Two Sides of the Same Coin: Transthyretin (TTR) as a Target or Drug Carrier for Drug (Bio)conjugates

Transthyretin (TTR) is a tetrameric protein present in plasma and cerebrospinal fluid that binds to thyroxine (T4) and retinol (vitamin A) to transport them across the blood–retina barrier and to the liver. Mutations on the TTR gene cause destabilization of the tetramer structure leading to misfolded monomers and aggregates, thus triggering several pathologies (i.e., cardiomyopathy and neurodegeneration). The stabilization of TTR tetramer architecture and the silencing of TTR gene expression represent viable therapeutic strategies for amyloidosis. Moreover, the TTR role as a delivery system using drug (bio)conjugates has increasingly been interrogated over the last years to facilitate the transport of different drugs displaying poor pharmacokinetic properties. In this Perspective, we highlight TTR two-faced features as a drug target and carrier, reporting the latest findings in TTR stabilization and its involvement as a drug carrier for the selective drug release on different receptors and cells, thus providing insights for future medicinal chemistry applications.