Isomerization of (Z,Z) to (E,E)1-bromo-2,5-bis-(3-hydroxycarbonyl-4-hydroxy)styrylbenzene in strong base: probes for amyloid plaques in the brain

A closer look at amyloid ligands, and what they tell us about protein aggregates

The accumulation of amyloid fibrils is characteristic of neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease. Detecting these fibrils with fluorescent or radiolabelled ligands is one strategy for diagnosing and better understanding these diseases. A vast number of amyloid-binding ligands have been reported in the literature as a result. To obtain a better understanding of how amyloid ligands bind, we have compiled a database of 3457 experimental dissociation constants for 2076 unique amyloid-binding ligands. These ligands target Aβ, tau, or αSyn fibrils, as well as relevant biological samples including AD brain homogenates. From this database significant variation in the reported dissociation constants of ligands was found, possibly due to differences in the morphology of the fibrils being studied. Ligands were also found to bind to Aβ(1-40) and Aβ(1-42) fibrils with similar affinities, whereas a greater difference was found for binding to Aβ and tau or αSyn fibrils. Next, the binding of ligands to fibrils was shown to be largely limited by the hydrophobic effect. Some Aβ ligands do not fit into this hydrophobicity-limited model, suggesting that polar interactions can play an important role when binding to this target. Finally several binding site models were outlined for amyloid fibrils that describe what ligands target what binding sites. These models provide a foundation for interpreting and designing site-specific binding assays.

Off-Cycle Processes in Pd-Catalyzed Cross-Coupling of Carboranes

Off-cycle processes in catalytic reactions can dramatically influence the outcome of the chemical transformation and affect its yield, selectivity, rate, and product distribution. While the generation of off-cycle intermediates can complicate reaction coordinate analyses or hamper catalytic efficiency, the generation of such species may also open new routes to unique chemical products. Recently, we reported the Pd-mediated functionalization of carboranes with a range of O-, N-, and C-based nucleophiles. By utilizing a Pd-based catalytic system supported by a biaryl phosphine ligand developed by Buchwald and co-workers, we discovered an off-cycle isomerization process ("cage-walking") that generates four regioisomeric products from a single halogenated boron cluster isomer. Here we describe how several off-cycle processes affect the regioisomer yield and distribution during Pd-catalyzed tandem cage-walking/cross-coupling. In particular, tuning the transmetallation step in the catalytic cycle allowed us to incorporate the cage-walking process into Pd-catalyzed cross-coupling of sterically unencumbered substrates, including cyanide. This work demonstrates the feasibility of using tandem cage-walking/cross-coupling as a unique low-temperature method for producing regioisomers of mono-substituted carboranes.

Atomistic investigation of an Iowa Amyloid-β trimer in aqueous solution

The self-assembly of Amyloid beta (Aβ) peptides are widely accepted to associate with Alzheimer's disease (AD) via several proposed mechanisms. Because Aβ oligomers exist in a complicated environment consisting of various forms of Aβ, including oligomers, protofibrils, and fibrils, their structure has not been well understood. The negatively charged residue D23 is one of the critical residues of the Aβ peptide as it is located in the central hydrophobic domain of the Aβ N-terminal and forms a salt-bridge D23-K28, which helps stabilize the loop domain. In the familial Iowa (D23N) mutant, the total net charge of Aβ oligomers decreases, resulting in the decrease of electrostatic repulsion between D23N Aβ monomers and thus the increase in their self-aggregation rate. In this work, the impact of the D23N mutation on 3Aβ_11–40 trimer was characterized utilizing temperature replica exchange molecular dynamics (REMD) simulations. Our simulation reveals that D23N mutation significantly enhances the affinity between the constituting chains in the trimer, increases the β-content (especially in the sequence 21–23), and shifts the β-strand hydrophobic core from crossing arrangement to parallel arrangement, which is consistent with the increase in self-aggregation rate. Molecular docking indicates that the Aβ fibril-binding ligands bind to the D23N and WT forms at different poses. These compounds prefer to bind to the N-terminal β-strand of the D23N mutant trimer, while they mostly bind to the N-terminal loop region of the WT. It is important to take into account the difference in the binding of ligands to mutant and wild type Aβ peptides in designing efficient inhibitors for various types of AD.

