Discovery of a Potent Degrader for Fibroblast Growth Factor Receptor 1/2

Selective degradation of FGFR1/2 overcomes antiestrogen resistance in ER+ breast cancer with FGFR1/2 alterations

FGFR1 amplification and FGFR1/2 activating mutations have been associated with antiestrogen resistance in estrogen receptor-positive (ER+) breast cancer. However, there are no approved FGFR1-targeted therapies for breast cancers harboring these alterations. In this study, we investigated the selective degradation of FGFR1/2 using the proteolysis-targeting chimera (PROTAC) DGY-09-192 as a novel therapeutic strategy in ER + breast cancers harboring FGFR1/2 somatic alterations. Treatment of ER+/FGFR1-amplified breast cancer cells and patient-derived xenografts with DGY-09-192 resulted in sustained degradation of FGFR1 in a proteasome-dependent manner and suppressed downstream signal transduction. The combination of DGY-09-192 and the ERα degrader fulvestrant resulted in complete cell growth arrest and tumor regression of ER+/FGFR1-amplified patients-derived xenografts. In addition, we tested the effect of DGY-09-192 on breast cancer cells expressing FGFR1N546K and FGFR2K659E hotspot kinase domain mutations as well as ER-negative breast cancer cells harboring FGFR2 gene amplification. Treatment with DGY-09-192 resulted in the degradation of mutant FGFR1/2 and blocked mutant receptor-induced signal transduction and antiestrogen resistance. Collectively, our study suggests that degradation of FGFR1/2, in combination with antiestrogens, can be leveraged as a therapeutic strategy in ER + breast cancers harboring FGFR1/2 driver alterations.

The landscape of FGFR-TACC fusion in adult glioblastoma: From bench to bedside

Glioblastoma (GBM) is a lethal central nervous system tumor, characterized by extensive genomic alterations and high intra-tumoral heterogeneity. Gene fusions, derived from chromosomal translocations, deletions, and inversions, were increasingly recognized as key carcinogenic events, with the highest frequency of FGFR-TACC fusion in glioblastoma. As reported, FGFR3-TACC3 fusion mostly coexists with wild-type IDH status, and associates with better prognosis. Mechanistically, FGFR3-TACC3 fusions can constitutively activate non-canonical FGFR downstream pathways, induce aneuploidy, and participate in mitochondrial metabolism, thereby promoting cell proliferation and tumorigenesis. These functions, whether based on FGFR3 phosphorylation or not, are predominantly attributed to the specific domain of TACC3 that involved in regulating the localization and activation of fusion products. Several preclinical studies and clinical trials are being performed to evaluate the efficacy and safety of the FGFR-TACC fusion as a personalised therapeutic target, including the treatments with tyrosine kinase inhibitors, metabolic inhibitors, HSP90 inhibitors, coiled-coil peptide-mimetics, and targeted protein degraders. A subset of populations with FGFR-TACC-positive glioblastoma, after refined molecular screening strategies, may benefit from targeted therapies. Despite major progress in biotechnology, our understanding on the role of fusion events in glioblastoma represented by the FGFR-TACC is still in its infancy. Here, we highlight recent progress on FGFR-TACC fusion in human glioblastoma, emphasizing their molecular mechanisms and potential clinical value.

Design, synthesis, and biological evaluation of novel FGFR1 PROTACs

Dysregulation of the fibroblast growth factor receptor 1 (FGFR1) signaling has prompted efforts to develop therapeutic agents, which is a carcinogenic driver of many cancers, including breast, prostate, bladder, and chronic myeloid leukemia. Despite significant progress in the development of potent and selective FGFR inhibitors, the long-term efficacy of these drugs in cancer therapy has been hampered by the rapid onset of acquired resistance. Therefore, more drug discovery strategies are needed to promote the development of FGFR-targeted drugs. Here, we discovered compound S2h, a compound that selectively and effectively degrades FGFR1 at nanomolar concentrations in KG1a cells (IC50 = 26.81 nM; DC50 = 39.78 nM), which incorporates an essential, nine atom-long linkers. The importance of linker length, composition, and tethering site proteolysis-targeting chimeras (PROTACs) design is emphasized, and slight modifications can significantly affect degradation potency. Meanwhile, it was verified that the degradation of FGFR1 protein at compound S2h was concentration- and time-dependent and that the protein degradation occurred through the ubiquitin-proteasome system (UPS). In summary, the newly designed heterobifunctional FGFR1 degrader, compound S2h, provides new ideas and references for the research of FGFR small-molecule degraders.

