A unique paradigm for a turn-ON near-infrared cyanine-based probe: Noninvasive intravital optical imaging of hydrogen peroxide

Article abstract of DOI:10.1021/ja203145v

The development of highly sensitive fluorescent probes in combination with innovative optical techniques is a promising strategy for intravital noninvasive quantitative imaging. Cyanine fluorochromes belong to a superfamily of dyes that have attracted sub

Full text of DOI:10.1021/ja203145v

ARTICLE
pubs.acs.org/JACS
A Unique Paradigm for a Turn-ON Near-Infrared Cyanine-Based Probe:
Noninvasive Intravital Optical Imaging of Hydrogen Peroxide
Naama Karton-Lifshin,† Ehud Segal,‡ Liora Omer,‡ Moshe Portnoy,† Ronit Satchi-Fainaro,*,‡ and
Doron Shabat*,†
†School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences and ‡Department of Physiology and Pharmacology,
Sackler School of Medicine, Tel Aviv University, Tel Aviv 69978, Israel
S Supporting Information
b
ABSTRACT: The development of highly sensitive fluorescent probes in
combination with innovative optical techniques is a promising strategy for
intravital noninvasive quantitative imaging. Cyanine fluorochromes belong to
a superfamily of dyes that have attracted substantial attention in probe design
for molecular imaging. We have developed a novel paradigm to introduce a
Turn-ON mechanism in cyanine molecules, based on a distinctive change in
their π-electronssystem. Our new cyaninefluorochromeis synthesized through
a simple two-step procedure and has an unprecedented high fluorescence
quantum yield of 16% and large extinction coefficient of 52 000 MÀ1cmÀ1. The
synthetic strategy allows one to prepare probes for various analytes by
introducing a specific triggering group on the probe molecule. The probe
was equipped with a corresponding trigger and demonstrated efficient imaging
of endogenous hydrogen peroxide, produced in an acute lipopolysaccharide-
inducedinflammation model in mice. This approachprovides, for the first time, anavailable methodologytoprepare modularmolecular
Turn-ON probes that can release an active cyanine fluorophore upon reaction with specific analyte.
’ INTRODUCTION
Optical imaging in the near-infrared (NIR) range enables
detection of molecular activity in vivo due to high penetration of
NIR photons through organic tissues and low autofluorescence
background.1À5 Cyanine dyes are widely employed as fluores-
cence labels for NIR imaging, since they are compounds with
Figure 1. Classic mechanism for a modular Turn-ON fluorescence probe.
large extinction coefficient and relatively high quantum yield.6
These qualities allow their fluorescence to penetrate deep tissues,
and thus, cyanines serve as attractive probes for intravital non-
specific Turn-ON probe to demonstrate noninvasive in vivo ima-
ging of endogenously produced hydrogen peroxide.
invasive imaging. To generate a Turn-ON system for a cyanine
molecule, a fluorescence resonance energy transfer7 (FRET)
A classic modular Turn-ON fluorescence probe is constructed
approachis usuallyapplied.8À12 Toincorporatea cyaninemolecule
of a dye molecule with a conjugated functional group (X) linked to
a trigger moiety (Figure 1). The masking of the functional group
reduces the conjugated π-electron system of the dye and thus turns
OFF the fluorescence of the probe. Removal of the trigger renews
the conjugation of the functional group with the π-electrons of the
dye and thereby turns ON the fluorescent signal of the probe.
Applying this Turn-ON mechanism in cyanine dyes is more
in a FRET-based probe, an additional dye must be included in the
molecular system. An alternative simpler approach, to turn ON a
fluorophore, could be achieved by applying changes in the pull-
push conjugated π-electron system of the dye. Such concept was
mostly demonstrated for dyes with UVÀvis fluorescence.13,14
Implementing reversible changes in the π-electron system of
difficult than for other common fluorescent dyes, such as coumarin
cyanine molecules is typically more complicated due to an absence
and fluorescein, since cyanine molecules have a polymethine
of a conjugated functional group. Here, we report a novel simple
strategy to introduce a Turn-ON mechanism in molecules with
cyanine spectroscopic characteristics. The approach is based on a
new design of a distinctive fluorochrome, which can be modularly
derivatized to generate different imaging probes for biological
research and clinical needs. We have applied our fluorochrome in a
π-electron system in between two nitrogen atoms that cannot be
masked by a trigger. To generate such a Turn-ON mechanism in
cyanine molecules, a new approach should be considered.
Received: April 17, 2011
Published: June 02, 2011
r
2011 American Chemical Society
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Figure 2. (I) Design of a cyanine-based Turn-ON probe with NIR fluorescence. (II) Synthesis of QCy7.
’ RESULTS AND DISCUSSION
The dye QCy7 was obtained as an orange crystalline powder.
