Synthesis and PET Imaging Biodistribution Studies of Radiolabeled Iododiflunisal, a Transthyretin Tetramer Stabilizer, Candidate Drug for Alzheimer's Disease.

The small-molecule iododiflunisal (IDIF) is a transthyretin (TTR) tetramer stabilizer and acts as a chaperone of the TTR-Amyloid beta interaction. Oral administration of IDIF improves Alzheimer's Disease (AD)-like pathology in mice, although the mechanism of action and pharmacokinetics remain unknown. Radiolabeling IDIF with positron or gamma emitters may aid in the in vivo evaluation of IDIF using non-invasive nuclear imaging techniques. In this work, we report an isotopic exchange reaction to obtain IDIF radiolabeled with 18F. [19F/18F]exchange reaction over IDIF in dimethyl sulfoxide at 160 °C resulted in the formation of [18F]IDIF in 7 ± 3% radiochemical yield in a 20 min reaction time, with a final radiochemical purity of >99%. Biodistribution studies after intravenous administration of [18F]IDIF in wild-type mice using positron emission tomography (PET) imaging showed capacity to cross the blood-brain barrier (ca. 1% of injected dose per gram of tissue in the brain at t > 10 min post administration), rapid accumulation in the liver, long circulation time, and progressive elimination via urine. Our results open opportunities for future studies in larger animal species or human subjects.

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-ß-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.

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.

Optimization of kinetic stabilizers of tetrameric transthyretin: A prospective ligand efficiency-guided approach.

In the past few years, attempts have been made to use decision criteria beyond Lipinski's guidelines (Rule of five) to guide drug discovery projects more effectively. Several variables and formulations have been proposed and investigated within the framework of multiparameter optimization methods to guide drug discovery. In this context, the combination of Ligand Efficiency Indices (LEI) has been predominantly used to map and monitor the drug discovery process in a retrospective fashion. Here we provide an example of the use of a novel application of the LEI methodology for prospective lead optimization by using the transthyretin (TTR) fibrillogenesis inhibitor iododiflunisal (IDIF) as example. Using this approach, a number of compounds with theoretical efficiencies higher than the reference compound IDIF were identified. From this group, ten compounds were selected, synthesized and biologically tested. Half of the compounds (5, 6, 7, 8 and 10) showed potencies in terms of IC50 inhibition of TTR aggregation equal or higher than the lead compound. These optimized compounds mapped within the region of more efficient candidates in the corresponding experimental nBEI-NSEI plot, matching their position in the theoretical optimization plane that was used for the prediction. Due to their upstream (North-Eastern) position in the progression lines of NPOL = 3 or 4 of the nBEI-NSEI plot, three of them (5, 6 and 8) are more interesting candidates than iododiflunisal because they have been optimized in the three crucial LEI variables of potency, size and polarity at the same time. This is the first example of the effectiveness of using the combined LEIs within the decision process to validate the application of the LEI formulation for the prospective optimization of lead compounds.

Oral Treatment with Iododiflunisal Delays Hippocampal Amyloid-ß Formation in a Transgenic Mouse Model of Alzheimer's Disease: A Longitudinal in vivo Molecular Imaging Study1.

BACKGROUND: Transthyretin (TTR) is a tetrameric, amyloid-ß (Aß)-binding protein, which reduces Aß toxicity. The TTR/Aß interaction can be enhanced by a series of small molecules that stabilize its tetrameric form. Hence, TTR stabilizers might act as disease-modifying drugs in Alzheimer's disease. OBJECTIVE: We monitored the therapeutic efficacy of two TTR stabilizers, iododiflunisal (IDIF), which acts as small-molecule chaperone of the TTR/Aß interaction, and tolcapone, which does not behave as a small-molecule chaperone, in an animal model of Alzheimer's disease using positron emission tomography (PET). METHODS: Female mice (AßPPswe/PS1A246E/TTR+/-) were divided into 3 groups (n = 7 per group): IDIF-treated, tolcapone-treated, and non-treated. The oral treatment (100 mg/Kg/day) was started at 5 months of age. Treatment efficacy assessment was based on changes in longitudinal deposition of Aß in the hippocampus (HIP) and the cortex (CTX) and determined using PET-[18F]florbetaben. Immunohistochemical analysis was performed at age = 14 months. RESULTS: Standard uptake values relative to the cerebellum (SUVr) of [18F]florbetaben in CTX and HIP of non-treated animals progressively increased from age = 5 to 11 months and stabilized afterwards. In contrast, [18F]florbetaben uptake in HIP of IDIF-treated animals remained constant between ages = 5 and 11 months and significantly increased at 14 months. In the tolcapone-treated group, SUVr progressively increased with time, but at lower rate than in the non-treated group. No significant treatment effect was observed in CTX. Results from immunohistochemistry matched the in vivo data at age = 14 months. CONCLUSION: Our work provides encouraging preliminary results on the ability of small-molecule chaperones to ameliorate Aß deposition in certain brain regions.