Orthogonal validation of PROTAC mediated degradation of the integral membrane proteins EGFR and c-MET

Dysregulation of integral membrane proteins (IMPs) has been linked to a myriad of diseases, making these proteins an attractive target in drug research. Whilst PROTAC technology has had a significant impact in scientific research, its application to IMPs is still limited. Limitations of the traditional approach of immunoblotting in PROTAC research include the low throughput compared to other methods, as well as a lack of spatial information for the target. Here we compare orthogonal antibody based approaches, i.e. immunoblotting, flow cytometry and immunofluorescence, to measure PROTAC mediated degradation of two established, endogenous targets, epidermal growth factor receptor (EGFR) and hepatocyte growth-factor receptor (c-MET). We discuss advantages and limitations of each methodology for the assessment of PROTAC efficacy on IMPs. Overall, we recommend the use of immunofluorescence and flow cytometry, for an increased accuracy with both a qualitative and quantitative insight into degradation efficacy and a critical distinction between cell membrane-localized and intracellular IMP protein pools.

Clinical advances and challenges in targeting FGF/FGFR signaling in lung cancer

Fibroblast growth factors (FGFs) and their receptors regulate numerous cellular processes, such as metabolism and signal transduction, but can also drive tumorigenesis. Specifically, in lung cancer, the overexpression of FGFs, as well as the amplification, mutation and fusion of FGFR genes, are closely linked to the initiation, progression and resistance of the disease, suggesting that targeting FGF/FGFR is an attractive therapeutic strategy for lung cancer treatment. Nintedanib, a multitarget tyrosine kinase inhibitor (TKI) used in combination with docetaxel, has shown some success as a second-line therapy for lung cancer. However, clinical trials evaluating other FGFR inhibitors have yielded mixed results, indicating substantial complexity in targeting aberrant FGF/FGFR signaling. In this review, we describe the aberrations in FGF/FGFR signaling in lung cancer and summarize the clinical efficacy of FGFR inhibitors, such as multitarget TKIs, selective FGFR-TKIs and biological agents. We also discuss various challenges associated with FGFR targeting in lung cancer, including precision patient selection, toxicity and resistance. Finally, we provide perspectives on future directions, namely, developing novel FGFR-targeting drugs, such as FGFR degraders and more specific FGFR-TKIs, adopting combination therapy and targeting FGFs.

Synthesis and identification of a selective FGFR2 degrader with potent antiproliferative effects in gastric cancer

Despite numerous efforts to develop FGFR inhibitors for cancer treatment, the widespread clinical application of currently available FGFR inhibitors has been significantly limited due to the serious side effects caused by poor selectivity and resistance. PROTAC technology, a method for protein degradation, has shown notable advantages over conventional inhibitors. In our study, we coupled Erdafitinib, a pan-FGFR inhibitor, with a CRBN binder to synthesize and identify an effective FGFR2 degrader, N5. Our findings demonstrated that N5 displayed notable specificity for FGFR2 and outstanding enzyme inhibitory capabilities, achieving an IC50 value of 0.08 nM against FGFR2, and strong antiproliferative activity, maintaining an inhibitory rate above 50% on gastric cancer cells at a concentration of 0.17 nM. Mechanistically, N5 induced gastric cancer cell cycle arrest at the G0/G1 phase and apoptosis by decreasing the levels of FGFR downstream proteins. Moreover, N5 demonstrated favorable pharmacokinetic characteristics with a bioavailability of 74.8% when administered intraperitoneally and effectively suppressed the growth of SNU16 xenograft tumors, exhibiting greater potency compared to the parental inhibitor Erdafitinib. This study lays the groundwork for developing and potentially applying therapeutic agents targeting FGFR2 degradation.

Targeted protein degradation: current molecular targets, localization, and strategies

Targeted protein degradation (TPD) has revolutionized drug discovery by selectively eliminating specific proteins within and outside the cellular context. Over the past two decades, TPD has expanded its focus beyond well-established targets, exploring diverse proteins beyond cancer-related ones. This evolution extends the potential of TPD to various diseases. Notably, TPD can target proteins at demanding locations, such as the extracellular matrix (ECM) and cellular membranes, presenting both opportunities and challenges for future research. In this review, we comprehensively examine the exciting opportunities in the burgeoning field of TPD, highlighting different targets, their cellular environment, and innovative strategies for modern drug discovery.

