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Cysteine cathepsins: their role in tumor progression and recent trends in the development of imaging probes.

Löser R, Pietzsch J - Front Chem (2015)

Bottom Line: The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging.After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response.In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiopharmaceutical and Chemical Biology, Institute of Radiopharmaceutical Cancer Research, Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany ; Department of Chemistry and Food Chemistry, Technische Universität Dresden Dresden, Germany.

ABSTRACT
Papain-like cysteine proteases bear an enormous potential as drug discovery targets for both infectious and systemic human diseases. The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging. After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response. In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

No MeSH data available.


Related in: MedlinePlus

Inhibitors that were used as experimental tools in studies that are referred to in this article.
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Figure 4: Inhibitors that were used as experimental tools in studies that are referred to in this article.

Mentions: Inhibitors based on small molecules that target papain-like cysteine proteases have contributed substantially to the understanding of the catalytic mechanisms, enzyme-substrate recognition and the biological functions of this class of proteolytic enzymes. Most of the known low molecular weight inhibitors of papain-like proteases are containing electrophilic entities, referred to as warheads, which give rise to irreversible or reversible covalent interactions with the active-site cysteine. The field of small-molecule based inhibitors has been surveyed by several excellent and comprehensive review articles that appeared in the recent years (Otto and Schirmeister, 1997; Leung et al., 2000; Lecaille et al., 2002; Powers et al., 2002; Abbenante and Fairlie, 2005; Frizler et al., 2010). Therefore, only a brief discussion of those inhibitor classes that served as lead structures for the design of imaging probes reviewed in the next section or those which were used as molecular tools for their biological evaluation will be given herein (Figure 4). Compound classes that exert inhibition by irreversible covalent modification of the active-site cysteine residue are peptide-derived halomethylketones, diazoketones, acyloxymethylketones, O-acyl hydroxamates, vinyl sulfones and epoxides, among other inhibitor chemotypes. Irreversible inhibitors of cysteine proteases have been reviewed in detail by Powers et al. The following remarks are mainly extracted from this reference and also kinetic data on the inhibition by some of the compounds in Figure 4 can be found therein (Powers et al., 2002). Peptidic chloromethyl ketones have been developed as inactivators of serine proteases but they are also potent cysteine protease inhibitors due to S-alkylation of the active-site cysteine. Their strong intrinsic reactivity results in modification of other biological thiols such as glutathione, even though in slower rates than their interaction with the targeted cysteine proteases. This drawback renders such inhibitors unsuitable for the application in cell-based and animal experiments. In contrast, fluoromethyl ketones such as compounds 1a and b are much less reactive toward thiols, due to the low energy of the carbon-fluorine bond. Despite their low inherent reactivity they are potent inactivators of cysteine proteases and show selectivity over serine proteases. Similarly, diazomethyl ketones can be considered to be selective for cysteine proteases, even though inactivation of serine proteases by this inhibitor chemotype has been reported for some instances. Their reactivity toward thiols of low molecular weight is negligible and some radiotracers have been developed on the basis of this inhibitor class (see Section 125I-labeled Compounds). Acyloxymethyl ketones (AOMK's; see compound 2 in Figure 4 for example) have been developed with the intention to create deactivated versions of chloromethyl ketones. This inhibitor chemotype has been shown to inactivate cysteine proteases selectively over other classes of proteolytic enzymes. In addition to the other reactive methyl ketones, they provide the opportunity of kinetic tuning upon varying the acyloxy leaving group. However, their potential susceptibility toward esterase-catalyzed degradation might compromise their applicability in vivo (Verdoes et al., 2013). Formal isoelectronic replacement of the methylene group in the latter inhibitor class leads to O-acyl hydroxamates, as represented by compound 3. This chemotype forms covalent complexes with the active-site cysteine in analogy to acyloxymethyl ketones. In contrast, peptide-derived O-acyl hydroxamates are capable to inactivate serine proteases, even though their mechanism of interaction may differ between these two classes of proteases (Brömme and Demuth, 1994). Mechanistically related to the aforementioned inhibitor classes are peptide-derived epoxides, among which the epoxysuccinyl peptides constitute the most important subclass. Differently to reactive methyl ketones, no nucleofuge is leaving the enzyme-inhibitor complex, as the attack of the active-site thiol results in epoxide opening. Their prototype is represented by the fungal secondary metabolite E64 (6), which has been shown to inactivate a broad spectrum of papain-like cysteine proteases. Replacement of the guanidinobutyl moiety by hydroxyphenyl and isobutyl led to JPM-565 (7a) and E64c (8a), respectively, without significant changes in the inhibitory potential. Replacement of the leucine residue in E64 (6) by isoleucine, C-terminal extension by proline and amidation of the carboxylic group results in CA074 (9a; Figure 4), which inactivates cathepsin B selectively over other cysteine cathepsins. Notably, epoxysuccinyl peptides can bind to the active site in the same or inverse orientation compared to the substrate binding mode. Esterification of the carboxylic groups in JPM-565, CA074, and E64 with simple alcohols renders these highly polar molecules membrane permeable (compounds 7b, 8b, and 9b in Figure 4). Despite the actual inhibitors can be released by the action of cellular esterases, the corresponding “prodrugs” still bear inhibitory potential but altered selectivity profiles compared to their “active” progenies, and cysteine cathepsin inactivation may occur faster than ester hydrolysis (Bogyo et al., 2000). Therefore, the results of cell-based experiments in which these esterified epoxysuccinyl peptides are used for pharmacological inhibition should be interpreted with care in terms of selectivity toward individual cysteine cathepsins. E64d (8b) has been demonstrated to be bioavailable upon oral administration. The spontaneous reactivity of epoxide inhibitors toward thiols has been shown to be low. A couple of radiotracers have been developed starting from epoxysuccinyl peptides (see Section 125I-labeled Compounds). Besides epoxysuccinyl peptides, peptidic inhibitors with epoxide warheads have been also created by replacing formally the terminal carboxylic group of N-terminal cleavage products by an oxirane ring, resulting in N-peptidyl-α-aminoalkyl epoxides. They interact with the active site by epoxide opening upon attack of the active-site sulfhydryl group at the methylene carbon. Concerning the two possible configurations at the methine carbon, the erythro diastereomers are more active than their threo-configured counterparts. In agreement with the altered electronic situation of the oxirane moiety, N-peptidyl-α-aminoalkyl epoxides are less potent inactivators of cysteine proteases compared to epoxysuccinyl derivatives. A tritium-labeled N-peptidyl-α-aminoalkyl epoxide has been reported (see Section Radiotracers Based on Fluorine-18, Radiocarbon Isotopes, and Tritium; and compound [3H]39 in Figure 11) (Albeck, 2000; Powers et al., 2002). Vinyl sulfones are cysteine-reactive moieties that give rise to nucleophilic addition instead of substitution. Their incorporation at the C-terminus of small peptides can lead to highly potent inactivators of cysteine proteases such as compounds 4 and 5 with limited inherent thiol reactivity. Compound 5 has been used as lead structure for radiotracer design (see Section 125I-labeled Compounds). Despite vinyl sulfones inactivate cysteine proteases selectively over serine proteases, the introduction of three leucine residues into position P1–P3 renders them capable of ether formation with the active-site threonine residue in proteasomes.


