Magnetic Resonance

Proton Chemical Exchange Dependent Saturation Transfer (CEST): Evaluation as a Mechanism for Non-Metal Based Exogenous MRI Contrast Agent

K.M. Ward, A.H. Aletras and R.S. Balaban
National Heart Lung and Blood Institute
National Institutes of Health, Bethesda MD, USA.

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Purpose

•To Develop an Exogenous Contrast Agent Based on Chemical Exchange Dependent Saturation Transfer.

•To Evaluate Different Chemical Exchange Sites That Could be Used in the Contrast Agents.  

General Experimental Conditions

•Spectroscopy at 300 MHz (Bruker) in 5 mm NMR tubes.

•All experiments conducted at 37^oC.

•pH buffers varied, however most comparisons made with 20 mM NaPO₄.

•Imaging conducted at 4.0 Tesla at 25 ^oC (GE whole body system).

Ideal CEST Agent

•1) Remains in slow exchange: t_caDw_ca>>1.

•2) Minimize t_ca to enhance saturation transfer:      Ms/Mo=[1-(1/t_ca)T_1sat].

•3) This suggests that the larger Dw_ca the better, since it will support a shorter t_ca without moving out of the slow exchange domain.

•4) The large Dw_ca will also improve specificity of the effect (field variations, water T_2, MT etc).

•Low toxicity and defined tissue distribution.•

Screen of Potential Exchangeable Sites

-OH: Sugars, polymers, Dextran* (small Dw (1.3-1.5ppm))
-NH2 : Amino acids (8) (tca too short at pH 7.0)
-Indole ring –NH: Nucleosides, pyrimidines, purines (good Dw 5-6.5 ppm, adjustable tca)
> 200 compounds evaluated

Barbituric Acid: Effect of Temperature

5,6 Dihydrouracil: Effect of pH

Ratiometric Determination of pH Independent of T1 or [Agent]

5,6 Dihydrouracil: Effect of [PO4]

Imaging Barbituric Acid Using CEST

Conclusions

Classes of proton exchange sites were found that could be used for markers of contrast agent concentration, pH or [PO4] using CEST. Several ring –NH exchange sites on were the most useful evaluated with large Dw and variable tca.

Current Limitations

Sensitivity: With the limit of tca and Dw the concentration requirements are on the order of 10 mM to achieve a 10% Ms effect Distribution: Most of the small molecules we have studied to date are quickly cleared to the urine. Toxicity:Relatively unknown. 50 mls of 150 mM Barbiturate has been injected in rabbits with no acute cardiovascular events.

Potential Solutions

Polymerization of exchange sites on single molecule. In practice this has been demonstrated for Dextran: 250 mM Dextrose Ms = 0.9 0.5 mM Dextran (70,000 mW) Ms = 0.85

(i.e. Efficiency is dependent on the number of sites/mole)
Increase proton chemical shift using local dipolar fields (metals). Example is the 70 ppm shift in deoxy-myoglobin. This could significantly extent the tca limit with regards the slow exchange limit. (C. Springer) Other nuclides, Xe.