Biochemical and Structural Evaluation of Highly Selective 2-Arylbenzoxazole-Based Transthyretin Amyloidogenesis Inhibitors

Transthyretin: the servant of many masters

Transthyretin (TTR) (formerly, thyroxine binding prealbumin) is an evolutionarily conserved serum and cerebrospinal fluid protein that transports holo-retinol-binding protein and thyroxine. Its serum concentration has been widely used to assess clinical nutritional status. It is also well known that wild-type transthyretin and approximately 100 different mutants give rise to a variety of forms of systemic amyloid deposition. It has been suspected and recently established that TTR can suppress the Alzheimer's disease phenotype in transgenic animal models of cerebral Abeta deposition. Thus, while TTR is a systemic amyloid precursor, in the brain it seems to have an anti-amyloidogenic effect. TTR is found in other organs as a result of local synthesis or transport, suggesting that it may have other, as yet undiscovered, functions. It is possible that its capacity to bind many classes of compounds allows it to serve as an endogenous detoxifier of molecules with potential pathologic effects.

The biochemistry of disease: desperately seeking syzygy

The elucidation of the precise molecular structure and dynamics of biological processes is the great work of biochemistry. From this, insights into the changes leading to process dysfunction or disease are derived, as well as the possible approaches to restore healthy function. Translating this information into effective and safe treatments for disease requires a coordinated interdisciplinary effort, a fusion of creativity and practicality, and a healthy dose of luck. Using several reviews in this volume as springboards, I discuss the broader issues of drug development, highlighting some recent successes and future directions. Such occurrences inspire awe but remain too rare for comfort.

Structurally simple inhibitors of lanosterol 14alpha-demethylase are efficacious in a rodent model of acute Chagas disease

We report structure-activity studies of a large number of dialkyl imidazoles as inhibitors of Trypanosoma cruzi lanosterol-14alpha-demethylase (L14DM). The compounds have a simple structure compared to posaconazole, another L14DM inhibitor that is an anti-Chagas drug candidate. Several compounds display potency for killing T. cruzi amastigotes in vitro with values of EC(50) in the 0.4-10 nM range. Two compounds were selected for efficacy studies in a mouse model of acute Chagas disease. At oral doses of 20-50 mg/kg given after establishment of parasite infection, the compounds reduced parasitemia in the blood to undetectable levels, and analysis of remaining parasites by PCR revealed a lack of parasites in the majority of animals. These dialkyl imidazoles are substantially less expensive to produce than posaconazole and are appropriate for further development toward an anti-Chagas disease clinical candidate.

Toward optimization of the second aryl substructure common to transthyretin amyloidogenesis inhibitors using biochemical and structural studies

Transthyretin (TTR) amyloidogenesis inhibitors are typically composed of two aromatic rings and a linker. We have previously established optimal structures for one aromatic ring and the linker. Herein, we employ a suboptimal linker and an optimal aryl-X substructure to rank order the desirability of aryl-Z substructures--using a library of 56 N-(3,5-dibromo-4-hydroxyphenyl)benzamides. Coconsideration of amyloid inhibition potency and ex vivo plasma TTR binding selectivity data reveal that 2,6, 2,5, 2, 3,4,5, and 3,5 substituted aryls bearing small substituents generate the most potent and selective inhibitors, in descending order. These benzamides generally lack undesirable thyroid hormone receptor binding and COX-1 inhibition activity. Three high-resolution TTR.inhibitor crystal structures (1.31-1.35 A) provide insight into why these inhibitors are potent and selective, enabling future structure-based design of TTR kinetic stabilizers.

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.

Toward optimization of the linker substructure common to transthyretin amyloidogenesis inhibitors using biochemical and structural studies

To develop potent and highly selective transthyretin (TTR) amyloidogenesis inhibitors, it is useful to systematically optimize the three substructural elements that compose a typical TTR kinetic stabilizer: the two aryl rings and the linker joining them. Herein, we evaluated 40 bisaryl molecules based on 10 unique linker substructures to determine how these linkages influence inhibitor potency and selectivity. These linkers connect one unsubstituted aromatic ring to either a 3,5-X 2 or a 3,5-X 2-4-OH phenyl substructure (X = Br or CH 3). Coconsideration of amyloid inhibition and ex vivo plasma TTR binding selectivity data reveal that direct connection of the two aryls or linkage through nonpolar E-olefin or -CH 2CH 2- substructures generates the most potent and selective TTR amyloidogenesis inhibitors exhibiting minimal undesirable binding to the thyroid hormone nuclear receptor or the COX-1 enzyme. Five high-resolution TTR.inhibitor crystal structures (1.4-1.8 A) provide insight into why such linkers afford inhibitors with greater potency and selectivity.

Quantification of the thermodynamically linked quaternary and tertiary structural stabilities of transthyretin and its disease-associated variants: the relationship between stability and amyloidosis

Urea denaturation studies were carried out as a function of transthyretin (TTR) concentration to quantify the thermodynamically linked quaternary and tertiary structural stability and to improve our understanding of the relationship between mutant folding energetics and amyloid disease phenotype. Urea denaturation of TTR involves at least two equilibria: dissociation of tetramers into folded monomers and monomer unfolding. To deal with the thermodynamic linkage of these equilibria, we analyzed concentration-dependent denaturation data by globally fitting them to an equation that simultaneously accounts for the two-step denaturation process. Using this method, the quaternary and tertiary structural stabilities of well-behaved TTR sequences, wild-type (WT) TTR and the disease-associated variant V122I, were scrutinized. The V122I variant is linked to late onset familial amyloid cardiomyopathy, the most common familial TTR amyloid disease. V122I TTR exhibits a destabilized quaternary structure and a stable tertiary structure relative to those of WT TTR. Three other variants of TTR were also examined, L55P, V30M, and A25T TTR. The L55P mutation is associated with the most aggressive familial TTR amyloid disease. L55P TTR has a complicated denaturation pathway that includes dimers and trimers, so globally fitting its concentration-dependent urea denaturation data yielded error-laden estimates of stability parameters. Nevertheless, it is clear that L55P TTR is substantially less stable than WT TTR, primarily because its tertiary structure is unstable, although its quaternary structure is destabilized as well. V30M is the most common mutation associated with neuropathic forms of TTR amyloid disease. V30M TTR is certainly destabilized relative to WT TTR, but like L55P TTR, it has a complex denaturation pathway that cannot be fit to the aforementioned two-step denaturation model. Literature data suggest that V30M TTR has stable quaternary structure but unstable tertiary structure. The A25T mutant, associated with central nervous system amyloidosis, is highly aggregation-prone and exhibits drastically reduced quaternary and tertiary structural stabilities. The observed differences in stability among the disease-associated TTR variants highlight the complexity and heterogeneity of TTR amyloid disease, an observation that has important implications for the treatment of these maladies.