Human transthyretin in complex with iododiflunisal: structural features associated with a potent amyloid inhibitor

Native State Stabilization of Amyloidogenic Proteins by Kinetic Stabilizers: Inhibition of Protein Aggregation and Clinical Relevance

Proteinopathies or amyloidoses are a group of life-threatening disorders that result from misfolding of proteins and aggregation into toxic insoluble amyloid aggregates. Amyloid aggregates have low clearance from the body due to the insoluble nature, leading to their deposition in various organs and consequent organ dysfunction. While amyloid deposition in the central nervous system leads to neurodegenerative diseases that mostly cause dementia and difficulty in movement, several other organs, including heart, liver and kidney are also affected by systemic amyloidoses. Regardless of the site of amyloid deposition, misfolding and structural alteration of the precursor proteins play the central role in amyloid formation. Kinetic stabilizers are an emerging class of drugs, which act like pharmacological chaperones to stabilize the native state structure of amyloidogenic proteins and to increase the activation energy barrier that is required for adopting a misfolded structure or conformation, ultimately leading to the inhibition of protein aggregation. In this review, we discuss the kinetic stabilizers that stabilize the native quaternary structure of transthyretin, immunoglobulin light chain and superoxide dismutase 1 that cause transthyretin amyloidoses, light chain amyloidosis and familial amyotrophic lateral sclerosis, respectively.

Transthyretin monomers: a new plasma biomarker for pre-symptomatic transthyretin-related amyloidosis

BACKGROUND

Genotyping and amyloid fibril detection in tissues are generally considered the diagnostic gold standard in transthyretin-related amyloidosis. Patients carry less stable TTR homotetramers prone to dissociation into non-native monomers, which rapidly self-assemble into oligomers and, ultimately, amyloid fibrils. Thus, the initial event of the amyloid cascade produces the smallest transthyretin species: the monomers. This creates engineering opportunities for diagnosis that remain unexplored.

METHODS

We hypothesise that molecular sieving represents a promising method for isolating and concentrating trace TTR monomers from the tetramers present in plasma samples. Subsequently, immunodetection can be utilised to distinguish monomeric TTR from other low molecular weight proteins within the adsorbed fraction. A two-step assay was devised (ImmunoSieve assay), combining molecular sieving and immunodetection for sensing monomeric transthyretin. This assay was employed to analyse plasma microsamples from 10 individuals, including 5 pre-symptomatic carriers of TTR-V30M, the most prevalent amyloidosis-associated TTR variant worldwide, and 5 healthy controls.

RESULTS

The ImmunoSieve assay enable sensitive detection of monomeric transthyretin in plasma microsamples. Moreover, the circulating monomeric TTR levels were significantly higher in carriers of amyloidogenic TTR mutation.

CONCLUSIONS

Monomeric TTR can function as a biomarker for evaluating disease progression and assessing responses to therapies targeted at stabilising native TTR.

Inhibitors of Transthyretin Amyloidosis: How to Rank Drug Candidates Using X-ray Crystallography Data

Amyloidosis is a group of protein misfolding diseases, which include spongiform encephalopathies, Alzheimer's disease and transthyretin (TTR) amyloidosis; all of them are characterized by extracellular deposits of an insoluble fibrillar protein. TTR amyloidosis is a highly debilitating and life-threatening disease. Patients carry less stable TTR homotetramers that are prone to dissociation into non-native monomers, which in turn rapidly self-assemble into oligomers and, ultimately, amyloid fibrils. Liver transplantation to induce the production of wild-type TTR was the only therapeutic strategy until recently. A promising approach to ameliorate transthyretin (TTR) amyloidosis is based on the so-called TTR kinetic stabilizers. More than 1000 TTR stabilizers have already been tested by many research groups, but the diversity of experimental techniques and conditions used hampers an objective prioritization of the compounds. One of the most reliable and unambiguous techniques applied to determine the structures of the TTR/drug complexes is X-ray diffraction. Most of the potential inhibitors bind in the TTR channel and the crystal structures reveal the atomic details of the interaction between the protein and the compound. Here we suggest that the stabilization effect is associated with a compaction of the quaternary structure of the protein and propose a scoring function to rank drugs based on X-ray crystallography data.

Investigation of the molecular basis of halogenated Schiff base derivative by combined crystallographic and computational studies

Halogenated Schiff base derivatives are gaining more popularity in supramolecular chemistry due to the synergistic effect of hydrogen and halogen-based noncovalent interactions, which helps to design novel therapeutic materials. In this work, we have examined the nature of molecular interactions to investigate the structure-functional relationship of a halogen-based derivative. The FTIR, HRMS and NMR spectroscopic techniques confirmed the formation of the desired novel Schiff base compound. Further, crystal structure studies showed an infinite 1D supramolecular chain formed by type-I halogen…halogen interaction. The Hirshfeld surface and enrichment ratio analyses were performed to visualize and assess the role of diverse interactions involved in crystal packing. The QTAIM, NCI, LOL and ELF studies were conducted extensively to comprehend the strength of interaction constructed based on electron density distribution. The global and local reactive indices were determined using DFT studies to analyze the molecular properties of the compound. Antibacterial activity against MRSA bacteria was performed and showed a good zone of inhibition. The docking analysis was performed for 1mwt protein and validated. The in silico molecular docking studies of the halogenated Schiff base structure with the penicillin-binding protein showed a good docking affinity of -7.5 kcal/mol and supported by in vitro studies. The ligand binding stability within the protein's active site was further demonstrated by molecular dynamics (MD) simulation studies for the Schiff base molecule.Communicated by Ramaswamy H. Sarma.

