Accessing Structurally Diverse Near-Infrared Cyanine Dyes for Folate Receptor-Targeted Cancer Cell Staining

Article abstract of DOI:10.1002/chem.201700026

Folate receptor (FR) targeting is one of the most promising strategies for the development of small-molecule-based cancer imaging agents considering that the FR is highly overexpressed on the surface of many cancer cell types. FR-targeted conjugates of near-infrared (NIR) emissive cyanine dyes are in advanced clinical trials for fluorescence-guided surgery and are valuable research tools for optical molecular imaging in animal models. Only a small number of promising conjugates has been evaluated so far. Analysis of structure–performance relations to identify critical factors modulating the performance of targeted conjugates is essential for successful further optimization. This contribution addresses the need for convenient synthetic access to structurally diverse NIR-emissive cyanine dyes for conjugation with folic acid. Structural variations were introduced to readily available cyanine precursors in particular via C?C-coupling reactions including Suzuki and (for the first time with these types of dyes) Sonogashira cross-couplings. Photophysical properties such as absorbance maxima, brightness, and photostability are highly dependent on the molecular structure. Selected modified cyanines were conjugated to folic acid for cancer cell targeting. Several conjugates display a favorable combination of high fluorescence brightness and photostability with high affinity to FR-positive cancer cells, and enable the selective imaging of these cells with low background.

Full text of DOI:10.1002/chem.201700026

A Journal of
Accepted Article
Title: Accessing Structurally Diverse Near-Infrared Cyanine Dyes for
Folate Receptor-Targeted Cancer Cell Staining
Authors: Sandra G. König and Roland Krämer
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To be cited as: Chem. Eur. J. 10.1002/chem.201700026
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Supported by

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Accessing Structurally Diverse Near-Infrared Cyanine Dyes for
Folate Receptor-Targeted Cancer Cell Staining
[a]
[a]
Sandra G. König and Roland Krämer*
Another highly promising strategy to achieve selectivity for
cancer cells is the chemical incorporation of folate derivatives as
Abstract: Folate receptor (FR) targeting is one of the most
promising strategies for the development of small-molecule
based cancer imaging agents since the FR is highly
overexpressed on the surface of many cancer cell types. FR-
targeted conjugates of NIR emissive cyanine dyes are in
advanced clinical trials for fluorescence-guided surgery and are
valuable research tools for optical molecular imaging in animal
models. Only a small number of promising conjugates has been
evaluated so far. Analysis of structure-performance relations to
identify critical factors modulating the performance of targeted
conjugates is essential for successful further optimization.
[6
]
targeting ligands.
This targeting moiety enables highly
selective labeling of cell types that overexpress folate receptors
[
7]
(FR), as observed in many cancer types, including ovary,
[
8-10]
cervix, kidney, lung, breast, and others.
molecules have proven useful as targeted drugs
selective labeling of cancer cells and tissues. Examples are the
Folate-conjugated
[
6,11-13]
or for
9
9m
[14-16]
FR-targeted radiotracer
Tc-EC20
or the fluorescent
markers Folate-FITC and OTL0038 (a pteroyl-linked, folate
[
17]
receptor targeted indotricarbocyanine dye), which have been
used for the intraoperative labeling of various carcinomas in
[18,19]
This contribution addresses the need for convenient synthetic
access to structurally diverse NIR-emissive cyanine dyes for
conjugation with folic acid. Structural variations were introduced
to readily available cyanine precursors in particular via C-C-
coupling reactions including Suzuki- and (for the first time with
these types of dyes) Sonogashira cross couplings.
Photophysical properties such as absorbance maxima,
brightness, and photostability are highly dependent on the
molecular structure. Selected modified cyanines were
conjugated to folic acid for cancer cell targeting. Several
humans and are in advanced clinical trials.
Further
examples for optical FR-targeted probes include silica
[20]
[21]
[22-24]
nanoparticles, europium complexes, and cyanine dyes.
The chemical structure of the fluorophore in targeted
indotricarbocyanine probes can dictate both in vitro and in vivo
[
25]
performance.
The synthesis of folate-indotricarbocyanine
conjugates typically involves challenging multistep procedures
which limits access to structurally diverse probes, in particular
with respect to modifications of the fluorophore. Finding more
versatile synthetic routes is seen as central to the further
development of this field.
conjugates display
a
favorable combination of high
fluorescence brightness and photostability with high affinity to
FR positive cancer cells, and enable the selective imaging of
these cells with low background.
Since intrinsic selectivity of fluorescent probes for certain
[
26,27]
tissues or cell types is only rarely observed,
the most
successful strategy for the introduction of targeting ligands relies
on the preparation of dye molecules with functional groups that
[
28]
allow for selective covalent binding to the fluorophore.