The UVÀvis spectrum exhibits two absorption peaks with
maximum at 460 and 570 nm (Figure 3A). As expected, the
fluorescence spectrum indeed shows emission peak in the NIR
region at a wavelength of 715 nm. The obtained large Stokes shift
between the excitation and the emission wavelengths of about
150 nm is a desirable feature for fluorescence probes, which assists
in increasing the signal-to-noise ratio.18,19 Next, we measured the
fluorescence spectrum of acetate 3 in comparison with that of
QCy7. Since acetate 3 is a masked derivative of QCy7, it can be
used to model the fluorescence activity of a probe with a specific
trigger. As predicted, the acetate derivative of QCy7 did not show
any fluorescence emission in the NIR region (Figure 3B, blue
plot).
With these results in hand, we turn to develop a specific
diagnostic probe based on our NIR fluorophore. It is well-known
that inflammatory events lead to oxidative stress and overproduc-
tion of hydrogen peroxide, which can damage living tissues.5,20
Thus, the development of optical probes that can image hydrogen
peroxide in vivo is a major challenge in the field of molecular
imaging. There are numerous reports describing the development
and evaluation of such imaging probes.21À24 These are commonly
based on boronic acid/ester as a specific masking group25À28 that
Our probe’s design is based on a new Cy7-like molecule, with a
conjugated functional group that can be masked, as presented in
Figure 2. Cy7 is a known heptamethine cyanine dye, which is
widely used for NIR imaging.15À17 The conjugated π-electron
system of Cy7 has a positive charge that is delocalized between
the two nitrogen atoms. Unlike Cy7, in phenol 1, the nitrogen
atoms have two positive charges, (one on each) and thus, the
conjugation pattern is significantly different. Deprotonation of
phenol 1 results in generation of phenolate 1a. The negative
charge of the phenolate can be delocalized toward one of the
nitrogen atoms to form quinones 1b or 1c and thereby eliminat-
ing one positive charge. The obtained quinone derivative QCy7
(the resonance hybrid of species 1a, 1b and 1c) has now similar
conjugation pattern to that of Cy7 and thus is expected to emit
NIR fluorescence. The incorporation of various protecting
groups to phenol 1 should allow masking of the NIR fluorescence
of QCy7 by a specific trigger moiety by which, upon its removal,
the free fluorophore will be obtained. QCy7 was synthesized by a
simple two-step procedure as presented in Figure 2. Commer-
cially available dialdehyde 2 was condensed with 2 equiv. of
indolium-iodide to give ester 3. The acetate group of 3 was
removed with potassium carbonate in methanol to afford QCy7.
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Figure 3. (A) Absorption and fluorescence spectra of QCy7 in PBS 7.4 [100 μM]. (B) NIR fluorescence spectra of QCy7 (red) in comparison to that of
Acetate 3 (blue) in PBS 7.4 [100 μM] (λex = 560 nm).
Figure 4. (I) Activation of probe 4 by hydrogen peroxide releases sulfo-QCy7. (II) Synthesis of probe 4.
can be removed upon reaction with hydrogen peroxide. Accord-
ingly, we designed molecular probe 4, which has phenylboronic
acid attached through an ether-linkage to a sulfonated derivative of
QCy7 (Figure 4). The purpose of sulfonated alkyl-chains is merely
to increase the aqueous solubility and to prevent aggregation of the
relatively hydrophobic QCy7 molecules. Incubation of hydrogen
peroxide with probe 4 under physiological conditions should result
in oxidation of the phenylboronic acid, which is followed by
hydrolysis and 1,6-elimination of p-quinone-methide to release
the active fluorophore sulfo-QCy7.
Probe 4 was synthesized as shown in Figure 4. Phenol 2 was
alkylated with previously synthesized27 benzyl-iodide 5 to gen-
erate ether 6. The aldehydes groups of 6 were condensed with
indolium-propane-sulfonate to give probe 4. Sulfo-QCy7 was
synthesized similarly as described for QCy7 (Figure 2) by using
indolium-3-propyl-sulfonate instead of indolium-iodide.
Sulfo-QCy7 presented similar spectroscopic behavior to that
of QCy7 with NIR fluorescence maxima at wavelength of 720 nm
(see Supporting Information). Figure 5A shows the typical cyan
color for solution of sulfo-QCy7 obtained in PBS 7.4. Expectedly,
due to the 150 nm blue Stokes shift, the masked form of sulfo-
QCy7, probe 4, had a yellow color in aqueous solution. When
imaged using NIR imaging camera, the solution of probe 4 is
clearly transparent while the solution of sulfo-QCy7 exhibits a
well-observed NIR fluorescence (Figure 5B). The fluorescence
quantum yield (16%) and the extinction coefficient (52 000
molÀ1 cmÀ1) of sulfo-QCy7 were measured as high values that
are adequate for in vivo measurements. The phenol’s pKa value of
sulfo-QCy7 (4.5) indicates the possibility of obtaining NIR
fluorescence also under relatively acidic conditions (see Support-
ing Information).