Design, synthesis and antitumor activity of a novel FGFR2-selective degrader to overcome resistance of the FGFR2V564F gatekeeper mutation based on a pan-FGFR inhibitor

Aberrant activation of fibroblast growth factor receptors (FGFRs) contributes to the development and progression of multiple types of cancer. Although many FGFR inhibitors have been approved by the FDA, their long-term therapeutic efficacy is hampered by acquired resistance to gatekeeper mutations and low subtype selectivity. FGFR2 has been found to be frequently amplified or mutated in many tumors. In this study, we designed several PROTACs with different E3 ligands based on LY2874455. By screening the length of the linker and the binding site in various degraders, we obtained a novel and highly efficient FGFR2-selective degrader 28e (DC50 = 0.645 nM, DCmax = 86 %). Compound 28e selectively degraded FGFR2 and essentially avoided degradation of FGFR1,3,4 isoforms (DC50 > 300 nM). Compound 28e significantly inhibited the proliferation of FGFR2-overexpressing cell lines, including KATOIII, SNU16, and AN3CA (IC50 = 0.794 nM/0.207 nM/4.626 nM), comparable to parental inhibitors. At the same time, the preferred compound showed superiority over the parental inhibitor in kinase inhibitory activity against the gatekeeper mutant isoform FGFR2V564F (IC50 = 0.121 nM). In summary, we identified 28e as a novel selective degrader of FGFR2 with high potency and high potential to overcome resistance to gatekeeper mutation. The discovery of 28e provides new evidence for the strategy of pan-inhibitor-based development of selective degrading agents.

Fibroblast growth receptor 1 is regulated by G-quadruplex in metastatic breast cancer

Limiting cellular plasticity is of key importance for the therapeutic targeting of metastatic breast cancer (MBC). Fibroblast growth receptor (FGFR) is a critical molecule in cellular plasticity and potent inhibitors of FGFR enzymatic activity have been developed, but kinase independent functions for this receptor also contribute to MBC progression. Herein, we evaluated several FGFR inhibitors and find that while FGFR-targeted kinase inhibitors are effective at blocking ligand-induced cell growth, dormant cells persist eventually giving rise to MBC progression. To more broadly target FGFR and cellular plasticity, we examined the FGFR1 proximal promoter, and found several sequences with potential to form G-quadruplex secondary structures. Circular dichroism was used to verify formation of G-quadruplex in the FGFR1 proximal promoter. Importantly, use of the clinical G-quadruplex-stabilizing compound, CX-5461, stabilized the FGFR1 G-quadruplex structures, blocked the transcriptional activity of the FGFR1 proximal promoter, decreased FGFR1 expression, and resulted in potent inhibition of pulmonary tumor formation. Overall, our findings suggest G-quadruplex-targeted compounds could be a potential therapeutic strategy to limit the cellular plasticity of FGFR1 overexpressing MBC.

Targeting focal adhesion kinase (FAK) for cancer therapy: FAK inhibitors, FAK-based dual-target inhibitors and PROTAC degraders

Focal adhesion kinase (FAK), a non-receptor tyrosine kinase, plays an essential role in regulating cell proliferation, migration and invasion through both kinase-dependent enzymatic function and kinase-independent scaffolding function. The overexpression and activation of FAK is commonly observed in various cancers and some drug-resistant settings. Therefore, targeted disruption of FAK has been identified as an attractive strategy for cancer treatment. To date, numerous structurally diverse inhibitors targeting distinct domains of FAK have been developed, encompassing kinase domain inhibitors, FERM domain inhibitors, and FAT domain inhibitors, with several FAK inhibitors advanced to clinical trials. Moreover, given the critical role of FAK scaffolding function in signal transduction, FAK-targeted PROTACs have also been developed. Although no current FAK-targeted therapeutics have been approved for the market, the combination of FAK inhibitors with other anticancer drugs has shown considerable promise in the clinic. This review provides an overview of current drug discovery strategies targeting FAK, including the development of FAK inhibitors, FAK-based dual-target inhibitors and proteolysis-targeting chimeras (PROTACs) in both literature and patent applications. Accordingly, their design and optimization process, mechanisms of action and biological activities are discussed to offer insights into future directions of FAK-targeting drug discovery in cancer therapy.

Clinical Developments and Challenges in Treating FGFR2-Driven Gastric Cancer

Recent advances in the treatment of gastric cancer (GC) with chemotherapy, immunotherapy, anti-angiogenic therapy and targeted therapies have yielded some improvement in survival outcomes; however, metastatic GC remains a lethal malignancy and amongst the leading causes of cancer-related mortality worldwide. Importantly, the ongoing molecular characterisation of GCs continues to uncover potentially actionable molecular targets. Among these, aberrant FGFR2-driven signalling, predominantly arising from FGFR2 amplification, occurs in approximately 3-11% of GCs. However, whilst several inhibitors of FGFR have been clinically tested to-date, there are currently no approved FGFR-directed therapies for GC. In this review, we summarise the significance of FGFR2 as an actionable therapeutic target in GC, examine the recent pre-clinical and clinical data supporting the use of small-molecule inhibitors, antibody-based therapies, as well as novel approaches such as proteolysis-targeting chimeras (PROTACs) for targeting FGFR2 in these tumours, and discuss the ongoing challenges and opportunities associated with their clinical development.

