Metabolic syndrome refers to a class of complex metabolic disorders, including adipose tissue accumulation, insulin resistance, diabetes, hyperinsulinemia, hypertension, and dyslipidemia, characterized by high levels of triglycerides (TG) and high-density lipoprotein cholesterol (HDL-c), which can further induce atherosclerosis and cardiovascular disease. Although the pathogenesis of the metabolic syndrome is not fully understood. Certainly, proteins, fats, and other important metabolites are closely associated with metabolic disorders, such as protein tyrosine phosphatase receptor δ (Ptprd), apolipoprotein E (ApoE), Apolipoprotein B (ApoB), Therefore, targeted regulation of specific protein levels may be an effective means to suppress metabolic syndrome.
The existing targeted drugs represented by small molecule drugs and biologics have shown remarkable therapeutic effects, but there are some limitations in drug resistance, toxicity, side effects, price, and scope of application. In addition, the confirmation of drug targets related to metabolic disorders and the discovery of lead compounds require a large amount of cost and a long time, which is difficult to meet the clinical drug needs of metabolic syndrome. Existing targeted drugs have many shortcomings, but this has prompted research institutions and pharmaceutical companies to look at new targeting technologies.
As an emerging therapeutic strategy in recent years, PROTAC and MG (molecular glue) achieve the degradation of target proteins through the ubiquitin-proteasome pathway, greatly reducing the level of pathogenic proteins. After more than 20 years of development, researchers have not only brought the targeted protein degradation technology to the treatment of cancer but also to the treatment of other diseases including metabolic disorders.
Traditional drugs for metabolic syndrome
1. Small molecule inhibitor or agonist
Small molecule inhibitors have been widely used in the clinical treatment of metabolic syndrome due to their good cell permeability and oral availability. Previous studies have shown that small-molecule drugs work mainly by occupying the active sites of the proteins that target them, which means that enzymes, receptors, and channel proteins are the main targets.
However, more than 70% of the human proteome is currently difficult to bind to small molecular compounds due to the lack of stable binding pockets or chemical structures. For example, apolipoprotein CIII (ApoC-III), Apolipoprotein (a) (Lp(a)), and ANGPTL3 (ANGPTL3), which are associated with metabolic disorders, are all involved in metabolic diseases but their catalytic and regulatory domains have not been clarified. Another example is the proprotein transporter subtilin 9 (PCSK9), which plays an important role in the development of atherosclerosis, but because its binding interface is too flat, the contact area of small molecules binding is limited. All of these are promising targets for the treatment of metabolic syndrome, but effective binding to these proteins cannot be achieved through small molecules at present.
On the other hand, drug resistance has become a difficult problem in the treatment of metabolic syndrome with small-molecule drugs. Studies have shown that inhibition of multidomain kinases may lead to compensatory feedback activation of their downstream signaling pathways. In addition, gene mutations, up-regulation or down-regulation of receptors, and activation of compensatory and bypass mechanisms may cause resistance to the original target. For example, statins competitively bind to the catalytic domain of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (HMGCR), but this induces the body to enhance transcription of the HMGCR gene, which in turn retards the degradation of the HMGCR protein. This can impair the effectiveness of statins and cause side effects such as type 2 diabetes.
In addition, studies have shown that the action of small-molecule inhibitors is dose- and time-dependent, which means that continuous high-dose administration may be required to achieve the desired effect, which may cause toxic side effects and make it difficult to enter the clinic. Due to the complex physiological and pathological mechanisms of metabolic syndrome, the corresponding treatment requires the regulation of lipid, glucose, and blood pressure homeostasis. There are no specific drugs used to treat metabolic syndrome. Hypoglycemic drugs, antihypertensive drugs, and lipid-regulating drugs are sometimes used in combination in the treatment of metabolic diseases. But combinations can lead to drug interactions and reduce patient compliance, ultimately decreasing efficacy and even increasing the toxicity of each drug.
