Nano assembly and encapsulation; a versatile platform for slowing the rotation of polyanionic Gd3+-based MRI contrast agents

摘要

Encapsulating discrete Gd³⁺ chelates in nano-assembled capsules (NACs) is a simple and effective method of preparing an MRI contrast agent capable of delivering a large payload of high relaxivity imaging agent. The preparation of contrast agent containing NACs had previously focussed preparations incorporating GdDOTP⁵⁻ into the internal aggregate. In this report we demonstrate that other Gd³⁺ chelates bearing overall charges as low as 2- can also be used to prepare NACs. This discovery opens up the possibility of using Gd³⁺ chelates that have inner-sphere water molecules that could further increase the relaxivity enhancement associated with the long τR that arises from encapsulation. However, encapsulation of the q = 1 chelate GdDTPA²⁻ did not give rise to a significant increase in relaxivity relative to encapsulation of the outer-sphere chelate GdTTHA³⁻. This leads us to the conclusion that in the NAC interior proton transport is not mediated by movement of whole water molecules and the enhanced relaxivity of Gd³⁺ chelate encapsulated within NACs arises primarily from second sphere effects.