ParaCEST Agents Encapsulated in Reverse Nano-Assembled Capsules (RACs): How Slow Molecular Tumbling Can Quench CEST Contrast.

Annah Farashishiko,Mauro Botta,Jacqueline R Slack,Mark Woods

doi:10.3389/fchem.2018.00096

Abstract

Although paraCEST is a method with immense scope for generating image contrast in MRI, it suffers from the serious drawback of high detection limits. For a typical discrete paraCEST agent the detection limit is roughly an order of magnitude higher than that of a clinically used relaxation agent. One solution to this problem may be the incorporation of a large payload of paraCEST agents into a single macromolecular agent. Here we report a new synthetic method for accomplishing this goal: incorporating a large payload of the paraCEST agent DyDOTAM3+ into a Reverse Assembled nano-Capsule. An aggregate can be generated between this chelate and polyacrylic acid (PAA) after the addition of ethylene diamine. Subsequent addition of polyallylamine hydrochloride (PAH) followed by silica nanoparticles generated a robust encapsulating shell and afforded capsule with a mean hydrodynamic diameter of 650 ± 250 nm. Unfortunately this encapsulation did not have the effect of amplifying the CEST effect per agent, but quenched the CEST altogether. The quenching effect of encapsulation could be attributed to the effect of slowing molecular tumbling, which is inevitable when the chelate is incorporated into a nano-scale material. This increases the transverse relaxation rate of chelate protons and a theoretical examination using Solomon Bloembergen Morgan theory and the Bloch equations shows that the increase in the transverse relaxation rate constant for the amide protons, in even modestly sized nano-materials, is sufficient to significantly quench CEST.

Highlights

In the nearly two decades since it was first demonstrated (Ward et al, 2000), exogenous Chemical Exchange Saturation Transfer (CEST) has become a topic of intense research interest
It is significant to note that the rate at which water molecules were able to diffuse between the solvent and the capsule interior was found to decrease with increasing size for both Nano-Assembled Capsules (NACs) and Cross-linked nano-Assembled Capsules (CACs) (Farashishiko et al, 2016, 2017)
The Reversed nano-Assembled Capsules (RACs) formed here are comparable in size to their NAC counterparts formed with R = 0.5, it seems reasonable to presume that the rate at which water exchanges in and out of these capsules is comparably rapid (Farashishiko et al, 2016)

Summary

INTRODUCTION

In the nearly two decades since it was first demonstrated (Ward et al, 2000), exogenous Chemical Exchange Saturation Transfer (CEST) has become a topic of intense research interest. When the resonance frequency of protons on the CEST agent is close to that of solvent water a significant amount of off-resonance direct saturation can occur This undesirable effect interferes with the observation of CEST but can be minimized by increasing the shift separation between the two exchanging pools (Zhang et al, 2003; Woods et al, 2006). ParaCEST agents are commonly derived from neutral ligands, such as the tetraamide derivatives of DOTA, and are intrinsically positively charged unless strategies are employed to alter the overall charge In this context we asked the question: can the electrostatic charges that hold these nano-capsule systems be reversed to permit the delivery of a large payload of polycationic (rather than polyanionic) chelates?.

RESULTS AND DISCUSSION

Evaluation of CEST Properties

CONCLUSIONS