FGFR-targeted therapeutics: clinical activity, mechanisms of resistance and new directions

Fibroblast growth factor (FGF) signalling via FGF receptors (FGFR1-4) orchestrates fetal development and contributes to tissue and whole-body homeostasis, but can also promote tumorigenesis. Various agents, including pan-FGFR inhibitors (erdafitinib and futibatinib), FGFR1/2/3 inhibitors (infigratinib and pemigatinib), as well as a range of more-specific agents, have been developed and several have entered clinical use. Erdafitinib is approved for patients with urothelial carcinoma harbouring FGFR2/3 alterations, and futibatinib and pemigatinib are approved for patients with cholangiocarcinoma harbouring FGFR2 fusions and/or rearrangements. Clinical benefit from these agents is in part limited by hyperphosphataemia owing to off-target inhibition of FGFR1 as well as the emergence of resistance mutations in FGFR genes, activation of bypass signalling pathways, concurrent TP53 alterations and possibly epithelial-mesenchymal transition-related isoform switching. The next generation of small-molecule inhibitors, such as lirafugratinib and LOXO-435, and the FGFR2-specific antibody bemarituzumab are expected to have a reduced risk of hyperphosphataemia and the ability to overcome certain resistance mutations. In this Review, we describe the development and current clinical role of FGFR inhibitors and provide perspective on future research directions including expansion of the therapeutic indications for use of FGFR inhibitors, combination of these agents with immune-checkpoint inhibitors and the application of novel technologies, such as artificial intelligence.

Breaking Bad Proteins-Discovery Approaches and the Road to Clinic for Degraders

Proteolysis-targeting chimeras (PROTACs) describe compounds that bind to and induce degradation of a target by simultaneously binding to a ubiquitin ligase. More generally referred to as bifunctional degraders, PROTACs have led the way in the field of targeted protein degradation (TPD), with several compounds currently undergoing clinical testing. Alongside bifunctional degraders, single-moiety compounds, or molecular glue degraders (MGDs), are increasingly being considered as a viable approach for development of therapeutics, driven by advances in rational discovery approaches. This review focuses on drug discovery with respect to bifunctional and molecular glue degraders within the ubiquitin proteasome system, including analysis of mechanistic concepts and discovery approaches, with an overview of current clinical and pre-clinical degrader status in oncology, neurodegenerative and inflammatory disease.

Recent progress in degradation of membrane proteins by PROTACs and alternative targeted protein degradation techniques

Targeted protein degradation (TPD) is one of the key strategies of current targeted cancer therapy, and it can eliminate some of the root causes of cancer, and effectively avoid drug resistance caused by traditional drugs. Proteolysis targeting chimera (PROTAC) is a hot branch of the TPD strategy, and it has been shown to induce the degradation of target proteins by activating the inherent ubiquitin-proteasome system (UPS) in tumor cells. PROTACs have been developed for more than two decades, and some of them have been clinically evaluated. Although most of the proteins degraded by PROTACs are intracellular, degradation of some typical membrane proteins has also been reported, such as epidermal growth factor receptor (EGFR), anaplastic lymphoma kinase (ALK), programmed death ligand 1 (PD-L1), and G-protein-coupled receptor (GPCR). In addition, some other effective membrane protein-degrading strategies have also emerged, such as antibody-based PROTAC (AbTAC), lysosome targeting chimera (LYTAC), molecular glue, and nanoparticle-based PROTAC (Nano-PROTAC). Herein, we discussed the advantages, disadvantages and potential applications of several important membrane protein degradation techniques. These techniques that we have summarized are insightful in paving the way for future development of more general strategies for membrane protein degradation.

The application of PROTAC in HDAC

Inducing protein degradation by proteolysis targeting chimera (PROTAC) has provided great opportunities for scientific research and industrial applications. Histone deacetylase (HDAC)-PROTAC has been widely developed since the first report of its ability to induce the degradation of SIRT2 in 2017. To date, ten of the eighteen HDACs (HDACs 1-8, HDAC10, and SIRT2) have been successfully targeted and degraded by HDAC-PROTACs. HDAC-PROTACs surpass traditional HDAC inhibitors in many aspects, such as higher selectivity, more potent antiproliferative activity, and the ability to disrupt the enzyme-independent functions of a multifunctional protein and overcome drug resistance. Rationally designing HDAC-PROTACs is a main challenge in development because slight variations in chemical structure can lead to drastic effects on the efficiency and selectivity of the degradation. In the future, HDAC-PROTACs can potentially be involved in clinical research with the support of the increased amount of in vivo data, pharmacokinetic evaluation, and pharmacological studies.

