Heparin Mimetics and Their Impact on Extracellular Matrix Protein Assemblies

Heparan sulfate is a crucial extracellular matrix component that organizes structural features and functional protein processes. This occurs through the formation of protein-heparan sulfate assemblies around cell surfaces, which allow for the deliberate local and temporal control of cellular signaling. As such, heparin-mimicking drugs can directly affect these processes by competing with naturally occurring heparan sulfate and heparin chains that then disturb protein assemblies and decrease regulatory capacities. The high number of heparan-sulfate-binding proteins that are present in the extracellular matrix can cause obscure pathological effects that should be considered and examined in more detail, especially when developing novel mimetics for clinical use. The objective of this article is to investigate recent studies that present heparan-sulfate-mediated protein assemblies and the impact of heparin mimetics on the assembly and function of these protein complexes.

Heparin, Heparan Sulphate and Sepsis: Potential New Options for Treatment

Sepsis is a life-threatening hyperreaction to infection in which excessive inflammatory and immune responses cause damage to host tissues and organs. The glycosaminoglycan heparan sulphate (HS) is a major component of the cell surface glycocalyx. Cell surface HS modulates several of the mechanisms involved in sepsis such as pathogen interactions with the host cell and neutrophil recruitment and is a target for the pro-inflammatory enzyme heparanase. Heparin, a close structural relative of HS, is used in medicine as a powerful anticoagulant and antithrombotic. Many studies have shown that heparin can influence the course of sepsis-related processes as a result of its structural similarity to HS, including its strong negative charge. The anticoagulant activity of heparin, however, limits its potential in treatment of inflammatory conditions by introducing the risk of bleeding and other adverse side-effects. As the anticoagulant potency of heparin is largely determined by a single well-defined structural feature, it has been possible to develop heparin derivatives and mimetic compounds with reduced anticoagulant activity. Such heparin mimetics may have potential for use as therapeutic agents in the context of sepsis.

Sulfonated non-saccharide molecules and human factor XIa: Enzyme inhibition and computational studies

Human factor XIa (FXIa) is a serine protease in the intrinsic coagulation pathway. FXIa has been actively targeted to develop new anticoagulants that are associated with a reduced risk of bleeding. Thousands of FXIa inhibitors have been reported, yet none has reached the clinic thus far. We describe here a novel class of sulfonated molecules that allosterically inhibit FXIa with moderate potency. A library of 18 sulfonated molecules was evaluated for the inhibition of FXIa using a chromogenic substrate hydrolysis assay. Only six molecules inhibited FXIa with IC₅₀ values of 4.6-29.5 μM. Michaelis-Menten kinetics indicated that sulfonated molecules are allosteric inhibitors of FXIa. Inhibition of FXIa by these molecules was reversed by protamine. The molecules also showed moderate anticoagulant effects in human plasma with preference to prolong activated partial thromboplastin time. Their binding to an allosteric site in the catalytic domain of FXIa was modeled to illustrate potential binding mode and potential important Arg/Lys residues. Particularly, inhibitor 16 (IC₅₀  = 4.6 µM) demonstrated good selectivity over a panel of serine proteases including those in the coagulation process. Inhibitor 16 did not significantly compromise the viability of three cell lines. Overall, the reported sulfonated molecules serve as a new platform to design selective, potent, and allosteric inhibitors of FXIa for therapeutic applications.

Antiviral Strategies Using Natural Source-Derived Sulfated Polysaccharides in the Light of the COVID-19 Pandemic and Major Human Pathogenic Viruses

Only a mere fraction of the huge variety of human pathogenic viruses can be targeted by the currently available spectrum of antiviral drugs. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak has highlighted the urgent need for molecules that can be deployed quickly to treat novel, developing or re-emerging viral infections. Sulfated polysaccharides are found on the surfaces of both the susceptible host cells and the majority of human viruses, and thus can play an important role during viral infection. Such polysaccharides widely occurring in natural sources, specifically those converted into sulfated varieties, have already proved to possess a high level and sometimes also broad-spectrum antiviral activity. This antiviral potency can be determined through multifold molecular pathways, which in many cases have low profiles of cytotoxicity. Consequently, several new polysaccharide-derived drugs are currently being investigated in clinical settings. We reviewed the present status of research on sulfated polysaccharide-based antiviral agents, their structural characteristics, structure-activity relationships, and the potential of clinical application. Furthermore, the molecular mechanisms of sulfated polysaccharides involved in viral infection or in antiviral activity, respectively, are discussed, together with a focus on the emerging methodology contributing to polysaccharide-based drug development.

Inhibition of Tumor-Host Cell Interactions Using Synthetic Heparin Mimetics

Low-molecular-weight heparin (LMWH) is the guideline-based drug for antithrombotic treatment of cancer patients, while its direct antitumor effects are a matter of ongoing debate. Although therapeutically established for decades, LMWH has several drawbacks mainly associated with its origin from animal sources. Aiming to overcome these limitations, a library of synthetic heparin mimetic polymers consisting of homo- and copolymers of sulfonated and carboxylated noncarbohydrate monomers has recently been synthesized via reversible addition-fragmentation chain transfer polymerization. These heparin mimetics were investigated for their capacities to interfere with simulated steps of tumor cell metastasis. Among them, homo- and copolymers from sodium 4-styrenesulfonate (poly(SSS)) with acrylic acid (poly(SSS-co-AA)) with an MW between 5 and 50 kDa efficiently attenuated cancer cell-induced coagulation and thus platelet activation and degranulation similar to or even better than LMWH. Furthermore, independent of anticoagulant activities, these polymers affected other metastasis-relevant targets with impressive affinities. Hence, they blocked heparanase enzymatic activity outmatching commercial heparins or a glycosidic drug candidate. Furthermore, these polymers bind P-selectin and the integrin VLA-4 similar to or even better than heparin, indicated by a biosensor approach and thus efficiently blocked melanoma cell binding to endothelium under blood flow conditions. This is the first report on the prospects of synthetic heparin mimetics as promising nontoxic compounds in oncology to potentially substitute heparin as an anticoagulant and to better understand its role as an antimetastatic drug.

