Abstract
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Decades after its discovery, positron emission tomography (PET)
remains the premier tool for imaging neurochemistry in living humans.
Technological improvements in radiolabeling methods, camera design,
and image analysis have kept PET in the forefront. In addition, the
use of PET imaging has expanded because researchers have developed
new radiotracers that visualize receptors, transporters, enzymes,
and other molecular targets within the human brain.
However,
of the thousands of proteins in the central nervous system
(CNS), researchers have successfully imaged fewer than 40 human proteins.
To address the critical need for new radiotracers, this Account expounds
on the decisions, strategies, and pitfalls of CNS radiotracer development
based on our current experience in this area.
We discuss the
five key components of radiotracer development for
human imaging: choosing a biomedical question, selection of a biological
target, design of the radiotracer chemical structure, evaluation of
candidate radiotracers, and analysis of preclinical imaging. It is
particularly important to analyze the market of scientists or companies
who might use a new radiotracer and carefully select a relevant biomedical
question(s) for that audience. In the selection of a specific biological
target, we emphasize how target localization and identity can constrain
this process and discuss the optimal target density and affinity ratios
needed for binding-based radiotracers. In addition, we discuss various
PET test–retest variability requirements for monitoring changes
in density, occupancy, or functionality for new radiotracers.
In the synthesis of new radiotracer structures, high-throughput,
modular syntheses have proved valuable, and these processes provide
compounds with sites for late-stage radioisotope installation. As
a result, researchers can manage the time constraints associated with
the limited half-lives of isotopes. In order to evaluate brain uptake,
a number of methods are available to predict bioavailability, blood–brain
barrier (BBB) permeability, and the associated issues of nonspecific
binding and metabolic stability. To evaluate the synthesized chemical
library, researchers need to consider high-throughput affinity assays,
the analysis of specific binding, and the importance of fast binding
kinetics. Finally, we describe how we initially assess preclinical
radiotracer imaging, using brain uptake, specific binding, and preliminary
kinetic analysis to identify promising radiotracers that may be useful
for human brain imaging. Although we discuss these five design components
separately and linearly in this Account, in practice we develop new
PET-based radiotracers using these design components nonlinearly and
iteratively to develop new compounds in the most efficient way possible.
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