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The Discovery of Small Molecule C5a Antagonists
The Discovery of Small Molecule C5a Antagonists Alan J. Hutchison and James E. Krause Neurogen Corporation, Branford, CT 06405, USA Contents 1. Introduction 2. The complement system, C5a and the C5a receptor 3. Peptide antagonists 3.1. Non-peptide antagonists 4. Conclusions References
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1. INTRODUCTION In host defense, the complement system of blood-borne proteins initiates a series of cellular and inflammatory responses to divergent stimuli. These can vary from infectious agents (e.g., bacteria, viruses and parasites) to biochemical, chemical and physical injury [1]. Excessive activation of the complement system in man results in elevated C5a, a glycosylated 74-amino acid protein that is associated with several acute and chronic clinical situations or disease states. Due to the pro-inflammatory nature of C5a receptor-mediated responses, this receptor has been investigated as a potential drug target by the pharmaceutical industry for some time [2]. Solid evidence, based on animal models, has only recently been generated to demonstrate the crucial involvement of the C5a receptor in disease pathogenesis or progression. For example, it has been shown in the K/B£N T cell receptor transgenic mouse model of inflammatory arthritis, that the arthritic state is critically dependent on the complement system and in particular the C5a receptor [3]. Further, several disease models in rat including an antigen-induced monarticular arthritis model, an acute limb ischemia-reperfusion model and a model of inflammatory bowel disease have been explored with peptide-based C5a receptor antagonists, and the results indicate promising activity of C5a receptor antagonists [4 – 6]. C5a receptor antagonists have been proposed to be useful in rheumatoid arthritis, respiratory distress syndrome, ischemia-reperfusion injury, sepsis, psoriasis, and inflammatory bowel disease [1 –3]. In this review, we provide an overview of the complement pathway in man and the role of the C5a effector arm in anaphylatoxin biology. Recent work on the identification of peptide and small molecule C5a receptor antagonists is summarized. ANNUAL REPORTS IN MEDICINAL CHEMISTRY, VOLUME 39 ISSN: 0065-7743 DOI 10.1016/S0065-7743(04)39011-1
q 2004 Elsevier Inc. All rights reserved
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2. THE COMPLEMENT SYSTEM, C5a AND THE C5a RECEPTOR The complement system consists of more than 30 serum and cellular proteins and encompasses three biochemical cascades (classic, mannose binding lectin and alternative pathways), that culminate in the activation C5 convertase, the enzyme that converts C5 to two products, C5a and C5b. The latter pathway and its multiple arms of activation are illustrated in Fig. 1. The specific molecular activators of the classical complement pathway are immune complexes; however, b-amyloid peptide has also been shown to interact with C1q under apparent physiological conditions to activate the cascade [7]. Complex polysaccharides activate the mannose binding lectin pathway, while microbial surfaces, IgA antibodies, certain Ig light chains, as well as
Fig. 1. Schematic illustration of the complement system. For the purpose of clarity, not all complement components are depicted. This figure is adapted from Ref. [3].
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foreign surfaces such as the tubing of the cardiopulmonary bypass-oxygenator activate the alternative pathway. In most animal species, there are in essence three effector arms of the complement pathway, the C5a receptor, the C3a receptor and the membrane attack complex. The two anaphylatoxins, C3a and C5a, activate G-protein coupled receptors and as such are the specific cellular effectors of the complement system. C3a and its receptor are also potential drug targets [8]. C5a is a glycosylated 74-amino acid containing protein that activates the G-protein coupled receptor (GPCR) known as CD88 or C5a receptor. C5b combines with C6, C7, C8 and C9 serum complement proteins to form C5b-9. The latter is also known as the membrane attack complex, and mediates lysis of invading microorganism membranes as well as clearing tissue debris from sites of tissue destruction or necrosis. The C5a receptor is expressed on neutrophils, mast cells, other cells of myeloid lineage, smooth muscle cells and endothelial cells. It initiates several biochemical cascades, including decreases in intracellular cAMP levels and increases of intracellular calcium levels, PI3 kinase activity and MAP kinase activity [2]. These biochemical effects of C5a receptor activation result in the classic neutrophil effects including chemotaxis, degranulation and cellular adhesion. More recently, receptor activation has been shown to influence cytokine and chemokine gene expression and even apoptosis, depending on the specific cellular site of C5a receptor expression [9,10].
