Hyperlipidemia is one of the most significant controllable risk factors for cardiovascular and cerebrovascular diseases such as myocardial infarction and stroke. These conditions have long ranked among the leading causes of death worldwide, posing a serious threat to public health. Dyslipidemia is not only directly linked to atherosclerosis but is also closely associated with metabolic disorders such as nonalcoholic fatty liver disease, type 2 diabetes, and chronic kidney disease.
Although statins have been widely used as first-line therapy, many patients still fail to achieve optimal lipid control due to insufficient efficacy or adverse reactions. This issue is particularly prominent among individuals with familial hypercholesterolemia or those intolerant to statins. Therefore, developing lipid-lowering drugs that are more effective, safer, and mechanistically diverse has become a key strategic direction for preventing cardiovascular diseases, reducing the burden of metabolic disorders, and improving patient quality of life.
PCSK9 as a Therapeutic Target
Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulatory protein secreted by hepatocytes. By binding to low-density lipoprotein receptors (LDLRs) and promoting their degradation, PCSK9 reduces the liver’s ability to clear cholesterol from the bloodstream, thereby increasing plasma low-density lipoprotein cholesterol (LDL-C) levels. Extensive genetic and clinical studies have demonstrated that inhibiting PCSK9 activity or expression can significantly reduce LDL-C levels and lower the risk of cardiovascular events. As a result, PCSK9 has become a major focus in lipid-lowering drug development in recent years. Multiple therapeutic strategies have been derived to target PCSK9, including monoclonal antibodies, siRNAs, peptides, small-molecule inhibitors, and even gene-editing approaches.
Monoclonal Antibody Therapies
Since 2015, six monoclonal antibody (mAb) drugs targeting PCSK9 have been launched globally. Two were developed by multinational companies: Alirocumab (Sanofi) and Evolocumab (Amgen). Four were developed by Chinese pharmaceutical companies: Tafolecimab (Innovent Biologics), Ebronucimab (Akeso Biopharma), Ongericimab (Junshi Biosciences), and Recaticimab (Hengrui Medicine).
These antibody drugs share highly similar mechanisms and efficacy profiles, with the main differences lying in dosing intervals – from biweekly subcutaneous injections initially, to once every 2-4 weeks, and now extended to every 8-12 weeks. The core logic behind mAb iterations has been to extend half-life, reduce dosing frequency, and improve patient compliance. Additional advantages, such as enhanced safety or broader indications, further strengthen their appeal. However, mAbs act directly on the downstream target protein PCSK9, and their clearance depends on systemic circulation levels. This inherently limits how much dosing frequency can be reduced and suggests that antibody-based therapies may face a performance plateau. Moreover, antibody production is complex and costly. As more similar products enter the market, price competition will intensify, posing a challenge for companies seeking to recoup substantial R&D investments and achieve profitability.
siRNA-Based Therapies
Compared with monoclonal antibodies, siRNA drugs targeting PCSK9 offer several distinct advantages:
- Upstream Mechanism: siRNA suppresses PCSK9 mRNA, blocking protein expression at its source for more complete inhibition.
- Liver-Specific Delivery: Using GalNAc conjugation technology, siRNA achieves precise delivery to hepatocytes, improving target specificity and minimizing systemic side effects.
- Low Dosing Frequency: siRNA drugs require infrequent administration, significantly enhancing patient adherence.
- High Safety and Lower Cost: They exhibit low immunogenicity, superior safety profiles, and can be synthesized chemically at scale, reducing manufacturing costs.
Given these benefits, PCSK9-targeting siRNA drugs may offer greater clinical value for patients. To date, one siRNA drug has been approved – Inclisiran (Novartis). Additionally, RBD7022 (Ribo Biotech) has entered Phase II clinical trials, and RN0191 (Dare Bioscience) has completed Phase I.
Nonetheless, Inclisiran also faces several limitations:
- It remains uncertain whether its LDL-C reduction translates into lower cardiovascular event risk.
- Its onset of action is slower than monoclonal antibodies, making it unsuitable for situations requiring rapid lipid control.
- The formulation requires cold-chain storage (2-8°C) and in-clinic administration, reducing convenience.
- Its efficacy is limited in patients with defective LDLR function.
- Injection-site reactions are relatively common.
Clearly, the next-generation siRNA drugs should aim for greater potency, faster onset, and stable formulations suitable for self-administration. Achieving this will require designing more precise sequences, more stable chemical modifications, and more efficient delivery systems.
Peptide-Based Drugs
Compared with small-molecule drugs, peptides can more accurately mimic the interactions between proteins and receptors. They exhibit high target specificity, strong selectivity, and are metabolized into amino acids, giving them an excellent safety profile. In contrast with large protein biologics, peptides have smaller molecular weights, better tissue penetration, lower immunogenicity, and simpler manufacturing processes. However, peptides also face several limitations – they are chemically unstable, easily degraded by proteases, and difficult to administer orally. The pioneer in the PCSK9 peptide field was Merck (MSD). Its peptide drug Enlicitide demonstrated significant lipid-lowering efficacy in Phase III clinical trials and is expected to become the world’s first oral PCSK9 inhibitor. But how was Enlicitide designed?
MSD collaborated with RARX to identify candidate peptides using mRNA display technology, leading to the discovery of the first hit peptide (Peptide 1). To reduce molecular size and complexity, researchers removed the N-terminal lasso-like “tail” region, generating Peptide 2 – which surprisingly showed enhanced potency rather than a loss of activity.