Sulfonated RAFT Copolymers as Heparin Mimetics: Synthesis, Reactivity Ratios, and Anticoagulant Activity

The glycosaminoglycan heparin is a clinically important anticoagulant drug, primarily used to reduce the risk of blood clots (thrombosis) during surgery. Despite its importance in medicine and its continuous use over many decades, heparin suffers from several limitations associated with its heterogeneity and its extraction from animal tissues. In order to address these limitations, reversible addition-fragmentation chain transfer polymerization is utilized to prepare a library of heparin mimetic copolymers from the sulfonated monomers sodium 4-styrene sulfonate, potassium-3-sulfopropyl acrylate, potassium-3-sulfopropyl methacrylate, and sodium-2-acrylamido-2-methyl-1-propane sulfonate. Copolymers are prepared using combinations of two different monomers in various ratios. Monomer reactivity ratios are also determined for some representative monomer combinations, and all polymers are characterized by ¹ H NMR spectroscopy and gel permeation chromatography. The anticoagulant activities of the copolymers are determined by activated partial thromboplastin time and thrombin clotting time assays and structure-activity relationships are explored.

Heparin mimetics with anticoagulant activity

Heparin, a sulfated polysaccharide belonging to the glycosaminoglycan family, has been widely used as an anticoagulant drug for decades and remains the most commonly used parenteral anticoagulant in adults and children. However, heparin has important clinical limitations and is derived from animal sources which pose significant safety and supply problems. The ever growing shortage of the raw material for heparin manufacturing may become a very significant issue in the future. These global limitations have prompted much research, especially following the recent well-publicized contamination scandal, into the development of alternative anticoagulants derived from non-animal and/or totally synthetic sources that mimic the structural features and properties of heparin. Such compounds, termed heparin mimetics, are also needed as anticoagulant materials for use in biomedical applications (e.g., stents, grafts, implants etc.). This review encompasses the development of heparin mimetics of various structural classes, including synthetic polymers and non-carbohydrate small molecules as well as sulfated oligo- and polysaccharides, and fondaparinux derivatives and conjugates, with a focus on developments in the past 10 years.

Heparin Mimetics: Their Therapeutic Potential

Heparin mimetics are synthetic and semi-synthetic compounds that are highly sulfated, structurally distinct analogues of glycosaminoglycans. These mimetics are often rationally designed to increase potency and binding selectivity towards specific proteins involved in disease manifestations. Some of the major therapeutic arenas towards which heparin mimetics are targeted include: coagulation and thrombosis, cancers, and inflammatory diseases. Although Fondaparinux, a rationally designed heparin mimetic, is now approved for prophylaxis and treatment of venous thromboembolism, the search for novel anticoagulant heparin mimetics with increased affinity and fewer side effects remains a subject of research. However, increasingly, research is focusing on the non-anticoagulant activities of these molecules. Heparin mimetics have potential as anti-cancer agents due to their ability to: (1) inhibit heparanase, an endoglycosidase which facilitates the spread of tumor cells; and (2) inhibit angiogenesis by binding to growth factors. The heparin mimetic, PI-88 is in clinical trials for post-surgical hepatocellular carcinoma and advanced melanoma. The anti-inflammatory properties of heparin mimetics have primarily been attributed to their ability to interact with: complement system proteins, selectins and chemokines; each of which function differently to facilitate inflammation. The efficacy of low/non-anticoagulant heparin mimetics in animal models of different inflammatory diseases has been demonstrated. These findings, plus clinical data that indicates heparin has anti-inflammatory activity, will raise the momentum for developing heparin mimetics as a new class of therapeutic agent for inflammatory diseases.

Reversal of abnormal collagen production in Crohn's disease intestinal biopsies treated with regenerating agents

BACKGROUND

Crohn's disease (CD) is characterised by inflammation, muscle layer overgrowth, and collagenous fibrosis of the intestinal tract, with no effective therapy against collagen accumulation.

AIMS

We quantified production of collagen in resection specimens from normal and CD patients and investigated the effect of regenerating agents (RGTAs) on collagen production. RGTAs are chemically substituted dextrans engineered to mimic the growth factor protecting effects of heparan sulphates. RGTAs have been shown to enhance tissue repair in various in vivo models and to modulate in vitro collagen phenotype differentially according to their structure.

PATIENTS

We studied intestinal biopsies from two groups of CD patients: treated with glucocorticoids (CD-GC group: 10 patients) or not treated (CD group: seven patients), and from seven control patients.

METHODS

After 24 hours of ex vivo incubation with (3H) proline, collagen I, III, and V were extracted by pepsin and quantitatively separated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Biosynthesis of each collagen type was quantified on radiolabelled isolated collagen.

RESULTS

Total intestinal collagen production in CD patients compared with controls was increased up to 3.5-fold overall (p<0.001). In particular, collagen III biosynthesis was enhanced by 6.2-fold (p<0.001) in CD patients. In the CD-GC group, collagen production abnormalities were less marked. RGTAs added to the incubation medium in the CD group decreased total collagen production by 50% and decreased collagen III synthesis by 76%.

CONCLUSION

This finding offers a rationale for using RGTAs in the treatment of intestinal fibrosis in CD, thus opening up a potential new therapeutic field for this family of drugs.