3. PEPTIDE ANTAGONISTS The search for potent small C5a peptide antagonists has proven to be a difficult challenge. Initial efforts in protein SAR focused on determining the minimum peptide fragment possessing potent binding affinity. The C-terminal octapeptide was identified as the shortest linear sequence possessing reasonable C5a binding affinity [11]. A major complication associated with C5a antagonists arises from the fact that historically, most C5a receptor active ligands display some partial agonist activity. In spite of this precedent, there has recently been significant progress in the development of pure antagonist C5a peptide ligands. The discovery that the relatively weak linear peptide antagonist MePhe-Lys-Pro-dCha-Trp-dArg, 1, is a pure antagonist has served as the springboard for the exploration of a series of small cyclic peptide analogs of C5a with potent C5a blocking activity [12 –15]. Compounds 2 and 3 have been reported to competitively displace 125I-rhC5a from rat, dog and human polymorphonuclear leukocytes (PMN’s) with IC50 values in the 40 – 200 nM range [16]. However, in a functional assay in human PMN’s the peptide 2 displayed insurmountable antagonism against C5a [17]. In addition, 2 was reported to block C5a- or endotoxin-induced neutropenia in the rat after iv administration [18]. Compound 3 was reported to be orally bioavailable with a 3 mg/kg po dose generating peak plasma levels of 300 ug/mL and a terminal half-life of about 70 min [19]. The latter analog was reported to have efficacy in a variety of disease models following oral and topical administration [4 –6,19 – 21]. It was also disclosed that 3, designated as PMX53, is currently being evaluated in Phase II clinical trials for rheumatoid arthritis and psoriasis [22]. In this connection, an improved, scalable synthesis of 3, suitable for large-scale production was reported [23].
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3.1. Non-peptide antagonists The first reports of non-peptide compounds with C5a blocking activity appeared in the early 1990s. A series of 4,6-diaminoquinolines exemplified by 4 and 5 possessed weak (IC50 values, 4– 40 mM) activity in inhibiting 125I-rhC5a binding as well as in antagonizing C5a-induced neutrophil activation [24]. Subsequent reports cast doubt on the specificity of these antagonists in view of the fact that the diaminoquinolines also inhibit the binding of the chemotactic agent IL-8 to its receptors [25].
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The optimization of screening hit 6, which displayed weak activity (IC50 ¼ 30 mM) in inhibiting 125I-rhC5a binding to its receptor was also documented [26]. These efforts resulted in the identification of RPR121154, 7. This biaryl guanidine showed a modest boost in binding affinity (IC50 ¼ 800 nM) and demonstrated functional antagonism of C5a in an oxidative burst assay at 30 mM.
Tetrahydro imidazopyridines 8, L-164,710 and benzodiazepines 9, L-747,981 were evaluated as C5a antagonist ligands [27,28]. Similar to the experience with early peptide ligands, 9 and related analogs were found to be partial to full agonists [28]. A further report from the same group described a hydantoin series of C5a ligands, the most potent of which is the analog 10. This compound is potent in a binding assay (IC50 ¼ 20 nM), but displays agonist properties in functional assays [29]. Extensive efforts to convert each of these lead series to potent and selective pure antagonists were ultimately unsuccessful. Thus, despite these promising early reports, none of the first generation non-peptide lead compounds proved to be a viable starting point for the generation of potent and selective low molecular weight C5a antagonists devoid of agonist activity and having suitable pharmacokinetic parameters.