2. Biological Agents
Given the bottlenecks and obstacles in small molecule drug development, biologics are becoming a hot area in drug development. Unlike small molecule-based enzyme inhibitors, biologics achieve target protein inhibition by inhibiting protein function and transcription. To date, researchers have developed a series of biologics with clinical value (Table 1). Most biologics, including monoclonal antibodies (Mabs), peptides, and vaccines, are based on protein-protein interactions that inhibit the function of target proteins.
| Year | Active ingredient (Trade name) | R&D company | Target | Indications | Adverse effect |
| Monoclonal antibody | |||||
| 2015 | Alirocumab (Praluent) | Regeneron and Sanofi | PCSK9 | HeFH or ASCVD | MI, stroke, ischemia-driven coronary revascularization |
| 2015 | Evolocumab (Repatha) | Amgen | |||
| 2021 | Evinacumab (Evkeeza) | Regeneron | ANGPTL3 | HoFH | Nasopharyngitis, infusion reactions, influenza-like illness, dizziness, rhinorrheas, nausea, pain in the extremity |
| Peptide | |||||
| 2005 | Exenatide (Byetta) | AstraZeneca | GLP-1 | T2D | Gastrointestinal side effects (nausea, diarrhea, vomiting, constipation, epigastric pain, etc.) |
| 2010 | Liraglutide (Victoza) | Novo Nordisk | |||
| 2014 | Albiglutide (Tanzeum) | GlaxoSmithKline | |||
| 2014 | Dulaglutide (Trulicity) | Eli Lilly | |||
| 2016 | Lixisenatide (Lyxumla) | Sanofi | |||
| 2017 | Semaglutide (Ozempic) | Novo Nordisk | |||
| 2020 | Setmelanotide (Imcivree) | Rhythm | MC4R | Genetic obesity | Skin pigmentation |
| Small interfering RNA | |||||
| 2021 | Inclisiran (Leqvio) | MDCO | PCSK9 | ASCVD or HeFH | Nasopharyngitis, influenza, episophea infections, back pain, gastroenteritis, injection reactions |
| Anti-sense oligonucleotide | |||||
| 2013 | Mipomersen | Genzyme | ApoB-100 | HoFH | Injection reactions, hepatic steatosis, hepatic enzyme elevation, flu-like symptoms |
For metabolic syndrome, in addition to approved drugs, many new biologics are gradually entering clinical trials. Not only are they effective in regulating glucose or lipid metabolism disorders, but most of them are safer than small-molecule drugs. Such as targeting activin II receptor (ACTRII) mAb bimagrumab, targeting glucagon-like peptide-1 (GLP-1) peptide tirzepatide, targeting PCSK9 AT04A vaccine, etc. However, poor cell permeability, low bioavailability, and short half-life mean that monoclonal antibodies need to be administered subcutaneously or intravenously with high frequency. Due to the characteristics of easy degradation, low stability in vivo, and not easy absorption in the intestine, polypeptide drugs need intravenous injection or infusion, which will greatly reduce patients’ compliance.
Meanwhile, monoclonal antibodies have been shown to have special side effects, immunogenicity, and varying degrees of injection reaction. Bococizumab, a humanized monoclonal antibody against PCSK9, was withdrawn from clinical trials due to its strong immunogenicity and high injection response. In addition, the ability of macromolecular drugs to bind to the target is weak, and the therapeutic effect may not be achieved when used alone.
With the development of genetic engineering, its related gene editing technology has been used in the field of medicine. Gene therapy techniques, including small interfering RNA (siRNA), microRNAs (miRNAs), and antisense oligonucleotides (ASO), have been reported or are under development. They inhibit the transcription of disease-causing proteins at the nucleic acid level by pairing with the bases of gene fragments expressing disease-causing proteins and ultimately silencing disease-causing genes.
siRNA and ASO have been reported to successfully silence PCSK9, ANGPTL3, Lp (a), ApoC-III, and ApoB-100, which can significantly reduce the risk of metabolic syndrome. However, the clinical application of biological agents is not ideal, and its high cost limits its wide application. Many miRNA drugs are routinely discontinued in clinical trials due to safety concerns.
In addition to common flu-like symptoms and adverse effects such as decreased platelet count, several studies have demonstrated that these antisense drugs may have cytotoxic and dose-dependent injection reactions at various sites. Delivery vehicle selection, off-target effect, and immunogenicity are still urgent problems to be solved.
Clinical applications are severely limited by difficulties in developing and quality control of biologics, high production, transport, and storage costs, and a lack of long-term safety data. Therefore, taking clinical practice as the center, further improving efficacy, safety, and drug resistance is the focus of future research and development of biological agents. In addition, there is a greater need for researchers to move beyond small molecule drugs and biologics to develop new therapeutic strategies to treat metabolic syndrome.
