.; Wierda, W.G.; Plunkett, W. Homoharringtonine reduced Mcl-1 expression and induced apoptosis in chronic lymphocytic leukemia. Blood 2011, 117, 156–164. [Google Scholar] [CrossRef] [Green Version]
  131. Qiu, X.; Li, Y.; Yu, B.; Ren, J.; Huang, H.; Wang, M.; Ding, H.; Li, Z.; Wang, J.; Bian, J. Discovery of selective CDK9 degraders with enhancing antiproliferative activity through PROTAC conversion. Eur. J. Med. Chem. 2021, 211, 113091. [Google Scholar] [CrossRef] [PubMed]
  132. Liang, S.; Hu, L.; Wu, Z.; Chen, Z.; Liu, S.; Xu, X.; Qian, A. CDK12: A Potent Target and Biomarker for Human Cancer Therapy. Cells 2020, 9, 1483. [Google Scholar] [CrossRef]
  133. Chen, H.H.; Wang, Y.C.; Fann, M.J. Identification and characterization of the CDK12/cyclin L1 complex involved in alternative splicing regulation. Mol. Cell. Biol. 2006, 26, 2736–2745. [Google Scholar] [CrossRef] [Green Version]
  134. Liang, K.; Gao, X.; Gilmore, J.M.; Florens, L.; Washburn, M.P.; Smith, E.; Shilatifard, A. Characterization of human cyclin-dependent kinase 12 (CDK12) and CDK13 complexes in C-terminal domain phosphorylation, gene transcription, and RNA processing. Mol. Cell. Biol. 2015, 35, 928–938. [Google Scholar] [CrossRef] [Green Version]
  135. Jiang, B.; Gao, Y.; Che, J.; Lu, W.; Kaltheuner, I.H.; Dries, R.; Kalocsay, M.; Berberich, M.J.; Jiang, J.; You, I.; et al. Discovery and resistance mechanism of a selective CDK12 degrader. Nat. Chem. Biol. 2021, 17, 1057–1064. [Google Scholar] [CrossRef]
  136. den Besten, W.; Lipford, J.R. Prospecting for molecular glues. Nat. Chem. Biol. 2020, 16, 1157–1158. [Google Scholar] [CrossRef]
  137. Schreiber, S.L. The Rise of Molecular Glues. Cell 2021, 184, 3–9. [Google Scholar] [CrossRef]
  138. Munn, J.D. Thalidomide and Congenital Malformations. Can. Med. Assoc. J. 1962, 86, 665. [Google Scholar] [PubMed]
  139. Kronke, J.; Udeshi, N.D.; Narla, A.; Grauman, P.; Hurst, S.N.; McConkey, M.; Svinkina, T.; Heckl, D.; Comer, E.; Li, X.; et al. Lenalidomide causes selective degradation of IKZF1 and IKZF3 in multiple myeloma cells. Science 2014, 343, 301–305. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  140. Asatsuma-Okumura, T.; Ando, H.; De Simone, M.; Yamamoto, J.; Sato, T.; Shimizu, N.; Asakawa, K.; Yamaguchi, Y.; Ito, T.; Guerrini, L.; et al. p63 is a cereblon substrate involved in thalidomide teratogenicity. Nat. Chem. Biol. 2019, 15, 1077–1084. [Google Scholar] [CrossRef]
  141. Donovan, K.A.; An, J.; Nowak, R.P.; Yuan, J.C.; Fink, E.C.; Berry, B.C.; Ebert, B.L.; Fischer, E.S. Thalidomide promotes degradation of SALL4, a transcription factor implicated in Duane Radial Ray syndrome. eLife 2018, 7, e38430. [Google Scholar] [CrossRef] [PubMed]