Cysteine cathepsins: their role in tumor progression and recent trends in the development of imaging probes.

Löser R, Pietzsch J - Front Chem (2015)

Inhibitors that were used as experimental tools in studies that are referred to in this article.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4477214&req=5

Figure 4: Inhibitors that were used as experimental tools in studies that are referred to in this article.
Mentions: Inhibitors based on small molecules that target papain-like cysteine proteases have contributed substantially to the understanding of the catalytic mechanisms, enzyme-substrate recognition and the biological functions of this class of proteolytic enzymes. Most of the known low molecular weight inhibitors of papain-like proteases are containing electrophilic entities, referred to as warheads, which give rise to irreversible or reversible covalent interactions with the active-site cysteine. The field of small-molecule based inhibitors has been surveyed by several excellent and comprehensive review articles that appeared in the recent years (Otto and Schirmeister, 1997; Leung et al., 2000; Lecaille et al., 2002; Powers et al., 2002; Abbenante and Fairlie, 2005; Frizler et al., 2010). Therefore, only a brief discussion of those inhibitor classes that served as lead structures for the design of imaging probes reviewed in the next section or those which were used as molecular tools for their biological evaluation will be given herein (Figure 4). Compound classes that exert inhibition by irreversible covalent modification of the active-site cysteine residue are peptide-derived halomethylketones, diazoketones, acyloxymethylketones, O-acyl hydroxamates, vinyl sulfones and epoxides, among other inhibitor chemotypes. Irreversible inhibitors of cysteine proteases have been reviewed in detail by Powers et al. The following remarks are mainly extracted from this reference and also kinetic data on the inhibition by some of the compounds in Figure 4 can be found therein (Powers et al., 2002). Peptidic chloromethyl ketones have been developed as inactivators of serine proteases but they are also potent cysteine protease inhibitors due to S-alkylation of the active-site cysteine. Their strong intrinsic reactivity results in modification of other biological thiols such as glutathione, even though in slower rates than their interaction with the targeted cysteine proteases. This drawback renders such inhibitors unsuitable for the application in cell-based and animal experiments. In contrast, fluoromethyl ketones such as compounds 1a and b are much less reactive toward thiols, due to the low energy of the carbon-fluorine bond. Despite their low inherent reactivity they are potent inactivators of cysteine proteases and show selectivity over serine proteases. Similarly, diazomethyl ketones can be considered to be selective for cysteine proteases, even though inactivation of serine proteases by this inhibitor chemotype has been reported for some instances. Their reactivity toward thiols of low molecular weight is negligible and some radiotracers have been developed on the basis of this inhibitor class (see Section 125I-labeled Compounds). Acyloxymethyl ketones (AOMK's; see compound 2 in Figure 4 for example) have been developed with the intention to create deactivated versions of chloromethyl ketones. This inhibitor chemotype has been shown to inactivate cysteine proteases selectively over other classes of proteolytic enzymes. In addition to the other reactive methyl ketones, they provide the opportunity of kinetic tuning upon varying the acyloxy leaving group. However, their potential susceptibility toward esterase-catalyzed degradation might compromise their applicability in vivo (Verdoes et al., 2013). Formal isoelectronic replacement of the methylene group in the latter inhibitor class leads to O-acyl hydroxamates, as represented by compound 3. This chemotype forms covalent complexes with the active-site cysteine in analogy to acyloxymethyl ketones. In contrast, peptide-derived O-acyl hydroxamates are capable to inactivate serine proteases, even though their mechanism of interaction may differ between these two classes of proteases (Brömme and Demuth, 1994). Mechanistically related to the aforementioned inhibitor classes are peptide-derived epoxides, among which the epoxysuccinyl peptides constitute the most important subclass. Differently to reactive methyl ketones, no nucleofuge is leaving the enzyme-inhibitor complex, as the attack of the active-site thiol results in epoxide opening. Their prototype is represented by the fungal secondary metabolite E64 (6), which has been shown