Cleavage agents for soluble oligomers of amyloid beta peptides

Neuroprotective peptide–macrocycle conjugates reveal complex structure–activity relationships in their interactions with amyloid β

Novel neuroprotective peptide–macrocycle conjugates exhibit complex, multifaceted structure–activity relationships in their interactions with amyloid β.

Attenuation of the aggregation and neurotoxicity of amyloid-β peptides by catalytic photooxygenation

Alzheimer's disease (AD), a progressive severe neurodegenerative disorder, is currently incurable, despite intensive efforts worldwide. Herein, we demonstrate that catalytic oxygenation of amyloid-β peptides (Aβ) might be an effective approach to treat AD. Aβ1-42 was oxygenated under physiologically-relevant conditions (pH 7.4, 37 °C) using a riboflavin catalyst and visible light irradiation, with modifications at the Tyr(10) , His(13) , His(14) , and Met(35) residues. The oxygenated Aβ1-42 exhibited considerably lower aggregation potency and neurotoxicity compared with native Aβ. Photooxygenation of Aβ can be performed even in the presence of cells, by using a selective flavin catalyst attached to an Aβ-binding peptide; the Aβ cytotoxicity was attenuated in this case as well. Furthermore, oxygenated Aβ1-42 inhibited the aggregation and cytotoxicity of native Aβ.

Target-directed catalytic metallodrugs

Most drugs function by binding reversibly to specific biological targets, and therapeutic effects generally require saturation of these targets. One means of decreasing required drug concentrations is incorporation of reactive metal centers that elicit irreversible modification of targets. A common approach has been the design of artificial proteases/nucleases containing metal centers capable of hydrolyzing targeted proteins or nucleic acids. However, these hydrolytic catalysts typically provide relatively low rate constants for target inactivation. Recently, various catalysts were synthesized that use oxidative mechanisms to selectively cleave/inactivate therapeutic targets, including HIV RRE RNA or angiotensin converting enzyme (ACE). These oxidative mechanisms, which typically involve reactive oxygen species (ROS), provide access to comparatively high rate constants for target inactivation. Target-binding affinity, co-reactant selectivity, reduction potential, coordination unsaturation, ROS products (metal-associated vs metal-dissociated; hydroxyl vs superoxide), and multiple-turnover redox chemistry were studied for each catalyst, and these parameters were related to the efficiency, selectivity, and mechanism(s) of inactivation/cleavage of the corresponding target for each catalyst. Important factors for future oxidative catalyst development are 1) positioning of catalyst reduction potential and redox reactivity to match the physiological environment of use, 2) maintenance of catalyst stability by use of chelates with either high denticity or other means of stabilization, such as the square planar geometric stabilization of Ni- and Cu-ATCUN complexes, 3) optimal rate of inactivation of targets relative to the rate of generation of diffusible ROS, 4) targeting and linker domains that afford better control of catalyst orientation, and 5) general bio-availability and drug delivery requirements.

A Co(III) complex cleaving soluble oligomers of h-IAPP in the presence of polymeric aggregates of h-IAPP

Soluble oligomers of human islet amyloid polypeptide (h-IAPP) are believed to be the pathogenic species for type 2 diabetes mellitus. In search of the peptide-cleavage agent cleaving oligomers of h-IAPP with low affinity for polymeric aggregates of h-IAPP, a chemical library was constructed by using the Ugi condensation. From the library, a Co(III) complex was discovered to cleave soluble oligomers of h-IAPP in the presence of polymeric aggregates of h-IAPP without being captured by the aggregates considerably. The peptide-cleavage agent inhibited apoptosis of INS-1 cell by h-IAPP even in the presence of preformed polymeric aggregates of h-IAPP. This suggests that target-selective peptide-cleavage agents may be applied clinically not only to diabetes but also to various other amyloid diseases.

