ChemiCal struCture and properties of low-moleCular furin inhibitors

The review is devoted to the analysis of the relationship between a chemical structure and properties of low-molecular weight inhibitors of furin, the most studied proprotein convertase, which is involved in the development of some pathologies, such as oncologic diseases, viral and bacterial infections, etc. The latest data concerning the influence of peptides, pseudo-peptides, aromatic and heterocyclic compounds, some natural ones such as flavonoids, coumarins, and others on enzyme inactivation are considered. The power of furin inhibition is shown to rise with the increasing number of positively charged groups in the structure of these compounds. Peptidomimetics (Ki = 5-8 pM) are shown to be the most effective furin inhibitors. The synthesized substances, however, have not been used in practical application yet. Nowadays it is very important to find more selective inhibitors, improve their stability, bioavailability and safety for the human organism.


ChemiCal struCture and properties of low-moleCular furin inhibitors
T. V. OSadchUk 1 , O. V. ShyByryN 1 , V. k. kIBIreV 1,2   1   Institute of Bioorganic chemistry and Petrochemistry, National academy of Sciences of Ukraine, kyiv; 2 Palladin Institute of Biochemistry, National academy of Sciences of Ukraine, kyiv; e-mail: osadchuk@bpci.kiev.ua The review is devoted to the analysis of the relationship between a chemical structure and properties of low-molecular weight inhibitors of furin, the most studied proprotein convertase, which is involved in the development of some pathologies, such as oncologic diseases, viral and bacterial infections, etc.The latest data concerning the influence of peptides, pseudo-peptides, aromatic and heterocyclic compounds, some natural ones such as flavonoids, coumarins, and others on enzyme inactivation are considered.The power of furin inhibition is shown to rise with the increasing number of positively charged groups in the structure of these compounds.Peptidomimetics (Ki = 5-8 pM) are shown to be the most effective furin inhibitors.The synthesized substances, however, have not been used in practical application yet.Nowadays it is very important to find more selective inhibitors, improve their stability, bioavailability and safety for the human organism.k e y w o r d s: furin, synthetic inhibitors, peptides, pseudo-peptides, derivatives of 2,5-dideoxystreptamine, enediynes, amidinohydrazones, flavonoids, coumarins.
F urin (EC 3.4.21.75), also known as PACE (Paired basic Amino acid Cleaving Enzyme) or SPCI (Subtilisin-like Proprotein Convertase I), belongs to the proprotein convertases (PCs) family, which is calcium-dependent serine endoproteinases involved in conversion of inactive precursors of proteins to their active forms [1][2][3].PCs are involved in many important biological processes, such as embryogenesis and homeostasis.However, along with the normal physiological processes, proprotein convertases are implicated in the development of some pathologies, such as viral and bacterial infections, oncologic diseases, metastasis, obesity, diabetes, Alzheimer disease, etc. [1][2][3].Therefore, these enzymes are considered to be promising targets for synthesis of inhibitors and development on their basis new therapeutic drugs [2,4,5].
Proteins and their fragments, natural components and low-molecular mass compounds with various structures have been studied in relation to their inhibitory effect on furin activity [6,7].Pro-tein, peptide and pseudo-peptide inhibitors have some disadvantages, in particular, large size, toxicity (in some cases), instability, low cell permeability.Therefore, the search for potential furin inhibitors among non-peptide low-molecular weight compounds is a current issue of biochemical investigation, since such compounds are shown to have high metabolic and proteolytic stability [6,8].Furthermore, they are easily synthesized, and using their structural modifications enables analysis relationships between structure and activity.
It has been four years since our article [7] about proprotein convertase inhibitors was published.During this period a series of interesting research works describing new classes of substances for inactivation of these enzymes has been published.
The aim of this review was to analyze not only the latest research, but also some significant articles (published earlier) focused on structure and efficiency of low-molecular inhibitors of furin.

