Local variations in tissue pH are often associated with many pathological conditions-including injury, inflammation, and ischemia, as well as a variety of cancers. While progress has been made, the creation of magnetic resonance imaging (MRI) agents capable of sensing pH but that also meet all of the desired traits- including high sensitivity, low minimum dose, high biological compatibility, and concentration-independent pH response-remains a persistent challenge. Based upon the work performed as part of the original AREA R15 support, the overall goal of the work proposed in this renewal application is to develop a novel class of MRI contrast agents based on dendron-functionalized super paramagnetic iron oxide nanoparticles ('dendron- SPIONs') that will provide highly sensitive pH responses in physiologically-relevant regimes, and to characterize the nature of those pH-sensitive responses. It is believed that most of the pH response results from transient, reversible clustering of the dendron-SPIONs modulated by an interplay between the SPION surface charge density and the solution ionic strength. Correspondingly, the Proposal has two Specific Aims: (1) to synthesize and characterize novel dendron-functionalized SPIONs designed to give strong pH-sensitive MR responses in physiologically relevant regimes (with varying values between pH~6-8);and (2) to perform fundamental studies of these novel dendron-SPIONs-in both chemically homogeneous and realistic cellular environments-to obtain a greater understanding of the origins of such pH-sensitive responses. In support of (1), a variety of new dendron-SPIONs will be synthesized (via multi-step organic synthesis and covalent dendron linkage to the nanocrystallite SPION cores via ligand-exchange methods) whose dendron functionalities with be designed to provide peak pH sensitivity at a given location between ~6-8;variation of the dendron generation should allow fine tuning of the magnitude and position of the pH inflection for a given dendron-SPION. MR measurements of aqueous 1H spin relaxation rates (R1, R2, R2*) as functions of SPION loading will allow systematic relaxivity measurement and characterization for a given dendron-SPION under various conditions (including pH, ionic strength, and magnetic field). In support of (2), the correlation between a given MR response and the physical SPION behavior will be studied. Echo-time dependent CPMG MR measurements of effective R2 values will provide an indication of the degree of SPION clustering present under a given set of conditions;these results will be interpreted in the context of a theoretical model (under development) and complimentary results obtained by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Furthermore, greater understanding of the pH-sensitive responses obtained in support of (2) will naturally be used as feedback for design and synthesis of next-generation pH-sensing SPIONs created in support of (1). Finally, experiments involving cell cultures will allow direct comparison of results obtained in multi-compartment, tissue-like environments with those obtained in chemically homogeneous environments.
Local variations in tissue pH are often associated with a range of diseases, including many cancers. Once successful, the proposed work should provide new insight into the design and mechanism-of-action of a novel class of pH-sensing agents, and furthermore, should lead directly to the development of novel MRI contrast agents that would allow the detailed mapping of pH gradients in the body for the improved visualization, diagnosis, and treatment of various malignancies (impact for other clinical situations where pH variance may be present is also envisioned). For example, it is anticipated that positive therapeutic measures in the treatment of cancers will manifest themselves first as changes in local metabolism, since the pH variance associated with such diseases is caused by problems with local metabolism. Thus, the ability to probe tissue pH in response to therapy promises to provide more rapid feedback than simple 'caliper-based'measurements of tumor dimensions (which can take much longer to manifest).
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