Development and Prospects of Furin Inhibitors for Therapeutic Applications

Furin, a serine protease enzyme located in the Golgi apparatus of animal cells, plays a crucial role in cleaving precursor proteins into their mature, active forms. It is ubiquitously expressed across various tissues, including the brain, lungs, gastrointestinal tract, liver, pancreas, and reproductive organs. Since its discovery in 1990, furin has been recognized as a significant therapeutic target, leading to the active development of furin inhibitors for potential use in antiviral, antibacterial, anticancer, and other therapeutic applications. This review provides a comprehensive overview of the progress in the development and characterization of furin inhibitors, encompassing peptides, linear and macrocyclic peptidomimetics, and non-peptide compounds, highlighting their potential in the treatment of both infectious and non-infectious diseases.

Recent advances in the application of parahydrogen in catalysis and biochemistry

Nuclear Magnetic Resonance (NMR) spectroscopy and Magnetic Resonance Imaging (MRI) are analytical and diagnostic tools that are essential for a very broad field of applications, ranging from chemical analytics, to non-destructive testing of materials and the investigation of molecular dynamics, to in vivo medical diagnostics and drug research. One of the major challenges in their application to many problems is the inherent low sensitivity of magnetic resonance, which results from the small energy-differences of the nuclear spin-states. At thermal equilibrium at room temperature the normalized population difference of the spin-states, called the Boltzmann polarization, is only on the order of 10-5. Parahydrogen induced polarization (PHIP) is an efficient and cost-effective hyperpolarization method, which has widespread applications in Chemistry, Physics, Biochemistry, Biophysics, and Medical Imaging. PHIP creates its signal-enhancements by means of a reversible (SABRE) or irreversible (classic PHIP) chemical reaction between the parahydrogen, a catalyst, and a substrate. Here, we first give a short overview about parahydrogen-based hyperpolarization techniques and then review the current literature on method developments and applications of various flavors of the PHIP experiment.

Parahydrogen-Induced Polarization of Amino Acids

Nuclear magnetic resonance (NMR) has become a universal method for biochemical and biomedical studies, including metabolomics, proteomics, and magnetic resonance imaging (MRI). By increasing the signal of selected molecules, the hyperpolarization of nuclear spin has expanded the reach of NMR and MRI even further (e.g. hyperpolarized solid-state NMR and metabolic imaging in vivo). Parahydrogen (pH2 ) offers a fast and cost-efficient way to achieve hyperpolarization, and the last decade has seen extensive advances, including the synthesis of new tracers, catalysts, and transfer methods. The portfolio of hyperpolarized molecules now includes amino acids, which are of great interest for many applications. Here, we provide an overview of the current literature and developments in the hyperpolarization of amino acids and peptides.

Triazole-Modified Peptidomimetics: An Opportunity for Drug Discovery and Development

Peptidomimetics play a fundamental role in drug design due to their preferential properties regarding natural peptides. In particular, compounds possessing nitrogen-containing heterocycles have been intensively studied in recent years. The triazolyl moiety incorporation decreases the molecule susceptibility to enzymatic degradation, reduction, hydrolysis, and oxidation. In fact, peptides containing triazole rings are a typical example of peptidomimetics. They have all the advantages over classic peptides. Both efficient synthetic methods and biological activity make these systems an interesting and promising object of research. Peptide triazole derivatives display a diversity of biological properties and can be obtained via numerous synthetic strategies. In this review, we have highlighted the importance of the triazole-modified peptidomimetics in the field of drug design. We present an overview on new achievements in triazolyl-containing peptidomimetics synthesis and their biological activity as inhibitors of enzymes or against cancer, viruses, bacteria, or fungi. The relevance of above-mentioned compounds was confirmed by their comparison with unmodified peptides.

Sunflower Trypsin Inhibitor-1 (SFTI-1): Sowing Seeds in the Fields of Chemistry and Biology

Nature-derived cyclic peptides have proven to be a vast source of inspiration for advancing modern pharmaceutical design and synthetic chemistry. The focus of this Review is sunflower trypsin inhibitor-1 (SFTI-1), one of the smallest disulfide-bridged cyclic peptides found in nature. SFTI-1 has an unusual biosynthetic pathway that begins with a dual-purpose albumin precursor and ends with the production of a high-affinity serine protease inhibitor that rivals other inhibitors much larger in size. Investigations on the molecular basis for SFTI-1's rigid structure and adaptable function have planted seeds for thought that have now blossomed in several different fields. Here we survey these applications to highlight the growing potential of SFTI-1 as a versatile template for engineering inhibitors, a prototypic peptide for studying inhibitory mechanisms, a stable scaffold for grafting bioactive peptides, and a model peptide for evaluating peptidomimetic motifs and platform technologies.