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The small molecules described thus far were all identified utilizing displacement binding assays. This strategy generally yielded weak ligands for the C5a receptor with partial to full agonist profiles. More recently, library screening with functional assays was successfully employed to identify pure C5a receptor antagonists. In the first account of this approach, structures were not disclosed [30]. However, the lead compounds were characterized as antagonists/inverse agonists and reported to inhibit Ca2þ flux and chemotaxis in a monocytic cell line (differentiated to a neutrophil-like state) with IC50s in the 0.3 –5 nM range. In addition, one of these newer generation antagonists blocked C5a-induced neutropenia in a primate model at oral doses as low as 1 mg/kg. These compounds were reported to show potent C5a blocking effects only at the human and primate C5a receptors. Following the initial report, several patent applications from the same research group claimed potent C5a antagonists [31 –33]. Some structural classes that are disclosed with specific examples include aminomethyl arylimidazoles, pyrazoles and pyridines such as 11, acylaminomethyl azabenzimidazoles such as 12, simple biarylcarboxamides such as 13, and tetrahydroisoquinoline derivatives such as 14. One compound, presumably within this patent estate, NGD 2000-1, is in Phase II clinical trials for asthma and rheumatoid arthritis [34].
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The most recent examples of small molecule C5a antagonists consist of diaryl ureas such as compound 15 and their isosteric amide analogs, exemplified by W-54011, 16 [35 – 37]. The potent C5a antagonist 16 displaces 125I-hC5a from human PMN’s with a Ki value of 2.2 nM. It has similar potency in inhibiting C5a-induced Ca2þ mobilization, chemotaxis and generation of reactive superoxide species in human neutrophils. Compound 16 also displays species specificity. It is a potent antagonist at human, primate and gerbil C5a receptors, whereas it is essentially inactive at mouse, rat, guinea pig, rabbit and dog C5a receptors [37]. In addition, 16 is active in a C5a-induced neutropenia model in the gerbil (3 –30 mg/kg, po).
4. CONCLUSIONS The generation of small molecule C5a antagonists has been a recalcitrant problem for medicinal chemists. However, recent reports suggest that several blueprints now exist for the potential solution of this elusive problem. At least two orally bioavailable small molecule C5a antagonists are reported to be in Phase II clinical trials, NGD 2000-1 and PMX53 [3]. Preliminary reports of the efficacy of PMX53 in psoriasis have been reported [23]. Thus, the long awaited proof of the clinical utility for this promising class of potential therapeutic agents may soon be available.