Calorimetric Studies of Binary and Ternary Molecular Interactions between Transthyretin, Aß 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 (Aß) 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, Aß(1-42) peptide, and TTR stabilizers using isothermal titration calorimetry (ITC). A TTR/Aß(1-42) (1:1) complex with a dissociation constant of Kd = 0.94 µM is formed; with IDIF (4), this constant improves up to Kd = 0.32 µM, indicating the presence of a ternary complex TTR/IDIF/Aß(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 Aß(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 Aß-peptides and prevent Aß 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 Aß-peptide sequestration capacity with the subsequent inhibition of AD symptoms as we have previously observed in mice.

Peptide Half-Life Extension: Divalent, Small-Molecule Albumin Interactions Direct the Systemic Properties of Glucagon-Like Peptide 1 (GLP-1) Analogues.

Noncovalent binding of biopharmaceuticals to human serum albumin protects against enzymatic degradation and renal clearance. Herein, we investigated the effect of mono- or divalent small-molecule albumin binders for half-life extension of peptides. For proof-of-principle, the clinically relevant glucagon-like peptide 1 (GLP-1) was functionalized with diflunisal, indomethacin, or both. In vitro, all GLP-1 analogues had subnanomolar GLP-1 receptor potency. Surface plasmon resonance revealed that both small molecules were able to confer albumin affinity to GLP-1 and indicated that affinity is increased for divalent analogues. In lean mice, the divalent GLP-1 analogues were superior to monovalent analogues with respect to control of glucose homeostasis and suppression of food intake. Importantly, divalent GLP-1 analogues showed efficacy comparable to liraglutide, an antidiabetic GLP-1 analogue that carries a long-chain fatty acid. Finally, pharmacokinetic investigations of a divalent GLP-1 analogue demonstrated a promising gain in circulatory half-life and absorption time compared to its monovalent equivalent.

Insights on the Interaction between Transthyretin and Aß in Solution. A Saturation Transfer Difference (STD) NMR Analysis of the Role of Iododiflunisal.

Several strategies against Alzheimer disease (AD) are directed to target Aß-peptides. The ability of transthyretin (TTR) to bind Aß-peptides and the positive effect exerted by some TTR stabilizers for modulating the TTR-Aß interaction have been previously studied. Herein, key structural features of the interaction between TTR and the Aß(12-28) peptide (3), the essential recognition element of Aß, have been unravelled by STD-NMR spectroscopy methods in solution. Molecular aspects related to the role of the TTR stabilizer iododiflunisal (IDIF, 5) on the TTR-Aß complex have been also examined. The NMR results, assisted by molecular modeling protocols, have provided a structural model for the TTR-Aß interaction, as well as for the ternary complex formed in the presence of IDIF. This basic structural information could be relevant for providing light on the mechanisms involved in the ameliorating effects of AD symptoms observed in AD/TTR± animal models after IDIF treatment and eventually for designing new molecules toward AD therapeutic drugs.

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 Aß 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 Aß, 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 Aß internalization was studied in human cerebral microvascular endothelial (hCMEC/D3) and hepatoma cells (HepG2), by flow cytometry. To investigate the fate of Aß at the blood-brain barrier, Aß efflux from hCMEC/D3 cells seeded on transwells was measured using ELISA. Further, to assess Aß 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 Aß. Furthermore, measurement of Aß efflux showed that stable or stabilized TTR increased Aß efflux from the basolateral to the apical side. Moreover, HepG2 cells incubated with Aß in the presence of WT TTR, but not L55P TTR, showed an increased number of lysosomes. Further, in the presence of WT TTR, Aß peptide colocalized with lysosomes, indicating that only stable TTR assists Aß internalization, leading to its degradation. Finally, we demonstrated that only stable TTR can increase LRP1 levels. CONCLUSION: TTR stabilization exerts a positive effect on Aß 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.