Amyloid beta peptide oligomers are believed to play key roles in Alzheimer's disease pathogenesis. D23N mutation significantly changes their structure and how they bind potential inhibitors.

Amyloid imaging: Past, present and future perspectives

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are Aβ amyloid plaques, and tau neurofibrillary tangles, along dendritic and synaptic loss and reactive gliosis. Functional and molecular neuroimaging techniques such as positron emission tomography (PET) using functional and molecular tracers, in conjuction with other Aβ and tau biomarkers in CSF, are proving valuable in the differential diagnosis of AD, as well as in establishing disease prognosis. With the advent of new therapeutic strategies, there has been an increasing application of these techniques for the determination of Aβ burden in vivo in the patient selection, evaluation of target engagement and assessment of the efficacy of therapeutic approaches aimed at reducing Aβ in the brain.

Isotope-labeling of the fibril binding compound FSB via a Pd-catalyzed double alkoxycarbonylation

We have synthesized two isotopically labeled variants of the β-amyloid binding compound FSB possessing (13)C-labels on the two terminal aryl carboxylic acid moieties. One of these was also fully deuterated on the olefinic spacers. The (13)C-isotope labeling was achieved applying a Pd-catalyzed methoxycarbonylation of the corresponding aryl chlorides with externally (ex situ) generated (13)C-labeled CO. Application of the Shirakawa-Hayashi protocol for the Pd-catalyzed reduction of a dialkyne intermediate using D(2)O allowed for the selective deuterium labeling of the two trans-C,C double bonds of FSB.

Synthesis and in vitro evaluation of fluorinated styryl benzazoles as amyloid-probes

The formation of proteinaceous aggregates is a pathognomonic hallmark of several neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. To date, the final diagnostic for these diseases can only be achieved by immunostaining of post-mortem brain tissues with the commonly used congo red and Thioflavin T/S amyloid-dyes. The interest in developing amyloid-avid radioprobes to be used for protein aggregates imaging by positron emission tomography has grown substantialy, due to the promise in assisting diagnosis of these disorders. To this purpose, the present work describes the synthesis and characterization of four novel fluorinated styryl benzazole derivatives 1-4 by means of the Wittig reaction, as well as their in vitro evaluation as amyloid-probing agents. All compounds were obtained as mixtures of geometric E and Z isomers, with the preferable formation of the E isomer. Photoisomerization reactions allowed for the maximization of the minor Z isomers. The authentic 1-4E/Z isomers were isolated after purification by column chromatography under dark conditions. Profiting from the fluorescence properties of the different geometric isomers of 1-4, their binding affinities towards amyloid fibrils of insulin, α-synuclein and β-amyloid peptide were also measured. These compounds share similarities with Thioflavin T, interacting specifically with fibrillary species with a red-shift in the excitation wavelengths along with an increase in the fluorescence emission intensity. Apparent binding constants were determined and ranged between 1.22 and 23.96 μM(-1). The present data suggest that the novel fluorinated styryl benzazole derivatives may prove useful for the design of (18)F-labeled amyloid radioprobes.

Amyloid PET Ligands for Dementia

The progressive nature of neurodegeneration suggests an age-dependent process that ultimately leads to synaptic failure and neuronal damage in cortical areas of the brain critical for memory and higher mental functions. The increasing age of the population in developed countries suggests that, if unchecked, these disorders will become increasingly prevalent. In the absence of specific biologic markers, direct pathologic examination of brain tissue still is the only definitive method for establishing a diagnosis of Alzheimer disease (AD) and other types of dementia. Pathologic hallmarks of AD are intracellular neurofibrillary tangles (NFT) and extracellular amyloid plaques. NFT are intraneuronal bundles of paired helical filaments mainly composed of the aggregates of an abnormally phosphorylated form of tau protein; neuritic plaques consist of dense extracellular aggregates of β-amyloid (Aβ), surrounded by reactive gliosis and dystrophic neurites. To date, all available evidence strongly supports the notion that an imbalance between the production and removal of Aβ leading to its progressive accumulation is central to the pathogenesis of AD. A growing understanding of the molecular mechanisms of Aβ formation, degradation, and neurotoxicity is being translated into new therapeutic approaches. Whereas AD is the most common cause of dementia in the elderly, postmortem studies have found dementia with Lewy Bodies and frontotemporal lobe degeneration each to account for about 20% of cases. Molecular neuroimaging techniques such as PET have been used for the in vivo assessment of molecular processes at their sites of action, permitting detection of subtle pathophysiological changes in the brain at asymptomatic stages The development of molecular imaging methods for noninvasively assessing disease-specific traits such as Aβ burden in AD is allowing early diagnosis at presymptomatic stages, more accurate differential diagnosis and, when available, the evaluation and monitoring of disease-modifying therapy.