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REFERENCES [1] S. C. Makrides, Pharmacol. Rev., 1998, 50, 59. [2] C. Gerard and N. P. Gerard, Ann. Rev. Immunol., 1994, 12, 775. [3] H. Ji, K. Ohmura, U. Mahmood, D. M. Lee, F. M. Hofhuis, S. A. Boackle, K. Takahashi, V. M. Holers, M. Walport, C. Gerard, A. Ezekowitz, M. C. Carroll, M. Brenner, R. Weissleder, J. S. Verbeek, V. Duchatelle, C. Degott, C. Benoist and D. Mathis, Immunity, 2002, 16, 157. [4] T. M. Woodruff, A. J. Strachan, N. Dryburgh, I. A. Shiels, R. C. Reid, D. P. Fairlie and S. M. Taylor, Arthritis Rheumat., 2002, 46, 2476. [5] T. M. Woodruff, T. V. Arumugam, I. A. Shiels, R. C. Reid, D. P. Fairlie and S. M. Taylor, J. Surg. Res., 2004, 116, 81. [6] T. M. Woodruff, T. V. Arumugam, I. A. Shiels, R. C. Reid, D. P. Fairlie and S. M. Taylor, J. Immunol., 2003, 171, 5514. [7] P. Tacnet-Delorme, S. Chevallier and G. J. Arlaud, J. Immunol., 2001, 167, 6374. [8] A. A. Humbles, B. Lu, C. A. Nilsson, C. Lilly, E. Israel, Y. Fujiwara, N. P. Gerard and C. Gerard, Nature, 2000, 406, 998. [9] E. P. Grant, D. Picarella, T. Burwell, T. Delaney, A. Croci, N. Avitahl, A. A. Humbles, J. C. Gutierrez-Ramos, M. Briskin, C. Gerard and A. J. Coyle, J. Exp. Med., 2002, 196, 1461. [10] R. F. Guo, M. Huber-Lang, X. Wang, X. Sarma, V. A. Padgaonkar, R. A. Craig, N. C. Riedemann, S. D. McClintock, T. Hlaing, M. M. Shi and P. A. Ward, J. Clin. Invest., 2000, 10, 1271. [11] M. Kawai, D. A. Quincy, B. Lane, K. W. Mollison, J. R. Luly and G. W. Carter, J. Med. Chem., 1991, 34, 2068. [12] J. A. DeMartino, Z. D. Konteatis, S. J. Siciliano, G. Van Riper, D. J. Underwood, P. A. Fischer and M. A. S. Springer, J. Biol. Chem., 1995, 270, 15966. [13] A. K. Wong, A. M. Finch, G. K. Pierens, D. J. Craik, S. M. Taylor and D. P. Fairlie, J. Med. Chem., 1998, 41, 3417. [14] A. M. Finch, A. K. Wong, N. J. Paczkowski, S. K. Wadi, D. J. Craik, D. P. Fairlie and S. M. Taylor, J. Med. Chem., 1999, 42, 1965. [15] S. Taylor and I. A. Shiels, WO Patent 033528-A1, 2003. [16] T. M. Woodruff, A. J. Strachan, S. D. Sanderson, P. N. Monk, A. K. Wong, D. P. Fairlie and S. D. Taylor, Inflammation, 2001, 25, 171. [17] N. J. Paczkowski, A. M. Finch, J. B. Whitmore, A. J. Short, A. K. Wong, P. N. Monk, S. A. Cain, D. P. Fairlie and S. D. Taylor, Br. J. Pharmacol., 1999, 128, 1461. [18] A. J. Short, A. K. Wong, A. M. Finch, G. Haaima, I. A. Shiels, D. P. Fairlie and S. D. Taylor, Br. J. Pharmacol., 1999, 125, 551. [19] A. J. Strachan, I. A. Shiels, R. C. Reid, D. P. Fairlie and S. M. Taylor, Br. J. Pharmacol., 2001, 134, 1778. [20] T. V. Arumugam, I. A. Shiels, T. M. Woodruff, R. C. Reid, D. P. Fairlie and S. M. Taylor, J. Surg. Res., 2002, 103, 260. [21] T. V. Arumugam, I. A. Shiels, A. J. Strachan, G. Abbenante, D. P. Fairlie and S. M. Taylor, Kidney Intl, 2003, 63, 134. [22] See www.promics.com and press releases therein. [23] R. C. Reid, G. Abbenante, S. M. Taylor and D. P. Fairlie, J. Org. Chem., 2003, 68, 4464. [24] T. J. Lanza, P. L. Durette, T. Rollins, S. Siciliano, D. N. Cianciarulo, S. V. Kobayashi, C. G. Caldwell, M. S. Springer and W. K. Hagmann, J. Med. Chem., 1992, 35, 252. [25] S. J. Siciliano, T. E. Rollins, J. Demartino, Z. Konteatis, L. Malkowitz, G. Van Riper, S. Bondy, H. Rosen and M. S. Springer, Proc. Natl Acad. Sci. USA, 1994, 91, 1214. [26] P. C. Astles, T. J. Brown, P. Cox, F. Halley, P. M. Lockey, C. McCarthy, I. M. McLay, T. N. Majid, A. D. Morley, B. Porter, A. J. Ratcliffe and R. J. A. Walsh, Bioorg. Med. Chem. Lett., 1997, 7, 907. [27] D. Kim, R. A. Rivero, S. E. deLaszlo, J. Tran, J. Fair, W. T. Ashton, S. M. Hutchins, L. Malkowitz, C. Molineaux, S. Siciliano, M. S. Springer, W. J. Greenlee and N. B. Mantlo, 210th ACS National Meeting, Chicago, IL, MEDI-086, 1995.
