The Pursuit of Theranostics with CEST MRI

Molecular imaging has often provided tantalizing potential for tracking theranostic agents during in vivo studies. Each in vivo imaging modality has its limitations, which has impeded the application of molecular imaging towards theranostics research. For example, Magnetic Resonance Spectroscopy (MRS) has been used to detect drug delivery with great molecular specificity due to the unique MR frequencies for each agent.1 However, MRS suffers from poor detection sensitivity, often requiring toxic concentrations of the agent. Magnetic Resonance Imaging (MRI) overcomes poor detection sensitivity by directly detecting water that is abundant in soft tissues. Theranostic agents can be labeled or packaged with metals that can influence the water signal detected with MRI.2 Unfortunately, many conditions affect MRI signals, so the specificity of detecting a metal-labeled agent is often lost when detecting the influence of the agent on the MRI signal of the water. Chemical Exchange Saturation Transfer (CEST) MRI is a hybrid of MRS and MRI that has evolved from the chemistry benchtop, to small animal imaging studies, to clinical radiology studies during the past century.3 Saturation at the specific MR frequency of a proton (a hydrogen atom) on a theranostic agent causes the proton’s coherent magnetization to disappear. This disappearance in MRI signal is then transferred to water after the agent’s proton undergoes chemical exchange with a water molecule. The resulting decrease in water signal can be detected with good sensitivity using standard MRI methods. Despite substantial interest in CEST MRI, concerns have been raised that the intermediate detection sensitivity and specificity may only result in inadequate sensitivity and specificity, resulting in very limited utility of this MRI method for non-invasively tracking theranostic agents during in vivo studies. A recent study by Prof. Guanshu Liu and colleagues at the Johns Hopkins University School of Medicine has shown promise that CEST MRI can successfully track a theranostic agent with adequate sensitivity and excellent specificity.4 Citicoline (cytidine-5’-diphosphocholine) is a neurodegenerative therapeutic agent that has shown significant neuroprotective effects afer ischemic stroke. Varying results have been attributed to variable delivery of citicoline, and various routes of administration have been proposed to improve delivery. Thus, there is a justifiable need to track the delivery of citicoline as a theranostic agent. Ideally, this agent should be tracked without labeling the molecule with an imaging probe, which inevitably raises concerns about altered efficacy and/or pharmacokinetics of the theranostic agent. Fortunately, citicoline contains cytosine that has previously been shown to act as a CEST agent. The amine group and two hydroxyl groups of citicoline have labile protons that readily exchange with water protons. Liu and colleagues showed that selective saturation at the MR frequencies of these specific protons, followed by the detection of decreased water signal, resulted in the sensitive detection of CEST signals from citicoline. Importantly, this CEST detection arises from the labile protons of the agent, obviating the need for labeling the molecule with an imaging probe. Therefore, this drug acts as a CEST agent, and thereby qualifies as a theranostic agent. As a major breakthrough in CEST MRI reseach, the investigators were able to show sufficiently sensitive detection of citicoline during CEST MRI studies with a rat model of transient focal cerebral ischemia. Initial studies in chemical solution showed that less than 1 mM of citicoline could be reliably Ivyspring