Effects of Congo red on aβ(1-40) fibril formation process and morphology

Alzheimer's disease (AD), an age-related neurodegenerative disorder, is the most common form of dementia, and the seventh-leading cause of death in the United States. Current treatments offer only symptomatic relief; thus, there is a great need for new treatments with disease-modifying potential. One pathological hallmark of AD is so-called senile plaques, mainly made up of β-sheet-rich assemblies of 40- or 42-residue amyloid β-peptides (Aβ). Hence, inhibition of Aβ aggregation is actively explored as an option to prevent or treat AD. Congo red (CR) has been widely used as a model antiamyloid agent to prevent Aβ aggregation. Herein, we report detailed morphological studies on the effect of CR as an antiamyloid agent, by circular dichroism spectroscopy, photo-induced cross-linking reactions, and atomic force microscopy. We also demonstrate the effect of CR on a preaggregated sample of Aβ(1-40). Our result suggests that Aβ(1-40) follows a different path for aggregation in the presence of CR.

Congo red and thioflavin-T analogs detect Abeta oligomers

Several small molecule ligands for amyloid-beta (Abeta) fibrils deposited in brain have been developed to facilitate radiological diagnosis of Alzheimer's disease (AD). Recently, the build-up of Abeta oligomers (AbetaO) in brain has been recognized as an additional hallmark of AD and may play a more significant role in early stages. Evidence suggests that quantitative assessment of AbetaO would provide a more accurate index of therapeutic effect of drug trials. Therefore, there is an urgent need to develop methods for efficient identification as well as structural analysis of AbetaO. We found that some well established amyloid ligands, analogs of Congo red and thioflavin-T (ThT), bind AbetaO with high affinity and detect AbetaO in vitro and in vivo. Binding studies revealed the presence of binding sites for Congo red- and thioflavin-T-analogs on AbetaO. Furthermore, these ligands can be used for imaging intracellular AbetaO in living cells and animals and as positive contrast agent for ultrastructural imaging of AbetaO, two applications useful for structural analysis of AbetaO in cells. We propose that by improving the binding affinity of current ligands, in vivo imaging of AbetaO is feasible by a 'signal subtraction' procedure. This approach may facilitate the identification of individuals with early AD.

Styryl-Based Compounds as Potential in vivo Imaging Agents for β-Amyloid Plaques

A group of styryl-based neutral compounds has been synthesized in this study for potential use as in vivo imaging agents for beta-amyloid plaques. Of 56 candidates, 14 compounds were found to label beta-amyloid plaques well on Alzheimer's disease (AD) human brain sections in vitro. The binding affinity to beta-amyloid fibrils was then determined by measuring the change in fluorescence intensity. Interestingly, we found that a class of quinaldine-styryl scaffold compounds displays specific binding to beta-amyloid fibrils. A representative compound, STB-8, was used in ex vivo and in vivo imaging experiments on an AD transgenic mouse model and demonstrated excellent blood-brain barrier (BBB) permeability and specific staining of the AD beta-amyloid plaques.

Cyanine dye–protein interactions: Looking for fluorescent probes for amyloid structures

We ascertained the ability to detect fibrillar beta-lactoglobulin (BLG) of a series of mono-, tri-, penta-, and heptamethinecyanines based on benzothiazole and benzimidazole heterocycles, and of benzothiazole squaraine. Fluorescence properties of these cyanine dyes were measured in the unbound state and in the presence of monomeric and fibrillar BLG and compared with those for the commercially available benzothiazole dye Thioflavin T. The correlation between the chemical nature of the dye molecules and the ability of dyes to bind aggregated proteins was established. We found that meso-substituted cyanines with amino substituents in heterocycle in contrast to the corresponding unsubstituted dyes have a binding preference to fibrillar BLG and a noticeable fluorescence response in the presence of the aggregated protein. For the squaraines and benzimidazole penthamethinecyanines studied, fluorescence emission increased both in the presence of native and fibrillar protein. The trimethinecyanines T-49 and SH-516 exhibit specifically increased fluorescence in the presence of fibrillar BLG. These dyes demonstrated the same or higher emission intensity and selectivity to aggregated BLG as Thioflavin T, and are proposed for application in selective fluorescent detection of aggregated proteins.

