A novel design platform for diaCEST agents

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Abstract Magnetic resonance imaging (MRI) is one of the most important clinical imaging modalities because it can provide high resolution images of soft tissues in a non-invasive manner. Gadolinium contrast media enhance image contrast by shortening the T1 and T2 relaxation times of tissue water protons. Alternatively, image contrast can be generated using chemical exchange saturation transfer (CEST) mechanism. CEST requires the existence of at least two slowly exchanging pools of protons with different NMR chemical shifts, and generates contrast by transferring saturated 1H spins from the small pool (agent) to the bulk (tissue) water pool. The key parameters in CEST are the exchange rate constant (kex) and the chemical shift (frequency) separation of the exchanging pools (Δω). CEST requires slow exchange condition, (kex ≤ Δω). CEST agents can be exogenous or endogenous diamagnetic molecules (diaCEST) or paramagnetic metal complexes (paraCEST) with labile protons. ParaCEST agents exhibit large frequency difference that easily satisfies the requirement for CEST but they have potential toxicity due to in vivo metal release. DiaCEST agents are metal free by their chemical nature but the chemical shift separation of diaCEST agents is usually less than 5 ppm from the bulk water. This small frequency separation poses severe limitations on current diaCEST agents such as poor selectivity due to the spectral overlap with other exchanging proton resonances, strong interference from tissue background magnetization transfer and direct water saturation. Here we propose to develop a new diaCEST platform technology that would overcome the shortcomings of current diaCEST agents by relying on slowly exchanging, highly downfield shifted (15 ppm from water) protons present in structurally constrained monoprotonated peri-naphthalene derivatives. Synthesis and in vitro characterization of the proposed agents will be accomplished in Specific Aim 1. The goal of Specific Aim 2 is to demonstrate the in vivo applicability of these probes in imaging experiments in mice. The proposed agents would offer several advantages over existing CEST agents including improved CEST efficiency, high selectivity without interference from proteins and fats, easier data analysis due to weaker tissue magnetization transfer effect and asymmetry, significantly reduced or eliminated direct water saturation, pH- independent CEST effect and improved safety profile due to metal free composition. This platform would allow the design and synthesis of non-specific agents without the confounding effect of pH as well as the construction of targeted and responsive MR agents through the modification of the naphthalene backbone.