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[28] K. L. Flanagan, L. L. Chang, W. T. Ashton, S. M. Hutchins, L. Malkowitz, C. Siciliano, M. S. Springer, W. J. Greenlee and N. B. Mantlo, 210th ACS National Meeting, Chicago, IL, MEDI-085, 1995. [29] S. E. de Laslo, E. E. Allen, B. Li, D. Ondeyka, R. Rivero, L. Malkowitz, C. Molineaux, S. J. Siciliano, M. A. Springer, W. J. Greenlee and N. B. Mantlo, Bioorg. Med. Chem. Lett., 1997, 7, 213. [30] A. J. Hutchison and K. R. Shaw, 220th ACS National Meeting, Washington, DC, MEDI321, 2000. [31] A. Thurkauf, X. Zhang, X. He, H. Zhao, J. Peterson, G. Maynard and R. Ohliger, WO Patent 049992-A2, 2002. [32] G. P. Luke, G. Maynard, S. Mitchell, A. Thurkauf, L. Xie, L. Zhang, S. Zhang, H. Zhao, B. L. Chenard, Y. Gao, B. Han and X. He, WO Patent 082829-A1, 2003. [33] A. Thurkauf, H. Zhao, S. Zhang and Y. Gao, WO Patent 018460-A1, 2004. [34] See www.nrgn.com and press releases therein. [35] S. Ishibuchi, H. Sumichika, K. Itoh and Y. Naka, WO Patent 014265-A1, 2002. [36] M. Nakamura, T. Kamahori, S. Ishibuchi, Y. Naka, H. Sumichika and K. Itoh, WO Patent 022556-A1, 2002. [37] H. Sumichika, K. Sakata, N. Sato, S. Takeshita, S. Ishibushi, M. Nakamura, T. Kamahori, S. Ehara, K. Itoh, T. Ohtsuka, T. Mishina, H. Komatsu and Y. Naka, J. Biol. Chem., 2002, 277, 49403.
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1. INTRODUCTION In host defense, the complement system of blood-borne proteins initiates a series of cellular and inflammatory responses to divergent stimuli. These can vary from infectious agents (e.g., bacteria, viruses and parasites) to biochemical, chemical and physical injury [1]. Excessive activation of the complement system in man results in elevated C5a, a glycosylated 74-amino acid protein that is associated with several acute and chronic clinical situations or disease states. Due to the pro-inflammatory nature of C5a receptor-mediated responses, this receptor has been investigated as a potential drug target by the pharmaceutical industry for some time [2]. Solid evidence, based on animal models, has only recently been generated to demonstrate the crucial involvement of the C5a receptor in disease pathogenesis or progression. For example, it has been shown in the K/B£N T cell receptor transgenic mouse model of inflammatory arthritis, that the arthritic state is critically dependent on the complement system and in particular the C5a receptor [3]. Further, several disease models in rat including an antigen-induced monarticular arthritis model, an acute limb ischemia-reperfusion model and a model of inflammatory bowel disease have been explored with peptide-based C5a receptor antagonists, and the results indicate promising activity of C5a receptor antagonists [4 – 6]. C5a receptor antagonists have been proposed to be useful in rheumatoid arthritis, respiratory distress syndrome, ischemia-reperfusion injury, sepsis, psoriasis, and inflammatory bowel disease [1 –3]. In this review, we provide an overview of the complement pathway in man and the role of the C5a effector arm in anaphylatoxin biology. Recent work on the identification of peptide and small molecule C5a receptor antagonists is summarized. ANNUAL REPORTS IN MEDICINAL CHEMISTRY, VOLUME 39 ISSN: 0065-7743 DOI 10.1016/S0065-7743(04)39011-1
q 2004 Elsevier Inc. All rights reserved
140
A.J. Hutchison and J.E. Krause
2. THE COMPLEMENT SYSTEM, C5a AND THE C5a RECEPTOR The complement system consists of more than 30 serum and cellular proteins and encompasses three biochemical cascades (classic, mannose binding lectin and alternative pathways), that culminate in the activation C5 convertase, the enzyme that converts C5 to two products, C5a and C5b. The latter pathway and its multiple arms of activation are illustrated in Fig. 1. The specific molecular activators of the classical complement pathway are immune complexes; however, b-amyloid peptide has also been shown to interact with C1q under apparent physiological conditions to activate the cascade [7]. Complex polysaccharides activate the mannose binding lectin pathway, while microbial surfaces, IgA antibodies, certain Ig light chains, as well as
Fig. 1. Schematic illustration of the complement system. For the purpose of clarity, not all complement components are depicted. This figure is adapted from Ref. [3].
