Zinc-sensitive MRI contrast agent detects differential release of Zn(II) ions from the healthy vs. malignant mouse prostate.

Many secretory tissues release Zn(II) ions along with other molecules in response to external stimuli. Here we demonstrate that secretion of Zn(II) ions from normal, healthy prostate tissue is stimulated by glucose in fasted mice and that release of Zn(II) can be monitored by MRI. An ∼50% increase in water proton signal enhancement is observed in T1-weighted images of the healthy mouse prostate after infusion of a Gd-based Zn(II) sensor and an i.p. bolus of glucose. Release of Zn(II) from intracellular stores was validated in human epithelial prostate cells in vitro and in surgically exposed prostate tissue in vivo using a Zn(II)-sensitive fluorescent probe known to bind to the extracellular surface of cells. Given the known differences in intracellular Zn(II) stores in healthy versus malignant prostate tissues, the Zn(II) sensor was then evaluated in a transgenic adenocarcinoma of the mouse prostate (TRAMP) model in vivo. The agent proved successful in detecting small malignant lesions as early as 11 wk of age, making this noninvasive MR imaging method potentially useful for identifying prostate cancer in situations where it may be difficult to detect using current multiparametric MRI protocols.

A second generation MRI contrast agent for imaging zinc ions in vivo.

A Zn2+ specific GdDOTA derivative containing two bis-(3-pyrazolyl) units was prepared and characterized. Unlike a previously reported Zn2+ binding agent, the new agent binds to human albumin both in the presence and absence of Zn2+.

Noninvasive MRI of ß-cell function using a Zn2+-responsive contrast agent.

Elevation of postprandial glucose stimulates release of insulin from granules stored in pancreatic islet ß-cells. We demonstrate here that divalent zinc ions coreleased with insulin from ß-cells in response to high glucose are readily detected by MRI using the Zn(2+)-responsive T(1) agent, GdDOTA-diBPEN. Image contrast was significantly enhanced in the mouse pancreas after injection of a bolus of glucose followed by a low dose of the Zn(2+) sensor. Images of the pancreas were not enhanced by the agent in mice without addition of glucose to stimulate insulin release, nor were images enhanced in streptozotocin-treated mice with or without added glucose. These observations are consistent with MRI detection of Zn(2+) released from ß-cells only during glucose-stimulated insulin secretion. Images of mice fed a high-fat (60%) diet over a 12-wk period and subjected to this same imaging protocol showed a larger volume of contrast-enhanced pancreatic tissue, consistent with the expansion of pancreatic ß-cell mass during fat accumulation and progression to type 2 diabetes. This MRI sensor offers the exciting potential for deep-tissue monitoring of ß-cell function in vivo during development of type 2 diabetes or after implantation of islets in type I diabetic patients.

A concentration-independent method to measure exchange rates in PARACEST agents.

The efficiency of chemical exchange dependent saturation transfer (CEST) agents is largely determined by their water or proton exchange kinetics, yet methods to measure such exchange rates are variable and many are not applicable to in vivo measurements. In this work, the water exchange kinetics of two prototype paramagnetic agents (PARACEST) are compared by using data from classic NMR line-width measurements, by fitting CEST spectra to the Bloch equations modified for chemical exchange, and by a method where CEST intensity is measured as a function of applied amplitude of radiofrequency field. A relationship is derived that provides the water exchange rate from the X-intercept of a plot of steady-state CEST intensity divided by reduction in signal caused by CEST irradiation versus 1/omega(1)(2), referred to here as an omega plot. Furthermore, it is shown that this relationship is independent of agent concentration. Exchange rates derived from omega plots using either high-resolution CEST NMR data or CEST data obtained by imaging agree favorably with exchange rates measured by the more commonly used Bloch fitting and line-width methods. Thus, this new method potentially allows access to a direct measure of exchange rates in vivo, where the agent concentration is typically unknown.

A new gadolinium-based MRI zinc sensor.

The properties of a novel Gd(3+)-based MRI zinc sensor are reported. Unlike previously reported Gd(3+)-based MRI contrast agents, this agent (GdL) differs in that the agent alone binds only weakly with human serum albumin (HSA), while the 1:2 GdL:Zn(2+) ternary complex binds strongly to HSA resulting in a substantial, 3-fold increase in water proton relaxivity. The GdL complex is shown to have a relatively strong binding affinity for Zn(2+) (K(D) = 33.6 nM), similar to the affinity of the Zn(2+) ion with HSA alone. The agent detects as little as 30 microM Zn(2+) in the presence of HSA by MRI in vitro, a value slightly more than the total Zn(2+) concentration in blood (approximately 20 microM). This combination of binding affinity constants and the high relaxivity of the agent when bound to HSA suggests that this new agent may be useful for detection of free Zn(2+) ions in vivo without disrupting other important biological processes involving Zn(2+).

Modulation of water exchange in europium(III) DOTA-tetraamide complexes via electronic substituent effects.

The chemical exchange saturation transfer (CEST) efficiency for a series Eu3+-based tetraamide complexes bearing p-substituents on a single coordinating pendant arm is highly sensitive to water exchange rates. The CEST effect increases in the order Me < MeO < F approximately CO2tBu < CN < H. These results show that CEST contrast can be modulated by changes in electron density at a single ligating atom, and this forms the basis of creating imaging agents that respond to chemical oxidation and reduction.