The Hanes‐Woolf linear QUESP method improves the measurements of fast chemical exchange rates with CEST MRI

CatalyCEST MRI can detect enzyme activity by monitoring the change in chemical exchange with water after a contrast agent is cleaved by an enzyme. Often these molecules use paramagnetic metals and are delivered with an additional non-responsive reference molecule. To improve this approach for molecular imaging, a single diamagnetic agent with enzyme-responsive and enzyme-unresponsive CEST signals was synthesized and characterized. The CEST signal from the aryl amide disappeared after cleavage of a dipeptidyl ligand with cathepsin B, while a salicylic acid moiety was largely unresponsive to enzyme activity. The ratiometric comparison of the two CEST signals from the same agent allowed for concentration independent measurements of enzyme activity. The chemical exchange rate of the salicylic acid moiety was unchanged after enzyme catalysis, which further validated that this moiety was enzyme-unresponsive. The temperature dependence of the chemical exchange rate of the salicylic acid moiety was non-Arrhenius, suggesting a two-step chemical exchange mechanism for salicylic acid. The good detection sensitivity at low saturation power facilitates clinical translation, along with the potentially low toxicity of a non-metallic MRI contrast agent. The modular design of the agent constitutes a platform technology that expands the variety of agents that may be employed by catalyCEST MRI for molecular imaging.

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“…4) (14). The same procedure was used to measure the chemical exchange rates of the product, 4-amino salicylic acid, and a Phe-Arg dipeptide.…”

Section: Resultsmentioning

confidence: 99%

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

Hingorani

Montano

Randtke

et al. 2015

Contrast Media & Molecular

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“…CEST spectra of each sample were obtained with a saturation power of 1.73 μT and saturation time of 2 sec, and with a saturation power of 2.45 μT and saturation times of 2 and 4 sec. These spectra were simultaneously fit with Bloch equations modified for chemical exchange because simultaneously fitting multiple CEST spectra has been shown to improve parameter estimations [23,24]. The base catalyzed exchange rate (k b ) and the rate of direct exchange with water (k 0 ) were fit using equation [1], assuming that the acid catalyzed exchange rate (k a ) is negligible at physiological pH [25].…”

Section: Methodsmentioning

confidence: 99%

Measuring Extracellular pH in a Lung Fibrosis Model with acidoCEST MRI

et al. 2014

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Purpose A feed-forward loop involving lactic acid production may potentially occur during the formation of idiopathic pulmonary fibrosis. To provide evidence for this feed-forward loop, we used acidoCEST MRI to measure the extracellular pH (pHe), while also measuring percent uptake of the contrast agent, lesion size, and the Apparent Diffusion Coefficient (ADC). Procedures We developed a respiration-gated version of acidoCEST MRI to improve the measurement of pHe and percent uptake in lesions. We also used T2-weighted MRI to measure lesion volumes and diffusion-weighted MRI to measure ADC. Results The longitudinal changes in average pHe and % uptake of the contrast agent was inversely related to reduction in lung lesion volume. The average ADC did not change during the time frame of the study. Conclusions The increase in pHe during the reduction in lesion volume indicates a role for lactic acid in the proposed feed-forward loop of IPF.

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“…A low saturation power can underestimate the reaction coordinate because incomplete saturation of each labile proton on an agent causes their CEST signal amplitudes to become more equivalent, so we performed in vivo studies at a relatively high 4-μT saturation power (Fig. S3d–f) [24]. The reaction coordinate showed only a minor dependence on temperature of 3.2 % °C −1 (Fig.…”

Section: Detection Of Klk6 Activity and Inhibition In Vitromentioning

confidence: 99%

Detection of Enzyme Activity and Inhibition during Studies in Solution, In Vitro and In Vivo with CatalyCEST MRI

et al. 2017

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Purpose The detection of enzyme activities and evaluation of enzyme inhibitors have been challenging with magnetic resonance imaging (MRI). To address this need, we have developed a diamagnetic, nonmetallic contrast agent and a protocol known as catalyCEST MRI that uses chemical exchange saturation transfer (CEST) to detect enzyme activity as well as enzyme inhibition. Procedures We synthesized a diamagnetic MRI contrast agent that has enzyme responsive and enzyme unresponsive CEST signals. We tested the ability of this agent to detect the activity of kallikrein 6 (KLK6) in biochemical solutions, in vitro and in vivo, with and without a KLK6 inhibitor. Results The agent detected KLK6 activity in solution and also detected KLK6 inhibition by antithrombin III. KLK6 activity was detected during in vitro studies with HCT116 colon cancer cells, relative to the detection of almost no activity in a KLK6-knockdown HCT116 cell line and HCT116 cells treated with antithrombin III inhibitor. Finally, strong enzyme activity was detected within an in vivo HCT116 tumor model, while lower enzyme activity was detected in a KLK6 knockdown tumor model and in the HCT116 tumor model treated with antithrombin III inhibitor. In all cases, comparisons of the enzyme responsive and enzyme unresponsive CEST signals were critical for the detection of enzyme activity. Conclusions This study has established that catalyCEST MRI with an exogenous diaCEST agent can evaluate enzyme activity and inhibition in solution, in vitro and in vivo.

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The Hanes‐Woolf linear QUESP method improves the measurements of fast chemical exchange rates with CEST MRI

Cited by 51 publications

References 26 publications

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

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

Measuring Extracellular pH in a Lung Fibrosis Model with acidoCEST MRI

Detection of Enzyme Activity and Inhibition during Studies in Solution, In Vitro and In Vivo with CatalyCEST MRI

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