furin as a member of proprotein convertase family and its role in biological processes
PCs constitute a family of proteolytic secretory enzymes, which are involved in processing of inactive protein precursors transforming them into "mature" forms through limited proteolysis of the polypeptide chain at one or several inner sites.PCs group includes nine proteinases: seven of which, namely, furin, PC1/3, PC2, PC4, PACE4, PC5/6 and PC7 are specific for positively charged basic amino acids, and two -for non-polar amino acid residues [2,3].Under their action proproteins are transformed into biologically active peptide hormones, receptors, growth factors, neuropeptides, regulators of cholesterol metabolism, and enzymes involved in growth maintaining, metabolism and other normal physiological process.PCs were shown to play an essential role not only in normal homeostasis, but also in development of various pathological processes [1][2][3].
One of the most studied enzymes of PCs family is furin, which was found in almost all studied human tissue and in all studied cell types.Furin expression and activity are required for development of such pathologies as cell transformation and tumor progression, metastasis and angiogenesis [1,9,10].Furin is also involved in development of Alzheimer's disease [11], non-Alzheimer brain amyloidosis [12], osteoarthritis [13], and atherosclerosis [14].Furthermore, bacterial toxins such as anthrax toxin [15], exotoxin Pseudomonas A, diphtheria toxin [16], Shiga toxin [17], dermonecrotic toxin Bordetella [18] are only activated after their cleavage by furin or other PCs of respective precursors.Furin plays an essential role in the spread of viral infections such as avian influenza (bird flu) [19], hepatitis B [20,21] and human immunodeficiency [22].
Furin is synthesized as a pre-proprotein, which contains: a signal peptide, a prodomain, subtilisin-like catalytic domain, middle P-domain, site enriched with cysteine residues, transmembrane "anchor" and cytoplasmic "tail".N-terminal prodomain, which consists of 81 amino acis residues (Gln27-Arg107), acts as a peculiar inhibitor [23,24].Profurin (104 kDa) becomes a proteolytic enzyme (98 kDa) after autocatalytic cleavage of its prodomain [25,26].It is accumulated in the trans-Golgi network (TGN) and circulated between the TGN, cell surface and endosomes, activating on this pathway large number of the target proteins [1].The enzyme active site contains (similar to other serine enzymes) triad of amino acids Ser, His and Asp, as well as Asn residue.Furin belongs to Ca 2+ -dependent type I transmembrane proteins.The enzyme identifies motif, enriched for basic amino acid cluster on the substrates, and enhances hydrolysis of the peptide chain, which contains sequences Lys-Arg or Arg-Arg at the C-terminal [27].It occurs, because substrate-binding site of the enzyme is shaped as a narrow canyon, rich in negatively charged Glu and Asp residues.It was shown by Shiryaev et al [28] that the presence of solely R-X-R /K/X-R motif in the substrate structure is insufficient for the realization of narrow furin specificity, since both shortrange P 4 -P 1 as well as long-range P 7 -P 6 interactions of the enzyme are important.
Blocking of the enzyme activity leads to reducing or complete termination of the pathological effects mentioned above, and it might become a promising treatment therapy.Therefore, inhibition of furin activity should be considered as a potential therapeutic strategy for the treatment of cancer, viral and bacterial infections, especially in short-term therapy [4].
To date, various furin inhibitors based on proteins, peptides and pseudo-peptides have been developed [6,7].However, high molecular mass of proteins, lack of selectivity, low cell permeability impede wide use of these compounds as pharmaceutical agents, although the search for furin inhibitors among both natural and recombinant proteins still continues.
For instance, in 2012 Zhu et al. [29] applied, dromedary VHH antibodies (so-called nanobodies), derived from dromedary heavy chain immunoglobulin, against catalytically active enzyme as specific furin inhibitor.These antibodies bound only to furin and not to other PCs.They inhibited cleavage of proteins, but did not block hydrolysis of low-molecular mass peptide substrates.This indicates that the mechanism for nanobodies action based on steric hindrance.Nanobodies were the first generation of non-competitive and specific furin inhibitors [29].
It is important to search selective low-molecular mass non-peptide furin inhibitors, which could be proteolytically more stable and available by synthetic methods [8].
In the early 2000's, polyarginines were found to inhibit furin [30,31].Ramos-Molina et al [32] demon strated that cyclic polyarginines, used as transduction reagents and containing tryptophan residues, were also able to inhibit intracellular proprotein Fig. 1.Chemical structure of pseudopeptides -the most effective furin inhibitors [35,36] 1 2 convertases activity.In particular, cyclic decapeptides containing alternating tryptophan and arginine residues in its ring (cyclo-[WRWRWRWRWC] or cyclo-[WRWRWRWRWR]), as well as cyclic hexapeptides, which contain a hydrophobic "tail" N-acetyltetratryptophan (Ac-WWWWcyclo[KRRRRR]) or dodecanoic acid residue (dodecanoyl-cyclo[KRRRRR]) outside the ring system were found to exhibit significant inhibitory effect on the furin activity both in vitro (k i = 0.1-1.0µM) and within cells.Importantly, these cyclopeptides did not exert any toxic effect on cells.Steinmetzer T. et al. made a great contribution to the development of highly potent furin inhibitors [33][34][35][36].They synthesized substrate-like peptides and peptidomimetics containing 4-amidino-benzylamide (4-amba) group at the molecule C-terminal and acyl residues with basic groups, for example, guanidine or amino groups at the N-terminal of sub-center P 5 .These compounds contained both natural and nonnatural amino acid residues at P 3 .To date, the most potent non-covalent furin inhibitor is peptidomimetic 1 (k i = 5.5 nM), which contains non-natural amino acid tert-leucine (Tle) residue at sub-center P 3 (Fig. 1).Peptide 2 with valine at P 3 sub-center exhibits reduced affinity for furin (k i = 8 pM).
The synthesized compounds were found to effectively inhibit other PCs such as PC1/3, PC4, PACE 4 and PC5/6 (k i = 1.7-810 nM), and in model experiments they exhibited high activity against anthrax and diphtheria toxins [35,36].