To evaluate the NIR fluorescence obtained by probe 4 upon
reaction with hydrogen peroxide under physiological conditions,
the probe was incubated in PBS 7.4 at 37 °C, in the presence or
absence of hydrogen peroxide, and the emitted NIR fluorescence
was measured by a spectrofluorometer. The results, presented in
Figure 5C, indicate full conversion of probe 4 to sulfo-QCy7
within 30 min (red plot). No NIR fluorescence is observed in the
absence of hydrogen peroxide (blue plot). The sensitivity of
probe 4 toward detection of hydrogen peroxide was then
evaluated. Figure 5D shows that the probe can straightforwardly
detect hydrogen peroxide concentrations below 1 μM.
Since the overproduction of hydrogen peroxide in vivo is
concerned with the development of numerous inflammatory
diseases, we wanted to investigate the ability of our Turn-ON
NIR-probe to image endogenously produced hydrogen peroxide
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Figure 5. Photograph of probe 4 and sulfo-QCy7 in PBS 7.4 [50 μM] solutions. (B) Photograph of probe 4 and sulfo-QCy7 in PBS 7.4 [50 μM]
solutions by NIR camera (excitation at wavelength 595 nm, cutting filter of >635 nm). (C) NIR fluorescence (λEx = 590 nm, λEm = 720 nm) emitted
upon incubation of probe 4 [50 μM] in the presence (red) or absence (blue) of hydrogen peroxide [50 μM] in PBS 7.4 at 37 °C. (D) NIR fluorescence
emitted of probe 4 [100 μM] upon incubation with the indicated concentrations of hydrogen peroxide.
Figure 6. In vivo imaging of exogenous hydrogen peroxide by probe 4. CRI Maestro image. Excitation at 595 nm, emission cutoff filter of 635 nm.
in a mouse model. Initially, as a control experiment, we injected a
premixed solution of probe 4 with hydrogen peroxide, into the
peritoneal cavity, and imaged the mice using the CRI Maestro
noninvasive fluorescence imaging system. Figure 6 clearly shows
that exogenously activated probe 4 can generate NIR fluores-
cence in vivo image. Mice treated only with the probe or with the
buffer did not show detectable NIR fluorescence at similar
imaging conditions.
Next, we wanted to test whether probe 4 can be used to
differentiate in vivo various concentrations of hydrogen peroxide.
Therefore, two different concentrations of hydrogen peroxide
were premixed with probe 4; immediately injected intramuscularly,
into the leg; and then imaged by the CRI Maestro noninvasive
fluorescence imaging system. Figure 7 shows that the intensity of
the NIR emitted fluorescence from probe 4 was indeed dependent
on the hydrogen peroxide initial concentration.
Finally, we tested the ability of our probe to visualize en-
dogenously produced hydrogen peroxide using noninvasive
imaging technique. We have used a previously described model of
acute inflammation in mice that can be induced by lipopolysaccharide
Figure 7. Probe 4-fluorescence dependency on hydrogen peroxide
concentration. CRI Maestro image. Excitation at 595 nm, emission
cutoff filter of 635 nm.
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Figure 8. In vivo imaging of endogenous hydrogen peroxide in the peritoneal cavity of mice during an LPS-induced inflammatory response, using probe 4.
(A) CRI Maestro image. Excitation at 595 nm, emission cutoff filter of 635 nm. Right mouse: 1 mL of 0.1 mg/mL LPS was injected into the peritoneal
cavity of mice, followed 6 h later by an ip injection of probe 4. Middle mouse: 1 mL PBS was injected into the peritoneal cavity of mice, followed 6 h later
by an ip injection of probe 4. Left mouse: negative control (PBS only). (B) Quantification of the NIR-fluorescence emission intensity from the mice
injected with probe 4 in the presence or absence of LPS-induced inflammation (i.e., H2O2). The total number of photons from the entire peritoneal
cavity was integrated and plotted compared with the control group (n = 5). The inset focuses on the NIR-fluorescence emission intensity obtained from
injections of PBS (control) or probe 4 in absence of LPS-induced inflammation.