Chlorinated Naringenin Analogues as Potential Inhibitors of Transthyretin Amyloidogenesis

Misfolding and aggregation of transthyretin are implicated in the fatal systemic disease known as transthyretin amyloidosis. Here, we report the development of a naringenin derivative bearing two chlorine atoms that will be efficacious for preventing aggregation of transthyretin in the eye. The amyloid inhibitory activity of the naringenin derivative was as strong as that of tafamidis, which is the first therapeutic agent targeting transthyretin in the plasma. X-ray crystal structures of the compounds in complex with transthyretin demonstrated that the naringenin derivative with one chlorine bound to the thyroxine-binding site of transthyretin in the forward mode and that the derivative with two chlorines bound to it in the reverse mode. An ex vivo competitive binding assay showed that naringenin derivatives exhibited more potent binding than tafamidis in the plasma. Furthermore, an in vivo pharmacokinetic study demonstrated that the dichlorinated derivative was significantly delivered to the eye.

Static discrete disorder in the crystal structure of iododiflunisal: on the importance of hydrogen bond, halogen bond and π-stacking interactions

We report a combined computational/crystallographic analysis focused on the static discrete disorder shown by the drug iododiflunisal.

Targeting transthyretin in Alzheimer's disease: Drug discovery of small-molecule chaperones as disease-modifying drug candidates for Alzheimer's disease

Transthyretin (TTR) has a well-established role in neuroprotection in Alzheimer's Disease (AD). We have setup a drug discovery program of small-molecule compounds that act as chaperones enhancing TTR/Amyloid-beta peptide (Aβ) interactions. A combination of computational drug repurposing approaches and in vitro biological assays have resulted in a set of molecules which were then screened with our in-house validated high-throughput screening ternary test. A prioritized list of chaperones was obtained and corroborated with ITC studies. Small-molecule chaperones have been discovered, among them our lead compound Iododiflunisal (IDIF), a molecule in the discovery phase; one investigational drug (luteolin); and 3 marketed drugs (sulindac, olsalazine and flufenamic), which could be directly repurposed or repositioned for clinical use. Not all TTR tetramer stabilizers behave as chaperones in vitro. These chemically diverse chaperones will be used for validating TTR as a target in vivo, and to select one repurposed drug as a candidate to enter clinical trials as AD disease-modifying drug.

Inhibitory activities of anthraquinone and xanthone derivatives against transthyretin amyloidogenesis

Transthyretin is a tetrameric protein which functions as a transporter of thyroxine and retinol-binding protein. Misfolding and amyloid aggregation of transthyretin are known to cause wild-type and hereditary transthyretin amyloidosis. Stabilization of the transthyretin tetramer by low molecular weight compounds is an efficacious strategy to inhibit the aggregation pathway in the amyloidosis. Here, we investigated the inhibitory activities of anthraquinone and xanthone derivatives against amyloid aggregation, and found that xanthone-2-carboxylic acid with one chlorine or methyl group has strong inhibitory activity comparable with that of diflunisal, which is one of the best known stabilizers of transthyretin. X-ray crystallographic structures of transthyretin in complex with the compounds revealed that the introduction of chlorine, which is buried in a hydrophobic region, is important for the strong inhibitory effect of the stabilizer against amyloidogenesis. An in vitro absorption, distribution, metabolism and elimination (ADME) study and in vivo pharmacokinetic study demonstrated that the compounds have drug-like features, suggesting that they have potential as therapeutic agents to stabilize transthyretin.

Diphenyl-Methane Based Thyromimetic Inhibitors for Transthyretin Amyloidosis

Thyromimetics, whose physicochemical characteristics are analog to thyroid hormones (THs) and their derivatives, are promising candidates as novel therapeutics for neurodegenerative and metabolic pathologies. In particular, sobetirome (GC-1), one of the initial halogen-free thyromimetics, and newly synthesized IS25 and TG68, with optimized ADME-Tox profile, have recently attracted attention owing to their superior therapeutic benefits, selectivity, and enhanced permeability. Here, we further explored the functional capabilities of these thyromimetics to inhibit transthyretin (TTR) amyloidosis. TTR is a homotetrameric transporter protein for THs, yet it is also responsible for severe amyloid fibril formation, which is facilitated by tetramer dissociation into non-native monomers. By combining nuclear magnetic resonance (NMR) spectroscopy, computational simulation, and biochemical assays, we found that GC-1 and newly designed diphenyl-methane-based thyromimetics, namely IS25 and TG68, are TTR stabilizers and efficient suppressors of TTR aggregation. Based on these observations, we propose the novel potential of thyromimetics as a multi-functional therapeutic molecule for TTR-related pathologies, including neurodegenerative diseases.

Diflunisal Derivatives as Modulators of ACMS Decarboxylase Targeting the Tryptophan-Kynurenine Pathway

In the kynurenine pathway for tryptophan degradation, an unstable metabolic intermediate, α-amino-β-carboxymuconate-ε-semialdehyde (ACMS), can nonenzymatically cyclize to form quinolinic acid, the precursor for de novo biosynthesis of nicotinamide adenine dinucleotide (NAD⁺). In a competing reaction, ACMS is decarboxylated by ACMS decarboxylase (ACMSD) for further metabolism and energy production. Therefore, the inhibition of ACMSD increases NAD⁺ levels. In this study, an Food and Drug Administration (FDA)-approved drug, diflunisal, was found to competitively inhibit ACMSD. The complex structure of ACMSD with diflunisal revealed a previously unknown ligand-binding mode and was consistent with the results of inhibition assays, as well as a structure-activity relationship (SAR) study. Moreover, two synthesized diflunisal derivatives showed half-maximal inhibitory concentration (IC₅₀) values 1 order of magnitude better than diflunisal at 1.32 ± 0.07 μM (22) and 3.10 ± 0.11 μM (20), respectively. The results suggest that diflunisal derivatives have the potential to modulate NAD⁺ levels. The ligand-binding mode revealed here provides a new direction for developing inhibitors of ACMSD.