Targeted degradation of extracellular secreted and membrane proteins

Targeted protein degradation (TPD) involving chimeric molecules has emerged as one of the most promising therapeutic modalities in recent years. Among various reported TPD strategies, proteolysis-targeting chimeras (PROTACs) stand out as a significant breakthrough in small-molecule drug discovery and have garnered the most attention to date. However, PROTACs are mainly capable of depleting intracellular proteins. Given that many important therapeutic targets such as cytokines, growth factors, and numerous receptors are membrane proteins or secreted extracellularly, there is interest in the development of novel strategies to degrade these protein categories. We review advances in this emerging area and provide insights to enhance the development of novel TPDs targeting extracellular proteins.

Degradation of extracellular and membrane proteins in targeted therapy: Status quo and quo vadis

Targeted protein degradation (TPD) strategies, such as proteolysis-targeting chimeras (PROTACs) only work for intracellular protein degradation because they involve the intracellular protein degradation machinery. Several new technologies have emerged in recent years for TPD of extracellular and membrane proteins. Even though some progress has been demonstrated in the extracellular and membrane protein degradation field, the application of these technologies is still in its infancy. In this review, we survey the therapeutic potential of existing technologies by summarizing and reviewing discoveries and hurdles in extracellular and membrane protein-of-interest (POI) degradation.

How many kinases are druggable? A review of our current understanding

There are over 500 human kinases ranging from very well-studied to almost completely ignored. Kinases are tractable and implicated in many diseases, making them ideal targets for medicinal chemistry campaigns, but is it possible to discover a drug for each individual kinase? For every human kinase, we gathered data on their citation count, availability of chemical probes, approved and investigational drugs, PDB structures, and biochemical and cellular assays. Analysis of these factors highlights which kinase groups have a wealth of information available, and which groups still have room for progress. The data suggest a disproportionate focus on the more well characterized kinases while much of the kinome remains comparatively understudied. It is noteworthy that tool compounds for understudied kinases have already been developed, and there is still untapped potential for further development in this chemical space. Finally, this review discusses many of the different strategies employed to generate selectivity between kinases. Given the large volume of information available and the progress made over the past 20 years when it comes to drugging kinases, we believe it is possible to develop a tool compound for every human kinase. We hope this review will prove to be both a useful resource as well as inspire the discovery of a tool for every kinase.

Past, present, and future of FGFR inhibitors in cholangiocarcinoma: from biological mechanisms to clinical applications

INTRODUCTION

Biliary tract carcinoma (BTC) is a heterogenous group of aggressive hepatic malignancies, second to hepatocellular carcinoma per prevalence. Despite clinical research advancement, the overall 5-year survival rate is just above 2%. With the identification of somatic core mutations in half of cholangiocarcinomas. In the intrahepatic subtype (iCCA), it is possible to target mutational pathways of pharmacological interest.

AREAS COVERED

Major attention has been drawn to fibroblast growth factor receptor (FGFR), especially the type 2 (FGFR2), found mutated in 10-15% of iCCAs. FGFR2 fusions became the target of novel tyrosine-kinase inhibitors investigated in clinical studies, showing promising results so as to gain regulatory approval by American and European committees in recent years. Such drugs demonstrated a better impact on the quality of life compared to standard chemotherapy; however, side effects including hyperphosphatemia, gastrointestinal, eye, and nail disorders are common although mostly manageable.

EXPERT OPINION

As FGFR inhibitors may soon become the new alternative to standard chemotherapy in FGFR-mutated cholangiocarcinoma, accurate molecular testing and monitoring of acquired resistance mechanisms will be essential. The possible application of FGFR inhibitors in first-line treatment, as well as in combination with current standard treatments, remains the next step to be taken.

Discovery of a Selective and Orally Bioavailable FGFR2 Degrader for Treating Gastric Cancer

Abnormal activation of fibroblast growth factor receptors (FGFRs) results in the development and progression of human cancers. FGFR2 is frequently amplified or mutated in cancers; therefore, it is an attractive target for tumor therapy. Despite the development of several pan-FGFR inhibitors, their long-term therapeutic efficacy is hindered by acquired mutations and low isoform selectivity. Herein, we report the discovery of an efficient and selective FGFR2 proteolysis-targeting chimeric molecule, LC-MB12, that incorporates an essential rigid linker. LC-MB12 preferentially internalizes and degrades membrane-bound FGFR2 among the four FGFR isoforms; this may promote greater clinical benefits. LC-MB12 exhibits superior potency in FGFR signaling suppression and anti-proliferative activity compared to the parental inhibitor. Furthermore, LC-MB12 is orally bioavailable and shows significant antitumor effects in FGFR2-dependent gastric cancer in vivo. Taken together, LC-MB12 is a candidate FGFR2 degrader for alternative FGFR2-targeting strategies and offers a promising starting point for drug development.