small molecule carbocyclic furin inhibitors
Among non-peptide low-molecular mass inhibi tors of furin, diterpene derivatives should be noted [37], which were found in andrographis paniculata plant, widely spread in Southeast Asia and India.The main components of this plant are andrographolide (3) and its derivatives (Fig. 2).
It was shown that these compounds exhibited a low inhibitory activity toward furin, PC1 and PC7.IC 50 values for the studied derivatives ranged from micromolar to millimolar concentrations.
Among the studied compounds, neoandrographolide 9 was found to be the most potent furin inhibitor (IC 50 = 53.5 µM), that is nearly 20-fold more effective than andrographolide itself 3 (IC 50 = 1.0 mM, and k i = 200 µM).Such difference might be due to the unique substitution by O-glycoside residue at the third carbon atom.This indicates that the specific chemical modification of the initial compound might lead to a molecule with higher capability to inactivate furin.This observation was supported by data obtained for succinic acid esters resulted from esterification of hydroxyl group at the C3 atom.
Jiao G.-S. et al. investigated 2.5-dideoxystreptamine derivatives as small molecule inhibitors of furin [38].Data as to effectiveness of these compounds to block furin are shown in Table 1.
It was shown that the most potent furin inhibitor was 2.5-dideoxystreptamine derivative with 4 guanidine substituents (compound 14).It was revealed that with increasing of substituent R 1 size (compounds 15 and 24 vs. 11), affinity for furin decreased.Derivatives containing two aryl substituents were found to be slightly more effective inhibitors than those containing more aryl substituents (compound 16 vs.23).Introduction of guanidine group into para-position also led to better results (compound 16 vs.17).The nature of linker, which binds the aromatic moiety and 2.5-dideoxystreptamine, also impacted the blocking capability.It was evidenced by a significant decrease in the inhibitory activity when oxygen atom of the ester group was substituted by a carbamate group (-NHCOO-) (compounds 25-27).
It was found that 2.5-dideoxystreptamine derivatives are competitive inhibitors of furin with k i values at nanomolar range that is comparable to the potency of protein-or peptide-like inhibitors.In addition, these compounds exert certain selectivity towards furin and PC5/6, therefore they can be used in therapy as anti-bacterial and anti-viral agents.
Analysis of the structure-activity relations in these compounds revealed there is no linear correlation between the inhibitory activity and amount of positively charged guanidine groups in their struc-Fig.2. chemical structures of non-peptide andrographolide-based furin inhibitors: 3 -andrographolide; 4 -14-dehydroandrografolide succinic acid monoester (daSM); 5 -succinic ester of andrographolide-1 (Sea-1); 6 -Sea-2; 7 -Sea-4; 8 -Sea-5; 9 -neoandrographolide [37]  tures.These findings are not consistent with data on inhibitory activity of polyarginine peptides, for which the correlation between inhibitory efficiency and the number of positive charges in a molecule was established [39].The differences in results could be explained by peculiarities of 3-dementional guanidine groups' arrangement relative to the aromatic ring and by nature of the linker between the 2.5-dideoxystreptamine core and aryl groups In 2015 Ramos-Molina et al [40] studied dependence of the anti-firin activity on structures of 2.5-dideoxystreptamine derivatives (Table 1) and bis-guanidinophenyl esters derivatives characterized by linkers with different hydrophobicity and contai ning two or four guanidine groups (Fig. 3, compounds 28-33).The authors assessed their activities directly in cells using the following methods: a) enzyme activity assay in the TGN (trans-Golgi In the third group of compounds (Table 1, compounds 12, 14, 15), effect of hydrophobicity of the mentioned derivatives was investigated.Finally, in the fourth group (Table 1, compounds 25-27), effect of guanidine group positions relative to the linker in the ortho-, meta-or para-positions was analyzed.