  142. Matyskiela, M.E.; Lu, G.; Ito, T.; Pagarigan, B.; Lu, C.-C.; Miller, K.; Fang, W.; Wang, N.-Y.; Nguyen, D.; Houston, J.; et al. A novel cereblon modulator recruits GSPT1 to the CRL4(CRBN) ubiquitin ligase. Nature 2016, 535, 252–257. [Google Scholar] [CrossRef] [PubMed]
  143. Sievers, Q.L.; Petzold, G.; Bunker, R.D.; Renneville, A.; Slabicki, M.; Liddicoat, B.J.; Abdulrahman, W.; Mikkelsen, T.; Ebert, B.L.; Thoma, N.H. Defining the human C2H2 zinc finger degrome targeted by thalidomide analogs through CRBN. Science 2018, 362, eaat0572. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  144. Zangari, M.; Elice, F.; Tricot, G. Immunomodulatory drugs in multiple myeloma. Expert Opin. Investig. Drugs 2005, 14, 1411–1418. [Google Scholar] [CrossRef]
  145. Quach, H.; Ritchie, D.; Stewart, A.K.; Neeson, P.; Harrison, S.; Smyth, M.J.; Prince, H.M. Mechanism of action of immunomodulatory drugs (IMiDS) in multiple myeloma. Leukemia 2010, 24, 22–32. [Google Scholar] [CrossRef] [Green Version]
  146. Bussiere, D.E.; Xie, L.; Srinivas, H.; Shu, W.; Burke, A.; Be, C.; Zhao, J.; Godbole, A.; King, D.; Karki, R.G.; et al. Structural basis of indisulam-mediated RBM39 recruitment to DCAF15 E3 ligase complex. Nat. Chem. Biol. 2020, 16, 15–23. [Google Scholar] [CrossRef]
  147. Du, X.; Volkov, O.A.; Czerwinski, R.M.; Tan, H.; Huerta, C.; Morton, E.R.; Rizzi, J.P.; Wehn, P.M.; Xu, R.; Nijhawan, D.; et al. Structural Basis and Kinetic Pathway of RBM39 Recruitment to DCAF15 by a Sulfonamide Molecular Glue E7820. Structure 2019, 27, 1625–1633. [Google Scholar] [CrossRef]
  148. Faust, T.B.; Yoon, H.; Nowak, R.P.; Donovan, K.A.; Li, Z.; Cai, Q.; Eleuteri, N.A.; Zhang, T.; Gray, N.S.; Fischer, E.S. Structural complementarity facilitates E7820-mediated degradation of RBM39 by DCAF15. Nat. Chem. Biol. 2020, 16, 7–14. [Google Scholar] [CrossRef]
  149. Slabicki, M.; Kozicka, Z.; Petzold, G.; Li, Y.D.; Manojkumar, M.; Bunker, R.D.; Donovan, K.A.; Sievers, Q.L.; Koeppel, J.; Suchyta, D.; et al. The CDK inhibitor CR8 acts as a molecular glue degrader that depletes cyclin K. Nature 2020, 585, 293–297. [Google Scholar] [CrossRef] [PubMed]
  150. Bettayeb, K.; Oumata, N.; Echalier, A.; Ferandin, Y.; Endicott, J.A.; Galons, H.; Meijer, L. CR8, a potent and selective, roscovitine-derived inhibitor of cyclin-dependent kinases. Oncogene 2008, 27, 5797–5807. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  151. Slabicki, M.; Yoon, H.; Koeppel, J.; Nitsch, L.; Roy Burman, S.S.; Di Genua, C.; Donovan, K.A.; Sperling, A.S.; Hunkeler, M.; Tsai, J.M.; et al. Small-molecule-induced polymerization triggers degradation of BCL6. Nature 2020, 588, 164–168. [Google Scholar] [CrossRef]
  152. Rana, S.; Natarajan, A. Small molecule induced polymerization of BCL6 facilitates SIAH1 mediated degradation. Signal Transduct. Target. Ther. 2021, 6, 142. [Google Scholar] [CrossRef]
Cancers 13 05506 g001 550
Figure 1. CDK inhibitors that are advanced to the clinic/clinical trials.
