Shanrong Zhang

Numerical solution of the Bloch equations provides insights into the optimum design of PARACEST agents for MRI.

Paramagnetic lanthanide complexes that display unusually slow water exchange between an inner sphere coordination site and bulk water may serve as a new class of MRI contrast agents with the use of chemical exchange saturation transfer (CEST) techniques. To aid in the design of paramagnetic CEST agents for reporting important biological indices in MRI measurements, we formulated a theoretical framework based on the modified Bloch equations that relates the chemical properties of a CEST agent (e.g., water exchange rates and bound water chemical shifts) and various NMR parameters (e.g., relaxation rates and applied B1 field) to the measured CEST effect. Numerical solutions of this formulation for complex exchanging systems were readily obtained without algebraic manipulation or simplification. For paramagnetic CEST agents of the type used here, the CEST effect is relatively insensitive to the bound proton relaxation times, but requires a sufficiently large applied B1 field to highly saturate the Ln3+‐bound water protons. This in turn requires paramagnetic complexes with large Ln3+‐bound water chemical shifts to avoid direct excitation of the exchanging bulk water protons. Although increasing the exchange rate of the bound protons enhances the CEST effect, this also causes exchange broadening and increases the B1 required for saturation. For a given B1, there is an optimal exchange rate that results in a maximal CEST effect. This numerical approach, which was formulated for a three‐pool case, was incorporated into a MATLAB nonlinear least‐square optimization routine, and the results were in excellent agreement with experimental Z‐spectra obtained with an aqueous solution of a paramagnetic CEST agent containing two different types of bound protons (bound water and amide protons). Magn Reson Med 53:790–799, 2005.

Renal and Systemic pH Imaging by Contrast-Enhanced MRI

Perturbations of renal and systemic pH accompany diseases of the kidney, such as renal tubular acidosis, and the ability to image tissue pH would be helpful to assess the extent and severity of such conditions. A dual‐contrast‐agent strategy using two gadolinium agents, the pH‐insensitive GdDOTP5− and the pH‐sensitive GdDOTA‐4AmP5−, has been developed to generate pH maps by MRI. The renal pharmacokinetics of the structurally dissimilar pH‐insensitive contrast agents GdDTPA2− and GdDOTP5− were found to be similar. On that basis, and on the basis of similarity of structure and charge, the renal pharmacokinetics of GdDOTP5− and GdDOTA‐4AmP5− were assumed to be identical. Dynamic T1‐weighted images of mice were acquired for 1 hr each following boluses of GdDOTP5− and GdDOTA‐4AmP5−. The time‐varying apparent concentration of GdDOTP5− and the time‐varying enhancement in longitudinal relaxation rate following GdDOTA‐4AmP5− were calculated for each pixel and used to compute pH images of the kidneys and surrounding tissues. MRI pH maps of control mice show acidic regions corresponding to the renal papilla, calyx, and ureter. Pretreatment of mice with the carbonic anhydrase inhibitor acetazolamide resulted in systemic metabolic acidosis and accompanying urine alkalinization that was readily detected by this dual‐contrast‐agent approach. Magn Reson Med 49:249–257, 2003.

High resolution pHe imaging of rat glioma using pH-dependent relaxivity

Previous studies using MR spectroscopy have shown that the extracellular pH (pHe) of tumors is acidic compared to normal tissues. This has a number of important sequelae that favor the emergence of more aggressive and therapy‐resistant tumors. New MRI methods based on pH‐sensitive T1 relaxivity are an attractive alternative to previous spectroscopic methods, as they allow improvements in spatial and temporal resolution. Recently, pH‐dependent GdDOTA‐4AmP5‐ and a pH‐independent analog, GdDOTP5‐, were used to image renal pH in mice. The current study has used a similar approach to image pHe in rat gliomas. Significant differences were observed compared to the renal study. First, the relaxivity of GdDOTP5‐ was found to be affected by the higher extracellular protein content of tumors. Second, the pixel‐by‐pixel analysis of the GdDOTP5‐ and GdDOTA‐4AmP5‐ pharmacokinetics showed significant dispersion, likely due to the temporal fluctuations in tumor perfusion. However, there was a robust correlation between the maximal enhancements produced by the two boluses. Therefore, to account for the local time‐courses differences, pHe maps were calculated at the time of maximal enhancement in each pixel. Finally, the comparison of the pHe and the time to maximal intensity maps revealed an inverse relationship between pHe and tumor perfusion. Magn Reson Med, 2006.