Pemigatinib for the treatment of myeloid/lymphoid neoplasms with FGFR1 rearrangement

INTRODUCTION

Myeloid/lymphoid neoplasms with fibroblast growth factor receptor-1 (FGFR1) rearrangements (MLN^FGFR1) are rare entities with aggressive features and poor prognosis. Presentation is heterogeneous, ranging from myeloproliferative neoplasms (with or without eosinophilia) to T-cell lymphoma and acute leukemia. Historical treatments have been guided by the presenting phenotype with induction chemotherapy frequently used. Pemigatinib is a FGFR1-3 tyrosine kinase inhibitor that has demonstrated high complete hematologic and cytogenetic response rates in MLN^FGFR1.

AREAS COVERED

We discuss the pathogenesis, presentation, and historical treatments for MLN^FGFR1, in addition to clinical data using pemigatinib and other targeted therapies. Discussion of the mechanism of action and adverse events are also included.

EXPERT OPINION

Pemigatinib represents a significant advance in the management of MLN^FGFR1. High rates of complete hematologic and cytogenetic response have been observed. While direct comparative data are unavailable, outcomes appear favorable compared to conventional approaches. Long term efficacy and tolerability are not yet known, and allogeneic hematopoietic stem cell transplant (alloHSCT) has been and continues to be the treatment with the highest chance of long term disease free survival in responding patients. Combinations of pemigatinib and chemotherapy, particularly for more aggressive phenotypes, warrant future investigation as does the use of pemigatinib maintenance following alloHSCT.

Advancing Strategies for Proteolysis-Targeting Chimera Design

Proteolysis-targeting chimeras (PROTACs) have shown great therapeutic potential by degrading various disease-causing proteins, particularly those related to tumors. Therefore, the introduction of PROTACs has ushered in a new chapter of antitumor drug development, marked by significant advances over recent years. Herein, we describe recent developments in PROTAC technology, focusing on design strategy, development workflow, and future outlooks. We also discuss potential opportunities and challenges for PROTAC research.

PROTACS: A technology with a gold rush-like atmosphere

Abnormally expressed or malfunctioning proteins may affect or even damage cells, leading to the onset of diseases. Proteolysis targeting chimera (PROTAC) technology has been proven to be a fresh therapeutic strategy, superior to conventional small molecule inhibitors for the treatment of diseases caused by pathogenic proteins. Unlike conventional small molecule inhibitors that are occupancy-driven, PROTACs are heterobifunctional small molecules with catalytic properties. They combine with E3 ligases and target proteins to form a ternary complex, rendering the target protein ubiquitous and subsequently degraded by the proteasome. This paper focuses first on significant events in the development of PROTAC technology from 2001 to 2022, followed by a brief overview of various PROTACs categorized by target proteins. In addition, the applications of PROTACs in the treatment of diseases and fundamental biology are also under discussion.

Prenylated flavonoids with significant anti-hepatoma activity from Daphne giraldii and effects on Fibroblast Growth Factor Receptor 1 (FGFR1)

We report here the orchestration of molecular ion networking (MoIN) and a set of computational and informatics assisted structural elucidation approaches in the discovery of 23 new prenyl-flavonoids and 13 known molecules from Daphne giraldii Nitsche (Thymelaeaceae), some of which possess significant bioactivity against hepatoma carcinoma. Daphnegiratriprenylone A (DPTP-A) represents the class of polyprenyl-flavonoids possessing a triprenyl substitution, and was identified with the guidance of mass spectrometry and nuclear magnetic resonance combined with computational approaches. This approach illustrates a paradigm shift in the application of computational tools for the direct assignment of new natural product structures and it was demonstrated to be reliable compared to conventional 2D-NMR techniques. Seventeen compounds exhibited potent and selective activity against Hep3B cells (IC50 ranging from 0.42 to 7.08 μM). Tyrosine kinase FGFR1 has emerged as a potential target of polyprenyl-flavonoids by a reverse pharmacophore mapping approach. We validated that the prenyl-flavonoids effectively inhibit FGFR1 using the Mobility Shift Assay, Western blot and molecular dynamics simulations, and the results suggest significant potency of the compounds towards FGFR1. These findings provide a new chemical class with strong links to traditional medicines, possessing reasonable safety for developing potential therapeutic agents for FGFR1-related diseases.

Progress of small molecules for targeted protein degradation: PROTACs and other technologies

Recent years have witnessed the rapid development of targeted protein degradation (TPD), especially proteolysis targeting chimeras. These degraders have manifested many advantages over small molecule inhibitors. To date, a huge number of degraders have been excavated against over 70 disease-related targets. In particular, degraders against estrogen receptor and androgen receptor have crowded into phase II clinical trial. TPD technologies largely expand the scope of druggable targets, and provide powerful tools for addressing intractable problems that can not be tackled by traditional small molecule inhibitors. In this review, we mainly focus on the structures and biological activities of small molecule degraders as well as the elucidation of mechanisms of emerging TPD technologies. We also propose the challenges that exist in the TPD field at present.