An Assay for Screening Potential Drug Candidates for Alzheimer's Disease That Act as Chaperones of the Transthyretin and Amyloid-β Peptides Interaction

The protein transthyretin (TTR) modulates amyloid-β (Aβ) peptides deposition and processing and this physiological effect is further enhanced by treatment with iododiflunisal (IDIF), a small-molecule compound (SMC) with TTR tetramer stabilization properties, which behaves as chaperone of the complex. This knowledge has prompted us to design and optimize a rapid and simple high-throughput assay that relies on the ability of test compounds to form ternary soluble complexes TTR/Aβ/SMC that prevent Aβ aggregation. The method uses the shorter Aβ(12-28) sequence which is cheaper and simpler to use while retaining the aggregation properties of their parents Aβ(1-40) and Aβ(1-42). The test is carried out in 96-plate wells that are UV monitored for turbidity during 6 h. Given its reproducibility, we propose that this test can be a powerful tool for efficient screening of SMCs that act as chaperones of the TTR/Aβ interaction that may led to potential AD therapies.

Repurposing Benzbromarone for Familial Amyloid Polyneuropathy: A New Transthyretin Tetramer Stabilizer

Transthyretin (TTR) is a homotetrameric protein involved in human amyloidosis, including familial amyloid polyneuropathy (FAP). Discovering small-molecule stabilizers of the TTR tetramer is a therapeutic strategy for these diseases. Tafamidis, the only approved drug for FAP treatment, is not effective for all patients. Herein, we discovered that benzbromarone (BBM), a uricosuric drug, is an effective TTR stabilizer and inhibitor against TTR amyloid fibril formation. BBM rendered TTR more resistant to urea denaturation, similarly to iododiflunisal (IDIF), a very potent TTR stabilizer. BBM competes with thyroxine for binding in the TTR central channel, with an IC50 similar to IDIF and tafamidis. Results obtained by isothermal titration calorimetry (ITC) demonstrated that BBM binds TTR with an affinity similar to IDIF, tolcapone and tafamidis, confirming BBM as a potent binder of TTR. The crystal structure of the BBM-TTR complex shows two molecules binding deeply in the thyroxine binding channel, forming strong intermonomer hydrogen bonds and increasing the stability of the TTR tetramer. Finally, kinetic analysis of the ability of BBM to inhibit TTR fibrillogenesis at acidic pH and comparison with other stabilizers revealed that benzbromarone is a potent inhibitor of TTR amyloidogenesis, adding a new interesting scaffold for drug design of TTR stabilizers.

Rational Design, Synthesis, Characterization and Evaluation of Iodinated 4,4'-Bipyridines as New Transthyretin Fibrillogenesis Inhibitors

The 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridine structure is proposed as a novel chemical scaffold for the design of new transthyretin (TTR) fibrillogenesis inhibitors. In the frame of a proof-of-principle exploration, four chiral 3,3',5,5'-tetrachloro-2-iodo-2'-substituted-4,4'- bipyridines were rationally designed and prepared from a simple trihalopyridine in three steps, including a Cu-catalysed Finkelstein reaction to introduce iodine atoms on the heteroaromatic scaffold, and a Pd-catalysed coupling reaction to install the 2'-substituent. The corresponding racemates, along with other five chiral 4,4'-bipyridines containing halogens as substituents, were enantioseparated by high-performance liquid chromatography in order to obtain pure enantiomer pairs. All stereoisomers were tested against the amyloid fibril formation (FF) of wild type (WT)-TTR and two mutant variants, V30M and Y78F, in acid mediated aggregation experiments. Among the 4,4'-bipyridine derivatives, interesting inhibition activity was obtained for both enantiomers of the 3,3',5,5'-tetrachloro-2'-(4-hydroxyphenyl)-2-iodo-4,4'-bipyridine. In silico docking studies were carried out in order to explore possible binding modes of the 4,4'-bipyridine derivatives into the TTR. The gained results point out the importance of the right combination of H-bond sites and the presence of iodine as halogen-bond donor. Both experimental and theoretical evidences pave the way for the utilization of the iodinated 4,4'-bipyridine core as template to design new promising inhibitors of TTR amyloidogenesis.

Calorimetric Studies of Binary and Ternary Molecular Interactions between Transthyretin, Aβ Peptides, and Small-Molecule Chaperones toward an Alternative Strategy for Alzheimer's Disease Drug Discovery

Transthyretin (TTR) modulates the deposition, processing, and toxicity of Abeta (Aβ) peptides. We have shown that this effect is enhanced in mice by treatment with small molecules such as iododiflunisal (IDIF, 4), a good TTR stabilizer. Here, we describe the thermodynamics of the formation of binary and ternary complexes among TTR, Aβ(1–42) peptide, and TTR stabilizers using isothermal titration calorimetry (ITC). A TTR/Aβ(1–42) (1:1) complex with a dissociation constant of K_d = 0.94 μM is formed; with IDIF (4), this constant improves up to K_d = 0.32 μM, indicating the presence of a ternary complex TTR/IDIF/Aβ(1–42). However, with the drugs diflunisal (1) or Tafamidis (2), an analogous chaperoning effect could not be observed. Similar phenomena could be recorded with the shorter peptide Aβ(12–28) (7). We propose the design of a simple assay system for the search of other chaperones that behave like IDIF and may become potential candidate drugs for Alzheimer’s disease (AD).