Figure 1. CDK inhibitors that are advanced to the clinic/clinical trials.
Cancers 13 05506 g001
Cancers 13 05506 g002 550
Figure 2. Schematic cartoon showing a PROTAC mechanism of action (created with BioRender.com).
Figure 2. Schematic cartoon showing a PROTAC mechanism of action (created with BioRender.com).
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Cancers 13 05506 g003 550
Figure 3. PROTACs evolution from bench to bedside (created with BioRender.com).
Figure 3. PROTACs evolution from bench to bedside (created with BioRender.com).
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Cancers 13 05506 g004 550
Figure 4. Chemical structures of different E3 ligands that have been used successfully to degrade a POI (• represent the site for linker conjugation).
Figure 4. Chemical structures of different E3 ligands that have been used successfully to degrade a POI (• represent the site for linker conjugation).
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Cancers 13 05506 g005 550
Figure 5. A flowchart highlighting the development of a PROTAC (created with BioRender.com).
Figure 5. A flowchart highlighting the development of a PROTAC (created with BioRender.com).
Cancers 13 05506 g005
Cancers 13 05506 g006 550
Figure 6. Structures of selective CDK2 (27, 29, and 31) and CDK4 (32, 33) PROTACs.
Figure 6. Structures of selective CDK2 (27, 29, and 31) and CDK4 (32, 33) PROTACs.
Cancers 13 05506 g006
Cancers 13 05506 g007 550
Figure 7. Structures of CDK6 (34, 36–41) and CDK8 (42) PROTACs.
Figure 7. Structures of CDK6 (34, 36–41) and CDK8 (42) PROTACs.
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Cancers 13 05506 g008 550
Figure 8. Structures of CDK9 PROTACs (44–46), CDK12 (47) PROTAC and a Cyclin K molecular glue (49).
Figure 8. Structures of CDK9 PROTACs (44–46), CDK12 (47) PROTAC and a Cyclin K molecular glue (49).
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Cancers 13 05506 g009 550
Figure 9. Mechanism of action of a prototypical molecular glue (created with BioRender.com).
Figure 9. Mechanism of action of a prototypical molecular glue (created with BioRender.com).
Cancers 13 05506 g009
Table
Table 1. CDK inhibitors in clinical trials.
Table 1. CDK inhibitors in clinical trials.
NCT No.ConditionsInterventionsPhase
04802759 aBCGDC-9545, Abemaciclib, Ipatasertib, GDC-0077, Ribociclib, EverolimusI, II
04607668 aMCCTrilaciclibIII
04585724 aBCAbemaciclib, Palbociclib, RibociclibI
04553133 aSCLC, NSCLC, TNBC, EGFR2, OCPF-07104091 monotherapy, PF-07104091 + Palbociclib, PF-07104091 + Palbociclib + LetrozoleII
04469764 aEC, OCAbemaciclib, Anastrozole, LetrozoleII
04282031 aST, BCBPI-1178 c, Fulvestrant, LetrozoleI, II
04247126 aST, BC, SCLCSY-5609 d, FulvestrantI
04116541 aSTHDM201, Ribociclib, Cabozantinib, AlectinibII
04049227 aOCAbemaciclib, LetrozoleI
04040205 aSTSAbemaciclibII
04010357 aSCLCAbemaciclibII
03959891 aBCIpatasertib, Fulvestrant, PalbociclibI
03740334 aALLRibociclib, Dexamethasone, EverolimusI
03675893 aECLetrozole, AbemaciclibII