Truncation of Huia versabilis Bowman-Birk inhibitor increases its selectivity, matriptase-1 inhibitory activity and proteolytic stability

A gradual truncation of the primary structure of frog skin-derived Huia versabilis Bowman-Birk peptidic inhibitor (HV-BBI) resulted in 18-times stronger inhibitor of matriptase-1 (peptide 6, K_i = 8 nm) in comparison to the full-length HV-BBI (K_i = 155 nm). Analogous increase in the inhibitory activity in correlation with the peptide length reduction was not observed in case of other serine proteases, bovine trypsin (K_i = 151 nm for peptide 6 and K_i = 120 nm for HV-BBI) and plasmin (K_i = 120 nm for peptide 6 and 82 nm for HV-BBI). Weaker binding affinity to these enzymes emphasized an inhibitory specificity of peptide 6. Molecular dynamic analysis revealed that the observed variations in the binding affinity of peptide 6 and HV-BBI with matriptase-1 are associated with the entropic differences of the unbound peptides. Moreover, several aspects explaining differences in the inhibition of matriptase-1 by peptide 6 (bearing the C-terminal amide group) and its two analogues, peptide 6∗ (having the C-terminal carboxyl group, K_i = 473 nm) and cyclic peptide 6∗∗ (K_i = 533 nm), both exhibiting more than 50-fold reduced inhibitory potency, were discovered. It was also shown that peptide 6 presented significantly higher resistance to proteolytic degradation in human serum than HV-BBI. Additional investigations revealed that, in contrast to some amphibian-derived inhibitors, HV-BBI and its truncated analogues do not possess bactericidal activity, thus they cannot be considered as bifunctional agents.

Specifically targeting cancer proliferation and metastasis processes: the development of matriptase inhibitors

Matriptase is a type II transmembrane serine protease, which has been suggested to play critical roles in numerous pathways of biological developments. Matriptase is the activator of several oncogenic proteins, including urokinase-type plasminogen activator (uPA), hepatocyte growth factor (HGF) and protease-activated receptor 2 (PAR-2). The activations of these matriptase substrates subsequently lead to the generation of plasmin, matrix metalloproteases (MMPs), and the triggers for many other signaling pathways related to cancer proliferation and metastasis. Accordingly, matriptase is considered an emerging target for the treatments of cancer. Thus far, inhibitors of matriptase have been developed as potential anti-cancer agents, which include small-molecule inhibitors, peptide-based inhibitors, and monoclonal antibodies. This review covers established literature to summarize the chemical and biochemical aspects, especially the inhibitory mechanisms and structure-activity relationships (SARs) of matriptase inhibitors with the goal of proposing the strategies for their future developments in anti-cancer therapy.

Binding Loop Substitutions in the Cyclic Peptide SFTI-1 Generate Potent and Selective Chymase Inhibitors

Chymase is a serine protease that is predominantly expressed by mast cells and has key roles in immune defense and the cardiovascular system. This enzyme has also emerged as a therapeutic target for cardiovascular disease due to its ability to remodel cardiac tissue and generate angiotensin II. Here, we used the nature-derived cyclic peptide sunflower trypsin inhibitor-1 (SFTI-1) as a template for designing novel chymase inhibitors. The key binding contacts of SFTI-1 were optimized by combining a peptide substrate library screen with structure-based design, which yielded several variants with potent activity. The lead variant was further modified by replacing the P1 Tyr residue with para-substituted Phe derivatives, generating new inhibitors with improved potency (K_i = 1.8 nM) and higher selectivity over closely related enzymes. Several variants were shown to block angiotensin I cleavage in vitro, highlighting their potential for further development and future evaluation as pharmaceutical leads.