(LPS).20 Mice were injected intraperitoneally (ip) with LPS (1 mL of
0.1 mg/mL), followed 6 h later by an additional ip injection of 400 μL
probe 4 (1 mM), and thereafter imaged by CRI Maestro noninvasive
fluorescence imaging system. Figure 8 shows that LPS-treated mice
injected with probe 4 generated a significantly greater intensity of
NIR fluorescence signal compared with non-LPS treated mice
injected with probe 4 or PBS. The slight NIR fluorescence observed
in mice treated only with probe 4 is attributed to basal levels of
hydrogen peroxide produced in living animals (Figure 8, inset).24 The
signal-to-noise ratio of the NIR fluorescence intensity observed by the
hydrogen peroxide imaging probe in mice was about 10-fold higher
compared with the control group. Such ratio should be adequate to
obtain strong contrast image. As far as we know, this is the highest
ratio obtained of a Turn-ON probe ever observed for an in vivo
hydrogen peroxide imaging probe.
To assess the photo and chemical stability of sulfo-QCy7
Figure 9. NIR fluorescence emitted upon incubation of sulfo-QCy7
under physiological conditions, its fluorescence decrease over
time was compared with that of the known dye Cy7. Both dyes
were incubated in PBS 7.4 at 37 °C and their NIR fluorescence
was monitored with spectrofluorometer. Figure 9 shows that
sulfo-QCy7 loses fluorescence in a similar rate to that of Cy7. It
should be noted that in the first few hours there is hardly any
decrease in the fluorescence intensity. Thus, in the time-scale of
the described in vivo imaging evaluations, sulfo-QCy7 exhibits
good fluorescence stability.
The boronate triggering group is efficiently removed upon
reaction with hydrogen peroxide and therefore can be introduced
to mask various fluorophores. That approach has been compre-
hensively demonstrated by the Chang group in probes designed
for detection and imaging of hydrogen peroxide.21 They have
also shown that the boronate-based probes are highly selective to
hydrogen peroxide among several other reactive oxygen species.
To demonstrate our probe’s capability to image biological
processes in vivo, we chose hydrogen peroxide as the analyte of
interest. In this regard, two elegant examples of chemilumines-
cent probes for imaging hydrogen peroxide in mice were recently
and Cy7 [50 μM] in PBS 7.4 at 37 °C vs time. Sulfo-QCy7: λEx
590 nm, λEm = 720 nm. Cy7: λEx = 700 nm, λEm = 780 nm.
=
reported.20,24 These probes exhibited highselectivity and sensitivity
and were effective for imaging hydrogen peroxide produced by the
inflammatory response to lipopolysaccharide or by testosterone-
mediated elevation of cell-proliferation in tumor xenografts.
As previously indicated, to develop methods that allow direct
and reliable assessment of analytes in vivo, a water-soluble and
NIR fluorescence-based probe having emission maxima between
650 and 900 nm is preferred. Previous efforts to design such Turn-
ON probes typically had limitations of low fluorescence quantum
yield, low extinction coefficientandcomplex synthesis.29,30 Because
of their superior spectroscopic characteristics, cyanine dyes have
received immense attention and can be considered as the main class
of NIR fluorescent probes for biological applications. Therefore,
there is an obvious need to develop effective Turn-ON probes,
which are based on these dyes. Our new cyanine fluorochrome is
synthesized through a simple two-step procedure and has a relative
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high fluorescence quantum yield and large extinction coefficient.
The probe’s design allows formation of a cyanine dye through π-
electrons relocation, upon removal of trigger by the appropriate
analyte. While most of the cyanine Turn-ON probes are based on
the FRET technique, there are only limited specific examples for
cyanine probes that involve changes with the π-electrons
conjugation.31,32 We have presented a modular approach for
cyanine-based probe, which suitably can be adjusted for imaging
of other factors such as enzymes, by introducing a specific substrate
as the trigger.
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’ CONCLUSIONS
In summary, we have developed a new paradigm for genera-
tion of novel class of Turn-ON NIR cyanine-based probes. For
the first time, there is an available methodology to prepare
modular molecular probes that can release an active cyanine
fluorophore upon reaction with specific analyte. The probe is
based on a new fluorochrome (QCy7), which is obtained upon
removal of a trigger moiety by the analyte of interest. A distinctive
change of π-electrons system leads to generation of a cyanine dye
with strong NIR fluorescence. The probe was demonstrated to
efficiently image endogenous hydrogen peroxide produced in an
acute inflammation model in mice. The synthesis of the QCy7 is
very simple and can be accustomed to prepare various probes for
detection and imaging of other analytes or enzymes. We envision
that QCy7-based probes will be commonly used as effective
research tools for noninvasive imaging for a range of important
biological factors in animals and, in the future, in humans.
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’ ASSOCIATED CONTENT
S
Supporting Information. Full experimental details, char-
b
acterization data of all new compounds, spectroscopic assay
conditions and in vivo experimental conditions. This material is
available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Authors
*ronitsf@post.tau.ac.il; chdoron@post.tau.ac.il.
’ ACKNOWLEDGMENT
D.S. thanks the Israel Science Foundation (ISF) and the
Binational Science Foundation (BSF) for financial support.
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