Multi-chromatic pH-activatable 19F-MRI nanoprobes with binary ON/OFF pH transitions and chemical-shift barcodes

Magnetic resonance imaging (MRI) is a powerful noninvasive imaging technique that has greatly impacted basic biological research as well clinical diagnosis of cancer and other diseases.[1] Conventional MR contrast agents are T1 (e.g. Gd-DTPA) or T2-based (e.g. iron oxide), which cause significant longitudinal or transverse relaxation of protons, respectively.[2] Despite their success in many biological applications, one potential limitation is the lack of multi-chromatic features that allows for simultaneous detection of multiple signals. Recently, 19F has received significant attention in MR imaging and spectroscopy studies.[3] Compared to 1H-MRI, 19F-MRI has little biological background due to the low levels of endogenous fluorine in the body. Moreover, 19F has 100% natural abundance and its gyromagnetic ratio (40.06 MHz/T) is second only to 1H, which makes it more sensitive for detection over other nuclei.[3f]

Imaging the tissue distribution of glucose in livers using a PARACEST sensor

Noninvasive imaging of glucose in tissues could provide important insights about glucose gradients in tissue, the origins of gluconeogenesis, or perhaps differences in tissue glucose utilization in vivo. Direct spectral detection of glucose in vivo by 1H NMR is complicated by interfering signals from other metabolites and the much larger water signal. One potential way to overcome these problems is to use an exogenous glucose sensor that reports glucose concentrations indirectly through the water signal by chemical exchange saturation transfer (CEST). Such a method is demonstrated here in mouse liver perfused with a Eu3+‐based glucose sensor containing two phenylboronate moieties as the recognition site. Activation of the sensor by applying a frequency‐selective presaturation pulse at 42 ppm resulted in a 17% decrease in water signal in livers perfused with 10 mM sensor and 10 mM glucose compared with livers with the same amount of sensor but without glucose. It was shown that livers perfused with 5 mM sensor but no glucose can detect glucose exported from hepatocytes after hormonal stimulation of glycogenolysis. CEST images of livers perfused in the magnet responded to changes in glucose concentrations demonstrating that the method has potential for imaging the tissue distribution of glucose in vivo. Magn Reson Med 60:1047–1055, 2008.

{DOTA‐bis(amide)}lanthanide Complexes: NMR Evidence for Differences in Water‐Molecule Exchange Rates for Coordination Isomers

Two derivatives of 1,4,7,10-tetraazacyclododecane with trans-acetate and trans-amide side-chain ligating groups have been prepared and their complexes with lanthanide cations examined by multinuclear NMR spectroscopy. These lanthanide complexes exist in aqueous solution as a mixture of slowly interconverting coordination isomers with 1H chemical shifts similar to those reported previously for the major (M) and minor (m) forms of the tetraacetate ([Ln(dota)]-) and tetraamide ([Ln(dtma)]3+) complexes. As in the [Ln(dota)]- and [Ln(dtma)]3+ complexes, the m/M ratio proved to be a sensitive function of lanthanide size and temperature. An analysis of 1H hyperfine shifts in spectra of the Yb3+ complexes revealed significant differences between the axial (D1) and non-axial (D2) components of the magnetic susceptibility tensor anisotropy in the m and M coordination isomers and the energetics of ring inversion and m <==> M isomerization as determined by two-dimensional exchange spectroscopy (EXSY). (17)O shift data for the Dy3+ complexes showed that both have one inner-sphere water molecule. A temperature-dependent (17)O NMR study of bulk water linewidths for solutions of the Gd3+ complexes provided direct evidence for differences in water exchange rates for the two coordination isomers. The bound-water lifetimes (tauM298) in the M and m isomers of the Gd3+ complexes ranged from 1.4-2.4 micros and 3-14 ns, respectively. This indicates that 1) the inner-sphere water lifetimes for the complexes with a single positive charge reported here are considerably shorter for both coordination isomers than the corresponding values for the [Gd(dtma)]3+ complex with three positive charges, and 2) the difference in water lifetimes for M and m isomers in these two series is magnified in the [Gd[dota-bis(amide)]] complexes. This feature highlights the remarkable role of both charge and molecular geometry in determining the exchange rate of the coordinated water.