The Discovery of Small Molecule C5a Antagonists
141
foreign surfaces such as the tubing of the cardiopulmonary bypass-oxygenator activate the alternative pathway. In most animal species, there are in essence three effector arms of the complement pathway, the C5a receptor, the C3a receptor and the membrane attack complex. The two anaphylatoxins, C3a and C5a, activate G-protein coupled receptors and as such are the specific cellular effectors of the complement system. C3a and its receptor are also potential drug targets [8]. C5a is a glycosylated 74-amino acid containing protein that activates the G-protein coupled receptor (GPCR) known as CD88 or C5a receptor. C5b combines with C6, C7, C8 and C9 serum complement proteins to form C5b-9. The latter is also known as the membrane attack complex, and mediates lysis of invading microorganism membranes as well as clearing tissue debris from sites of tissue destruction or necrosis. The C5a receptor is expressed on neutrophils, mast cells, other cells of myeloid lineage, smooth muscle cells and endothelial cells. It initiates several biochemical cascades, including decreases in intracellular cAMP levels and increases of intracellular calcium levels, PI3 kinase activity and MAP kinase activity [2]. These biochemical effects of C5a receptor activation result in the classic neutrophil effects including chemotaxis, degranulation and cellular adhesion. More recently, receptor activation has been shown to influence cytokine and chemokine gene expression and even apoptosis, depending on the specific cellular site of C5a receptor expression [9,10].
3. PEPTIDE ANTAGONISTS The search for potent small C5a peptide antagonists has proven to be a difficult challenge. Initial efforts in protein SAR focused on determining the minimum peptide fragment possessing potent binding affinity. The C-terminal octapeptide was identified as the shortest linear sequence possessing reasonable C5a binding affinity [11]. A major complication associated with C5a antagonists arises from the fact that historically, most C5a receptor active ligands display some partial agonist activity. In spite of this precedent, there has recently been significant progress in the development of pure antagonist C5a peptide ligands. The discovery that the relatively weak linear peptide antagonist MePhe-Lys-Pro-dCha-Trp-dArg, 1, is a pure antagonist has served as the springboard for the exploration of a series of small cyclic peptide analogs of C5a with potent C5a blocking activity [12 –15]. Compounds 2 and 3 have been reported to competitively displace 125I-rhC5a from rat, dog and human polymorphonuclear leukocytes (PMN’s) with IC50 values in the 40 – 200 nM range [16]. However, in a functional assay in human PMN’s the peptide 2 displayed insurmountable antagonism against C5a [17]. In addition, 2 was reported to block C5a- or endotoxin-induced neutropenia in the rat after iv administration [18]. Compound 3 was reported to be orally bioavailable with a 3 mg/kg po dose generating peak plasma levels of 300 ug/mL and a terminal half-life of about 70 min [19]. The latter analog was reported to have efficacy in a variety of disease models following oral and topical administration [4 –6,19 – 21]. It was also disclosed that 3, designated as PMX53, is currently being evaluated in Phase II clinical trials for rheumatoid arthritis and psoriasis [22]. In this connection, an improved, scalable synthesis of 3, suitable for large-scale production was reported [23].
142
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3.1. Non-peptide antagonists The first reports of non-peptide compounds with C5a blocking activity appeared in the early 1990s. A series of 4,6-diaminoquinolines exemplified by 4 and 5 possessed weak (IC50 values, 4– 40 mM) activity in inhibiting 125I-rhC5a binding as well as in antagonizing C5a-induced neutrophil activation [24]. Subsequent reports cast doubt on the specificity of these antagonists in view of the fact that the diaminoquinolines also inhibit the binding of the chemotactic agent IL-8 to its receptors [25].