to inactivate a broad spectrum of papain-like cysteine proteases. Replacement of the guanidinobutyl moiety by hydroxyphenyl and isobutyl led to JPM-565 (7a) and E64c (8a), respectively, without significant changes in the inhibitory potential. Replacement of the leucine residue in E64 (6) by isoleucine, C-terminal extension by proline and amidation of the carboxylic group results in CA074 (9a; Figure 4), which inactivates cathepsin B selectively over other cysteine cathepsins. Notably, epoxysuccinyl peptides can bind to the active site in the same or inverse orientation compared to the substrate binding mode. Esterification of the carboxylic groups in JPM-565, CA074, and E64 with simple alcohols renders these highly polar molecules membrane permeable (compounds 7b, 8b, and 9b in Figure 4). Despite the actual inhibitors can be released by the action of cellular esterases, the corresponding “prodrugs” still bear inhibitory potential but altered selectivity profiles compared to their “active” progenies, and cysteine cathepsin inactivation may occur faster than ester hydrolysis (Bogyo et al., 2000). Therefore, the results of cell-based experiments in which these esterified epoxysuccinyl peptides are used for pharmacological inhibition should be interpreted with care in terms of selectivity toward individual cysteine cathepsins. E64d (8b) has been demonstrated to be bioavailable upon oral administration. The spontaneous reactivity of epoxide inhibitors toward thiols has been shown to be low. A couple of radiotracers have been developed starting from epoxysuccinyl peptides (see Section 125I-labeled Compounds). Besides epoxysuccinyl peptides, peptidic inhibitors with epoxide warheads have been also created by replacing formally the terminal carboxylic group of N-terminal cleavage products by an oxirane ring, resulting in N-peptidyl-α-aminoalkyl epoxides. They interact with the active site by epoxide opening upon attack of the active-site sulfhydryl group at the methylene carbon. Concerning the two possible configurations at the methine carbon, the erythro diastereomers are more active than their threo-configured counterparts. In agreement with the altered electronic situation of the oxirane moiety, N-peptidyl-α-aminoalkyl epoxides are less potent inactivators of cysteine proteases compared to epoxysuccinyl derivatives. A tritium-labeled N-peptidyl-α-aminoalkyl epoxide has been reported (see Section Radiotracers Based on Fluorine-18, Radiocarbon Isotopes, and Tritium; and compound [3H]39 in Figure 11) (Albeck, 2000; Powers et al., 2002). Vinyl sulfones are cysteine-reactive moieties that give rise to nucleophilic addition instead of substitution. Their incorporation at the C-terminus of small peptides can lead to highly potent inactivators of cysteine proteases such as compounds 4 and 5 with limited inherent thiol reactivity. Compound 5 has been used as lead structure for radiotracer design (see Section 125I-labeled Compounds). Despite vinyl sulfones inactivate cysteine proteases selectively over serine proteases, the introduction of three leucine residues into position P1–P3 renders them capable of ether formation with the active-site threonine residue in proteasomes.

Bottom Line: The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging.After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response.In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

View Article: PubMed Central - PubMed

Affiliation: Department of Radiopharmaceutical and Chemical Biology, Institute of Radiopharmaceutical Cancer Research, Helmholtz-Zentrum Dresden-Rossendorf Dresden, Germany ; Department of Chemistry and Food Chemistry, Technische Universität Dresden Dresden, Germany.

ABSTRACT
Papain-like cysteine proteases bear an enormous potential as drug discovery targets for both infectious and systemic human diseases. The considerable progress in this field over the last two decades has also raised interest in the visualization of these enzymes in their native context, especially with regard to tumor imaging. After a short introduction to structure and general functions of human cysteine cathepsins, we highlight their importance for drug discovery and development and provide a critical update on the current state of knowledge toward their involvement in tumor progression, with a special emphasis on their role in therapy response. In accordance with a radiopharmaceutical point of view, the main focus of this review article will be the discussion of recently developed fluorescence and radiotracer-based imaging agents together with related molecular probes.

No MeSH data available.


Related in: MedlinePlus