Radiochemical examination of transthyretin (TTR) brain penetration assisted by iododiflunisal, a TTR tetramer stabilizer and a new candidate drug for AD

It is well settled that the amyloidogenic properties of the plasma protein transporter transthyretin (TTR) can be modulated by compounds that stabilize its native tetrameric conformation. TTR is also present in cerebrospinal fluid where it can bind to Aβ-peptides and prevent Aβ aggregation. We have previously shown that treatment of Alzheimer’s Disease (AD) model mice with iododiflunisal (IDIF), a TTR tetramer stabilizing compound, prevents AD pathologies. This evidence positioned IDIF as a new lead drug for AD. In dissecting the mechanism of action of IDIF, we disclose here different labeling strategies for the preparation of ¹³¹I-labeled IDIF and ¹³¹I- and ¹²⁴I-labeled TTR, which have been further used for the preparation of IDIF-TTR complexes labeled either on the compound or the protein. The biodistribution of all labeled species after intravenous administration has been investigated in mice using ex vivo and in vivo techniques. Our results confirm the capacity of TTR to cross the blood brain barrier (BBB) and suggest that the formation of TTR-IDIF complexes enhances BBB permeability of both IDIF and TTR. The increased TTR and IDIF brain concentrations may result in higher Aβ-peptide sequestration capacity with the subsequent inhibition of AD symptoms as we have previously observed in mice.

Tegsedi (Inotersen): An Antisense Oligonucleotide Approved for the Treatment of Adult Patients with Hereditary Transthyretin Amyloidosis

Tegsedi (Inotersen) is a chemically modified antisense oligonucleotide that inhibits the hepatic production of transthyretin (TTR). Several single-point mutations in TTR destabilize its structure, leading to the aggregation and accumulation of amyloid deposits in the nervous system, heart, kidneys and eyes. In July 2018, Tegsedi was approved by the European Commission for use in adults with stage one and two polyneuropathies. Later on, in October 2018, the FDA and Health Canada also approved its use for the treatment of the polyneuropathy of hereditary transthyretin-mediated amyloidosis (hATTR) in adults in the U.S. and Canada. Tegsedi was developed by Ionis Pharmaceuticals, the company that holds the global marketing license, together with its subsidiary Akcea Therapeutics.

Transthyretin stability is critical in assisting beta amyloid clearance- Relevance of transthyretin stabilization in Alzheimer's disease

BACKGROUND

The absence of transthyretin (TTR) in AD mice decreases brain Aβ clearance and reduces the low-density lipoprotein receptor-related protein 1 (LRP1). It is possible that neuroprotection by TTR is dependent on its tetramer structural stability, as studies using TTR mutants showed that unstable L55P TTR has low affinity for Aβ, and TTR tetrameric stabilizers such as iododiflunisal ameliorate AD features in vivo.

METHODS

We firstly investigated TTR folding status in human plasma measuring the resistance to urea denaturation. The importance of TTR stability on Aβ internalization was studied in human cerebral microvascular endothelial (hCMEC/D3) and hepatoma cells (HepG2), by flow cytometry. To investigate the fate of Aβ at the blood-brain barrier, Aβ efflux from hCMEC/D3 cells seeded on transwells was measured using ELISA. Further, to assess Aβ colocalization with lysosomes, Lysotracker was used. Moreover, levels of LRP1 were assessed in the liver and plasma of mice with different TTR backgrounds or treated with iododiflunisal.

RESULTS

We showed that TTR stability is decreased in AD and that WT TTR and drug-stabilized L55P TTR are able to increase uptake of Aβ. Furthermore, measurement of Aβ efflux showed that stable or stabilized TTR increased Aβ efflux from the basolateral to the apical side. Moreover, HepG2 cells incubated with Aβ in the presence of WT TTR, but not L55P TTR, showed an increased number of lysosomes. Further, in the presence of WT TTR, Aβ peptide colocalized with lysosomes, indicating that only stable TTR assists Aβ internalization, leading to its degradation. Finally, we demonstrated that only stable TTR can increase LRP1 levels.

CONCLUSION

TTR stabilization exerts a positive effect on Aβ clearance and LRP1 levels, suggesting that TTR protective role in AD is dependent on its stability. These results provide relevant information for the design of TTR-based therapeutic strategies for AD.

Identification of Transthyretin Fibril Formation Inhibitors Using Structure-Based Virtual Screening

Transthyretin (TTR) is the primary carrier for thyroxine (T₄ ) in cerebrospinal fluid and a secondary carrier in blood. TTR is a stable homotetramer, but certain factors, genetic or environmental, could promote its degradation to form amyloid fibrils. A docking study using crystal structures of wild-type TTR was planned; our aim was to design new ligands that are able to inhibit TTR fibril formation. The computational protocol was thought to overcome the multiple binding modes of the ligands induced by the peculiarity of the TTR binding site and by the pseudosymmetry of the site pockets, which generally weaken such structure-based studies. Two docking steps, one that is very fast and a subsequent step that is more accurate, were used to screen the Aldrich Market Select database. Five compounds were selected, and their activity toward inhibiting TTR fibril formation was assessed. Three compounds were observed to be actives, two of which have the same potency as the positive control, and the other was found to be a promising lead compound. These results validate a computational protocol that is able to archive information on the key interactions between database compounds and TTR, which is valuable for supporting further studies.