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The optimization of screening hit 6, which displayed weak activity (IC50 ¼ 30 mM) in inhibiting 125I-rhC5a binding to its receptor was also documented [26]. These efforts resulted in the identification of RPR121154, 7. This biaryl guanidine showed a modest boost in binding affinity (IC50 ¼ 800 nM) and demonstrated functional antagonism of C5a in an oxidative burst assay at 30 mM.
Tetrahydro imidazopyridines 8, L-164,710 and benzodiazepines 9, L-747,981 were evaluated as C5a antagonist ligands [27,28]. Similar to the experience with early peptide ligands, 9 and related analogs were found to be partial to full agonists [28]. A further report from the same group described a hydantoin series of C5a ligands, the most potent of which is the analog 10. This compound is potent in a binding assay (IC50 ¼ 20 nM), but displays agonist properties in functional assays [29]. Extensive efforts to convert each of these lead series to potent and selective pure antagonists were ultimately unsuccessful. Thus, despite these promising early reports, none of the first generation non-peptide lead compounds proved to be a viable starting point for the generation of potent and selective low molecular weight C5a antagonists devoid of agonist activity and having suitable pharmacokinetic parameters.
144
A.J. Hutchison and J.E. Krause
The small molecules described thus far were all identified utilizing displacement binding assays. This strategy generally yielded weak ligands for the C5a receptor with partial to full agonist profiles. More recently, library screening with functional assays was successfully employed to identify pure C5a receptor antagonists. In the first account of this approach, structures were not disclosed [30]. However, the lead compounds were characterized as antagonists/inverse agonists and reported to inhibit Ca2þ flux and chemotaxis in a monocytic cell line (differentiated to a neutrophil-like state) with IC50s in the 0.3 –5 nM range. In addition, one of these newer generation antagonists blocked C5a-induced neutropenia in a primate model at oral doses as low as 1 mg/kg. These compounds were reported to show potent C5a blocking effects only at the human and primate C5a receptors. Following the initial report, several patent applications from the same research group claimed potent C5a antagonists [31 –33]. Some structural classes that are disclosed with specific examples include aminomethyl arylimidazoles, pyrazoles and pyridines such as 11, acylaminomethyl azabenzimidazoles such as 12, simple biarylcarboxamides such as 13, and tetrahydroisoquinoline derivatives such as 14. One compound, presumably within this patent estate, NGD 2000-1, is in Phase II clinical trials for asthma and rheumatoid arthritis [34].
The Discovery of Small Molecule C5a Antagonists
145
The most recent examples of small molecule C5a antagonists consist of diaryl ureas such as compound 15 and their isosteric amide analogs, exemplified by W-54011, 16 [35 – 37]. The potent C5a antagonist 16 displaces 125I-hC5a from human PMN’s with a Ki value of 2.2 nM. It has similar potency in inhibiting C5a-induced Ca2þ mobilization, chemotaxis and generation of reactive superoxide species in human neutrophils. Compound 16 also displays species specificity. It is a potent antagonist at human, primate and gerbil C5a receptors, whereas it is essentially inactive at mouse, rat, guinea pig, rabbit and dog C5a receptors [37]. In addition, 16 is active in a C5a-induced neutropenia model in the gerbil (3 –30 mg/kg, po).
4. CONCLUSIONS The generation of small molecule C5a antagonists has been a recalcitrant problem for medicinal chemists. However, recent reports suggest that several blueprints now exist for the potential solution of this elusive problem. At least two orally bioavailable small molecule C5a antagonists are reported to be in Phase II clinical trials, NGD 2000-1 and PMX53 [3]. Preliminary reports of the efficacy of PMX53 in psoriasis have been reported [23]. Thus, the long awaited proof of the clinical utility for this promising class of potential therapeutic agents may soon be available.
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