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\begin{document}

\title{Commentary on ``PET imaging of microglia by targeting macrophage
colony-stimulating~ factor 1 receptor (CSF1R)''}



\author[1]{SRSTT Radiopharmaconnect}%
\affil[1]{Radiopharmaconnect}%


\vspace{-1em}



  
  \date{May 27, 2024}


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Horti, A. G. \emph{et al.}
\textbf{\href{https://www.pnas.org/content/116/5/1686}{PET imaging of
microglia by targeting macrophage colony-stimulating factor 1 receptor
(CSF1R)}.} Proc Natl Acad Sci U S A 116, 1686-1691,
doi:10.1073/pnas.1812155116 (2019).

Written by \href{https://twitter.com/igorcamargof?lang=en}{Igor Camargo
Fontana}

\par\null

\textbf{1.~ ~ ~Abstract:}

Neuroinflammation is involved in several brain neurodegenerative
diseases, such as Alzheimer's Disease (AD) and Multiple Sclerosis
(MS)\textsuperscript{1,2}. Horti, A. G. \emph{et al.} (2019) developed
{[}\textsuperscript{11}C{]}CPPC, a Positron Emission Tomography (PET)
radiotracer with high specificity for the macrophage colony-stimulating
factor 1 receptor (CSF1R), essentially expressed in microglia. In
addition, the authors discussed {[}\textsuperscript{11}C{]}CPPC
application in models of neuroinflammation, AD and MS. Here, this
commentary will be focused on reviewing the steps for
{[}\textsuperscript{11}C{]}CPPC radiosynthesis, and discussing its
application in animal models of neuroinflammation and a mouse model of
AD.

~

\textbf{2.~~~} \textbf{Synthesis}

The precursor (Pre-CPPC) was prepared in a four-step reaction.
Radiosynthesis (Figure 1) of {[}\textsuperscript{11}C{]}CPPC was
obtained by using {[}\textsuperscript{11}C{]}CH\textsubscript{3}I, with
a radiochemical yield of 21 \selectlanguage{ngerman}± 8\% and high radiochemical purity
(\textgreater{} 95\%).

\par\null

~\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Presentation1/Presentation1}
\caption{{Radiosynthesis of {[}\textsuperscript{11}C{]}CPPC.
{\label{209581}}%
}}
\end{center}
\end{figure}

\textbf{~}

\par\null\par\null

\textbf{3.~~}~ \textbf{Results:}

The brain regional distribution of {[}\textsuperscript{11}C{]}CPPC was
evaluated using healthy C57BL/6J mice. Animals were sacrificed at 5, 15,
30 and 60 min and \%SUV calculated.~

\selectlanguage{ngerman}·~~~~~~ Major {[}\textsuperscript{11}C{]}CPPC uptake was seen in the
frontal cortex with 150\% SUV, between 5-15 min after radiotracer
injection;

·~~~~~~ Blocking studies demonstrated the specific binding of
{[}\textsuperscript{11}C{]}CPPC only when correcting brain uptake for
radioactivity concentration in blood (SUVR);

·~~~~~~ Results indicated a 20\% decrease on radioactivity in mice
treated with high doses of CPPC blocker;

·~~~~~~ The {[}\textsuperscript{11}C{]}CPPC brain uptake has a small
significant reduction in microglia-depleted mice;~

The {[}\textsuperscript{11}C{]}CPPC brain\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Presentation2/Presentation2}
\caption{{\textbf{CSF1R expression and
{[}}\textsuperscript{\textbf{11}}\textbf{C{]}CPPC binding in healthy and
disease. A --} CSF1R expression and {[}\textsuperscript{11}C{]}CPPC
binding are increased in LPS-induced or disease-related~
neuroinflammatory conditions. \textbf{B -} Physicochemical properties
and specific binding in animal models suggest great potential for
{[}\textsuperscript{11}C{]}CPPC clinical translation.
{\label{118737}}%
}}
\end{center}
\end{figure}

\par\null

~

\textbf{Biodistribution of
{[}}\textsuperscript{\textbf{11}}\textbf{C{]}CPPC in LPS-induced murine
models of neuroinflammation}

*~~~~~ Two LPS-induced neuroinflammation models were performed:
administration via intraperitoneal (i.p.) and intracranial (i.c.);

*~~~~~ The \%SUV of {[}\textsuperscript{11}C{]}CPPC uptake was increased
in both LPS models~ compared to control mice;

*~~~~~ In blocking studies \%SUV was only significant in i.c. LPS model;

*~~~~~ For i.p. LPS model, brain \%SUV had to be corrected by the
radioactivity concentration in blood.

A previous work published by Illig, C. R. et al. \textsuperscript{3},
describes the pharmacological properties of CPPC and other CSF1R
specific inhibitors. The best molecules were tested in arthritis mouse
model, showing efficacy by reducing bone erosion, cartilage damage and
inflammation. These data indicate that the CSF1R inhibitors may interact
throughout the body before reaching the brain. Indeed, this may explain
the need for blood correction to obtain significant results in blocking
studies involving a systemic inflammation mouse model (i.p. LPS).

\textbf{~}

\textbf{A Murine model of Alzheimer's Disease:}

*~~~~~ {[}\textsuperscript{11}C{]}CPPC was tested in a mouse model of
AD, using mice overexpressing the human amyloid precursor protein (APP)
with Swedish and Indiana APP mutations;

*~~~~~ {[}\textsuperscript{11}C{]}CPPC uptake was increased in AD mice
compared to wild-type (WT) littermates controls mice;

*~~~~~ Highest {[}\textsuperscript{11}C{]}CPPC uptake was in cortex and
hippocampus, two brain areas affected in AD \textsuperscript{4}.