This
can either be achieved by preparing asymmetric dye backbones
or by using symmetric dyes with a central chlorine substituent.
Introduction
[
29]
Functionalization of the latter, e.g. via SRN1 reactions,
is a
For numerous bioimaging applications, near-infrared (NIR)
fluorescent probes have proven to be superior to chromophores
with fluorescence in the visible spectrum mainly due to two
important key advantages: First, background signals from
cellular auto-fluorescence are minimal in the NIR-region and
second, the penetration depth of NIR-radiation is significantly
widely used approach for the preparation of dyes with only one
additional targeting ligand or two different targeting moieties. At
[30,31]
[32,33]
[34,35]
the central position, amines,
alcohols,
and thiols
can easily be introduced. However, despite the fact that the
resulting probes often present interesting photophysical
[36]
properties, for example large Stokes shifts, several limitations
[
1]
higher, therefore enabling imaging of deeper tissue. Amongst
the spectrum of NIR dyes, indotricarbocyanines are particularly
promising platforms for biocompatible probes. This is highlighted
by the FDA-approved fluorescent contrast agent indocyanine
have to be considered. For example, their chemical stability is
often not ideal since some of these linkages can easily be
attacked by strong nucleophiles. Additionally, susceptibility to
reactions with biothiols has been reported, especially for
thioenolether derivatives and enolethers of phenols bearing
[
2]
green (ICG) and derivatives thereof, such as cypate or
[
3]
SIDAG.
Recently, we demonstrated the applicability of such probes for
[37]
additional electron-withdrawing substituents. Since it has been
reported that the introduction of a robust C-C-bond at the central
[4]
the selective and fast labeling of live cancer cells or selective
position of the chromophore significantly improves chemical
[5]
cell labeling in presence of high zinc(II)-concentrations.
[38,39]
stability,
we considered this modification as highly promising.
Now, in an effort to characterize the influence of C-C coupled
substituents in detail, we prepared a series of structurally
diverse cyanine derivatives and investigated their photophysical
properties as well as their optical stability. We also present the
first examples of Sonogashira couplings at the central position
of heptamethine indotricarbocyanine chromophores. The
resulting derivatives are all highly water-soluble and cover a
wavelength range between 731 nm and 816 nm. Furthermore,
[a]
Dr. S. G. König, Prof. Dr. R. Krämer
Institute of Inorganic Chemistry
University of Heidelberg
Im Neuenheimer Feld 274, 69120 Heidelberg
E-mail: kraemer@aci.uni-heidelberg.de
[
40]
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
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suitable chromophores were selected and modified with folate
as targeting moiety. All resulting probes were successfully
applied in selective cancer cell staining.
For this reason, we were particularly interested in the
preparation of dye backbones with terminal alkyne substituents.
However, the Sonogashira coupling of TMS-acetylene to 1
turned out to be more challenging than for the arylacetylenes
that will subsequently be discussed. While the latter were
3 2 2
coupled to 1 using [Pd(PPh ) Cl ] with yields of up to 85%, the
Results and Discussion
same conditions yielded only traces of 3 together with an
unexpected annulated compound (4; 30%; see Fig. 1c (left) and
Fig. S1 (ESI) for further details on the structure). When using
When planning this study, we aimed at the clarification of
several aspects. First, we wanted to characterize the influence
of different - mainly C-C coupled
Therefore, the asymmetric backbone 1 (Fig. 1a; see ESI for
synthetic procedures) was chosen as starting material for dyes
50 mol% CuI instead of 20 mol%, it was possible to isolate 3, but
- substituents in detail.
4
remained the main product (65% after purification). After
changing the catalyst to XPhos Pd G2 (5 mol%) and XPhos
ligand (5 mol%), 3 was obtained in 22% yield while the formation
of 4 was suppressed. Subsequent deprotection of 3 with CsF
yielded 5, which contains a terminal alkyne.
Next, we wanted to investigate whether aromatic systems with
different electronic characteristics influence the photophysical
properties due to potential electronic interaction caused by the
increased rotational flexibility of the alkynyl group. Therefore, we
prepared several new derivatives via Sonogashira coupling of 1
with phenylacetylene (6), 4-tolylacetylene (7), 4-ethynylanisole
2-13 (Fig. 1a) because this backbone already contains a
carboxylic acid group for further derivatization with targeting
ligands. As an example for aryl-substituted compounds, the
asymmetric, phenyl-substituted derivative 2 was prepared via
3 4
Suzuki C-C cross coupling using [Pd(PPh ) ] as catalyst in water.