Buried treasure: biosynthesis, structures and applications of cyclic peptides hidden in seed storage albumins

Covering: 1999 up to the end of 2017The small cyclic peptide SunFlower Trypsin Inhibitor-1 (SFTI-1) from sunflower seeds is the prototypic member of a novel family of natural products. The biosynthesis of these peptides is intriguing as their gene-encoded peptide backbone emerges from a precursor protein that also contains a seed storage albumin. The peptide sequence is cleaved out from the precursor and cyclised by the albumin-maturing enzymatic machinery. Three-dimensional solution NMR structures of a number of these peptides, and of the intact precursor protein preproalbumin with SFTI-1, have now been elucidated. Furthermore, the evolution of the family has been described and a detailed understanding of the biosynthetic steps, which are necessary to produce cyclic SFTI-1, is emerging. Macrocyclisation provides peptide stability and thus represents a key strategy in peptide drug development. Consequently the constrained structure of SFTI-1 has been explored as a template for protein engineering, for tuning selectivity towards clinically relevant proteases and for grafting in sequences with completely novel functions. Here we review the discovery of the SFTI-1 peptide family, their evolution, biosynthetic origin, and structural features, as well as highlight the potential applications of this unique class of natural products.

Linking inhibitor motions to proteolytic stability of sunflower trypsin inhibitor-1

The remarkable capability of an enzyme isn't only determined by its active site but also controlled by the environment. To unravel the environment role in catalysis, the dynamic motions as well as the static mechanism need to be studied. In this work, QM/MM MD simulations were employed to study the proteolysis process of SFTI-1 and BiKF, which revealed that a combination of static non-bonded interactions and dynamic motions along the reaction coordinate can account for the different hydrolysis rates between them. A comparison among SFTI-1 and three analogs with similar non-bonded interactions further revealed a positive correlation between the mobility of inhibitors and the hydrolysis rates. Apart from the cyclic backbone and disulfide bond, intramolecular hydrogen bonds also increase the rigidity of the backbone of inhibitors, and therefore hinder inhibitor motions to resist proteolysis. These new detailed mechanistic insights suggest the need to consider inhibitor motions in the rational design of peptide inhibitors.

Besides the non-bonded interactions, inhibitor motions especially rotation of the scissile bond also influence proteolytic stability.

Amino Acid Scanning at P5' within the Bowman-Birk Inhibitory Loop Reveals Specificity Trends for Diverse Serine Proteases

Sunflower trypsin inhibitor-1 (SFTI-1) is a 14-amino acid cyclic peptide that shares an inhibitory loop with a sequence and structure similar to a larger family of serine protease inhibitors, the Bowman-Birk inhibitors. Here, we focus on the P5' residue in the Bowman-Birk inhibitory loop and produce a library of SFTI variants to characterize the P5' specificity of 11 different proteases. We identify seven amino acids that are generally preferred by these enzymes and also correlate with P5' sequence diversity in naturally occurring Bowman-Birk inhibitors. Additionally, we show that several enzymes have divergent specificities that can be harnessed in engineering studies. By optimizing the P5' residue, we improve the potency or selectivity of existing inhibitors for kallikrein-related peptidase 5 and show that a variant with substitutions at 7 of the scaffold's 14 residues retains a similar structure to SFTI-1. These findings provide new insights into P5' specificity requirements for the Bowman-Birk inhibitory loop.

Hit evaluation of an α-helical peptide: Ala-scan, truncation and sidechain-to-sidechain macrocyclization of an RNA polymerase Inhibitor

RNA polymerase (RNAP) remains a relatively underexplored target with only rifampicin and fidaxomicin in clinical use. Hence, the concurrent rise in bacterial resistance rate urges the search for novel RNAP inhibitors with a novel mode of action. In this work, we investigated the impact of several systematic modifications including sidechain-to-sidechain macrocylization in the α-helical content and biological activity of a previously identified inhibitory sigma factor fragment. Ala-scan results, peptide truncation from both the N- and C-terminus and modifications inspired by other RNAP inhibitors revealed novel structure activity relationships but did not yield a superior sequence. Additionally, four insertion points for non-natural amino acids bearing side chains required for macrocylization were explored. Linear precursors showed improved stabilization of the α-helical content compared to the original sequence as demonstrated by circular dichroism (CD) spectroscopy. However, this increase in α-helicity did not translate into improved biological activity. Instead, complete abolishment of RNAP inhibitory activity occurred. We hypothesize three possible reasons for such a discrepancy and offer the basis for further optimization efforts for this peptidic RNAP inhibitor.