Ferulic acid and benzothiazole dimer derivatives with high binding affinity to β-amyloid fibrils

New ferulic acid and benzothiazole dimer derivatives were synthesized and evaluated by in vitro competition assay using [(125)I]TZDM for their specific binding affinities to Abeta fibrils. In particular, 4a showed the most excellent binding affinity (K(i)=0.53 nM), compared to PIB (K(i)=0.77 nM), for benzothiazole binding sites of Abeta(1-42) fibrils. This result suggests a possibility of a potential AD diagnostic probe for detection of Abeta fibrils.

Bis-styrylpyridine and bis-styrylbenzene derivatives as inhibitors for Aβ fibril formation

New bis-styrylpyridine and bis-styrylbenzene derivatives were designed and synthesized. These 34 compounds were evaluated by Abeta fibril formation inhibitory assay using thioflavin T as a dye (named ThT assay). Most of them showed excellent inhibitory activities for Abeta fibril formation at IC50 of 0.1-2.7 microM which is comparable to curcumin (IC50 of 0.8 microM). Among them, nine compounds were screened for their cytotoxicities on HT-22 cell by MTT assay at 1, 10, and 50 microM. In particular, I-7 and II-2 exhibited the best combination of inhibitory activity and compound cytotoxicity.

Imaging Alzheimer's disease pathology: one target, many ligands

Over the past five years there has been a surge of interest in using positron emission tomography (PET) to determine the in vivo density of the senile plaque, a key pathological feature of Alzheimer's disease. The development of the tracers [(11)C]-PIB, [(11)C]-SB13 and [(18)F]-FDDNP has coincided with drug strategies aimed at altering the brain metabolism of amyloid-beta peptides. The evolution of these novel ligands serves not only as an excellent example of how rapidly imaging technologies can progress but also as a reminder that the fundamental biological knowledge, which is necessary to fully interpret the PET data, can be left trailing behind.

La lunga attesa: towards a molecular approach to neuroimaging and therapeutics in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are Aβ amyloid plaques, neurofibrillary tangles (NFT), synaptic loss and reactive gliosis. Current diagnosis of AD is made by clinical, neuropsychologic, and neuroimaging assessments. Routine structural neuroimaging evaluation with computed tomography (CT) and magnetic resonance imaging (MRI) is based on non-specific features such as atrophy, a late feature in the progression of the disease, hence the crucial importance of developing new approaches for early and specific recognition at the prodromal stages of AD. Functional neuroimaging techniques such as functional magnetic resonance imaging (fMRI), magnetic resonance spectroscopy (MRS), positron emission tomography (PET) and single photon emission computed tomography (SPECT), possibly in conjuction with other related Aβ biomarkers in plasma and CSF, could prove to be valuable in the differential diagnosis of AD, as well as in assessing prognosis. With the advent of new therapeutic strategies aimed at reducing the Aβ amyloid burden in the brain, there is increasing interest in the development of MRI contrast agents and PET and SPECT radioligands that will permit the assessment of Aβ amyloid burden in vivo. - ma dov'è / la lenta processione di stagioni / che fu un'alba infinita e senza strade, / dov'è la lunga attesa e qual è il nome / del vuoto che ci invade. - Eugenio Montale.

Characterisation of the binding of amyloid imaging tracers to rodent Abeta fibrils and rodent-human Abeta co-polymers

Despite the application of amyloid imaging agents such as PIB, SB13, and FDDNP in Alzheimer's disease (AD) patients, the successful use of these agents in transgenic mice models of AD has not been reported to date. As a first step in understanding the behaviour of these ligands in transgenic models of AD, we have investigated in a series of in vitro ligand binding assays the interaction of selected agents, including PIB, FDDNP, SB13, and BSB, with amyloid fibrils produced from rodent Abeta(1-40) (roAbeta) peptide. The data indicate that the ligand binding affinities together with the pattern and number of binding sites on the roAbeta fibrils are broadly conserved with that reported previously for human Abeta(1-40) (huAbeta) fibrils. However, characterisation of huAbeta fibrils formed in the presence of increasing amounts of roAbeta (1, 5, 10% w/w) demonstrated a dose-dependent reduction in the number of high affinity [(3)H]Me-BTA-1 binding sites such that at the highest amount of roAbeta the specific signal was reduced by approximately 95%. These studies suggest that (i) the presence of small amounts of roAbeta in huAbeta fibrils has the potential to cause subtle ultrastructural alterations in the polymers and (ii) the weak binding signal observed in vivo in the transgenic mouse models of AD may in part be due to the decreased number of high affinity binding sites on the Abeta fibrils.