Since indotricarbocyanines with alkynyl-substituents at the
central position have not been investigated before, we studied
the influence of the latter on the photophysical properties as well
as potential changes of stability. Additionally, it has to be noted
(
(
8) and 4-nitrophenylacetylene (9) using either [Pd(PPh
10 mol%; all mentioned derivatives except 9) or XPhos Pd G2
3 2 2
) Cl ]
that terminal alkynes allow for
a large variety of further
derivatization strategies, e.g. via azide-alkyne-click reactions,
which would open up the possibility of orthogonal dye
derivatization.
with additional XPhos ligand (both 10 mol%; only compound 9)
in combination with CuI (20 mol%) and DIPEA (Fig. 1a; see ESI
for synthetic procedures).
A
B
.
C
H
Cl
7
31
734
755
761
784
800
802
815 nm
Figure 1. Structures of the modified dyes based on the asymmetric backbone 1 (A) or the symmetric backbone 14 (B) which were obtained via Suzuki coupling
2, 15), Sonogashira coupling (4, 6-10, 16), catalytic dehalogenation (12), or nucleophilic substitution (13). C) Schematic illustration of the influence of different
(
substituents on the absorbance maxima (arbitrary scale; colors are derived from the colors of the compounds in aqueous solution).
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1
3
2
5
11
With the alkynyl derivatives in hand, we were additionally
interested in 1,3-butadiynyl structures. Fortunately, our protocol
for the preparation of alkyne substituted conjugates also allowed
for the preparation of 10, and the deprotected compound 11 was
obtained after treatment of 10 with CsF.
4
12
1
6
A 1.0
1
2
4
5
6
0
0
0
0
0
.8
.6
.4
.2
.0
1
1
2
3
1
1
Because of the necessity of an unsubstituted reference
compound for comparison, chromophore 12 was prepared via
3 4
catalytic dehalogenation of 1 with [Pd(PPh ) ]. Additionally, since
several literature-known cyanine-based probes contain
a
nitrogen atom connected to the central position, we prepared 13
via nucleophilic substitution of the central chloro-substituent with
aniline.
500
600
700
Wavelength [nm]
800
B
In contrast to the asymmetric backbone 1 which can only be
purified by preparative HPLC, we also prepared compounds
based on the symmetric backbone 14 (Fig. 1b), where no HPLC
purification is necessary. In order to introduce a carboxylic acid
group
carboxyethyl)phenylboronic acid and Suzuki conditions to obtain
5 as well as 4-ethinylbenzoate and Sonogashira conditions to
for
further
derivatization,
we
used
4-(2-
1
Figure 2. a) The normalized UV/Vis absorbance spectra of selected
compounds (1, 2, 4-6, 11, 12, 13) in PBS (pH 7.4) demonstrate the influence
of different substituents at R (Fig. 2b, left) on the photophysical properties
obtain 16. Pleasingly, it was even possible to obtain pure 15
1
without chromatographic purification.
2
(R = COOH). b) Structures of the dye backbones.
Photophysical properties and photostability
After having obtained a large variety of compounds with
substituents differing in their steric and electronic properties, we
characterized the photophysical properties of the new
derivatives. We found that their absorbance maxima cover a
range of almost 100 nm (Fig. 1c). In relation to the
dehalogenated reference compound 12, the most blue-shifted
absorbance maxima were observed for the annulated derivative
Table 1. Photophysical properties of the dyes 1-19. All values were
determined using 0.25 - 0.75 µM dye solutions at 20 °C. Values for
fluorescence emission are based on
λEx = 705 nm (1, 2, 12-15, 17-19),
λ
Ex = 680 nm (4) or Ex = 740 nm (3, 5-11, 16). Quantum yields were
λ
[
41]
f
determined using ICG (φ = 0.13 in DMSO) as standard. Dye backbones are
apresented in Fig. 2b.
compounds are based on Fig. 2b (left), R = COOH; compounds are based
2
b
2
on Fig. 2b (left), R = SO
c
H; compounds are based on Fig. 2b (right).
3
4
(731 nm) and the aniline-modified chromophore 13 (734 nm).
1
R
λ
max
λ
max
-1ε
[M cm
φ
f
Brightness
(ε∙φ
In contrast, the butadiynyl-modified compound 11 has an
absorbance maximum at 816 nm, which corresponds to a
bathochromic shift of 61 nm with respect to the dehalogenated
compound 12 (Fig. 2a). All photophysical properties of the new
fluorophores and their parent compounds are summarized in
Table 1, the corresponding backbones are depicted in Fig. 2b.