Matriptase-2: monitoring and inhibiting its proteolytic activity

Matriptase-2 (MT2) is a membrane-anchored proteolytic enzyme. It acts as the proteolytic key regulator in human iron homeostasis. A high expression level can lead to iron overload diseases, whereas mutations in the gene encoding MT2, TMPRSS6, may result in various forms of iron deficiency anemia. Recently, MT2 has been reported as a positive prognostic factor in breast and prostate cancers. However, the exact functions of MT2 in various pathophysiological conditions are still not fully understood. In this review, we describe the synthetic tools designed and synthesized to regulate or monitor MT2 proteolytic activity and present the latest knowledge about the role of MT2 in iron homeostasis and cancer.

Small Peptides as Modulators of Serine Proteases

Serine proteases play critical roles in many physiological and pathological processes, and are proven diagnostic and therapeutic targets in a number of clinical indications. Suppression of the aberrant proteolytic activities of these proteases has been clinically used for the treatments of relevant diseases. Polypeptides with 10-20 residues are of great interests as medicinal modulators of serine proteases, because these peptides demonstrate the characteristics of both small molecule drugs and macromolecular drugs. In this review, we summarized the recent development of peptide-based inhibitors against serine proteases with potent inhibitory and high specificity comparable to monoclonal antibodies. In addition, we also discussed the strategies of enhancing plasma half-life and bioavailability of peptides in vivo, which is the main hurdle that limits the clinical translation of peptide-based drugs. This review advocates new avenue for the development of effective serine protease inhibitors and highlights the prospect of the medicinal use of these inhibitors.

Engineering potent mesotrypsin inhibitors based on the plant-derived cyclic peptide, sunflower trypsin inhibitor-1

Plants produce a diverse range of peptides and proteins that inhibit the activity of different serine proteases. The value of these inhibitors not only stems from their native role(s) in planta, but they are also regarded as promising templates for inhibitor engineering. Interest in this field has grown rapidly in recent years, particularly for therapeutic applications. The serine protease mesotrypsin has been implicated in several cancers, but is a challenging target for inhibitor engineering as a number of serine protease inhibitors that typically display broad-range activity show limited activity against mesotrypsin. In this study, we use a cyclic peptide isolated from sunflower seeds, sunflower trypsin inhibitor-1 (SFTI-1), as a scaffold for engineering potent mesotrypsin inhibitors. SFTI-1 comprises 14-amino acids and is a potent inhibitor of human cationic trypsin (K_i = 30 ± 0.8 pM) but shows 165,000-fold weaker activity against mesotrypsin (K_i = 4.96 ± 0.2 μM). Using an inhibitor library based on SFTI-1, we show that the inhibitor's P2' residue (Ile) is a key contributor to SFTI-1's limited activity against mesotrypsin. Substituting P2' Ile with chemically diverse amino acids, including non-canonical aromatic residues, produced new inhibitor variants that maintained a similar structure to SFTI-1 and showed marked improvements in activity (exceeding 100-fold). An assessment of the activity of the new inhibitors against closely-related trypsin paralogs revealed that the improved activity against mesotrypsin was accompanied by a loss in activity against off-target proteases, such that several engineered variants showed comparable activity against mesotrypsin and human cationic trypsin. Together, these findings identify potent mesotrypsin inhibitors that are suitable for further optimisation studies and demonstrate the potential gains in activity and selectivity that can be achieved by optimising the P2' residue, particularly for engineered SFTI-based inhibitors.

The Antiviral Potential of Host Protease Inhibitors

The replication of numerous pathogenic viruses depends on host proteases, which therefore emerged as potential antiviral drug targets. In some cases, e.g., for influenza viruses, their function during the viral propagation cycle is relatively well understood, where they cleave and activate viral surface glycoproteins. For other viruses, e.g., Ebola virus, the function of host proteases during replication is still not clear. Host proteases may also contribute to the pathogenicity of virus infection by activating proinflammatory cytokines. For some coronaviruses, human proteases can also serve in a nonproteolytical fashion simply as receptors for virus entry. However, blocking of such protein-protein contacts is challenging, because receptor surfaces are often flat and difficult to address with small molecules. In contrast, many proteases possess well-defined binding pockets. Therefore, they can be considered as well-druggable targets, especially, if they are extracellularly active. The number of their experimental crystal structures is steadily increasing, which is an important prerequisite for a rational structure-based inhibitor design using computational chemistry tools in combination with classical medicinal chemistry approaches. Moreover, host proteases can be considered as stable targets, and their inhibition should prevent rapid resistance developments, which is often observed when addressing viral proteins. Otherwise, the inhibition of host proteases can also affect normal physiological processes leading to a higher probability of side effects and a narrow therapeutic window. Therefore, they should be preferably used in combination therapies with additional antiviral drugs. This strategy should provide a stronger antiviral efficacy, allow to use lower drug doses, and minimize side effects. Despite numerous experimental findings on their antiviral activity, no small-molecule inhibitors of host proteases have been approved for the treatment of virus infections, so far.