Targeting Different Transthyretin Binding Sites with Unusual Natural Compounds

Misfolding and aggregation of the transthyretin (TTR) protein leads to certain forms of amyloidosis. Some nutraceuticals, such as flavonoids and natural polyphenols, have recently been investigated as modulators of the self-assembly process of TTR, but they generally suffer from limited bioavailability. To discover innovative and more bioavailable natural compounds able to inhibit TTR amyloid formation, a docking study was performed using the crystallographic structure of TTR. This computational strategy was projected as an ad hoc inspection of the possible relationship between binding site location and modulation of the assembly process; interactions with the as-yet-unexplored epigallocatechin gallate (EGCG) sites and with the thyroxine (T4) pocket were simultaneously analyzed. All the compounds studied seem to prefer the traditional T4 binding site, but some interesting results emerged from the screening of an in-house database, used for validating the computational protocol, and of the Herbal Ingredients Targets (HIT) catalogue available on the ZINC database.

Tetrabromobisphenol A Is an Efficient Stabilizer of the Transthyretin Tetramer

Amyloid formation of the human plasma protein transthyretin (TTR) is associated with several human disorders, including familial amyloidotic polyneuropathy (FAP) and senile systemic amyloidosis. Dissociation of TTR's native tetrameric assembly is the rate-limiting step in the conversion into amyloid, and this feature presents an avenue for intervention because binding of an appropriate ligand to the thyroxin hormone binding sites of TTR stabilizes the native tetrameric assembly and impairs conversion into amyloid. The desired features for an effective TTR stabilizer include high affinity for TTR, high selectivity in the presence of other proteins, no adverse side effects at the effective concentrations, and a long half-life in the body. In this study we show that the commonly used flame retardant tetrabromobisphenol A (TBBPA) efficiently stabilizes the tetrameric structure of TTR. The X-ray crystal structure shows TBBPA binding in the thyroxine binding pocket with bromines occupying two of the three halogen binding sites. Interestingly, TBBPA binds TTR with an extremely high selectivity in human plasma, and the effect is equal to the recently approved drug tafamidis and better than diflunisal, both of which have shown therapeutic effects against FAP. TBBPA consequently present an interesting scaffold for drug design. Its absorption, metabolism, and potential side-effects are discussed.

Modifications of the 7-Hydroxyl Group of the Transthyretin Ligand Luteolin Provide Mechanistic Insights into Its Binding Properties and High Plasma Specificity

Amyloid formation of the plasma protein transthyretin (TTR) has been linked to familial amyloid polyneuropathy and senile systemic amyloidosis. Binding of ligands within its natural hormone binding site can stabilize the tetrameric structure and impair amyloid formation. We have recently shown that the flavonoid luteolin stabilizes TTR in human plasma with a very high selectivity. Luteolin, however, is inactivated in vivo via glucuronidation for which the preferred site is the hydroxy group at position 7 on its aromatic A-ring. We have evaluated the properties of two luteolin variants in which the 7-hydroxy group has been exchanged for a chlorine (7-Cl-Lut) or a methoxy group (7-MeO-Lut). Using an in vitro model, based on human liver microsomes, we verified that these modifications increase the persistence of the drug. Crystal structure determinations show that 7-Cl-Lut binds similarly to luteolin. The larger MeO substituent cannot be accommodated within the same space as the chlorine or hydroxy group and as a result 7-MeO-Lut binds in the opposite direction with the methoxy group in position 7 facing the solvent. Both 7-Cl-Lut and 7-MeO-Lut qualify as high-affinity binders, but in contrast to luteolin, they display a highly non-specific binding to other plasma components. The binding of the two conformations and the key-interactions to TTR are discussed in detail. Taken together, these results show a proof-of-concept that the persistence of luteolin towards enzymatic modification can be increased. We reveal two alternative high-affinity binding modes of luteolin to TTR and that modification in position 7 is restricted only to small substituents if the original orientation of luteolin should be preserved. In addition, the present work provides a general and convenient method to evaluate the efficacy of TTR-stabilizing drugs under conditions similar to an in vivo environment.

Tuning transthyretin amyloidosis inhibition properties of iododiflunisal by combinatorial engineering of the nonsalicylic ring substitutions

Two series of iododiflunisal and diflunisal analogues have been obtained by using a two step sequential reaction solution-phase parallel synthesis. The synthesis combined an aqueous Suzuki-Miyaura cross-coupling and a mild electrophilic aromatic iodination step using a new polymer-supported iodonium version of Barluenga's reagent. From a selected set of 77 noniodinated and 77 iodinated diflunisal analogues, a subset of good transthyretin amyloid inhibitors has been obtained with improved turbidimetry inhibition constants, high binding affinity to transthyretin, and good selectivity for TTR compared to other thyroxine binding proteins.

Novel drugs targeting transthyretin amyloidosis

Transthyretin amyloidosis (ATTR) is either a hereditary disease related to a mutation in the transthyretin gene that leads to neuropathy and/or cardiomyopathy or an acquired disease of the elderly that leads to restrictive cardiomyopathy. The prevalence of this disease is higher than once thought and awareness is likely to increase amongst physicians and in particular cardiologists. Until recently there have been no treatment options for this disease except to treat the heart failure with diuretics and the neuropathy symptomatically. However, there are several emerging pharmacologic therapies designed to slow or stop the progression of ATTR. This article reviews novel therapeutic drugs that work at different points in the pathogenesis of this disease attempting to change its natural history and improve outcomes.

