Developing MRI sensors for monitoring Zn2+ using iCEST

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Metal ions play a crucial role in myriad biological processes, and the ability to monitor real-time changes in metal ion levels is essential for understanding a variety of physiological events. Although imaging of dynamic changes in metal ion levels is applicable in vitro using optical-based responsive dyes, which are limited by low tissue penetration, noninvasive detection of free metal ions in vivo in deep tissues remains a formidable challenge. MRI-based sensors allow the longitudinal study of the same subject with unlimited tissue penetration, which may be useful for studying dynamic biological processes, such as the occurrence and progression of diseases and the efficacy of suggested therapeutics. Moreover, and importantly, MRI sensors have the potential for clinical translation. This proposal aims to design, synthesize, characterize, and optimize fluorinated chelates for Zn2+ imaging using a recently developed approach called ion Chemical Exchange Saturation Transfer (iCEST)-MRI. We have recently shown that using a fluorinated derivative of a calcium chelate, 5,52-difluoro BAPTA (5F-BAPTA), allows the detection of low, biologically relevant concentrations of calcium using iCEST. As a first step, a series of potential iCEST sensors for Zn2+ imaging will be synthesized and studied to characterize their ability to monitor labile zinc ions. Properties such as the dissociation constant (Kd), the frequency offset between bound and free chelate in the 19F-NMR spectrum (¿), the exchange rate (kex), the specificity for Zn2+ (compared to competitive ions), and contrast enhancement, which are all critical for the design of iCEST agents, will be examined. Then, the limitations of the preferred probes, such as the detectability levels of the probe and the dynamic range of detectable Zn2+ concentrations, will be examined. As an example of a potential application, we will capitalize on the ample experience of our research group in pancreatic islet cells as a therapeutic strategy for diabetes. Since insulin secretion from pancreatic beta cells is accompanied by a high concentration of zinc that is released into the extracellular space, monitoring Zn2+ is considered a biomarker for pancreatic beta cell function. We will monitor the Zn2+ release from therapeutic pancreatic islet cells upon the addition of glucose and correlate these levels (detected by the iCEST methodology) with insulin secretion, i.e., their functionality and therapeutic capabilities. Upon completion of this study, we anticipate establishing a new approach to imaging labile Zn2+ in biological systems using MRI, which will be applicable to a broad spectrum of biomedical applications and will launch a novel strategy for imaging biologically relevant metal ions.