Nano Assembly and Encapsulation; A Versatile Platform for Slowing the Rotation of Polyanionic Gd3+-Based MRI Contrast Agents

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 on 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