Synthesis of novel diflunisal hydrazide-hydrazones as anti-hepatitis C virus agents and hepatocellular carcinoma inhibitors.

Hepatitis C virus (HCV) infection is a main cause of chronic liver disease, leading to liver cirrhosis and hepatocellular carcinoma (HCC). The objective of our research was to develop effective agents against viral replication. We have previously identified the hydrazide-hydrazone scaffold as a promising hepatitis C virus (HCV) and hepatocelluler inhibitor. Herein we describe the design a number of 2',4'-difluoro-4-hydroxy-N'-(arylmethylidene) biphenyl-3-carbohydrazide (3a-t) as anti-HCV and anticancer agents. Results from evaluation of anti-HCV activity indicated that most of the synthesized hydrazone derivatives inhibited viral replication in the Huh7/Rep-Feo1b and Huh 7.5-FGR-JCI-Rluc2A reporter systems. Antiproliferative activities of increasing concentrations of 2',4'-difluoro-4-hydroxy-N'-(2-pyridyl methylidene)biphenyl-3-carbohydrazide 3b and diflunisal (2.5-40 µM) were assessed in liver cancer cell lines (Huh7, HepG2, Hep3B, Mahlavu, FOCUS and SNU-475) with sulforhodamine B assay for 72 h. Compound 3b with 2-pyridinyl group in the hydrazone part exhibited promising cytotoxic activity against all cell lines with IC50 values of 10, 10.34 16.21 4.74, 9.29 and 8.33 µM for Huh7, HepG2, Hep3B, Mahlavu, FOCUS and SNU-475 cells, respectively, and produced dramatic cell cycle arrest at SubG1/G0 phase as an indicator of apoptotic cell death induction.

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.

Transthyretin stabilization by iododiflunisal promotes amyloid-ß peptide clearance, decreases its deposition, and ameliorates cognitive deficits in an Alzheimer's disease mouse model.

Alzheimer's disease (AD) is the most common form of dementia and now represents 50-70% of total dementia cases. Over the last two decades, transthyretin (TTR) has been associated with AD and, very recently, a novel concept of TTR stability has been established in vitro as a key factor in TTR/amyloid-ß (Aß) interaction. Small compounds, TTR stabilizers (usually non-steroid anti-inflammatory drugs), bind to the thyroxine (T4) central binding channel, increasing TTR tetrameric stability and TTR/Aß interaction. In this work, we evaluated in vivo the effects of one of the TTR stabilizers identified as improving TTR/Aß interaction, iododiflunisal (IDIF), in Aß deposition and other AD features, using AßPPswe/PS1A246E transgenic mice, either carrying two or just one copy of the TTR gene (AD/TTR+/+ or AD/TTR+/-, respectively), available and characterized in our laboratory. The results showed that IDIF administered orally bound TTR in plasma and stabilized the protein, as assessed by T4 displacement assays, and was able to enter the brain as revealed by mass spectrometry analysis of cerebrospinal fluid. TTR levels, both in plasma and cerebrospinal fluid, were not altered. In AD/TTR+/- mice, IDIF administration resulted not only in decreased brain Aß levels and deposition but also in improved cognitive function associated with the AD-like neuropathology in this mouse model, although no improvements were detectable in the AD/TTR+/+ animals. Further, in AD/TTR+/- mice, Aß levels were reduced in plasma suggesting TTR promoted Aß clearance from the brain and from the periphery. Taken together, these results strengthen the importance of TTR stability in the design of therapeutic drugs, highlighting the capacity of IDIF to be used in AD treatment to prevent and to slow the progression of the disease.

Human serum albumin-based design of a diflunisal prodrug.

