Concentration-independent MRI of pH with a dendrimer-based pH-responsive nanoprobe

Abstract

The measurement of extracellular pH (pHe ) has significant clinical value for pathological diagnoses and for monitoring the effects of pH-altering therapies. One of the major problems of measuring pHe with a relaxation-based MRI contrast agent is that the longitudinal relaxivity depends on both pH and the concentration of the agent, requiring the use of a second pH-unresponsive agent to measure the concentration. Here we tested the feasibility of measuring pH with a relaxation-based dendritic MRI contrast agent in a concentration-independent manner at clinically relevant field strengths. The transverse and longitudinal relaxation times in solutions of the contrast agent (GdDOTA-4AmP)44 -G5, a G5-PAMAM dendrimer-based MRI contrast agent in water, were measured at 3 T and 7 T magnetic field strengths as a function of pH. At 3 T, longitudinal relaxivity (r1 ) increased from 7.91 to 9.65 mM(-1) s(-1) (on a per Gd(3+) basis) on changing pH from 8.84 to 6.35. At 7 T, r1 relaxivity showed pH response, albeit at lower mean values; transverse relaxivity (r2 ) remained independent of pH and magnetic field strengths. The longitudinal relaxivity of (GdDOTA-4AmP)44 -G5 exhibited a strong and reversible pH dependence. The ratio of relaxation rates R2 /R1 also showed a linear relationship in a pH-responsive manner, and this pH response was independent of the absolute concentration of (GdDOTA-4AmP)44 -G5 agent. Importantly, the nanoprobe (GdDOTA-4AmP)44 -G5 shows pH response in the range commonly found in the microenvironment of solid tumors.

Keywords: dendritic agent; magnetic resonance imaging; noninvasive pH measurement; responsive agent.

Figure 1

Schematic view of Gd³⁺ chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraaminophosphonate (DOTA-4AmP⁸⁻) inaG5PAMAM dendrimer.

Figure 2

Longitudinal and transverse relaxivities (r₁ and r₂, mM⁻¹ s⁻¹) as a function of pH (6.35–8.84) in liquid phantoms measured at 3 T (triangles) and 7 T (diamonds), with linear regression fits to the data. Transverse relaxivity does not vary significantly with pH, or with field strength. The slopes of the longitudinal relaxivities do not differ between the field strengths.

Figure 3

The calculated ratio of R₂/R₁ for (Gd-DOTA-4AmP)₄₄-G5 at four different concentrations and three different pH values at 7 T: top line, pH = 8.84, middle line, pH = 7.40, bottom line, pH = 6.96. The ratio R₂/R₁ does not vary significantly with concentration of contrast agent at the lower pH values, but there is a small and significant variation at the higher pH, which lies outside the range of values usually encountered in tumor tissues.

Figure 4

R₁ (A) and R₂ (B) maps generated from T₁ and T₂-weighted images of phantoms containing 0.16 mM Gd₄₄-G5 at six different pH values. Average R₁ and R₂ values were calculated from hand-drawn ROIs. R₂/R₁ ratio maps at six different pH values were also generated (C).

Figure 5

R₂/R₁ versus pH plots. Left-hand ordinate, 3 T values; right-hand ordinate, 7 T values. The pH dependence of the relaxation rate ratio is shown with regression lines. Upper line (diamonds), 3 T data; bottom line (triangles), 7 T data. The 95% confidence intervals are plotted for both linear regressions.