T a b l e 1. Inhibition constants (k i ) for 2.5-dideoxystreptamine derivatives [38]
The bis-guanidinophenyl esters structures, which demonstrate the effect of nature of the linker on anti furin activity of the compounds 28-33 are shown in Fig. 3.
The performed comprehensive study emphasized the importance of synthesis of compartmentspecific furin inhibitors, and found that the best correlation between the k i values determined in vitro and anti furin activity determined inside cells was observed for furin functioning on the cell surface.Authors believed that they succeeded to identify the compounds, which possess a wide spectrum of biological activities without marked toxicity.
These compounds can be referred to aromatic bicyclic guanidines.They exhibited inhibitory activity toward furin with k i > 15 µM, whereas their Fig. 3. Structure and activity of bis-guanidinophenyl esters (G -guanidine group) [40] 28 30

31
inhibitory activity toward protein convertase PC2 was much lower -3.3-10.0µM.Among the studied compounds based on pyrrolidinebispiperazine, inhibitory activity toward furin was found to be higher than 25 µM that is not significant result.But these substances showed remarkable results with the PC2 inactivation (k i = 0.54-0.66µM) and were found to be first established selective inhibitors of this enzyme.To obtain potential furin inhibitors on the basis of these compounds, further researches of their structural modifications are required.Podsiadlo P. et al. [42] showed that some stable complexes of zinc and copper can effectively block furin (Table 2).Structures of terpyridine derivatives that are constituents of chelate complexes are presented in Fig. 5. Free chelating molecules did not inactivate the enzyme.Compounds, type of Cu(TTP) Cl 2 and Zn(TTP)Cl 2 , blocked furin activity with the IC 50 values ranged from 5 to 10 µM.Free Zn 2+ -ions exhibited less inactivating effect than copper complex compounds.Although, Cu 2+ -ions were more effective than chelate complex Cu(TTP)Cl 2 .In general, Cu-ions had a greater ability to bind to the enzyme active site owing to their coordination with catalytic residues.Inhibition was competitive and irreversible.The effectiveness of inhibition depended on the nature of the substituent in chelate complex.These substances, in general, are stable and promi sing for research on bioterrorist protection.

non-peptide heterocyclic and aromatic enediynes
Basak et al. [43] presented an innovative strategy for the synthesis of effective PCs inhibitors on the basis of aromatic and heterocyclic derivatives (so-called enediynes).The approach was based on incorporation of enediynyl amino acid (Eda) into a polypeptide chain QQVAKRRTKR ↓ DVYQE at the P 1 -P 1 ′•site (marked with arrow), which correspond to amino acid 98-112 sequence of human furin prodomain [43].The Bergman cycloaromatization reaction [44] led to a free radical generation resulting in a sufficiently high reactivity of these derivatives in the enzyme inactivation.Insertion of Eda also contributed to the formation of polypeptide chain β-turns.As a result, a potent furin inhibitor (IC 50 ~ 40 nM)   In the work [43] it was shown for the first time that various aromatic enediynyl derivatives exhibi-ted inactivating effect on furin as well as on PC5 or PC7 activities with IC 50 values ranging from 8.5 to 193 µM depending on the enzyme nature and structure of enediynyl derivatives.The most effective inhibitor was found to be a compound composed of 12-membered heterocycle with bis-1,4 nitrogen atoms and two benzene side chains (Fig. 6, compound 44).