Imaginem oblivionis: the prospects of neuroimaging for early detection of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterised by the gradual onset of dementia. The pathological hallmarks of the disease are A beta amyloid plaques, neurofibrillary tangles, and reactive gliosis. Current diagnosis of AD is made by clinical, neuropsychologic, and neuroimaging assessments. Routine structural neuroimaging evaluation is based on non-specific features such as atrophy, a late feature in the progression of the disease, hence the crucial importance of developing new approaches for early and specific recognition at the prodromal stages of AD. Functional neuroimaging techniques such as positron emission tomography (PET) and single photon emission computed tomography (SPECT) could prove to be valuable in the differential diagnosis of AD, as well as in assessing prognosis. With the advent of new therapeutic strategies aimed at reducing the A beta amyloid burden in the brain, there is increasing interest in the development of PET and SPECT radioligands that will permit the assessment of A beta amyloid burden in vivo. From this, the prospect of specific preclinical diagnosis arises, possibly in conjunction with other related A beta biomarkers in plasma and CSF.

Design, synthesis, and structure–activity relationship of novel thiophene derivatives for β-amyloid plaque imaging

Novel 2,5-diphenylthiophene derivatives were synthesized and structure activity relationship with regard to Abeta plaque binding was studied. Binding affinities of these compounds were found to range from 3.9 to >1000 nM, depending on the substitution patterns on the phenyl ring. The fluoroethyl-substituted thiophene derivatives showed excellent binding affinities. These compounds may be useful for the development of novel PET tracers for the imaging of beta-amyloid plaques in the brain of patients with Alzheimer's disease.

Visualizing Pathology Deposits in the Living Brain of Patients with Alzheimer's Disease

One of the major neuropathological changes characteristic of Alzheimer's disease (AD) are deposits of beta-amyloid plaques and neurofibrillary tangles in neocortical and subcortical regions of the AD brain. The histochemical detection of these lesions in postmortem brain tissue is necessary for definitive diagnosis of AD. Methods for their in vivo detection would greatly aid the diagnosis of AD in early stages when neuronal loss and related functional impairment are still limited and also open the opportunity for effective therapeutic interventions. Positron emission tomography (PET) using an appropriate radiolabeled imaging probe with high binding affinity for these lesions is one of such techniques. We have developed 2-(1-{6-[(2-[F-18]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malononitrile ([F-18]FDDNP), a naphthalene-based radiofluorinated PET imaging probe with binding affinity for amyloid and amyloid-like structures, and applied it for in vivo brain imaging of patients with Alzheimer's disease and cognitively normal controls with PET. Analysis of in vivo [F-18]FDDNP imaging data using Logan plot graphical analysis with the cerebellum as a reference region was performed, and the binding levels in several areas of neocortex were determined. We observed increased levels of [F-18]FDDNP binding in patients in several neocortical regions in Alzheimer's disease compared with the cerebellum. In contrast, control subjects have uniformly low levels of [F-18]FDDNP binding in all areas, which is comparable to that of cerebellum.