The cyclooxygenase-2 inhibitor, diflunisal, is used in the clinic for its anti-inflammatory activity. About 99% of a dose of diflunisal is unavailable for reaction with the target enzyme, because diflunisal strongly binds to human serum albumin (HSA). To reduce the binding affinity of diflunisal to albumin, we designed and synthesized the prodrug acetyldiflunisal. The crystal structure of HSA complexed with fatty acid and acetyldiflunisal revealed that acetyldiflunisal binds to the IIA subdomain and that upon binding, it acetylates lysine 199. Mass spectrometry confirmed that acetyldiflunisal acetylates Lys199. The acetylated albumin had twofold weaker binding affinity for diflunisal as demonstrated by fluorescence quenching. Reduced binding affinity means that diflunisal is more easily released from acetylated albumin into the circulation. Therefore, lower doses of acetyldiflunisal compared to diflunisal will be required. Taken together, our results not only provide a template for design of HSA-based prodrugs, but also pave the way toward more effective use of diflunisal in the clinic.

Stability of the transthyretin molecule as a key factor in the interaction with a-beta peptide--relevance in Alzheimer's disease.

Transthyretin (TTR) protects against A-Beta toxicity by binding the peptide thus inhibiting its aggregation. Previous work showed different TTR mutations interact differently with A-Beta, with increasing affinities correlating with decreasing amyloidogenecity of the TTR mutant; this did not impact on the levels of inhibition of A-Beta aggregation, as assessed by transmission electron microscopy. Our work aimed at probing differences in binding to A-Beta by WT, T119M and L55P TTR using quantitative assays, and at identifying factors affecting this interaction. We addressed the impact of such factors in TTR ability to degrade A-Beta. Using a dot blot approach with the anti-oligomeric antibody A11, we showed that A-Beta formed oligomers transiently, indicating aggregation and fibril formation, whereas in the presence of WT and T119M TTR the oligomers persisted longer, indicative that these variants avoided further aggregation into fibrils. In contrast, L55PTTR was not able to inhibit oligomerization or to prevent evolution to aggregates and fibrils. Furthermore, apoptosis assessment showed WT and T119M TTR were able to protect against A-Beta toxicity. Because the amyloidogenic potential of TTR is inversely correlated with its stability, the use of drugs able to stabilize TTR tetrameric fold could result in increased TTR/A-Beta binding. Here we showed that iododiflunisal, 3-dinitrophenol, resveratrol, [2-(3,5-dichlorophenyl)amino] (DCPA) and [4-(3,5-difluorophenyl)] (DFPB) were able to increase TTR binding to A-Beta; however only DCPA and DFPB improved TTR proteolytic activity. Thyroxine, a TTR ligand, did not influence TTR/A-Beta interaction and A-Beta degradation by TTR, whereas RBP, another TTR ligand, not only obstructed the interaction but also inhibited TTR proteolytic activity. Our results showed differences between WT and T119M TTR, and L55PTTR mutant regarding their interaction with A-Beta and prompt the stability of TTR as a key factor in this interaction, which may be relevant in AD pathogenesis and for the design of therapeutic TTR-based therapies.

Synthesis and biological evaluation of amide derivatives of diflunisal as potential anti-tumor agents.

To discover the new medicinal activity, the structure of diflunisal has been modified. Forty amide derivatives of diflunisal were synthesized starting from diflunisal in three steps. Their inhibition growth rate of human lung cancer cell (A549) and human endometrial adenocarcinoma cell (Ishikawa) in vitro was evaluated. The preliminary assay results showed that compounds 6j, 7o and 8c exhibited good anti-tumor activities and excellent selectivity for the Ishikawa cell, may be potential anti-tumor agents.

Synthesis of some novel heterocyclic compounds derived from diflunisal hydrazide as potential anti-infective and anti-inflammatory agents.

