Recent studies on the aggregation pathway of TTR into amyloid fibrils point to a fibrillogenesis model which involves several steps such as dissociation of the tetramer, changes on monomer conformation, aggregation of conformationally modified monomers into non-fibrillar oligomers that latter form protofibrils and further elongate into mature fibrils. This mechanism along with the fact that binding of thyroid hormones to TTR results in tetramer stabilization, suggests that inhibition of amyloid fibril formation can be accomplished by small molecule compounds sharing structural similarities with T4. Indeed this Rapamycin hypothesis has been confirmed by the identification of several families of compounds that, by binding to TTR, stabilize the ground state of the protein to an extent which is proportional to the dissociation constants. The most common molecular features on this range of inhibitors is that they are composed of two aromatic rings bearing halogen substituents in one moiety and hydrophilic functions in the second which give rise to structures as diverse as tetrahydroquinolines, dihydropyridines, benzodiazepines, phenoxazines, stilbenes and benzoxazoles. Thyroid hormones are the only human biochemicals presenting multiple iodine atoms in their molecules. Blake and co-workers were the first to describe that in each TTR binding site there are six pockets capable of accomodate an iodine atom. Indeed, when T4 binds TTR, four of these six pockets become occupied by the iodine atoms of the hormone molecule resulting in a close steric fit between the ligand and the binding site. Therefore, iodine atoms are crucial for the binding mode of thyroid hormones to TTR, making an important contribution to the protein-hormone interactions that stabilise the complex. In spite of this evidence, up to our knowledge, none of the potential newly designed TTR amyloid inhibitors have taken advantage of the potential benefits of incorporating iodine atoms to mimick the iodine-assisted binding mode of thyroid hormones. Accordingly, the aim of the present investigation was to provide initial evidences for the hypothesis that iodine atom addition to already known TTR inhibitors could produce more potent TTR fibrillogenesis inhibitors. Salicylates look particularly interesting as drug candidates due to their long therapeutic tradition and wide clinical applications. Owing that a number of salicylate analogues have also been postulated as good TTR amyloid inhibitors and because the salicylic core is RWJ 64809 amenable to electrophilic iodination, a salicylate was chosen as a model template to test this hypothesis. The positioning of iododiflunisal in the TTR channel is exclusively in the forward mode, this is, with the difluorophenyl ring occupying the inner part of the cavity and the salicylic ring the outer part. This is a common feature among other inhibitors having a biphenyl core molecule. The same forward mode is also the single disposition that is seen in both 23b and 22b structures which show almost coincident spatial ring disposition. In both cases, the compounds are located further inside in the cavity than iododiflunisal. In sharp contrast, diflunisal is observed in the pocket sharing two orientations with equal probabilities, the one described as forward and a totally opposite where the rings swap positions that is called reverse mode. The iodine atom in the iododiflunisal complex establishes close hydrophobic interactions with Leu17, Thr106, Ala108, Thr119 and Val121, thus, occupying the HBP1 pocket which is the outermost and more hydrophobic HBP. The innermost HBP pockets, HBP3 and HBP39, in turn, closely interact with the fluorine atoms of the difluorophenyl ring. A further stabilizing interaction is found between the carbonyl group of Thr106 and iodine which closely resembles an halogen bond. Similar but more optimized interactions than in 
the iododiflunisal complex are observed for the iodine atom in both crystal structures of 23b and 22b complexes.