An overview of drugs currently under investigation for the treatment of transthyretin-related hereditary amyloidosis

INTRODUCTION

Transthyretin (TTR)-related hereditary amyloidosis is an adult-onset, dominantly inherited, systemic neurodegenerative disease endemic in some populations. Stabilization of the native structure of TTR by small-molecule ligands has recently proved effective in slowing neurological progression. Two drugs, tafamidis and diflunisal, are now available for most patients, particularly in the early stage of the disease. However, this disorder remains life threatening with several unmet needs. There are great expectations for a number of novel agents undergoing investigation.

AREAS COVERED

The authors review the current investigational drugs for the treatment of TTR amyloidosis according to the different steps of the fibrillogenesis process they target. Innovative approaches include suppression of TTR secretion, prevention of TTR misfolding by stronger stabilizers identified through structure-based design and high-throughput screening methodologies as well as the redirection of pathogenic aggregates toward nontoxic species and reabsorption of deposits through amyloid disrupters and immunotherapy.

EXPERT OPINION

Suppression of TTR synthesis by antisense oligonucleotides and small-interfering RNA is presently one of the most promising therapeutic approaches. However, well-designed clinical trials are required to establish their safety and efficacy compared with liver transplantation, tafamidis and diflunisal. With a longer time frame, it may be possible to develop combination therapies that target multiple steps of the aggregation process that could provide the best long-life effective treatments for this devastating disease.

Inhibition of amyloid formation

Amyloid is aggregated protein in the form of insoluble fibrils. Amyloid deposition in human tissue-amyloidosis-is associated with a number of diseases including all common dementias and type II diabetes. Considerable progress has been made to understand the mechanisms leading to amyloid formation. It is, however, not yet clear by which mechanisms amyloid and protein aggregates formed on the path to amyloid are cytotoxic. Strategies to prevent protein aggregation and amyloid formation are nevertheless, in many cases, promising and even successful. This review covers research on intervention of amyloidosis and highlights several examples of how inhibition of protein aggregation and amyloid formation has been achieved in practice. For instance, rational design can provide drugs that stabilize a native folded state of a protein, protein engineering can provide new binding proteins that sequester monomeric peptides from aggregation, small molecules and peptides can be designed to block aggregation or direct it into non-cytotoxic paths, and monoclonal antibodies have been developed for therapies towards neurodegenerative diseases based on inhibition of amyloid formation and clearance.

Structure-based design of kinetic stabilizers that ameliorate the transthyretin amyloidoses

Small molecules that bind to normally unoccupied thyroxine (T(4)) binding sites within transthyretin (TTR) in the blood stabilize the tetrameric ground state of TTR relative to the dissociative transition state and dramatically slow tetramer dissociation, the rate-limiting step for the process of amyloid fibril formation linked to neurodegeneration and cell death. These so-called TTR kinetic stabilizers have been designed using structure-based principles and one of these has recently been shown to halt the progression of a human TTR amyloid disease in a clinical trial, providing the first pharmacologic evidence that the process of amyloid fibril formation is causative. Structure-based design has now progressed to the point where highly selective, high affinity TTR kinetic stabilizers that lack undesirable off-target activities can be produced with high frequency.

Crystal structure of iodotyrosine deiodinase, a novel flavoprotein responsible for iodide salvage in thyroid glands

The flavoprotein iodotyrosine deiodinase (IYD) salvages iodide from mono- and diiodotyrosine formed during the biosynthesis of the thyroid hormone thyroxine. Expression of a soluble domain of this membrane-bound enzyme provided sufficient material for crystallization and characterization by x-ray diffraction. The structures of IYD and two co-crystals containing substrates, mono- and diiodotyrosine, alternatively, were solved at resolutions of 2.0, 2.45, and 2.6 A, respectively. The structure of IYD is homologous to others in the NADH oxidase/flavin reductase superfamily, but the position of the active site lid in IYD defines a new subfamily within this group that includes BluB, an enzyme associated with vitamin B(12) biosynthesis. IYD and BluB also share key interactions involving their bound flavin mononucleotide that suggest a unique catalytic behavior within the superfamily. Substrate coordination to IYD induces formation of an additional helix and coil that act as an active site lid to shield the resulting substrate.flavin complex from solvent. This complex is stabilized by aromatic stacking and extensive hydrogen bonding between the substrate and flavin. The carbon-iodine bond of the substrate is positioned directly over the C-4a/N-5 region of the flavin to promote electron transfer. These structures now also provide a molecular basis for understanding thyroid disease based on mutations of IYD.

Iodine atoms: a new molecular feature for the design of potent transthyretin fibrillogenesis inhibitors

The thyroid hormone and retinol transporter protein known as transthyretin (TTR) is in the origin of one of the 20 or so known amyloid diseases. TTR self assembles as a homotetramer leaving a central hydrophobic channel with two symmetrical binding sites. The aggregation pathway of TTR into amiloid fibrils is not yet well characterized but in vitro binding of thyroid hormones and other small organic molecules to TTR binding channel results in tetramer stabilization which prevents amyloid formation in an extent which is proportional to the binding constant. Up to now, TTR aggregation inhibitors have been designed looking at various structural features of this binding channel others than its ability to host iodine atoms. In the present work, greatly improved inhibitors have been designed and tested by taking into account that thyroid hormones are unique in human biochemistry owing to the presence of multiple iodine atoms in their molecules which are probed to interact with specific halogen binding domains sitting at the TTR binding channel. The new TTR fibrillogenesis inhibitors are based on the diflunisal core structure because diflunisal is a registered salicylate drug with NSAID activity now undergoing clinical trials for TTR amyloid diseases. Biochemical and biophysical evidence confirms that iodine atoms can be an important design feature in the search for candidate drugs for TTR related amyloidosis.

