A Frequency-Selective pH-Responsive paraCEST Agent

Abstract

Paramagnetic chemical exchange saturation transfer (paraCEST) agents are well-suited for imaging tissue pH because the basis of CEST, chemical exchange, is inherently sensitive to pH. Several previous pH-sensitive paraCEST agents were based on an exchanging Ln³⁺ -bound water molecule as the CEST antenna but this design often added additional line-broadening to the bulk water signal due to T₂ exchange. We report herein a pH-sensitive paraCEST agent that lacks an inner-sphere water molecule but contains one Ln-bound -OH group for CEST activation. The Yb³⁺ complex, Yb(1), displayed a single, highly shifted CEST peak originating from the exchangeable Yb-OH proton, the frequency of which changed over the biologically relevant pH range. CEST images of phantoms ranging in pH from 6 to 8 demonstrate the potential of this agent for imaging pH. Initial rodent imaging studies showed that Gd(1) remains in the vascular system much longer than anticipated but is cleared slowly via renal filtration.

Keywords: contrast agents; imaging agents; lanthanide complexes; pH sensing; paraCEST.

Figure 1.

¹H NMR spectrum of 15 mM Yb(1) in H₂O (26° C, pH 6.1, 9.4 T); inset. Zoomed ¹H NMR spectra of Yb(1) at different pH values.

Figure 2.

Plots of ³¹P chemical shift versus pH for each of the three phosphonate groups in Yb(1) (15 mM Yb(1), 161.9 MHz, 25° C).

Figure 3.

a) T_2w images of Dy(1) and Dy(2) in water (9.4 T, pH 7, 25°C). b) Plots of R₂ versus [Dy(1)] and [Dy(2)] (9.4 T, pH 7.2, 37°C).

Figure 4.

a) CEST spectra of 20 mM Yb(1) in water and in 2% agarose (solid line), and in water (dotted line) (B₁ = 15 μT, pH 7, 25°C, 9.4 T); b) CEST spectra of 17 mM Yb(1) (blue line) and Yb(2) (red line) in plasma at pH 7.3 (B₁ = 15 μT, 37° C, 9.4 T).

Figure 5.

a) CEST spectra of Yb(1) at different pH values (9.4T, 25°C); b) Chemical shift versus pH calibration curve at 25°C and a fit of those data to the protonation constants of the phosphonate groups (solid line); c) The false colored CEST images of 25 mM Yb(1) at different pH values; d) Linear plot showing the agreement between the CEST measured pH compared to electrode measurements.

Figure 6.

a) Axial T₁ weighted images of a mouse before and after injection of 0.07mmol/kg Gd(1); b) Dynamic plots of normalized signal intensity in blood in the left ventricle (ROI) versus time for Gd(1) and Gd(2) (Prohance) after bolus injection of identical amounts of each agent.

Chart 1.

The ligands discussed in this work.

Scheme 1.

Synthesis of 1,4,7-tris(methylenephosphonic acid)-10-(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane (1).