Hyperpolarized 15 N-pyridine Derivatives as pH-Sensitive MRI Agents

Highly sensitive MR imaging agents that can accurately and rapidly monitor changes in pH would have diagnostic and prognostic value for many diseases. Here, we report an investigation of hyperpolarized 15N-pyridine derivatives as ultrasensitive pH-sensitive imaging probes. These molecules are easily polarized to high levels using standard dynamic nuclear polarization (DNP) techniques and their 15N chemical shifts were found to be highly sensitive to pH. These probes displayed sharp 15N resonances and large differences in chemical shifts (Δδ >90 ppm) between their free base and protonated forms. These favorable features make these agents highly suitable candidates for the detection of small changes in tissue pH near physiological values.

In Vivo Magnetic Resonance Imaging of Tissue pH Using a Novel pH-Sensitive Contrast Agent, GdDOTA-4AmP

The pH of bodily fluids affects the organism in many ways, especially through its effects on cellular and plasma proteins. Maintenance of acid-base homeostasis is therefore critical, and occurs at several levels. The most immediate and local response to an acid or alkali load is through chemical processes, including intracellular and extracellular buffers. However buffering capacity is typically limited, and must be backed up by physiologic responses to the acid-base imbalance. These physiologic processes can be at the cellular level, such as feed-back changes in metabolism, and at the systemic level, involving adaptive changes to the excretion of volatile acids by the lungs and fixed acids by the kidneys (1). Pathologically altered renal physiology can manifest with perturbations in systemic and renal pH. Hereditary defects and acquired deficiencies in renal tubular ion transport systems can result in systemic metabolic acidosis or alkalosis (1,2). Novel gene therapies have shown promise for the treatment of some of these diseases, but there are still problems with heterogeneous gene delivery to and loss of the corrective gene from the target tissue over time (1,2). Methodologies to image the spatial distribution of renal pH would have considerable biomedical and clinical relevance in such cases, by enabling the non-invasive assessment of disease extent, progression, and response to therapy. Recently, a pH-sensitive gadolinium-based contrast agent, GdDOTA-4AmP5 , was introduced (1). We have employed a dual contrast agent method to image tissue pH in vivo by MRI using GdDOTA-4AmP5 .

Silencing of phosphonate-gadolinium magnetic resonance imaging contrast by hydroxyapatite binding

Rationale and Objectives:GdDOTP5− is a highly charged, bone-seeking paramagnetic complex that could potentially detect bone lesions by magnetic resonance imaging (MRI). To date, its pharmacokinetics, effects on organ relaxivity, and interaction with hydroxyapatite (HA) has not been described. Methods:Liver, kidney, and bone MRI images were obtained on male white rabbits after the administration of GdDOTP5− or a gold standard MRI contrast agent, GdDTPA2−. Parallel in vitro experiments quantified the effect of HA binding on GdDOTP5− -induced changes in relaxivity. Results:The 2 compounds showed similar MRI enhancements in visceral tissues, but no enhancement of bone was evident with GdDOTP5− despite confirmation of bone and HA binding of the radioactive 153SmDOTP5− and 111InDOTP5− derivatives. In vitro experiments demonstrated that GdDOTP5−-induced changes in relaxivity were silenced upon HA binding but could be recovered by acid elution of the complex. Conclusions:HA binding assays revealed that GdDOTP5− is essentially MR silent when bound to bone, likely because of the exclusion of all outer sphere water molecules from the surface of the complex. These data suggest a novel strategy for creating highly sensitive, switchable MRI contrast agents.

Gd3+ complexes with slowly exchanging bound-water molecules may offer advantages in the design of responsive MR agents

Zhang S, Wu K, Sherry AD. Gd3+ complexes with slowly exchanging bound-water molecules may offer advantages in the design of responsive MR agents. Invest Radiol 2001;36:82–86. rationale and objectives. Slow water exchange in Gd3+ complexes is generally considered detrimental to their use as MR contrast agents. The objective of this work was to demonstrate how this feature may serve as a useful template for the design of responsive MR agents. methods.Lanthanide (Ln) complexes of two 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA)–tetraamide phosphonate (1) and phosphonate ester (2) ligands were studied by multinuclear (1H, 13C, 31P, and 17O) nuclear MR spectroscopy. results.The inner-sphere water lifetime in the Ln(2) complexes was much longer (&tgr;M298 = 0.8–1.3 ms) than in the corresponding Ln(1) complexes. This allowed direct detection of the bound-water molecule in europium(2) in water at 40° C by 1H nuclear MR. The water relaxivity of gadolinium(2) was independent of pH between 8.5 and 6.0, whereas the relaxivity of gadolinium(1) increased more than twofold in this pH range. conclusions.T1-weighted images of phantoms containing gadolinium(1) at different pH values demonstrate the efficacy of this complex as a pH-sensitive MR contrast agent.