Since poor stability is the primary obstacle for oral peptide drugs, early stability testing was deemed essential. Researchers assessed Peptide 2’s stability in gastrointestinal proteases and whole blood and performed early pharmacokinetic studies via intravenous administration.
Results showed that Peptide 2 was unstable in chymotrypsin, elastase, and trypsin, but stable in pepsin. The four amide bonds formed by the five N-terminal amino acids were identified as key metabolic sites. In mouse whole blood, the half-life of Peptide 2 was approximately one hour, with primary metabolism occurring at the amide bond between Proline (position 8) and Cysteine (position 9). The peptide also showed a short in vivo half-life and moderately high clearance in mice.
Suspecting that instability resulted from the action of proline peptidase or a related enzyme, researchers substituted proline with α-methylproline, yielding Peptide 3. This modification significantly improved blood stability and increased potency eightfold. However, Peptide 3 remained unstable against protease degradation and continued to display a short in vivo half-life and high clearance. A co-crystal structure of Peptide 3 bound to PCSK9 was then obtained and analyzed. Structural elucidation revealed that Peptide 3 adopted a ring-like “donut” conformation:
- The third amino acid (5F-Trp) was positioned outside the ring, while the fourth 5F-Trp faced inward, with its fluorine atom precisely inserted into a small pocket on the PCSK9 surface.
- The side chains of amino acids 1, 5, and 6 were solvent-exposed and did not directly interact with the protein.
- The seventh amino acid side chain fit into a small groove on the protein surface, and its phenolic hydroxyl group was identified as a potential modification site to improve physicochemical properties.
- The benzene ring in the thioether linker lay directly over the Ile369 side chain, contributing directly to binding affinity.
- Five key hydrogen bonds were identified:
Gly(2) NH with Ser381 side-chain OH
Gly(2) carbonyl with Ser381 main-chain NH
5F-Trp(3) NH with Phe379 carbonyl
5F-Trp(4) carbonyl with Phe379 NH
Asp(5) carbonyl with Thr377 side-chain OH
This co-crystal structure elucidated the binding mode, providing a clear structural basis for further optimization. Subsequent optimization focused on three major strategies:
- Improving amide bond stability by introducing methyl groups or cyclization near the bond.
- Linking nearby groups in the 3D structure to stabilize the bioactive conformation.
- Optimizing non-essential binding groups to enhance physicochemical properties and overall druggability.
Through multiple rounds of design and screening, researchers ultimately obtained Enlicitide. Systematic optimization details have been published elsewhere and are not elaborated here; interested readers may consult the original literature.
Small-Molecule Inhibitors
The most attractive features of small-molecule inhibitors are their oral bioavailability, reversibility, low cost, and easy combination with other drugs. Although they may not completely replace monoclonal antibodies or siRNA therapies, small molecules can extend potent lipid-lowering therapy beyond specialized injection-based and cold-chain settings, making treatment more accessible and widespread, especially in primary care and home environments.
The frontrunner in this field is AstraZeneca, whose small-molecule drug Laroprovstat has completed Phase II clinical trials. The results showed good safety, dose-dependent LDL-C reduction, and no restriction by fasting or fed state. The drug entered Phase III trials in May 2025, showing great promise. Unlike antibody or peptide inhibitors, Laroprovstat does not directly block the PCSK9-LDLR protein-protein interaction. Instead, it binds to a non-classical small pocket within the C-terminal domain (CTD) of PCSK9. By altering the protein’s conformation, it weakens PCSK9’s ability to direct LDLR toward lysosomal degradation, allowing LDLR to be recycled and thereby lowering LDL-C levels.
Although no dedicated publication has detailed Laroprovstat’s design rationale, it is known to have been discovered through fragment-based drug design (FBDD). Researchers performed multiple rounds of fragment screening on PCSK9’s CTD, identified an initial fragment binder, and then used structural biology, medicinal chemistry, and computer-aided drug design to grow the fragment and establish a lead compound. Through multiple rounds of structure-activity relationship (SAR) optimization, they finally achieved a candidate balancing oral bioavailability and in vivo efficacy – Laroprovstat.
PCSK9 small-molecule inhibitors have attracted widespread attention. Since the Laroprovstat patent was published, numerous companies have rushed to develop “fast-follow” candidates. To circumvent existing patents, various creative strategies have emerged. Among these, Novartis and Hansoh Pharma have taken the lead, primarily using heterocycle substitution to maintain potency while improving pharmacokinetic properties.
In Laroprovstat’s patent, the blue-boxed region—a pyrimidine ring—is rigidly protected, making it the easiest site for modification. Novartis substituted this with five-membered nitrogen-containing heterocycles and filed a new patent (WO2023084449), though no further development was reported. Whether the pyrimidine ring can be modified effectively remains uncertain.
Meanwhile, the red-boxed region—protected as aryl, heteroaryl, or heterocyclic groups—was overly broad and likely unpatentable. Subsequent claims narrowed protection to specific mono- or bicyclic heteroaryl structures, listing explicit examples. Hansoh Pharma employed a novel bicyclic scaffold substitution and filed WO2024078620, with its lead compound HS-10510 already in Phase I clinical trials. As Laroprovstat advances through Phase III, more small-molecule contenders are expected to appear. Which will falter and which will succeed remains to be seen.
Finally, gene-editing approaches targeting PCSK9 are still in early research stages. Whether they can eventually be translated into clinical therapies will depend on extensive further validation.
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