Radiosynthesis of 123I-labelled benzimidazoles as novel single-photon emission computed tomography tracers for the histamine H3 receptor

Article abstract of DOI:10.1002/jlcr.1899

The histamine H3 receptor is implicated in many neurological and psychological disorders and is therefore of interest as a target for pharmacological intervention. The drug development process can be facilitated by using a radiotracer for this receptor in

Full text of DOI:10.1002/jlcr.1899

Note
Received 28 February 2011,
Revised 8 April 2011,
Accepted 13 April 2011
Published online 19 July 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI: 10.1002/jlcr.1899
Radiosynthesis of 123I‐labelled
benzimidazoles as novel single‐photon
emission computed tomography tracers
for the histamine H3 receptor
a
b
b
*
Sue Champion, Jonathan Gross, Albert J Robichaud,
and Sally Pimlottc
The histamine H3 receptor is implicated in many neurological and psychological disorders and is therefore of interest
as a target for pharmacological intervention. The drug development process can be facilitated by using a radiotracer
for this receptor in conjunction with single‐photon emission computed tomography or positron emission tomography
imaging studies, such as drug occupancy studies. This study investigates methods for the radiosynthesis of three
compounds to be evaluated as novel radiotracers for the histamine H3 receptor. The radiosyntheses of the three target
compounds, 123I‐WYE230949, 123I‐WYE126734 and 123I‐WYE127044 were evaluated using electrophilic iododesilylation
and iododestannylation of the corresponding trimethylsilyl and tributylstannyl precursors. All three compounds could
be produced in high radiochemical yield by electrophilic iododestannylation. By contrast, only two of the three
(WYE230949 and WYE126734) could be produced by electrophilic iododesilylation. This is because of the fact that the
trimethylsilyl precursors are less reactive than the stannyl precursors. Electrophilic iododestannylation of the tributylstannyl
precursors is therefore the method of choice for radiosynthesis of these compounds because all three target compounds
could be produced in high radiochemical yield.
Keywords: radiosynthesis; histamine H3 receptor; radioiodination; single photon emission computed tomography;
iododestannylation; iododesilylation
Introduction
Material and methods
Reagents
The release and synthesis of the neurotransmitter histamine
is regulated by a presynaptic G protein‐coupled histamine H3
autoreceptor.1 The histamine H3 receptor also acts as a
heteroreceptor regulating other neurotransmitters such as
acetylcholine,2,3 noradrenaline,4 dopamine5 and serotonin.6 It
has been suggested that histamine H3 receptor therapeutics
may have utility in a number of neurological and psycho-
logical disorders including schizophrenia, attention deficit
hyperactivity disorder, Alzheimer’s disease and obesity.7–9
This project aimed to develop a single‐photon emission com-
puted tomography imaging agent for the histamine H3 receptor
in order to facilitate the development of novel H3 therapeutics.
Our three candidate molecules WYE230949 (1), WYE126734 (2)
and WYE127044 (3) (Figure 1) were chosen from a library of
compounds within Wyeth Research’s histamine H3 receptor drug
development programme.10
No‐Carrier added sodium [123I]‐iodide in dilute (0.05 M) NaOH was
purchased from GE Healthcare, Amersham, Bucks, UK. Other
reagents and chemicals were purchased from Riedel de Haën and
Sigma–Aldrich, and were used without further purification.
Precursor compounds and cold standards were obtained from
Wyeth Research, Princeton, NJ, USA. Peracetic acid was prepared
the day prior to reaction by mixing together 2 ml of 35%
hydrogen peroxide solution and 1 ml glacial acetic acid, and
leaving to stand overnight at room temperature.
aInstitute of Neuroscience and Psychology, University of Glasgow, West of
Scotland Radionuclide Dispensary, Western Infirmary, Glasgow, G11 6NT, UK
bWyeth Research Chemical Sciences, Princeton, NJ, 08551, USA
Iododestannylation and iododesilylation with 123I were inves-
tigated as routes to obtain the target radiolabelled compounds. cSchool of Medicine, University of Glasgow, Glasgow, UK
Both of these are established methods for introducing radio-
*Correspondence to: Sue Champion, Institute of Neuroscience and Psychology,
University of Glasgow, West of Scotland Radionuclide Dispensary, Western
Infirmary, Glasgow, UK, G11 6NT.
iodine (and other radiohalogens) into a target molecule.11 It was
anticipated that one or both of these methods would afford the
target compounds in a good radiochemical yield.
E‐mail: sue.champion@glasgow.ac.uk
J. Label Compd. Radiopharm 2011, 54 674–677
Copyright © 2011 John Wiley & Sons, Ltd.

