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allows for the distinction between drugs and
metabolites while maintaining the spatial localiza-
tion of these compounds.
IMS of drugs and other small molecules has the
added challenge in that there is redundancy of
nominal masses observed in the lower molecular
weight part of the spectrum. Often several
compounds within a tissue section can have the
same nominal mass as the molecule of interest
andmaynotbeabletoberesolvedbylowermass
resolu ionanalyzerssuchasquadrupoleand
time-of-
this approach in the analysis of positron emission
tomography (PET) tracers in the brains and
kidneys of mice.
73
Two compounds, 3,5-dichloro-
N
-[(2
S
)-1-ethylpyrrolidin-2-yl]methyl-2-hydroxy-
6-methoxybenzamide (raclopride) or 7-cholor-3-
methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-
8-ol (SCH 23390) were administered by injection
at different doses through the tail veins of rats.
Animals were sacri
ced at 1, 5, or 30 minutes
post dose and the brains and kidneys dissected
and snap frozen. Midpoint sections of the organs
were collected and CHCA matrix was deposited
by dry coating, which allowed for desorption
and ionization of both compounds without the
delocalization that can occur with wet, solvent-
based matrix application. Both compounds
were imaged as intact parent masses along with
MS/MS imaging. Raclopride was imaged by
monitoring the transition
m/z
347 to
m/z
129 and
m/z
111 while SCH 23390 was imaged by moni-
toring the transition
m/z
288 to
m/z
179. Images
of parent compounds collected on both qTOF
and FT-ICR instruments showed excellent corre-
lation with MS/MS images of the compounds
con
ight-based analyzers. Three different
approaches can be taken to circumvent this issue.
The
first is to use a much higher resolution mass
analyzer such as a Fourier transform ion cyclotron
resonance (FT-ICR) instrument that can allow for
the distinction of compounds separated by frac-
tions of one mass unit and con
dently validate
the identity of a compound by accurate mass.
68,70
A second approach is to carry out the imaging in
an MS/MS mode by following a speci
c transition
from the parent mass to a fragment ion of the
compound.
17,71
A third approach is to use ion
mobility mass spectrometry to separate molecules
of interest from other isobaric compounds based
on their gas phase conformations and capture cross
sectional areas.
11,72
Castellino et al. have shown the applicability
of the FT-ICR approach in the imaging of several
drugs and metabolites in animal tissue.
68
Organs
were collected from dogs dosed with lapatinib
and subjected to imaging using a Bruker solariX
12T FT-ICR mass spectrometer. Through use of
this high-resolution instrument, investigators
were able to distinguish two different metabo-
lites, GW006 and M2, which differ in mass by
0.013 amu and display different spatial distribu-
tions within the liver of the dosed dog. This work
has led to the identi
rming their localization and identities
(
Figure 4
). Raclopride was found to be at highest
levels in the brain at 1 minute post dose with an
80% drop in concentration by 30 minutes post
dose as evidenced by decreased signal intensity.
Similar results were obtained in the kidney. This
method was also used to quantitate raclopride
in the tissue sections by use of a serial dilution
of the compound on control tissue section to
a generated calibration curve based on signal
response. Raclopride was determined to be at
a concentration of 60 nM at 1 minute post 7.5
mg/kg dose and a concentration of 15 nM at 1
minute post 2 mg/kg dose.
cation of a total of 21
different metabolites in dog liver sections.
68
Alternatively, selected reaction monitoring
through the application of MS/MS imaging can
be used in the accurate determination of the spatial
localization and quantitation of drug compounds
in tissue sections. Goodwin et al. have employed
3D IMAGING
An exciting application of imaging mass
spectrometry is the ability to generate three-
dimensional volumes of biomolecules within an
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