Targeted protein degradation techniques
1. PROTAC
First reported in 2001, PROTAC is a novel therapy that degrades target proteins by hijacking the ubiquitin-proteasome system (UPS). From its initial compound structure based on chimeric peptides, PROTAC has evolved into a small molecule permeable to cells, with increasing interest in research and significant clinical translational potential. Following the first two anticancer PROTACs to enter clinical trials in 2019, several other small-molecule depressants are entering the clinic to treat various diseases.
Once PROTAC enters the cell, POI is induced to be close to E3 ligase. The target head at one end of the chimera will specifically bind to the POI, and the ligand at the other end will raise E3 ubiquitin ligase, which will direct the polyubiquitination of the target protein by forming the target protein-Protac-E3 ubiquitin ligase terpolymer and be eliminated by the 26 S proteasome. After the degradation of the target protein, PROTAC can dissociate and enter the next degradation cycle.
Whereas traditional small-molecule drugs only act on the protein kinase domain and work by blocking the enzyme function of the protein, PROTAC acts as a catalyst. Only the activity of binding to target proteins and hijacking E3 ligase can destroy all target protein functions through the UPS pathway. By disrupting rather than simply inhibiting the target protein, PROTAC can not only avoid adverse reactions caused by off-target but also overcome drug resistance caused by traditional small-molecule therapies.
In contrast to biologics, PROTAC does not need to regulate target proteins by manipulating mRNA or genomic DNA as nucleic acid drugs do, making it difficult for cells to gain sufficient time to initiate compensatory mechanisms. These hypoimmunogenic chimeras trigger reversible and rapid POI depletion with negligible effects on the genome and transcriptome and have more potential drug properties.
Due to the cyclic degradation ability of PROTAC, the equivalent PROTAC molecule does not need to maintain a higher concentration in vivo, and can function at a lower dose and frequency of administration, to a certain extent avoiding the off-target effect caused by high dose. In addition, in vivo experiments have shown that PROTAC molecule improves the therapeutic safety window and has lower toxic side effects than classical small molecule drugs.
The concept of MG was first proposed in the early 1990s. Immunosuppressants cyclosporine A (CsA), FK506, and rapamycin were subsequently found to act as protein-protein interaction (PPI) enhancers to induce the formation of terpolymer complexes, as demonstrated by structural biology.
This mechanism of chemically induced protein recombination has led to great interest in the small molecule MG. The first MG depressant was discovered in 2014, when thalidomide-like immunomodulatory drugs (IMids) were found to bind to CRBN, a substrate receptor for E3 ubiquitin ligase, inducing the degradation of specific proteins. Further studies showed that the thalidomide analogs bind to shallow hydrophobic pockets on the CRBN surface and drive protein-protein interactions.
Like PROTAC, MG achieves protein degradation by hijacking the ubiquitin-proteasome pathway. MG, however, does not require a complex chimeric double ligand structure, only binding activity to either E3 ubiquitase or the target protein (and of course, to ensure that it enhances PPI after binding) (Fig 1).
The major advantage of MGS is that they can take advantage of collective binding pockets formed by bringing two protein interfaces together. The proximity of previously untargetable proteins to E3 ubiquitases may create new pockets that bind to small molecules. As small molecules, MG also has more drug properties, that is, better oral bioavailability and pharmacokinetic (PK)/pharmacodynamic (PD) indicators, and most MG meet Lipinski’s five-point rule.
3. ATTEC
ATTEC was first proposed by Professor Lu Boxun of Fudan University in 2019. By using high-throughput screening, we identified two small molecules from a library of 375 small molecule compounds that can bind both LC3 protein and mutant mHTT protein. Studies have shown that these small molecules rely on phage autophagy to selectively degrade mHTT without affecting the level of wild-type HTT, showing the potential to treat various symptoms of Huntington’s disease. ATTEC molecules function in a manner independent of ubiquitination. This principle of pulling the target protein toward LC3 and using phagocytes for degradation has broad applicability. In the follow-up study, the team designed LD⋅ATTEC that can be used to degrade lipid droplets (LD), which showed the great potential of ATTEC to degrade non-protein macromolecules. The development of ATTEC provides a new strategy for the treatment of similar diseases such as Huntington’s disease and metabolic syndrome.
References
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