Mechanism of peptide hydrolysis by co-catalytic metal centers containing leucine aminopeptidase enzyme: a DFT approach

In this density functional theory study, reaction mechanisms of a co-catalytic binuclear metal center (Zn1-Zn2) containing enzyme leucine aminopeptidase for two different metal bridging nucleophiles (H(2)O and -OH) have been investigated. In addition, the effects of the substrate (L-leucine-p-nitroanilide → L-leucyl-p-anisidine) and metal (Zn1 → Mg and Zn2 → Co, i.e., Mg1-Zn2 and Mg1-Co2 variants) substitutions on the energetics of the mechanism have been investigated. The general acid/base mechanism utilizing a bicarbonate ion followed by this enzyme is divided into two steps: (1) the formation of the gem-diolate intermediate, and (2) the cleavage of the peptide bond. With the computed barrier of 17.8 kcal/mol, the mechanism utilizing a hydroxyl nucleophile was found to be in excellent agreement with the experimentally measured barrier of 18.7 kcal/mol. The rate-limiting step for reaction with L-leucine-p-nitroanilide is the cleavage of the peptide bond with a barrier of 17.8 kcal/mol. However, for L-leucyl-p-anisidine all steps of the mechanism were found to occur with similar barriers (18.0-19.0 kcal/mol). For the metallovariants, cleavage of the peptide bond occurs in the rate-limiting step with barriers of 17.8, 18.0, and 24.2 kcal/mol for the Zn1-Zn2, Mg1-Zn2, and Mg1-Co2 enzymes, respectively. The nature of the metal ion was found to affect only the creation of the gem-diolate intermediate, and after that all three enzymes follow essentially the same energetics. The results reported in this study have elucidated specific roles of both metal centers, the nucleophile, indirect ligands, and substrates in the catalytic functioning of this important class of binuclear metallopeptidases.

Minding metals: tailoring multifunctional chelating agents for neurodegenerative disease

Neurodegenerative diseases like Alzheimer's and Parkinson's disease are associated with elevated levels of iron, copper, and zinc and consequentially high levels of oxidative stress. Given the multifactorial nature of these diseases, it is becoming evident that the next generation of therapies must have multiple functions to combat multiple mechanisms of disease progression. Metal-chelating agents provide one such function as an intervention for ameliorating metal-associated damage in degenerative diseases. Targeting chelators to adjust localized metal imbalances in the brain, however, presents significant challenges. In this perspective, we focus on some noteworthy advances in the area of multifunctional metal chelators as potential therapeutic agents for neurodegenerative diseases. In addition to metal chelating ability, these agents also contain features designed to improve their uptake across the blood-brain barrier, increase their selectivity for metals in damage-prone environments, increase antioxidant capabilities, lower Abeta peptide aggregation, or inhibit disease-associated enzymes such as monoamine oxidase and acetylcholinesterase.

Metallotherapeutics: novel strategies in drug design

A new paradigm for drug activity is presented, which includes both recognition and subsequent irreversible inactivation of therapeutic targets. Application to both RNA and protein biomolecules has been demonstrated. In contrast to RNA targets that are subject to strand scission chemistry mediated by ribose H-atom abstraction, proteins appear to be inactivated either through oxidative damage to amino acid side chains around the enzyme active site, or by backbone hydrolysis.

Target-selective peptide-cleaving catalysts as a new paradigm in drug design

This tutorial review describes the evolution of peptide-hydrolyzing metal catalysts towards artificial metalloproteases cleaving target proteins selectively. The catalytic cleavage of the backbone of a protein related to a disease may effect a cure. In particular, a new therapeutic option for amyloid diseases such as Alzheimer's disease, diabetes and Parkinson's disease has been presented. The new paradigm of drug design based on artificial metalloproteases should be of interest to researchers in the areas of biomimetic chemistry, as well as medicinal chemistry.

Targeting proteins with metal complexes

Unique properties of metal complexes, such as structural diversity, adjustable ligand exchange kinetics, fine-tuned redox activities, and distinct spectroscopic signatures, make them exciting scaffolds not only for binding to nucleic acids but increasingly also to proteins as non-traditional targets. This feature article discusses recent trends in this field. These include the use of chemically inert metal complexes as structural scaffolds for the design of enzyme inhibitors, new strategies for inducing selective coordination chemistry at the protein binding site, recent advances in the development of catalytic enzyme inhibitors, and the design of metal complexes that can inject electrons or holes into redox enzymes. A common theme in many of the discussed examples is that binding selectivity is at least in part achieved through weak interactions between the ligand sphere and the protein binding site. These examples hint to an exciting future in which "organic-like" molecular recognition principles are combined with properties that are unique to metals and thus promise to yield compounds with novel and unprecedented properties.