Labeling of the scrapie-associated prion protein in vitro and in vivo

Prion diseases are a group of infectious neurodegenerative diseases that affect both animals and humans. A characteristic of prion diseases is the aggregation and accumulation of a disease-associated isoform of the prion protein in the brains of infected individuals. The amyloid imaging probe (trans,trans)-1-bromo-2,5-bis-(3-hydroxycarbonyl-4-hydroxy)styrylbenzene (BSB) has shown potential in the diagnosis of other amyloid disorders and we hypothesized that this compound would be effective in labeling prion protein plaques in vitro and in vivo. To test this, we compared BSB fluorescence to prion protein immunostaining on infected and uninfected brain tissue sections from scrapie-infected hamsters. We found that both methods labeled the same plaques in infected tissues while not substantially staining uninfected tissues. To test the potential of BSB as an in vivo label for prion aggregates, we perfused scrapie-infected animals with BSB and observed BSB labeled plaques co-stained with an anti-prion protein antibody. These results suggest that BSB may have use as a diagnostic tool for prion diseases. We were unable to detect BSB staining in preclinical scrapie-infected hamsters suggesting that the diagnostic potential of BSB could be limited in cases of prion disease that do not have plaques either due to a preclinical lack of pathology or disease agents like sporadic Creutzfeldt-Jakob disease (CJD), which generally lack prion plaques. However, BSB may be a useful for prion diseases where plaques are present, such as clinical variant CJD.

A comparison of amyloid fibrillogenesis using the novel fluorescent compound K114

Proteinaceous inclusions with amyloidogenic properties are a common link between many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Histological and in vitro studies of amyloid fibrils have advanced the understanding of protein aggregation, and provided important insights into pathogenic mechanisms of these neurodegenerative brain amyloidoses. The classical amyloid dyes Congo Red (CR) and thioflavin T and S, have been used extensively to detect amyloid inclusions in situ. These dyes have also been utilized to monitor the maturation of amyloid fibrils assembled from monomer subunits in vitro. Recently, the compound (trans,trans)-1-bromo-2,5-bis-(3- hydroxycarbonyl-4-hydroxy)styrylbenzene (BSB), derived from the structure of CR, was shown to bind to a wide range of amyloid inclusions in situ. More importantly it was also used to label brain amyloids in live animals. Herein, we show that an analogue of BSB, (trans,trans)-1-bromo-2,5-bis-(4-hydroxy)styrylbenzene (K114), recognizes amyloid lesions, and has distinctive properties which allowed the quantitative monitoring of the formation of amyloid fibrils assembled from the amyloid-beta peptide, alpha-synuclein, and tau.

Dimethylamino-fluorenes: ligands for detecting beta-amyloid plaques in the brain

Formation of beta-amyloid (Abeta) plaques in the brain is a major contributing factor in the pathogenesis of Alzheimer's disease (AD). Detection of Abeta plaques in the brain will be potentially useful in early diagnosis and monitoring the progression of the disease. A series of novel Abeta aggregate-specific ligands based on fluorenes, which are simple and rigid tricyclic molecules, are synthesized and characterized. Starting with 2- or 3-aminofluorenes, 1a-1f, the amino group was converted to the N,N-dimethylamino group (2a-2f) in excellent yield. It was found that 7-iodo-2-N,N-dimethylaminofluorene (2f) showed an extremely high binding affinity to preformed Abeta40 aggregates (K(i) = 0.9 nM). In vitro autoradiography study using brain sections obtained from transgenic mice (Tg2576) with [(125)I]2f showed exquisitely high specific binding to Abeta plaques. The same section also displayed an equivalent labeling when stained by Thioflavin-S, a commonly used fluorescent dye for Abeta plaques. When [(125)I]2f was injected intravenously into normal mice, it exhibited an excellent brain uptake. Taken together the data suggest that [(125)I]2f may be useful as an in vivo imaging agent to detect Abeta plaques in the brain.

Aspartic proteases involved in Alzheimer's disease

Alzheimer's disease afflicts every tenth human aged over 65. Despite the dramatic progress that has been made in understanding the disease, the exact cause of Alzheimer's disease is still unknown. Most gene mutations associated with Alzheimer's disease point at the same culprits: amyloid precursor protein and ultimately amyloid beta. The enigmatic proteases alpha-,beta-, and gamma-secretase are the three executioners of amyloid precursor protein processing, and disruption of their delicate balance is suspected to result in Alzheimer's disease. Significant progress has been made in the selective control of these proteases, regardless of the availability of structural information. Not even the absence of a robust cell-free assay for gamma-secretase could hamper the identification of nonpeptidic inhibitors of this enzyme for long. Within five years, four distinctly different structural moieties were developed and the first drug candidates are in clinical trials. Unfortunately, selective inhibition of amyloid beta formation remains a crucial issue because fundamental fragments of the gamma-secretase complex are important for other signaling events. This problem makes beta-secretase inhibition and alpha-secretase induction even more appealing.