Herein, aged APP-mice (16 months) were used to test
{[}\textsuperscript{11}C{]}CPPC uptake. At this age, these animals
present high insoluble amyloid-beta (A\selectlanguage{greek}β\selectlanguage{english})~ content widely distributed in
the brain \textsuperscript{5}. In fact, recent evidence suggests
microglial activation as an early stage in AD pathology, which occurs
even before the formation of mature insoluble A\selectlanguage{greek}β \selectlanguage{english}plaques
\textsuperscript{6,7}. In keeping with this,
{[}\textsuperscript{11}C{]}CPPC could be differentially taken up in
different stages of amyloidosis. A PET longitudinal study with APP-mice
and {[}\textsuperscript{11}C{]}CPPC, would be a very important step to
elucidate initial inflammatory changes in AD\textsuperscript{8}.

~

\textbf{A nonhuman primate LPS model of neuroinflammation:}

*~~~~~ A systemic administration of LPS increased the distribution
volume in the whole brain compared to control baboons;

*~~~~~ The increase on distribution volume in the brain was fully
blocked by injecting non-radioactive CPPC;

*~~~~~ Blood sample analysis detected the presence of two hydrophilic
metabolites, which were shown to minimally penetrate the brain;

~

\textbf{4.~~~} \textbf{Conclusions:}

It is well known that most PET radiotracers aiming the CNS fail due to
its inability to cross the blood-brain barrier. The innovative PET
radiotracer {[}\textsuperscript{11}C{]}CPPC for imaging CSF1R discussed
in this commentary, presented good brain penetrance, a moderate
heterogenous distribution and good clearance.
{[}\textsuperscript{11}C{]}CPPC is obtained with sufficient
radiochemical yield, purity and specific activity, essential features
for clinical translation. Of note, {[}\textsuperscript{11}C{]}CPPC has
high specificity in mouse and baboon models of neuroinflammation.
Additionally, in murine models of AD, there was significant increase on
{[}\textsuperscript{11}C{]}CPPC binding compared to controls. Together
these results indicate {[}\textsuperscript{11}C{]}CPPC as a potential
tool to help on the diagnosis of brain diseases involving
neuroinflammation, such as AD and MS. However, an important question
remains: would {[}\textsuperscript{11}C{]}CPPC be useful for detecting
microglia activation and neuroinflammation in the early stages of AD,
before the symptomatic phase?

\textbf{Take home message:}

\textbf{What would I like to see on the follow up of this paper?}

The application of {[}\textsuperscript{11}C{]}CPPC targeting CSF1R in
neuroinflammation-linked diseases has great potential for clinical
translation. Currently, the inaccuracy of diagnosis and treatment for
neurodegenerative diseases is a great matter of discussion. In AD, it
seems that neuroinflammatory changes may emerge even earlier than
detectable canonical A\selectlanguage{greek}β \selectlanguage{english}and tau pathology. If
{[}\textsuperscript{11}C{]}CPPC could detect this initial
neuroinflammatory alterations with topographical resolution could help
to detect the earliest AD-related changes, opening a new window for
therapeutic intervention.

~

\textbf{References:}

1~~~~~~~~~ Calsolaro, V. \& Edison, P. Neuroinflammation in Alzheimer's
disease: Current evidence and future directions. \emph{Alzheimers
Dement} \textbf{12}, 719-732, doi:10.1016/j.jalz.2016.02.010 (2016).

2~~~~~~~~~ Datta, G. \emph{et al.} Neuroinflammation and its
relationship to changes in brain volume and white matter lesions in
multiple sclerosis. \emph{Brain} \textbf{140}, 2927-2938,
doi:10.1093/brain/awx228 (2017).

3~~~~~~~~~ Illig, C. R. \emph{et al.} Discovery of novel FMS kinase
inhibitors as anti-inflammatory agents. \emph{Bioorg Med Chem Lett}
\textbf{18}, 1642-1648, doi:10.1016/j.bmcl.2008.01.059 (2008).

4~~~~~~~~~ Reilly, J. F. \emph{et al.} Amyloid deposition in the
hippocampus and entorhinal cortex: quantitative analysis of a transgenic
mouse model. \emph{Proc Natl Acad Sci U S A} \textbf{100}, 4837-4842,
doi:10.1073/pnas.0330745100 (2003).

5~~~~~~~~~ Melnikova, T. \emph{et al.} Reversible pathologic and
cognitive phenotypes in an inducible model of Alzheimer-amyloidosis.
\emph{J Neurosci} \textbf{33}, 3765-3779,
doi:10.1523/JNEUROSCI.4251-12.2013 (2013).

6~~~~~~~~~ Boza-Serrano, A., Yang, Y., Paulus, A. \& Deierborg, T.
Innate immune alterations are elicited in microglial cells before plaque
deposition in the Alzheimer's disease mouse model 5xFAD. \emph{Sci Rep}
\textbf{8}, 1550, doi:10.1038/s41598-018-19699-y (2018).

7~~~~~~~~~ Kreisl, W. C. Discerning the relationship between microglial
activation and Alzheimer's disease. \emph{Brain} \textbf{140},
1825-1828, doi:10.1093/brain/awx151 (2017).

8~~~~~~~~~ Fan, Z., Brooks, D. J., Okello, A. \& Edison, P. An early and
late peak in microglial activation in Alzheimer's disease trajectory.
\emph{Brain} \textbf{140}, 792-803, doi:10.1093/brain/aww349 (2017).

\par\null

~



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