A self‐calibrating PARACEST MRI contrast agent that detects esterase enzyme activity

Abstract

The CEST effect of many PARACEST MRI contrast agents changes in response to a molecular biomarker. However, other molecular biomarkers or environmental factors can influence CEST, so that a change in CEST is not conclusive proof for detecting the biomarker. To overcome this problem, a second control CEST effect may be included in the same PARACEST agent, which is responsive to all factors that alter the first CEST effect except for the biomarker to be measured. To investigate this approach, a PARACEST MRI contrast agent was developed with one CEST effect that is responsive to esterase enzyme activity and a second control CEST effect. The ratio of the two CEST effects was independent of concentration and T1 relaxation, so that this agent was self-calibrating with respect to these factors. This ratiometric method was dependent on temperature and was influenced by MR coalescence as the chemical exchange rates approached the chemical shifts of the exchangable protons as temperature was increased. The two CEST effects also showed evidence of having different pH dependencies, so that this agent was not self-calibrating with respect to pH. Therefore, a self-calibrating PARACEST MRI contrast agent can more accurately detect a molecular biomarker such as esterase enzyme activity, as long as temperature and pH are within an acceptable physiological range and remain constant.

A single diamagnetic catalyCEST MRI contrast agent that detects cathepsin B enzyme activity by using a ratio of two CEST signals

“…Other peptide sequences can be incorporated into the synthesis scheme to create catalyCEST MRI contrast agents that sense other proteases (26). Other classes of enzymes that modify an aryl functional group may also be detected using this platform technology, such as esterases (7), phosphatases (27), and sulfatases (28). The salicylic acid moiety may be replaced by other molecules that have intramolecular hydrogen bonding that causes highly shifted CEST signals and are less likely to undergo slow degradation (11,17,29).…”

“…1B). To date, CEST agents for catalyCEST MRI have been developed that detect the activity of caspase-3 (2), urokinase Plasminogen Activator (1,3), cathepsin D (4), transglutaminase (5), β-galactosidase (6), esterase (7), protein kinase A (8), and cytosine deaminase enzymes (9). …”

The reactivity of macrocyclic Fe(II) paraCEST MRI contrast agents towards biologically relevant anions, cations, oxygen or peroxide

“…Ln(III) paraCEST agents have also been reported that respond to pH [7][8][9] and to biologically relevant anions including phosphate, citrate, and phosphate esters [10,11]. Enzyme activation of paraCEST agents is another area of interest [12].…”

Vision 20/20: Single photon counting x‐ray detectors in medical imaging

“…By injecting two agents simultaneously, one with target receptors and labeled by one element and the other without receptors and labeled by another element, and imaging both simultaneously, the agent without receptor can be used as a control. 120 This will solve the problems with the interpretation.…”