Paramagnetic lanthanide complexes as PARACEST agents for medical imaging

Abstract

This tutorial review examines the fundamental aspects of a new class of contrast media for MRI based upon the chemical shift saturation transfer (CEST) mechanism. Several paramagnetic versions called PARACEST agents have shown utility as responsive agents for reporting physiological or metabolic information by MRI. It is shown that basic NMR exchange theory can be used to predict how parameters such as chemical shift, bound water lifetimes, and relaxation rates can be optimized to maximize the sensitivity of PARACEST agents.

Fig. 1

The Boltzmann distribution of spins in a magnetic field results in a bulk magnetization of the sample, represented by a vector along the z-axis (left). Application of a presaturation pulse increases the number of spins aligned against the magnetic field, reducing the magnitude of the bulk magnetization vector. This ultimately reduces the signal intensity of an NMR or MRI experiment.

Fig. 2

A presaturation pulse applied to Pool B alters the distribution of spins in this pool. If the R₁ of Pool B is fast relative to the rate of chemical exchange with Pool A then the spins promoted to the high energy state will relax back to the equilibrium Boltzmann distribution and a normal NMR spectrum will be obtained. However, if chemical exchange is the faster process then the population densities of the high energy level will equilibrate altering the distribution of spins in Pool A, reducing the bulk magnetization of this pool. The result is that not only is the signal intensity of Pool B reduced but also that of Pool A.

Fig. 3

CEST spectra of 125 mM (blue), 62.5 mM (red), and 31.25 mM (green) solutions of barbituric acid recorded at 300 MHz, pH 7.0 and 37 °C.

Fig. 4

The CEST spectra of a) a 30 mM solution EuDOTA-4AmCE³⁺ recorded at 270 MHz and 25 °C; the peak at +50 ppm reflects exchange with the coordinated water molecule, b) DyDOTAM³⁺ recorded at 400 MHz and 25 °C; the peak at +80 ppm reflects exchange with the amide protons while the peak at −720 ppm reflects the coordinated water molecule, c) a 35 mM solution of EuCNPHC³⁺ recorded at 270 MHz and 25 °C; the three peaks at 16, 14 and 12 ppm reflect exchange with each of the hydroxyl protons.

Fig. 5

A simulated ¹H NMR spectrum showing the coordinated water proton resonances of EuDOTA-4AmC⁻ (+50 ppm) and TbDOTA-4AmC⁻ (−600 ppm) in water (resonances arising from the ligand were omitted for clarity). Below this is a schematic representation of the result of selectively irradiating these resonances on samples containing EuDOTA-4AmC⁻, TbDOTA-4AmC⁻ and a mixture of the two. It is seen that each complex may be activated selectively regardless of the presence or absence of the other complex.

Fig. 6

The ratio between the CEST effects arising from the coordinated water molecule (blue line) and the amide protons (red line) of PrDOTA-4AmC⁻ may be used to determine pH without knowing the concentration of the complex in solution. Data taken from reference .

Fig. 7

The CEST spectra of 10 mM solution of EuDOTA-4AmCE³⁺ recorded at 400 MHz, pH 7.3, B₁ = 714 Hz and different temperatures (top), from reference ; and the ratiometric determination of temperature using PrDOTA-4AmC⁻ comparing the water (blue) and amide protons (red), the ratio ST(H₂O)/ST(NH) is shown in black (bottom).

Fig. 8

The behaviour of PARACEST agents designed to sense the presence of glucose and zinc. Top: The effect of glucose on the CEST effect of a 10 mM solution of EuDTMA-2PB³⁺, in the absence (red) and presence (blue) of 10 mM glucose (25 °C, pH 7.4, B₁ = 1000 Hz). Bottom: The CEST spectra of a 20 mM solution of EuDOTAMPy³⁺ in the absence (red) and presence (blue) of 20 mM Zn²⁺ (25 °C, pH 8, B₁ = 1000 Hz).

Fig. 9

The magnitude of the CEST effect calculated for typical parameters for a 20 mM solution of a PARACEST agent (T_1bulk = 2.5 s, T_1bound = 0.2 s, Δω = 10 kHz) as a function of the water exchange rate (1/τ_M) for different presaturation powers (B₁). The advantage of selecting an appropriate water exchange rate is evident.

Fig. 10

The water exchange rate in TmDOTMA⁻ is too fast for the complex to be used as a PARACEST agent. However, encapsulation of the complex in a liposome means TmDOTMA⁻ can behave as a shift reagent that acts only on the water molecules trapped inside the liposome. Exchange of these shifted water molecules across the liposome membrane is slow, which means that the entire liposome can be used as a PARACEST agent.

Chart 1