S. Champion et al.
Scheme 2. Radiolabelling via iododesilylation.
Table 1. Radiochemical purity (%) determined by HPLC
analysis of radiolabelling reaction mixtures
Method of labelling
Iododestannylation
Precursor
Radiochemical purity by
HPLC (%)
1a
2a
3a
1b
2b
3b
72 10 (n = 3)
80 4 (n = 4)*
77 5 (n = 6)
66 12 (n = 4)
48 4 (n = 4)*
negligible
Figure 1. Structures of target molecules.
Iododesilylation
HPLC methods
Analytical HPLC was performed using a Phenomenex 4 µm
Hydro‐RP 80Å column (150 × 4.6 mm) (Phenomenex, Inc.,
Torrance, CA, USA) with 10 mm guard cartridge, UV 254 nm
and flow 1 ml/minute. Radiodetection was carried out using a
Radiomatic 500R series radiodetector and the mobile phase was
0.1% trifluoroacetic acid in water:0.1% trifluoroacetic acid in
acetonitrile The identity of each of the three radioactive
compounds was confirmed by co‐elution of the product with
the cold standard compound by analytical HPLC.
*p < 0.05 between iododestannylation and iododesilylation
followed by oxidant (Scheme 2). Acetic acid and 80:20 ethanol:
acetic acid were investigated as a solvent for the precursor and
peracetic acid and chloramine‐T solution in varying volumes
were investigated as oxidants. The reaction was mixed via
vortex, and the time and temperature of the reaction were
investigated. The reaction mixture was analysed by analytical
HPLC to determine the incorporation yield.
Radiolabelling with 123I via iododestannylation
To a v‐vial containing 10–80 MBq of Na[123I] in 50 µl of 0.05 M
NaOH was added 20 µl of 1 M HCl, 0.3 mg of the precursor (1a–3a)
in 100 µl ethanol and 50 µl chloramine‐T solution (1 mg/ml)
(Scheme 1). The reaction was mixed via vortex and incubated at
room temperature for 5 minutes. The reaction mixture was
analysed by analytical HPLC to determine the incorporation yield.
Results and discussion
Electrophilic radioiododestannylation of the tributyltin precur-
sors as shown in Scheme 1 resulted in high (>70%) incorporation
yields of all three target compounds (1–3) after a 5 minutes
room temperature radiolabelling reaction (Table 1). These yields
represent the optimal conditions, and HPLC analysis showed
that the radiolabelled compounds 1–3 co‐eluted with samples
of the respective cold standards.
Radiolabelling with 123I via iododesilylation
To a v‐vial containing 10–80 MBq of Na[123I] in 50 µl of 0.05 M
NaOH was added 0.3 mg of precursor (1b–3b) in 100 µl solution
Scheme 1. Radiolabelling via iododestannylation.
J. Label Compd. Radiopharm 2011, 54 674–677
Copyright © 2011 John Wiley & Sons, Ltd.
www.jlcr.org