natural compounds containing oxygen 5.1. flavonoids
Among the oxygen-containing furin inhibitors, natural flavonoids, such as baicalein, chrysin, oroxylin A and its glycoside were studied (Fig. 7, compounds 46-49) [45].These derivatives are the main components of Oroxylum indicum plant.It was pre-Fig.7. Flavonoids, which block the furin and other PCs activities viously found that baicalein and its glycoside (7-D-βglycoside), known as baicalin, the major component of another medicinal plant Scutellaria baicalensis, effectively blocked cancer cell growth in vitro [46].Authors hypothesized that anti-cancer properties of flavonoids related to their ability to inhibit PCs activi ty.Clear correlation between furin inactivation Data on cancer cell proliferation and migration revealed that baicalein and oroxylin A glycoside were more effective (among tested flavonoids) in suppressing of growth and migration of pathogenic cells.It was also found that baicalein (compound 46), which did not have the carbohydrate moiety in its structure, inhibited more effectively growth and proliferation of cancer cells than its glycoside -baicalin (compound 50).
Majumbar et al. [45] demonstrated that these flavonoids exhibit inhibitory effect against furin and other PCs.k i and IC 50 values were found to be in the range of 5-35 µM.Thus, oroxylin A was the most potent furin inhibitor with k i = 5.0 µM that is 5-and 7-fold higher than for baicalein and chrysin, respectively.Its anticancer effectiveness was lower than that of baicalein and oroxylin A glycoside.This difference might result from the involvement, besides furin, other PCs (particularly PCSK6) in the process of tumor growth.A comparative analysis of the inhibitory effectiveness against furin, PCSK4, PCSK5 and PCSK7 revealed that only baicalein has selectivi ty for PCSK4.
Previously, it was found that other members of the flavonoid family, namely baicalin, rutin, naringin and methyl hesperidin (Fig. 7, compounds 50-53) also inhibited activity of furin with k i values ranged from 80 to 200 µM.Since rutin inhibited the furin activity via reversible competitive mechanism [47], we can assume that this is mediated by direct binding of the inhibitor molecule to the enzyme active site.The interaction can occur owing to hydrogen bonds between hydroxyl groups of glycosylated fragment and amino acid residues of the enzyme active site.This finding is consistent with the results on inhibition of furin by andrographolide 3 and glycosylated derivative 9.The effectiveness of inhibition depends on 3-D-dementional position of the glycoside residue that could provide efficient interaction with furin binding center.

furin inactivation by components of natural products: coumarins, quinonoids and iridoids
Among the small molecule inhibitors of furin, derivatives 54-62 of dicoumarol, which is used in clinic as an anticoagulant and is notable for its high bioavailability, low toxicity and high cell permeability, were studied [48] (Table 3).

T a b l e 3. Furin inhibitors based on dicoumarol [48]
Compound R- These compounds were selected after thorough screening of their furin inhibitory effect given k i values from 1.04 (compound 54) to 185 µM (compound 62).4-Hydroxycoumarin, warfarin and 3-(α-acetonyl-phenyl)-4-hydroxycoumarin, which can be considered as structural units of dicoumarol derivative molecules, were found to be capable of inhibiting the enzyme only in millimolar concentrations.That is, the unique dicoumarol structure is the cause for their more effective furin inhibition.
Furin inactivation can occur both at the cell surface (in case of anthrax toxin) and in the secretory pathways (blocking of metastasis growth factor -membrane type-1 matrix metalloproteinase (MT1-MMP)) at the micromolar concentrations, and mechanism for the inhibition was shown to be noncompetitive.Inhibition of the enzyme activity by compounds 56, 57 and 62 were reversible, indicating the lack of non-specific reactions between these derivatives and the enzyme.However, selectivity for furin was not observed, since these compounds also exhibited inhibitory effect on rat PC PACE4 and human PC5/6 and PC7.
k i and IC 50 values for furin inhibition on the cell surface as well as for intracellular processing were found to be in the low micromolar range.These values significantly differed from those for furin inhibitors of peptide and protein structure, which were ~ 1000-10 000 k i .For comparison, IC 50 values for blocking of the intracellular processing by compounds 54, 55 and 57 were ~ 4 k i , 17 k i , and 1 k i , respectively.This highly effective inhibition can be associated with dicoumarol ability to block both intracellular enzyme activity and enzyme activity on the cell surface.
Dicoumarols are used in medicine as anticoagulants of indirect action, which makes them an attractive platform for structural modifications.Structural optimization of dicoumarol-based inhibitors is required to achieve selectivity for furin and other PCs.The search for more effective inhibitors will give an opportunity to investigate thoroughly dicoumarol binding sites, as well as the mechanism for non-competitive inactivation of furin.
Compounds 67-69 (Table 4) exhibited relatively low furin inhibiting effect.Though, notably, they were able to inhibit the PCs activity, furin in particular, as it was found for iridoids 67, 68 and quinone 69.