Direct and indirect mechanisms of KLK4 inhibition revealed by structure and dynamics

The kallikrein-related peptidase (KLK) family of proteases is involved in many aspects of human health and disease. One member of this family, KLK4, has been implicated in cancer development and metastasis. Understanding mechanisms of inactivation are critical to developing selective KLK4 inhibitors. We have determined the X-ray crystal structures of KLK4 in complex with both sunflower trypsin inhibitor-1 (SFTI-1) and a rationally designed SFTI-1 derivative to atomic (~1 Å) resolution, as well as with bound nickel. These structures offer a structural rationalization for the potency and selectivity of these inhibitors, and together with MD simulation and computational analysis, reveal a dynamic pathway between the metal binding exosite and the active site, providing key details of a previously proposed allosteric mode of inhibition. Collectively, this work provides insight into both direct and indirect mechanisms of inhibition for KLK4 that have broad implications for the enzymology of the serine protease superfamily, and may potentially be exploited for the design of therapeutic inhibitors.

Steering the azido-tetrazole equilibrium of 4-azidopyrimidines via substituent variation - implications for drug design and azide-alkyne cycloadditions

This paper focuses on an interesting constitutional isomerism called azido-tetrazole equilibrium which is observed in azido-substituted N-heterocycles. We present a systematic investigation of substituent effects on the isomer ratio within a 2-substituted 4-azidopyrimidine model scaffold. NMR- and IR-spectroscopy as well as X-ray crystallography were employed for thorough analysis and characterization of synthesized derivatives. On the basis of this data, we demonstrate the possibility to steer this valence tautomerism towards the isomer of choice by means of substituent variation. We show that the tetrazole form can act as an efficient disguise for the corresponding azido group masking its well known reactivity in azide-alkyne cycloadditions (ACCs). In copper(I)-catalyzed AAC reactions, substituent-stabilized tetrazoles displayed a highly decreased or even abolished reactivity whereas azides and compounds in the equilibrium were directly converted. By use of an acid sensitive derivative, we provide, to our knowledge, the first experimental basis for a possible exploitation of this dynamic isomerism as a pH-dependent azide-protecting motif for selective SPAAC conjugations in aqueous media. Finally, we demonstrate the applicability and efficiency of stabilized tetrazolo[1,5-c]pyrimidines for Fragment-Based Drug Design (FBDD) in the field of quorum sensing inhibitors.

CuAAC: An Efficient Click Chemistry Reaction on Solid Phase

Click chemistry is an approach that uses efficient and reliable reactions, such as Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC), to bind two molecular building blocks. CuAAC has broad applications in medicinal chemistry and other fields of chemistry. This review describes the general features and applications of CuAAC in solid-phase synthesis (CuAAC-SP), highlighting the suitability of this kind of reaction for peptides, nucleotides, small molecules, supramolecular structures, and polymers, among others. This versatile reaction is expected to become pivotal for meeting future challenges in solid-phase chemistry.

Improving the Selectivity of Engineered Protease Inhibitors: Optimizing the P2 Prime Residue Using a Versatile Cyclic Peptide Library

Standard mechanism inhibitors are attractive design templates for engineering reversible serine protease inhibitors. When optimizing interactions between the inhibitor and target protease, many studies focus on the nonprimed segment of the inhibitor's binding loop (encompassing the contact β-strand). However, there are currently few methods for screening residues on the primed segment. Here, we designed a synthetic inhibitor library (based on sunflower trypsin inhibitor-1) for characterizing the P2' specificity of various serine proteases. Screening the library against 13 different proteases revealed unique P2' preferences for trypsin, chymotrypsin, matriptase, plasmin, thrombin, four kallikrein-related peptidases, and several clotting factors. Using this information to modify existing engineered inhibitors yielded new variants that showed considerably improved selectivity, reaching up to 7000-fold selectivity over certain off-target proteases. Our study demonstrates the importance of the P2' residue in standard mechanism inhibition and unveils a new approach for screening P2' substitutions that will benefit future inhibitor engineering studies.