Three novel series of 2',4'-difluoro-4-hydroxybiphenyl-3-carboxylic acid derivatives namely 4-substituted-1,2,4-triazoline-3-thiones (4a-g); 2-substituted-1,3,4-thiadiazoles (5a-g) and 2-substituted-1,3,4-oxadiazoles (6a-g) have been synthesized. Twenty-one of the newly synthesized compounds were tested against various bacteria, fungi, yeast species and virus. In addition, we have replaced the carboxylic acid group of diflunisal with heterocycles and the anti-inflammatory activity of heterocycles reported here. Compound (5d) showed activity against Escherichia coli A1 and Streptococcus pyogenes ATCC-176 at a concentration of 31.25 microg/mL, whereas cefepime, the drug used as standard, has been found less active against the bacteria mentioned above. Compound (4b) has exhibited activity against Aspergillus variecolor and Trichophyton rubrum at a concentration of 31.25 and 15.25 microg/mL, whereas Amphotericin B, the drug used as standard, has been found less active against the yeast and fungi. The highest antiviral activity was found in the 1,3,4-thiadiazole derivative (5a) having a methyl group at 2nd position against Sindbis virus at 9.6 microg/mL. Compound (4c) exhibited the highest anti-inflammatory activity (73.03%) whereas diflunisal, the drug used as standard, has been found less active (24.16%). Compound (5f) presented similar antinociceptive activity with the standard drug (paw withdrawal latency was 19.21 s compared to that of diflunisal which was 19.14s, in hot plate test).

Cell based screening of inhibitors of transthyretin aggregation.

The amyloidoses are the extracellular subset of a group of diseases in which in vivo protein misfolding leads to a pathologic gain of function, i.e., aggregation leading to protein deposition, with subsequent tissue damage. Wild-type and mutant transthyretins (TTR) are the etiologic agents in prototypic systemic amyloidoses. We describe a cell-based assay that measures the cytotoxicity of physiologic concentrations of the amyloidogenic Val30Met TTR variant (V30M TTR) using cells of the same lineage as the in vivo tissue target of amyloid deposition. We have utilized the assay to screen small molecules for their capacity to inhibit the TTR-induced cell damage. We compared the inhibitory activity of each compound with its ability to prevent TTR fibril formation in vitro. Our results emphasize the importance of screening compounds under physiologic conditions. Moreover, if a common conformational intermediate is responsible for cell death in all the amyloid diseases, the cell-based assay has the potential to aid in the discovery of compounds useful in the treatment of amyloidoses caused by other misfolded proteins as well as those caused by TTR.

Allosteric modulation of [3H]EBOB binding to GABAA receptors by diflunisal analogues.

Allosteric modulatory effects of 12 biphenyl derivatives of diflunisal and two fenamates were studied on A-type receptors of GABA (GABAAR) via [3H]4'-ethynylbicycloorthobenzoate (EBOB) binding to synaptic membrane preparations of rat forebrain. A simplified ternary allosteric model was used to determine binding affinities of the compounds and the extents of cooperativity with GABA. Structure activity analysis revealed that 4-hydroxy substituents of the biphenyls contribute to their micromolar binding affinities more than 3-carboxyl groups. Electron-withdrawing fluorinated substituents, especially in ortho position, were also advantageous. These factors also strongly enhanced the cooperativity with GABA binding. The correlation between displacing potency of the allosteric agents and cooperativity with GABA suggests that these processes are associated with common mechanisms. The pharmacological relevance of these interactions is discussed. These data help to differentiate the structural requirements of these agents to act on GABAergic neurotransmission versus nonsteroidal anti-inflammatory effects.

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

Ex vivo and in vitro studies have revealed the remarkable amyloid inhibitory potency and specificity of iododiflunisal in relation to transthyretin [Almeida, Macedo, Cardoso, Alves, Valencia, Arsequell, Planas and Saraiva (2004) Biochem. J. 381, 351-356], a protein implicated in familial amyloidotic polyneuropathy. In the present paper, the crystal structure of transthyretin complexed with this diflunisal derivative is reported, which enables a detailed analysis of the protein-ligand interactions. Iododiflunisal binds very deep in the hormone-binding channel. The iodine substituent is tightly anchored into a pocket of the binding site and the fluorine atoms provide extra hydrophobic contacts with the protein. The carboxylate substituent is involved in an electrostatic interaction with the N(zeta) of a lysine residue. Moreover, ligand-induced conformational alterations in the side chain of some residues result in the formation of new intersubunit hydrogen bonds. All these new interactions, induced by iododiflunisal, increase the stability of the tetramer impairing the formation of amyloid fibrils. The crystal structure of this complex opens perspectives for the design of more specific and effective drugs for familial amyloidotic polyneuropathy patients.