Targeting FGFRs for tumor therapy: current status and novel strategies

The FGF receptors (FGFRs) belong to a family of receptor tyrosine kinases. Abundant evidence shows that FGFRs are closely related to tumor cell invasion and angiogenesis. Hence, targeted modulation of FGFRs has become an effective strategy for cancer treatment. Recently, the development of small-molecule inhibitors targeting FGFRs has been extensively studied, and three inhibitors have been approved for marketing. Based on the clinical problems with the current inhibitors, there is a need to develop novel inhibitors and technologies to address the pitfalls. This review summarizes recent advances in small-molecule inhibitors targeting FGFRs, focusing on structure-activity relationships. Moreover, recent progress of novel technologies are summarized to provide a reference for promoting the application of drugs targeting FGFRs in tumor therapy.

Development of selective FGFR1 degraders using a Rapid synthesis of proteolysis targeting Chimera (Rapid-TAC) platform

Proteolysis Targeting Chimera (PROTAC) has emerged as a novel therapeutic strategy. The major bottleneck for the development of PROTACs is the need to screen multiple parameters to create an effective degrader. It often involves the synthesis of dozens to hundreds of compounds one by one through a tedious process. We have developed a two-stage approach that allows for the rapid synthesis of PROTACs (Rapid-TAC) using preassembled building blocks to screen multiple parameters simultaneously. We herein report the application of this method to the development of PROTACs for FGFR, a challenging membrane protein target. In the first stage, we prepared 24 potential PROTACs quickly from a hydrazide-containing FGFR inhibitor and our previously reported VHL and CRBN ligand library bearing various linkers and an aldehyde functional group. These 24 PROTACs were then directly used for screening in cellular assay for protein degradation. Multiple hits were identified from the initial screening. We then prepared the corresponding stable analogues by replacing the hydrolytic labile acylhydrazone motif with an amide in the second stage. Among them, PROTAC LG1188 was identified as a potent and selective FGFR1 degrader.

Proteolysis Targeting Chimeras (PROTACs): A Perspective on Integral Membrane Protein Degradation

Targeted protein degradation (TPD) is a promising therapeutic modality to modulate protein levels and its application promises to reduce the "undruggable" proteome. Among TPD strategies, Proteolysis TArgeting Chimera (PROTAC) technology has shown a tremendous potential with attractive advantages when compared to the inhibition of the same target. While PROTAC technology has had a significant impact in scientific research, its application to degrade integral membrane proteins (IMPs) is still in its beginnings. Among the 15 compounds having entered clinical trials by the end of 2021, only two targets are membrane-associated proteins. In this review we are discussing the potential reasons which may underlie this, and we are presenting new tools that have been recently developed to solve these limitations and to empower the use of PROTACs to target IMPs.

Discovery of a Potent and Selective Degrader for USP7

The tumor suppressor p53 is the most frequently mutated gene in human cancer and more than half of cancers contain p53 mutations. The development of novel and effective therapeutic strategies for p53 mutant cancer therapy is a big challenge and highly desirable. Ubiquitin-specific protease 7 (USP7), also known as HAUSP, is a deubiquitinating enzyme and proposed to stabilize the oncogenic E3 ubiquitin ligase MDM2 that promotes the proteosomal degradation of p53. Herein, we report the design and characterization of U7D-1 as the first selective USP7-degrading Proteolysis Targeting Chimera (PROTAC). U7D-1 showed selective and effective USP7 degradation, and maintained potent cell growth inhibition in p53 mutant cancer cells, with USP7 inhibitor showing no activity. These data clearly demonstrated the practicality and importance of PROTAC as a preliminary chemical tool for investigating USP7 protein functions and a promising method for potential p53 mutant cancer therapy.

Receptor Tyrosine Kinases Amplified in Diffuse-Type Gastric Carcinoma: Potential Targeted Therapies and Novel Downstream Effectors

Simple Summary

Diffuse-type gastric carcinoma (DGC) is an aggressive subtype of gastric carcinoma with an extremely poor prognosis due to frequent peritoneal metastasis and high probability of recurrence. Its pathogenesis is poorly understood, and consequently, no effective molecular targeted therapy is available. The importance of oncogenic receptor tyrosine kinase (RTK) signaling has been recently demonstrated in the malignant progression of DGC. In particular, RTK gene amplification appears to accelerate peritoneal metastasis. In this review, we provide an overview of RTK gene amplification in DGC and the potential of related targeted therapies.