High quality binding modes in docking ligands to proteins

Multiple near-optimal conformations of protein-ligand complexes provide a better chance for accurate representation of biomolecular interactions, compared with a single structure. We present ISE-dock--a docking program which is based on the iterative stochastic elimination (ISE) algorithm. ISE eliminates values that consistently lead to the worst results, thus optimizing the search for docking poses. It constructs large sets of such poses with no additional computational cost compared with single poses. ISE-dock is validated using 81 protein-ligand complexes from the PDB and its performance was compared with those of Glide, GOLD, and AutoDock. ISE-dock has a better chance than the other three to find more than 60% top single poses under RMSD = 2.0 A and more than 80% under RMSD = 3.0 A from experimental. ISE alone produced at least one 3.0 A or better solutions among the top 20 poses in the entire test set. In 98% of the examined molecules, ISE produced solutions that are closer than 2.0 A from experimental. Paired t-tests (PTT) were used throughout to assess the significance of comparisons between the performances of the different programs. ISE-dock provides more than 100-fold docking solutions in a similar time frame as LGA in AutoDock. We demonstrate the usefulness of the large near optimal populations of ligand poses by showing a correlation between the docking results and experiments that support multiple binding modes in p38 MAP kinase (Pargellis et al., Nat Struct Biol 2002;9:268-272] and in Human Transthyretin (Hamilton, Benson, Cell Mol Life Sci 2001;58:1491-1521).

Heart transplantation for homozygous familial transthyretin (TTR) V122I cardiac amyloidosis

Heart failure is the usual cause of death in patients with amyloid cardiomyopathy. The commonest form of hereditary cardiac amyloidosis is associated with the Val122Ile variant of transthyretin (TTR), which is carried by 3-4% of the African American population. Here, we report the outcome of the first cardiac transplantation in a patient with TTR V122I. A 59-year-old Caribbean man presented with biventricular failure. Other than previous bilateral carpel tunnel syndrome, he had been well and had no evidence of extracardiac amyloidosis. An endomyocardial biopsy demonstrated amyloid of TTR type. Sequencing of TTR gene indicated homozygosity for V122I. He underwent cardiac transplantation and 3 years later, remains well with no evidence of allograft or systemic amyloid deposition.

Iodination of salicylic acid improves its binding to transthyretin

Transthyretin (TTR) is a plasma homotetrameric protein associated with senile systemic amyloidosis and familial amyloidotic polyneuropathy. In theses cases, TTR dissociation and misfolding induces the formation of amyloidogenic intermediates that assemble into toxic oligomeric species and lead to the formation of fibrils present in amyloid deposits. The four TTR monomers associate around a central hydrophobic channel where two thyroxine molecules can bind simultaneously. In each thyroxine binding site there are three pairs of symmetry related halogen binding pockets which can accommodate the four iodine substituents of thyroxine. A number of structurally diverse small molecules that bind to the TTR channel increasing the protein stability and thereafter inhibiting amyloid fibrillogenesis have been tested. In order to take advantage of the high propensity to interactions between iodine substituents and the TTR channel we have identified two iodinated derivatives of salicylic acid, 5-iodosalicylic acid and 3,5-diiodosalicylic acid, available commercially. We report in this paper the relative binding affinities of salicylic acid and the two iodinated derivatives and the crystal structure of TTR complexed with 3,5-diiodosalicylic acid, to elucidate the higher binding affinity of this compound towards TTR.

Design of mechanism-based inhibitors of transthyretin amyloidosis: studies with biphenyl ethers and new structural templates

Transthyretin (TTR), a tetrameric thyroxine (T4) carrier protein, is associated with a variety of amyloid diseases. In this study, we explore the potential of biphenyl ethers (BPE), which are shown to interact with a high affinity to its T4 binding site thereby preventing its aggregation and fibrillogenesis. They prevent fibrillogenesis by stabilizing the tetrameric ground state of transthyretin. Additionally, we identify two new structural templates (2-(5-mercapto-[1,3,4]oxadiazol-2-yl)-phenol and 2,3,6-trichloro-N-(4H-[1,2,4]triazol-3-yl) represented as compounds 11 and 12, respectively, throughout the manuscript) exhibiting the ability to arrest TTR amyloidosis. The dissociation constants for the binding of BPEs and compound 11 and 12 to TTR correlate with their efficacies of inhibiting amyloidosis. They also have the ability to inhibit the elongation of intermediate fibrils as well as show nearly complete (>90%) disruption of the preformed fibrils. The present study thus establishes biphenyl ethers and compounds 11 and 12 as very potent inhibitors of TTR fibrillization and inducible cytotoxicity.

Models for binding cooperativities of inhibitors with transthyretin

Here, molecular dynamics (MD) simulations are performed to study the differences of binding channel shapes of TTR with two inhibitors, flufenamic acid (FLU) and one kind of N-phenyl phenoxazine (BPD). The asymmetries of global structure including the central binding channel are found to be intrinsic. Moreover, the conformational changes of the binding channel are responsible for negative cooperativity (NC) or independent cooperativity (IC) of ligands. The results suggested a possible binding mechanism addressing NC of FLU and IC of BPD. For FLU, when the first ligand binds with TTR, it leads to expansion of the second binding site which may weaken the interaction of the second FLU with TTR. But for BPD, the first ligand's binding changes the second site's shape slightly, the second ligand has similar binding ability with TTR in the second site like the first binding event.

