Rafoxanide, a salicylanilide anthelmintic, interacts with human plasma protein transthyretin

Transthyretin (TTR) is a carrier protein for thyroid hormone thyroxine (T₄) in plasma, placental cytosol, and cerebrospinal fluid. While the potential toxicity of small molecules that compete with T₄ for binding to TTR should be carefully studied, these small molecules can also serve as anti-ATTR amyloidosis drugs by stabilizing the TTR structure. Here, we demonstrated that rafoxanide, an EU-approved anthelmintic drug for domesticated animals, binds to the T₄-binding site of TTR. An intrinsic fluorescence quenching assay showed that rafoxanide also binds to the thyroid hormone-related proteins, including serum albumin and thyroid hormone receptor β. Rafoxanide strongly inhibited TTR amyloidogenesis in fibrillization assay, but the binding of rafoxanide to TTR was interfered with in human plasma, probably due to interactions with thyroid hormone-related proteins. Protein crystallography provided clues for the optimization of binding affinity and selectivity. Our findings emphasize the importance of considering rafoxanide as both a possible thyroid-disrupting chemical and a lead compound for the development of new ATTR amyloidosis inhibitors.