Human Transthyretin with a Tailored Gd(III) Complex as a High-Relaxivity MRI Contrast Agent

Protein-based biomaterials are attractive platforms for medical applications, as they can combine drug delivery, targeting, and diagnostic imaging. In this work, human transthyretin (TTR), a 55 kDa homotetrameric plasma protein with drug-carrier potential, was functionalized with the paramagnetic complex Gd-C4-IA to generate a protein-based MRI contrast agent. The gadolinium(III) complex features a DOTA-like macrocyclic scaffold with a propionic carboxyamide arm for fast water exchange and an iodoacetamide linker for covalent attachment to the protein cysteine residue. Successful conjugation was confirmed via ¹H NMR paramagnetic broadening and ICP-MS. Analysis of the ¹H nuclear magnetic relaxation dispersion (NMRD) profiles at 25 and 37 °C indicated a substantial enhancement in relaxivity relative to that of the free complex. Quantitative analysis reveals that the relaxation mechanism is dominated by nanosecond reorientation times, optimized for clinical magnetic fields, though partially averaged by fast local dynamics of the paramagnetic tag. The combination of fast water exchange, favorable rotational dynamics, and multivalent Gd(III) loading accounts for the remarkable relaxivity gain. Given the physiological role of TTR, this conjugate represents a promising biocompatible platform for combined diagnostic and drug delivery payloads. Furthermore, these data underscore the potential of Gd-C4-IA as a highly efficient paramagnetic tag for functionalizing protein-based biomaterials.