The highest quantum yields and fluorescence brightness values
were found for the dehalogenated backbone 12 and for 15,
which was obtained via Suzuki coupling. A very low quantum
yield was observed for the aniline-modified chromophore 13,
which also shows an unusually low extinction coefficient
compared to other cyanine dyes. Interestingly, the photophysical
properties of all arylalkynyl-modified compounds (6-9, 16) are
quite similar, suggesting that the para-substituent only has a
subtle influence. Only the nitro-compound 9 really stands out, as
it does not show any fluorescence at all.
-1
]
(Abs)
(Em)
f
)
[
nm]
[nm]
a
1
a
784
761
814
781
200 000
276 000
0.05
0.09
10 000
24 840
2
a
3
a
805
731
821
751
197 000
167 000
0.02
0.12
3 940
4
annulated
derivative
20 040
a
5
6
800
802
820
822
226 000
237 000
0.02
0.02
4 520
4 740
a
a
a
a
7
8
9
800
798
808
820
820
-
217 000
246 000
252 000
0.02
0.02
-
4 340
4 920
-
a
1
1
1
0
1
2
816
815
755
828
825
780
180 000
276 000
293 000
0.01
0.01
0.12
1 800
2 760
35 160
a
a
a
Apart from different substituents at the central position of the
chromophore, we were also interested in the influence of the
cyclohexenyl ring, since the well-known ICG and many other
1
3
734
802
129 000
0.02
2 580
b
b
14
782
756
800
775
214 000
207 000
0.06
0.14
12 840
28 980
commercially available cyanine dyes do not contain
a
15
cyclohexenyl ring, but an open polymethine chain. Therefore,
fluorophores 17, 18, and 19 (Fig. 2, B) were prepared for
comparison, where 17 presents the structural equivalent of the
dehalogenated compound 12 but without the cyclohexenyl ring,
while 19 (which is based on 18) was prepared to mimic the
overall charge of 15. Surprisingly and in contrast to previously
b
c
1
6
800
820
231 000
0.03
6 930
1
1
1
7
c
-
-
-
750
754
758
775
780
778
270 000
250 000
268 000
0.13
0.10
0.13
35 000
25 000
34 840
8
9
c
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Table 2. Photophysical properties of the folate-modified fluorescent probes
20-24. All values were determined using 0.25 - 0.75 µM dye solutions at
20 °C. Values for fluorescence emission are based on an excitation
wavelength of λEx = 705 nm (20, 21, 22) or λEx = 740 nm (23, 24). Quantum
[
41]
yields were determined using ICG (φ
Assumed refractive index of human plasma: 1.351.
f
= 0.13 in DMSO)
as standard.
[
42]
Solvent
λ
max
λ
(Em)
max
ε
φ
f
Brightness
(ε·φ
-1
-1
(Abs)
[M cm ]
f
)
[nm] [nm]
2
0
1
PBS
DMSO
Plasma
PBS
Plasma
PBS
757
781
795
781
787
787
782
825
820
272 000
210 000
262 000
258 000
254 000
227 000
298 000
305 000
0.12
0.29
0.12
0.12
0.09
0.13
0.02
0.02
32 640
60 900
31 440
26 400
26 800
29 510
5 960
767
763
761
764
762
810
805
2
22
23
24
PBS
PBS
6 100
same stability when compared with each other, while only 8
appeared to be slightly more stable. This is interesting since
these observations for our compounds differ from previously
Figure 3. Structures of the folate-modified dyes 20-24 which were prepared
via amide bond formation between 17 (20), 19 (21), 15 (22), 16 (23) or 8 (24)
and γ-modified folate. TFA counter ions result from purification via semi-
preparative HPLC.
[
39]
reported results
that propose an enhancement of
photostability for a cyclohexenyl ring and a C-C coupled
modification at the central position of the chromophore in
comparison to the open-chain dye ICG.
[
38]
reported findings,
their quantum yields and fluorescence
brightness values are almost the same as the values we
obtained for 3 and 15 (Table 1). We can therefore not confirm an
enhancing effect of rigidifying rings within the polymethine chain
on fluorescence quantum yields for the compounds investigated
in this study.