De novo discovery of bioactive cyclic peptides using bacterial display and flow cytometry

Cyclic peptides are increasingly desired for their enhanced stability and pharmacologic properties. Due to their limited conformational flexibility, cyclic peptides with C-to-N-terminal peptide bond and a disulfide bridge can confer high target binding affinity and resistance to proteolytic enzymes. Challenging drug targets including protein interaction surfaces can be successfully targeted using peptides rather than small molecules or proteins. Peptides, capable of antibody-like affinities with increased potency, can be designed to fill in the gap between small molecules and larger proteins. However, cysteine-rich peptides with several disulfide bonds have limitations in production and purification. Therefore, we devised a strategy to identify cyclic peptides with single disulfide connectivity that offers desired properties along with ease in synthesis and production. Here, de novo design of cyclic peptides is demonstrated through screening of peptide libraries using bacterial display and cell sorting. Herein, a step-by-step protocol is presented to design and screen diverse peptide libraries to identify cyclic peptides with desired specificity and affinity towards arbitrary target proteins.

Disulfide-bridged peptide macrobicycles from nature

This review highlights known disulfide-bridged peptide bicycles and the studies on their unique structural and biological features.

Toward the first class of suicide inhibitors of kallikreins involved in skin diseases

The inhibition of kallikreins 5 and 7, and possibly kallikrein 14 and matriptase, (that initiates the kallikrein proteolytic cascade) constitutes an innovative way to treat some skin diseases such as Netherton syndrome. We present here the inhibitory properties of coumarin-3-carboxylate derivatives against these enzymes. Our small collection of these versatile organic compounds was enriched by newly synthesized derivatives in order to obtain molecules selective against one, two, three enzymes or acting on the four ones. We evidenced a series of compounds with IC50 values in the nanomolar range. A suicide mechanism was observed against kallikrein 7 whereas the inactivation was either definitive (suicide type) or transient for kallikreins 5 and 14, and matriptase. Most of these potent inhibitors were devoid of cytotoxicity toward healthy human keratinocytes. In situ zymography investigations on skin sections from human kallikrein 5 transgenic mouse revealed significant reduction of the global proteolytic activity by several compounds.

Potent and specific inhibition of the biological activity of the type-II transmembrane serine protease matriptase by the cyclic microprotein MCoTI-II

Matriptase is a type-II transmembrane serine protease involved in epithelial homeostasis in both health and disease, and is implicated in the development and progression of a variety of cancers. Matriptase mediates its biological effects both via as yet undefined substrates and pathways, and also by proteolytic cleavage of a variety of well-defined protein substrates, several of which it shares with the closely-related protease hepsin. Development of targeted therapeutic strategies will require discrimination between these proteases. Here we have investigated cyclic microproteins of the squash Momordica cochinchinensis trypsin-inhibitor family (generated by total chemical synthesis) and found MCoTI-II to be a high-affinity (Ki 9 nM) and highly selective (> 1,000-fold) inhibitor of matriptase. MCoTI-II efficiently inhibited the proteolytic activation of pro-hepatocyte growth factor (HGF) by matriptase but not by hepsin, in both purified and cell-based systems, and inhibited HGF-dependent cell scattering. MCoTI-II also selectively inhibited the invasion of matriptase-expressing prostate cancer cells. Using a model of epithelial cell tight junction assembly, we also found that MCoTI-II could effectively inhibit the re-establishment of tight junctions and epithelial barrier function in MDCK-I cells after disruption, consistent with the role of matriptase in regulating epithelial integrity. Surprisingly, MCoTI-II was unable to inhibit matriptase-dependent proteolytic activation of prostasin, a GPI-anchored serine protease also implicated in epithelial homeostasis. These observations suggest that the unusually high selectivity afforded by MCoTI-II and its biological effectiveness might represent a useful starting point for the development of therapeutic inhibitors, and further highlight the role of matriptase in epithelial maintenance.

