2 research outputs found
Synthesis, Radiolabeling, and Characterization of Plasma Protein-Binding Ligands: Potential Tools for Modulation of the Pharmacokinetic Properties of (Radio)Pharmaceuticals
The
development of (radio)Āpharmaceuticals with favorable pharmacokinetic
profiles is crucial for allowing the optimization of the imaging or
therapeutic potential and the minimization of undesired side effects.
The aim of this study was, therefore, to evaluate and compare three
different plasma protein binders (PPB-01, PPB-02, and PPB-03) that
are potentially useful in combination with (radio)Āpharmaceuticals
to enhance their half-life in the blood. The entities were functionalized
with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)
chelator via a l-lysine and Ī²-alanine linker moiety
using solid-phase peptide chemistry and labeled with <sup>177</sup>Lu (<i>T</i><sub>1/2</sub> = 6.65 days), a clinically established
radiometal. The binding capacities of these radioligands and <sup>177</sup>LuāDOTA were evaluated using human plasma and solutions
of human serum albumin (HSA), human Ī±<sub>1</sub>-acid glycoprotein
(Ī±<sub>1</sub>-AGP), and human transthyretin (hTTR) by applying
an ultrafiltration assay. <sup>177</sup>LuāDOTAāPPB-01
and <sup>177</sup>LuāDOTAāPPB-02 bound to a high and
moderate extent to human plasma proteins (>90% and ā¼70%,
respectively),
whereas the binding to hTTR was considered negligible (<10%). <sup>177</sup>LuāDOTAāPPB-03 showed almost complete binding
to human plasma proteins (>90%) with a high fraction bound to hTTR
(ā¼50%). Plasma protein binding of the <sup>177</sup>LuāDOTA
complex, which was used as a control, was not observed (<1%). <sup>177</sup>LuāDOTAāPPB-01 and <sup>177</sup>LuāDOTAāPPB-02
were both displaced (>80%) from HSA by ibuprofen, specific for
Sudlowās
binding site II and coherent with the aromatic structures, and >80%
by their respective binding entities. <sup>177</sup>LuāDOTAāPPB-03
was displaced from hTTR by the site-marker l-thyroxine (>60%)
and by its binding entity PPB-03* (>80%). All three radioligands
were
investigated with regard to the in vivo blood clearance in normal
mice. <sup>177</sup>LuāDOTAāPPB-01 showed the slowest
blood clearance (<i>T</i><sub>1/2,Ī²</sub>: >15
h)
followed by <sup>177</sup>LuāDOTAāPPB-03 (<i>T</i><sub>1/2,Ī²</sub>: ā¼2.33 h) and <sup>177</sup>LuāDOTAāPPB-02
(<i>T</i><sub>1/2,Ī²</sub>: ā¼1.14 h), which
was excreted relatively fast. Our results confirmed the high affinity
of the 4-(4-iodophenyl)-butyric acid entity (PPB-01) to plasma proteins,
while replacement of the halogen by an ethynyl entity (PPB-02) reduced
the plasma protein binding significantly. An attractive approach is
the application of the transthyretin binder (PPB-03), which shows
high affinity to hTTR. Future studies in our laboratory will be focused
on the application of these binding entities in combination with clinically
relevant targeting agents for diagnostic and therapeutic purposes
in nuclear medicine
Reduced <sup>18</sup>FāFolate Conjugates as a New Class of PET Tracers for Folate Receptor Imaging
5-Methyltetrahydrofolate
(5-MTHF), a reduced folate form, is the
biologically active folate involved in many different metabolic processes.
To date, there are no studies available in the literature on <sup>18</sup>F-labeled 6<i>S</i>- and 6<i>R</i>-5-MTHF
radiotracers for imaging folate receptor (FR)-Ī±-positive tissues.
Therefore, the goal of this study was to synthesize four <sup>18</sup>F-labeled 5-MTHF derivatives conjugated at either the Ī±- or
Ī³-carboxylic functionality of glutamate and to assess their
suitability for FR-targeting. Organic syntheses of the precursors
and the four reference compounds, namely, 6<i>S</i>-Ī±,
6<i>S</i>-Ī³, 6<i>R</i>-Ī±, and 6<i>R</i>-Ī³-click-fluoroethyl-5-MTHF, were carried out in
low to moderate overall chemical yields. The radiosyntheses of the
Ī±- and Ī³-conjugated <sup>18</sup>F-labeled folate derivatives
were accomplished in approximately 100 min, low radiochemical yields
(1ā7% d.c.) and high molar activities (139ā245 GBq/Ī¼mol).
Radiochemically pure tracers were obtained after the addition of a
mixture of antioxidants consisting of sodium ascorbate and l-cysteine. <i>In vitro</i>, all four 5-MTHF conjugates
showed similar binding affinities to FR-Ī± (IC<sub>50</sub> =
17.7ā24.0 nM), whereas folic acid showed a significantly higher
binding affinity to the FR-Ī±. Cell uptake and internalization
experiments with KB cells demonstrated specific uptake and internalization
of the radiofolate conjugates. Metabolite studies in mice revealed
high <i>in vivo</i> stability of the radiotracers in mice.
Biodistribution and positron emission tomography (PET) imaging studies
in FR-positive KB tumor-bearing mice demonstrated that the 6<i>S</i>- and 6<i>R</i>-5-MTHF conjugates exhibited a
different accumulation pattern in various organs including the kidneys
and the liver, whereas no significant differences in radioactivity
accumulation in the kidneys and the liver were found for both the
Ī±- and Ī³-conjugated diastereoisomers. Despite the considerably
lower binding affinities of the 5-MTHF derivatives compared to the
corresponding folic acid conjugates similar high KB tumor uptake was
observed for all the folate conjugates investigated (8ā11%
IA/g). Based on these results, we conclude that <sup>18</sup>F-labeled
5-MTHF conjugates are a promising new class of radiotracers for targeting
FR-positive tumor tissues