aromatic compounds containing positively charged substituents
It has previously been shown that the presence of one or more of guanidine groups in the structure of non-peptide inhibitors facilitated their interaction with the furin active site [38].Given the structure of the enzyme binding site determined by X-ray analysis [49,50], it could be expected that most effective inhibitors of furin would be extended low-molecular compounds.Moreover, these compounds should contain several positively charged substituents, which would provide effective interaction with Glu and Asp residues of the enzyme binding site.
Analysis of furin inhibition effect of aromatic compounds containing amidinohydrazone groups (Table 5, compounds 70-91) [51][52][53] revealed that compounds with one positively charged group in molecule inhibited only slightly furin activity.The presence of two such groups resulted in more effective binding with the enzyme.It was assumed that one positively charged group might bind to the S 1 pocket of the enzyme, and the second one -to the S 2 sub-site.Substitution of hydrogen in amino group of compound 70 by positively charged groups (compounds 74-78) led to increase in the inhibitory activity.The most efficient furin inhibitor, among the studied derivatives, contained two amidino- It was shown in 2015 that non-peptide furin inhibitors can be designed on the calix [4]arene platform [55].This macrocycle CX3im derivative (Table 6), which contains positively charged N-methylimidazole groups at the upper rim, inhibited furin with k i = 58µM.
Moreover, the magnitude of the effect depended on hydrophobicity of the groups localized at the lower rim of calixarene cup.
Thus, in recent years the different classes of compounds that are capable of inhibiting the PCs (furin in particular) activities have been established.It should be noted that the sources of PCs inhibitors have often been natural substances.Such substances have a wide spectrum of biological activity associated with their ability to inactivate enzymes.
It has been shown that potential furin inhibitors should contain several positively charged groups for effective interaction with negatively charged glutamic and aspartic amino acid residues on the surface of the enzyme binding pocket.
In general, effectiveness of furin inhibition by non-peptide compounds is inferior to power of bioengineered α1-PDX protein with k i ~ 600 pM [56].hydrazone and one guanidine groups at the linker (compound 76).Although derivative 80 with four amidinohydrazone substituents exhibited significant inhibition (k i = 0.58 µM), it should be noted that increase in the number of positively charged groups to 4 did not significantly impact enzyme inactivation.
It was shown in work [54] that naphthofluorescein (known as CCG 8294) inhibited furin activity with a value of IC 50 = 22 µM.The results of computer modelling and docking showed that its conformation in the enzyme active site was similar to conformation of baicalein (compound 46) -well known furin inhibitor.Naphthofluorescein also blocked the matrix metalloprotease proMT1-MMP processing and aggressiveness of human fibrosarcoma cells (HT1080).
µΜ 0.169±0.0090.089±0.0220.404±0.0180.022±0.0020.006±0.0020.069±0.0040.012±0.003>100 0.042±0.003>100 >100 T. V. Osadchuk, O. V. Shybyryn, V. k. kibirev l e 1. Continuation network) using created chimeric substrate CHO-GRAPfurin; b) testing of HT-1080 fibrosarcoma cells migration to assess furin processing of matrix metalloproteinases (in the TGN), c) measurement of Pseudonomas exotoxin A cytotoxicity on CHO-DG44cells to assess the furin activity in the endocytic pathway; d) testing of anthrax cytotoxicity on macrophage RAW 264.7 cells to examine the furin activity on the cell surface.Additionally, the effect of the synthesized compounds on cell survival was studied.The tested compounds were divided into four groups.In the first group (Table 1, compounds 10, 11, 16, 17), effect of amount of guanidine groups and their relative position in the molecule on furin was studied.In the second group of inhibitors (Table 1, compounds 18-20, 22), effect of the presence of heteroatoms (such as nitrogen) in benzene ring of 2,5-dideoxy streptamine derivatives was assessed.

T a b l e 5 T a b l e 6 . 3 T
The structure of calix[4]arenes and their effect on furin activity at pH 7.3[55] .V. Osadchuk, O. V. Shybyryn, V. k. kibirevIn recent years, based on Phac-RVR-4-Amba, peptidomimetics 1 and 2, the most effective synthetic competitive inhibitors of furin, have been developed (k i ~ 5-8 pM)[35,36].Despite numerous publications concerning development and the study of the PCs inhibitors' properties, still only limited data on their clinical trials have been represented[57].An important issue is the selectivity of these com-pounds, in particular, synthesis of compartmentspecific furin inhibitors, and their safety to the human body.It is vital that these compounds would have greater stability, bioavailability and improved pharmacokinetic characteristics.Along these lines, the search for effective PCs inhibitors among lowmolecular mass compounds remains a current area of research.