Evaluation of Enzyme Substrate Radiotracers as PET/MRS Hybrid Imaging Agents

19 F MRI Probes for Multimodal Imaging*

Magnetic resonance imaging (MRI) is one of the most powerful imaging tools today, capable of displaying superior soft-tissue contrast. This review discusses developments in the field of ¹⁹ F MRI multimodal probes in combination with optical fluorescence imaging (OFI), ¹ H MRI, chemical exchange saturation transfer (CEST) MRI, ultrasonography (USG), X-ray computed tomography (CT), single photon emission tomography (SPECT), positron emission tomography (PET), and photoacoustic imaging (PAI). In each case, multimodal ¹⁹ F MRI probes compensate for the deficiency of individual techniques and offer improved sensitivity or accuracy of detection over unimodal counterparts. Strategies for designing ¹⁹ F MRI multimodal probes are described with respect to their structure, physicochemical properties, biocompatibility, and the quality of images.

Identification of AV-1451 as a Weak, Nonselective Inhibitor of Monoamine Oxidase

[¹⁸F]AV-1451 is one of the most widely used radiotracers for positron emission tomography (PET) imaging of tau protein aggregates in neurodegenerative disorders. While the radiotracer binds with high affinity to tau neurofibrillary tangles, extensive clinical studies have simultaneously revealed off-target tracer accumulation in areas of low tau burden such as the basal ganglia and choroid plexus. Though there are a number of possible reasons for this accumulation, it is often attributed to off-target binding to monoamine oxidase (MAO). In this paper, we investigate the association between [¹⁸F]AV-1451 and MAO through (i) enzyme inhibition assays, (ii) autoradiography with postmortem tissue samples, and (iii) nonhuman primate PET imaging. We confirm that [¹⁸F]AV-1451 is a weak inhibitor of MAO-A and -B and that MAO inhibitors can alter binding of [¹⁸F]AV-1451 in autoradiography and in vivo PET imaging.