S. Champion et al.
Table 2. Summary of radiolabelling reactions of precursor 3b via iododesilylation
Oxidant
Added amount of oxidant (µl) Temperature (°C) Time (minutes) Radiochemical purity by HPLC (%)*
Peracetic acid
Chloramine‐T
(5 mg/ml)
15
20
50
65
65
65
65
65
65
82
82
20
45
45
20
30
45
20
20
0
7
7
9
7
0
0
0
50
100
100
50
100
*n = 1 for each reaction condition.
Compounds 1 and 2 could also be produced by heating with reacting aromatic ring of 3 deactivates the ring towards
their respective trimethylsilyl precursors with chloramine‐T in electrophilic attack and substitution at the ortho position (the
acetic acid, as shown in Scheme 2. The time, temperature and site of the leaving group on 3) is less electronically favoured under
amount of oxidant were investigated, and the optimal yields these conditions. The combination of a less electronically
(Table 1) were obtained using 50 µl of 5 mg/ml chloramine‐T favoured reaction with the relatively shorter and stronger
solution at 65 °C for 45 minutes. There was no significant difference carbon‐silicon bond compared with the carbon‐tin bond could
in the incorporation yield of 1 between iododestannylation and account for the difference in reactivity between 3a and 3b.
iododesilylation; however, the yield of 2 by iododesilylation was
significantly lower ( p < 0.05, two‐tailed t‐test) than that achieved
by iododestannylation. In addition, the radioiododesilylation
reactions took much longer (45 minutes vs 5 minutes for radio-
iododestannylation), and required heating to 65 °C.
Labelling of precursor 3b by electrophilic iododesilylation
proved problematic. Using the same methodology as for the
other trimethylsilyl precursors (solution of the precursor in acetic
acid and heating at 65 °C for 45 minutes) was unsuccessful and
Conclusions
The three target compounds, WYE230949 (1), WYE126734 (2)
and WYE127044 (3) have all been efficiently radiolabelled via
electrophilic iodostannylation. 123I‐WYE230949 (1) and 123I‐
WYE126734 (2) can also be synthesised by electrophilic
iododesilylation, but this failed for WYE127044 (3). In the case
of electrophilic radioiodesilylation, the reaction takes much
resulted in little or no product. Changes of the amount of oxidant
longer and requires heating because of the greater stability of
added, the oxidant itself, the reaction time and/or temperature
the silyl precursor. Therefore, radiosynthesis of the target
did not have any effect on the incorporation yield (Table 2). The
compounds 1–3 via electrophilic iododestannylation is the
method of choice.
solvent was changed to 20% acetic acid in ethanol and a
radioactive peak other than iodide was formed. However, this
radioactive peak did not co‐elute with cold standard. Synthesis
of 123I‐WYE127044 (3) by iododesilylation of precursor 3b was
therefore unsuccessful.
Acknowledgements
This work was supported by an award (Ref: NS_AU_084) from
the Translational Medicine Research Collaboration – a consor-
tium made up of the Universities of Aberdeen, Dundee,
Edinburgh and Glasgow, the four associated HNS Health Boards
(Grampian, Tayside, Lothian and Greater Glasgow & Clyde),
Scottish Enterprise and Wyeth Pharmaceutical. Dr. S. Champion
is funded by the Scottish Imaging Network: A Platform for
Scientific Excellence (SINAPSE).
Because of the different bond lengths of the C‐Sn (2.16 Å) and
C‐Si bonds (1.85 Å) , and consequently bond strength (192 kJ/mol
and 318 kJ/mol, respectively, C‐Sn and C‐Si bond), different
reactivities can be expected.12 As such, tributyltin precursors are
more reactive than the corresponding trimethylsilyl analogues, as
has been noted previously.13,14 Thus, whilst compounds 1 and 2
can be synthesised by electrophilic iododesilylation, the reaction
conditions are harsher and reaction times longer than for the
corresponding iododestannylation. This difference in reactivity
between the stannyl and silyl precursors seems to be particularly
pronounced for compound 3.
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undergoes efficient radioiodination. It is therefore unlikely that
the low reactivity of the silyl precursor 3b is due to steric effects.
Ipso‐iododemetallation is an analogous mechanistic process to
electrophilic aromatic substitution,15 and therefore the iodode-
metallation reaction will likewise be affected by the presence of
other substituents on the aromatic ring. The carbonyl group in the
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