Abstract

Gastric cancer (GC) is a major cause of cancer-related death worldwide. Patients with an aggressive subtype of GC, known as diffuse-type gastric carcinoma (DGC), have extremely poor prognoses. DGC is characterized by rapid infiltrative growth, massive desmoplastic stroma, frequent peritoneal metastasis, and high probability of recurrence. These clinical features and progression patterns of DGC substantially differ from those of other GC subtypes, suggesting the existence of specific oncogenic signals. The importance of gene amplification and the resulting aberrant activation of receptor tyrosine kinase (RTK) signaling in the malignant progression of DGC is becoming apparent. Here, we review the characteristics of RTK gene amplification in DGC and its importance in peritoneal metastasis. These insights may potentially lead to new targeted therapeutics.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin-proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article "PROTACs: great opportunities for academia and industry" in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020-2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

Signaling Pathway and Small-Molecule Drug Discovery of FGFR: A Comprehensive Review

Targeted therapy is a groundbreaking innovation for cancer treatment. Among the receptor tyrosine kinases, the fibroblast growth factor receptors (FGFRs) garnered substantial attention as promising therapeutic targets due to their fundamental biological functions and frequently observed abnormality in tumors. In the past 2 decades, several generations of FGFR kinase inhibitors have been developed. This review starts by introducing the biological basis of FGF/FGFR signaling. It then gives a detailed description of different types of small-molecule FGFR inhibitors according to modes of action, followed by a systematic overview of small-molecule-based therapies of different modalities. It ends with our perspectives for the development of novel FGFR inhibitors.

E3 ligase ligand chemistries: from building blocks to protein degraders

In recent years, proteolysis-targeting chimeras (PROTACs), capable of achieving targeted protein degradation, have proven their great therapeutic potential and usefulness as molecular biology tools. These heterobifunctional compounds are comprised of a protein-targeting ligand, an appropriate linker, and a ligand binding to the E3 ligase of choice. A successful PROTAC induces the formation of a ternary complex, leading to the E3 ligase-mediated ubiquitination of the targeted protein and its proteasomal degradation. In over 20 years since the concept was first demonstrated, the field has grown substantially, mainly due to the advancements in the discovery of non-peptidic E3 ligase ligands. Development of small-molecule E3 binders with favourable physicochemical profiles aided the design of PROTACs, which are known for breaking the rules of established guidelines for discovering small molecules. Synthetic accessibility of the ligands and numerous successful applications led to the prevalent use of cereblon and von Hippel-Lindau as the hijacked E3 ligase. However, the pool of over 600 human E3 ligases is full of untapped potential, which is why expanding the artillery of E3 ligands could contribute to broadening the scope of targeted protein degradation. In this comprehensive review, we focus on the chemistry aspect of the PROTAC design process by providing an overview of liganded E3 ligases, their chemistries, appropriate derivatisation, and synthetic approaches towards their incorporation into heterobifunctional degraders. By covering syntheses of both established and underexploited E3 ligases, this review can serve as a chemistry blueprint for PROTAC researchers during their future ventures into the complex field of targeted protein degradation.

Fibroblast growth factor receptor fusions in cancer: opportunities and challenges

Fibroblast growth factors (FGFs) and their receptors (FGFRs) play critical roles in many biological processes and developmental functions. Chromosomal translocation of FGFRs result in the formation of chimeric FGFR fusion proteins, which often cause aberrant signaling leading to the development and progression of human cancer. Due to the high recurrence rate and carcinogenicity, oncogenic FGFR gene fusions have been identified as promising therapeutic targets. Erdafitinib and pemigatinib, two FGFR selective inhibitors targeting FGFR fusions, have been approved by the U.S. Food and Drug Administration (FDA) to treat patients with urothelial cancer and cholangiocarcinoma, respectively. Futibatinib, a third-generation FGFR inhibitor, is under phase III clinical trials in patients with FGFR gene rearrangements. Herein, we review the current understanding of the FGF/FGFRs system and the oncogenic effect of FGFR fusions, summarize promising inhibitors under clinical development for patients with FGFR fusions, and highlight the challenges in this field.

Discovery and Characterisation of Highly Cooperative FAK-Degrading PROTACs

Focal adhesion kinase (FAK) is a key mediator of tumour progression and metastasis. To date, clinical trials of FAK inhibitors have reported disappointing efficacy for oncology indications. We report the design and characterisation of GSK215, a potent, selective, FAK-degrading Proteolysis Targeting Chimera (PROTAC) based on a binder for the VHL E3 ligase and the known FAK inhibitor VS-4718. X-ray crystallography revealed the molecular basis of the highly cooperative FAK-GSK215-VHL ternary complex, and GSK215 showed differentiated in-vitro pharmacology compared to VS-4718. In mice, a single dose of GSK215 induced rapid and prolonged FAK degradation, giving a long-lasting effect on FAK levels (≈96 h) and a marked PK/PD disconnect. This tool PROTAC molecule is expected to be useful for the study of FAK-degradation biology in vivo, and our results indicate that FAK degradation may be a differentiated clinical strategy versus FAK inhibition for the treatment of cancer.