03593915 bMSAlvocidib + Decitabine, or AzacitidineI, II
03455270 bBCG1T48, PalbociclibI
03439735 aBCAI and PalbociclibII
03386929 bNSCLCAvelumab e, Axitinib, PalbociclibI, II
03363893 aSTCT7001, CT7001 + FulvestrantI, II
03310879 aCancerAbemaciclibII
03285412 aBCRibociclib, ETII
03280563 aBCAtezolizumab f, Bevacizumab g, Entinostat,
Exemestane, Fulvestrant
Ipatasertib, Tamoxifen, Abemaciclib		I, II
03227328 aBCCDK4/6 inhibitor + ET, chemotherapy + ETII
03220178 aBCPalbociclib, Fulvestrant, Anastrozole, Letrozole,
Exemestane		IV
03170206 aLCBinimetinib, PalbociclibI, II
03155997 aBCAbemaciclib, ETIII
03110744 aBCPalbociclibII
02712723 aBCLetrozole, RibociclibII
02644460 aBT, ST, ES, BCAbemaciclibI
02632045 aBCLEE-011 (Ribociclib), FulvestrantII
02626507 aBCGedatolisib, Faslodex, Palbociclib, ZoladexI
02603679 aBCPaclitaxel, Tamoxifen + Palbociclib, AI + Palbociclib, Goserelin + AI + Palbociclib,II
02599714 bBCAZD2014, Palbociclib, FulvestrantI
02592083 bBCTamoxifen, Goserelin, PalbociclibII
02586675 bBCTamoxifen, Ribociclib, GoserelinI
02503709 bSTAT7519, OnalespibI
02499146 bBCPalbociclib, LetrozoleI
02095132 bBTAdavosertib, Irinotecan HydrochlorideI, II
01864746 bHRPalbociclibIII
01723774 aBCPD0332991, Anastrozole, GoserelinII
01676753 bBC, TNBCDinaciclib, Pembrolizumab hI
01522989 bSTPD-0332991, 5-FU, OxaliplatinI
01434316 aSTDinaciclib, VeliparibI
a recruiting; b active, not recruiting; c undisclosed CDK4/6 inhibitor; d undisclosed CDK7 inhibitor; e human IgG1 anti-PDL1 monoclonal antibody; f an anti-programmed death-ligand 1 antibody; g a humanized monoclonal antibody that binds to all VEGF-A isoforms; h a humanized antibody inhibiting PD-1 receptor; Non-small Cell Lung Cancer (NSCLC); Small Cell Lung Cancer (SCLC); Triple Negative Breast Cancer (TNBC); Breast Cancer (BC); Lung Cancer (LC); Myelodysplastic Syndromes (MS); Central Nervous System (CNS); Acute Lymphoblastic Leukemia (ALL); Solid Tumor (ST); Ewing Sarcoma (ES); Metastatic Colorectal Cancer (MCC); Brain Tumor (BT); Bone Cancer (BC); Endometrial Carcinoma (EC); Deficiency Disorder (DD); Soft Tissue Sarcoma (STS); Aromatase Inhibitor (AI); Endocrine therapy (ET).
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Rana, S.; Mallareddy, J.R.; Singh, S.; Boghean, L.; Natarajan, A. Inhibitors, PROTACs and Molecular Glues as Diverse Therapeutic Modalities to Target Cyclin-Dependent Kinase. Cancers 2021, 13, 5506. https://doi.org/10.3390/cancers13215506

AMA Style

Rana S, Mallareddy JR, Singh S, Boghean L, Natarajan A. Inhibitors, PROTACs and Molecular Glues as Diverse Therapeutic Modalities to Target Cyclin-Dependent Kinase. Cancers. 2021; 13(21):5506. https://doi.org/10.3390/cancers13215506

Chicago/Turabian Style

Rana, Sandeep, Jayapal Reddy Mallareddy, Sarbjit Singh, Lidia Boghean, and Amarnath Natarajan. 2021. "Inhibitors, PROTACs and Molecular Glues as Diverse Therapeutic Modalities to Target Cyclin-Dependent Kinase" Cancers 13, no. 21: 5506. https://doi.org/10.3390/cancers13215506

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