Artificial proteases toward catalytic drugs for amyloid diseases

We have proposed catalytic drugs based on artificial proteases as a new paradigm in drug design. Catalytic cleavage of the backbone of a protein related to a disease may effect a cure. Catalytic drugs can be designed even for proteins lacking active sites. Soluble oligomers of amyloid β-42 peptide (Aβ42) are implicated as the primary toxic species in amyloid diseases such as Alzheimer's disease (AD). Cleavage of Aβ42 included in an oligomer may provide a novel method for reduction of Aβ42 oligomers, offering a new therapeutic option. The Co(III) complex of cyclen was used as the catalytic center for peptide hydrolysis. Binding sites of the catalysts that recognize the target were searched by using various chemical libraries. Four compounds were selected as cleavage agents for the oligomers of Aβ42. After reaction with the cleavage agents for 36 h at 37 °C and pH 7.50, up to 30 mol % of Aβ42 (4.0 μM) was cleaved, although the target oligomers existed as transient species. Considerable activity was manifested at the concentrations of the agents as low as 100 nM.

Versatile synthesis of head group functionalized phospholipids via oxime bond formation

A method for introduction of various head groups on phospholipid frameworks via oxime bond formation has been developed for the synthesis of cyclen-Cu(II), pyrene, naphthalene, and other headgroup functionalized phospholipids that can cleave the membrane protein, hemagglutinin.

Proteolytic activity of Co(III) complex of 1-oxa-4,7,10-triazacyclododecane: a new catalytic center for peptide-cleavage agents

Catalytic drugs based on target-selective artificial proteases have been proposed as a new paradigm in drug design. Peptide-cleavage agents selective for pathogenic proteins of Alzheimer's disease, type 2 diabetes mellitus or Parkinson's disease have been prepared using the Co(III) aqua complex (Co(III)cyclen) of 1,4,7,10-tetraazacyclododecane as the catalytic center. In the present study, the Co(III) aqua complex (Co(III)oxacyclen) of 1-oxa-4,7,10-triazacyclododecane was examined in search of an improved catalytic center for peptide-cleavage agents. An X-ray crystallographic study of [Co(oxacyclen)(CO(3))](ClO(4)), titration of Co(III)oxacyclen, and kinetic studies on the cleavage of albumin, gamma-globulin, lysozyme, and myoglobin by Co(III)oxacyclen were carried out. Considerably higher proteolytic activity was observed for Co(III)oxacyclen in comparison with Co(III)cyclen, indicating that better target-selective artificial metalloproteases would be obtained using Co(III)oxacyclen as the catalytic center. The improved proteolytic activity was attributed to either steric effects or the increased Lewis acidity of the Co(III) center. The kinetic data also predicted that side effects due to the cleavage of nontarget proteins by a catalytic drug based on Co(III)oxacyclen would be insignificant.

Sequestration of copper from beta-amyloid promotes selective lysis by cyclen-hybrid cleavage agents

Decelerated degradation of beta-amyloid (Abeta) and its interaction with synaptic copper may be pathogenic in Alzheimer disease. Recently, Co(III)-cyclen tagged to an aromatic recognition motif was shown to degrade Abeta in vitro. Here, we report that apocyclen attached to selective Abeta recognition motifs (KLVFF or curcumin) can capture copper bound to Abeta and use the Cu(II) in place of Co(III) to become proteolytically active. The resultant complexes interfere with Abeta aggregation, degrade Abeta into fragments, preventing H2O2 formation and toxicity in neuronal cell culture. Because Abeta binds Cu in amyloid plaques, apocyclen-tagged targeting molecules may be a promising approach to the selective degradation of Abeta in Alzheimer disease. The principle of copper capture could generalize to other amyloidoses where copper is implicated.

Cleavage agents for soluble oligomers of human islet amyloid polypeptide

Soluble oligomers of human islet amyloid polypeptide (h-IAPP) are implicated in the initiation of beta-cell apoptosis leading to type 2 diabetes mellitus (T2DM). Cleavage of the h-IAPP included in an oligomer may provide a novel method for reducing the level of h-IAPP oligomers, offering a new therapeutic option for T2DM. From the combinatorial library of triazine derivatives prepared by exploiting the Co(III) complex of cyclen as the cleavage center for peptide bonds, eight compounds were selected as cleavage agents for oligomers of h-IAPP. After reaction with cleavage agents for 36 h at 37 degrees C and pH 7.50, up to 20 mol% of h-IAPP (initial concentration: 4.0 microM) was cleaved, although the target oligomers existed as transient species. Considerable activity was manifested at agent concentrations as low as 100 nM.