Manganese(III) porphyrin oligomers as high-relaxivity MRI contrast agents

Manganese(III) porphyrins (Mn^IIIPs) as MRI contrast agents (CAs) have drawn particular attention due to their high longitudinal relaxivity (r₁) and unique biodistribution. In this work, two Mn^IIIP-based oligomers, MnPD and MnPT, were designed to further improve the relaxivity with ease of synthesis. The two compounds were fully characterized and their nuclear magnetic relaxation dispersion (NMRD) profiles were acquired with a fast field cycling NMR relaxometer. Both of the compounds exhibited extended high molar r₁ at high fields, higher than that of Gd-DTPA, the first clinical gadolinium(III)-based MRI CA. The r₁ value of per manganese atom increased with the increasing number of Mn^IIIP building blocks, suggesting rotational correlation time (τ_R) played dominant role in the r₁ dispersion. The toxicity of the two Mn^IIIPs and the imaging effectiveness were estimated in vitro and in vivo. With good biocompatibility, significant contrast enhancement, and complete excretion in 24 h, MnPD and MnPT are both promising for high field clinical applications. The applied strategy also potentially provided a facile approach for creation of more Mn^IIIP oligomer as efficient T₁ MRI CAs.