Genetic microheterogeneity of human transthyretin detected by IEF

Mutations of the human transthyretin (TTR) gene have attracted medical interest as a cause of amyloidosis. Recently, we have described in detail an electrophoretic procedure with PAGE followed by IEF in urea gradients for the study of the microheterogeneity of TTR monomers (Altland, K., Winter, P., Sauerborn, M. K., Electrophoresis 1999, 20, 1349-1364). In this paper, we present a study on 49 different mutations of TTR including 33 that result in electrically neutral amino acid substitutions. The aims of the investigation were to test the sensitivity of the procedure to detect TTR variants in patients with TTR amyloidosis and their relatives and to identify some common characteristics that could explain the amyloidogenicity of these variants. We found that all tested amyloidogenic mutations could be detected by our method with the exception of those for which the corresponding variant was absent in plasma samples. Most of the electrically neutral amyloidogenic TTR variants had in common a reduced conformational stability of monomers by the activity of protons and urea. For three variants, e.g. TTR-F64L, TTR-I107V and TTR-V122I, the monomers had a conformational stability close to that of normal monomers but we found experimental and structural arguments for a weakening of the monomer-monomer contact. All types of amyloidogenic mutations affected the stability of TTR tetramers.

Structural basis for the protective role of sulfite against transthyretin amyloid formation

Transthyretin (TTR) is a plasma protein, which under conditions not yet completely understood, aggregates forming amyloid deposits that occur extracellularly. It is a protein composed of four identical subunits. Each monomer has a single cysteine residue (Cys10), which in the plasma is reduced (Cys-SH), oxidized (Cys-SO3-), sulfonated (Cys-S-SO3-) or bound to various sulfhydryls. There is evidence that these chemical modifications of the SH group alter the stability and the amyloidogenic potential of the protein. The sulfonated form was found to enhance the stability of the native conformation of TTR, avoiding misassembly of the protein leading to amyloid. Consequently, the potential treatment of TTR-type amyloidosis by sulfite has been suggested. The structure of TTR pre-incubated with sulfite at physiological pH, was determined by X-ray crystallography to provide structural insight for the stabilizing effect of sulfite. Each subunit has a beta-sandwich conformation, with two four stranded beta-pleated sheets (DAGH and CBEF) and a small alpha-helix between strands. The sulfonated cysteines have two sulfite oxygens involved in intramonomer hydrogen bonds that bridge Cys10, the amino acid immediately before beta-strand A, to the amino acids immediately after the edge beta-strand D. Implications of the newly observed interactions in the inhibition of fibril formation are discussed in light of the recent structural models of TTR amyloid fibrils.

Structural and functional analysis of PucM, a hydrolase in the ureide pathway and a member of the transthyretin-related protein family

The ureide pathway, which produces ureides from uric acid, is an essential purine catabolic process for storing and transporting the nitrogen fixed in leguminous plants and some bacteria. PucM from Bacillus subtilis was recently characterized and found to catalyze the second reaction of the pathway, hydrolyzing 5-hydroxyisourate (HIU), a product of uricase in the first step. PucM has 121 amino acid residues and shows high sequence similarity to the functionally unrelated protein transthyretin (TTR), a thyroid hormone-binding protein. Therefore, PucM belongs to the TTR-related proteins (TRP) family. The crystal structures of PucM at 2.0 A and its complexes with the substrate analogs 8-azaxanthine and 5,6-diaminouracil reveal that even with their overall structure similarity, homotetrameric PucM and TTR are completely different, both in their electrostatic potential and in the size of the active sites located at the dimeric interface. Nevertheless, the absolutely conserved residues across the TRP family, including His-14, Arg-49, His-105, and the C-terminal Tyr-118-Arg-119-Gly-120-Ser-121, indeed form the active site of PucM. Based on the results of site-directed mutagenesis of these residues, we propose a possible mechanism for HIU hydrolysis. The PucM structure determined for the TRP family leads to the conclusion that diverse members of the TRP family would function similarly to PucM as HIU hydrolase.

The binding of xanthone derivatives to transthyretin

A series of xanthone derivatives, isolated from Calophyllum teysmannii var. inophylloide, have been evaluated for their binding affinity to transthyretin. Transthyretin is a plasma protein involved in the transport of thyroxine (T4) and also implicated in amyloid diseases. Using competition-binding studies with the protein natural ligand T4, we have identified one prenylated xanthone with a very strong affinity to transthyretin. Molecular docking simulations show that the flexible tail of the prenylated xanthone could allow favorable molecular interactions. Since this xanthone may play a role in the thyroxine metabolism and/or over the pathogenic process associated with the amyloid disease, these results may be explored for the design of new ligands.

The effect of iodide and chloride on transthyretin structure and stability

Transthyretin amyloid formation occurs through a process of tetramer destabilization and partial unfolding. Small molecules, including the natural ligand thyroxine, stabilize the tetrameric form of the protein, and serve as inhibitors of amyloid formation. Crucial for TTR's ligand-binding properties are its three halogen-binding sites situated at the hormone-binding channel. In this study, we have performed a structural characterization of the binding of two halides, iodide and chloride, to TTR. Chlorides are known to shield charge repulsions at the tetrameric interface of TTR, which improve tetramer stability of the protein. Our study shows that iodides, like chlorides, provide tetramer stabilization in a concentration-dependent manner and at concentrations approximately 15-fold below that of chlorides. To elucidate binding sites of the halides, we took advantage of the anomalous scattering of iodide and used the single-wavelength anomalous dispersion (SAD) method to solve the iodide-bound TTR structure at 1.8 A resolution. The structure of chloride-bound TTR was determined at 1.9 A resolution using difference Fourier techniques. The refined structures showed iodides and chlorides bound at two of the three halogen-binding sites located at the hydrophobic channel. These sites therefore also function as halide-binding sites.