Overexpression of matriptase correlates with poor prognosis in esophageal squamous cell carcinoma

Matriptase is one of the type II transmembrane serine proteases and is known to be involved in cancer progression. Increased matriptase expression has been reported in a variety of human cancers, and its association with poor prognosis has been highlighted in some cancer types. However, its exact role in cancer progression and its effect on patient survival in esophageal squamous cell carcinoma (ESCC) are still unclear. We performed immunohistochemical staining of matriptase in 171 ESCC samples after antibody validation and evaluated the association of its expression with clinicopathological parameters and prognosis. High matriptase expression was observed in 38.6 % (66/171) of ESCC samples and more frequently in N3 stage and in poorly differentiated tumors. Both overall survival (OS) and disease-free survival (DFS) were significantly lower for patients with high expression of matriptase than for patients with low expression (5-year OS rate, 38.6 vs 55.3 %; p=0.034 and 5-year DFS rate, 30.5 vs 49.4 %; p=0.007). High matriptase expression was an independent prognostic factor for OS [hazard ratio (HR), 1.65 (95 % confidence interval (CI), 1.01-2.68); p=0.045] and for DFS [HR, 1.79 (95 % CI, 1.14-2.81); p=0.012]. In conclusion, higher expression of matriptase is an independent prognostic factor involved in the progression of ESCC, which suggests that matriptase is a factor in ESCC tumor progression and also a potential molecular therapeutic target.

Click chemistry in peptide-based drug design

Click chemistry is an efficient and chemoselective synthetic method for coupling molecular fragments under mild reaction conditions. Since the advent in 2001 of methods to improve stereochemical conservation, the click chemistry approach has been broadly used to construct diverse chemotypes in both chemical and biological fields. In this review, we discuss the application of click chemistry in peptide-based drug design. We highlight how triazoles formed by click reactions have been used for mimicking peptide and disulfide bonds, building secondary structural components of peptides, linking functional groups together, and bioconjugation. The progress made in this field opens the way for synthetic approaches to convert peptides with promising functional leads into structure-minimized and more stable forms.

Ahp cyclodepsipeptides: the impact of the Ahp residue on the "canonical inhibition" of S1 serine proteases

S1 serine proteases are by far the largest and most diverse family of proteases encoded in the human genome. Although recent decades have seen an enormous increase in our knowledge, the biological functions of most of these proteases remain to be elucidated. Chemical inhibitors have proven to be versatile tools for studying the functions of proteases, but this approach is hampered by the limited availability of inhibitor scaffold structures with the potential to allow rapid discovery of selective, noncovalent small-molecule protease inhibitors. The natural product class of Ahp cyclodepsipeptides is an unusual class of small-molecule canonical inhibitors; the incorporation of protease cleavage sequences into their molecular scaffolds enables the design of specific small-molecule inhibitors that simultaneously target the S and S' subsites of the protease through noncovalent mechanisms. Their synthesis is tedious, however, so in this study we have investigated the relevance of the Ahp moiety for achieving potent inhibition. We found that although the Ahp residue plays an important role in inhibition potency, appropriate replacement with β-hydroxy amino acids results in structurally less complex derivatives that inhibit serine proteases in the low micromolar range.

High-affinity cyclic peptide matriptase inhibitors

BACKGROUND

Sunflower trypsin inhibitor-1 (SFTI-1) and Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II) are potent protease inhibitors comprising a cyclic backbone.

RESULTS

Elucidation of structure-activity relationships for SFTI-1 and MCoTI-II was used to design inhibitors with enhanced inhibitory activity.

CONCLUSION

An analog of MCoTI-II is one of the most potent inhibitors of matriptase.

SIGNIFICANCE

These results provide a solid basis for the design of selective peptide inhibitors of matriptase with therapeutic potential. The type II transmembrane serine protease matriptase is a key activator of multiple signaling pathways associated with cell proliferation and modification of the extracellular matrix. Deregulated matriptase activity correlates with a number of diseases, including cancer and hence highly selective matriptase inhibitors may have therapeutic potential. The plant-derived cyclic peptide, sunflower trypsin inhibitor-1 (SFTI-1), is a promising drug scaffold with potent matriptase inhibitory activity. In the current study we have analyzed the structure-activity relationships of SFTI-1 and Momordica cochinchinensis trypsin inhibitor-II (MCoTI-II), a structurally divergent trypsin inhibitor from Momordica cochinchinensis that also contains a cyclic backbone. We show that MCoTI-II is a significantly more potent matriptase inhibitor than SFTI-1 and that all alanine mutants of both peptides, generated using positional scanning mutagenesis, have decreased trypsin affinity, whereas several mutations either maintain or result in enhanced matriptase inhibitory activity. These intriguing results were used to design one of the most potent matriptase inhibitors known to date with a 290 pm equilibrium dissociation constant, and provide the first indication on how to modulate affinity for matriptase over trypsin in cyclic peptides. This information might be useful for the design of more selective and therapeutically relevant inhibitors of matriptase.