An important parameter for cell imaging is the photostability of
fluorophores. Therefore, a comparative analysis was carried out
by irradiating 1 µM solutions in PBS with a strong 500 W
halogen lamp. This concentration was selected to ensure that
the majority of dye molecules would be present as monomers,
since indocyanines tend to form aggregates even at low
concentrations. Time-course fluorescence measurements
Folate-modified probes
Since our ultimate goal was to identify suitable new fluorescent
dyes for folate-targeted cancer cell labelling, we selected the
most promising compounds from each series of substitution
positions for further derivatization with folic acid. Compounds 17,
19, and 15 were selected due to their excellent fluorescence
brightness values. Furthermore, 15 is the most easily accessible
compound as it requires only five synthetic steps which work in
good yields and since there is no need for purification by
preparative HPLC. Additionally, we selected two arylalkyne-
modified derivatives: 16 was chosen since it exhibits the highest
fluorescence brightness of this group of compounds while 8 was
most stable towards irradiation. Using amide bond formation,
these fluorophores were coupled to folate via a linker attached at
its γ-position (see ESI for synthetic procedures). This position
was selected for derivatization since it has been reported that
γ-modified folate binds more efficiently to the folate receptor
(
summarized in Fig. S5, ESI) showed the fastest decomposition
for the chlorine-substituted compound 1, closely followed by the
annulated derivative 10. In contrast, compounds with an open
polymethine chain (17 and SIDAG, which we prepared as
reference) were most stable against this type of irradiation. The
derivatives with aryl-, alkynyl- and arylalkynyl-substituents at the
central position of the chromophore and a cyclohexenyl ring
were between these two extremes and displayed almost the
[
43]
than the corresponding α-modified derivatives.
Figure 4. Representative fluorescence microscopy images of live cells that were labeled with the folate-modified probes 20-24 (100 nM in PBS, 15 min, 37 °C). NIR
fluorescence is shown in red, nuclei were stained with Hoechst 33342 (blue). Upper panel: Cancer cells (HeLa), lower panel: normal cells (HDFa).
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Additionally, a mixture of live HeLa cells and HDFa cells was
prepared (ratio approx. 1/100) and treated with 20 and Hoechst
33342 as described above (Fig. 5). In this image, only 5 out of
ca. 155 cells were labeled, which clearly underlines selectivity
for the cancer cells.
In order to determine the amount of unspecific binding of the
folate-dye-conjugates, we simultaneously incubated compounds
20-22 with a 100-fold molar excess of folic acid. In all cases,
binding was almost completely blocked by the excess of folic
acid, which indicates an extremely low amount of non-specific
binding (Fig. 5d). Assuming that binding to other cellular targets
d
is negligible, K values of about 8 and 7 nM, respectively, would
derive for the best binders 20 and 22. The affinity of 21 is
significantly lower (about 28 nM). High affinity of 22 may indicate
that the dye backbone does almost not interfere with receptor
binding in this case. In contrast, 21 has a lower binding affinity,
which can possibly be explained by electrostatic (due to different
distribution of the negative charge in contrast to compound 22)
or steric repulsion. Values in the low nanomolar range are in
accordance with those reported for other folate probes (for
example, values between 7.0 nM and 64 nM have been reported
by Kelderhouse et al. using similar experimental conditions on
Figure 5. Representative fluorescence microscopy images of a mixture of
HeLa cells and human dermal fibroblasts (HDFa) in a ratio of approx. 1/100
per well. Nuclei were stained with Hoechst 33342; cells were subsequently
treated with 20 (100 nM in PBS, 15 min, 37 °C). A) Fluorescence emission of
the nuclei (blue, UV-channel), B) fluorescence emission of 20 (red, NIR-
channel), C) overlay of UV- and NIR-fluorescence channels. D) Relative
affinities of 20, 21, and 22 to adherent HeLa cells using flow cytometry. Cells
were labeled with dye solutions in PBS in the absence (black, red, and blue) or
presence (orange, pink, and turquoise) of a 100-fold excess of folic acid (FA).
Error bars represent ± standard deviation.
[45]
KB cells).
Finally, to further characterize the probes’ binding to the folate
receptors on the cell surface, confocal microscopy images were
recorded directly after incubation of live HeLa cells with folate-
modified probes (Fig. S10, ESI). These images clearly show
bright fluorescence originating from the cell membrane while
only weak fluorescence was detected inside the cells.
The resulting probes are depicted in Fig. 3; their photophysical
properties in PBS and human plasma are summarized in
Table 2. In all cases, attachment of the folate moiety only had a
small influence on fluorescence brightness and maxima.
However, when investigating the photostability of the folate-dye
conjugates (Fig. S6 and S7, ESI), we found that all folate-
modified probes were significantly more stable towards Conclusions
irradiation than their parent compounds. These observations can
possibly be explained by a scavenging effect of the folate moiety
for reactive species that arise during irradiation.
Folate receptor targeted indotricarbocyanine probes are
promising tools in cancer imaging. Their challenging multistep
synthesis limits access to structurally diverse dye backbones for
optimization of the application-dependent performance. To
create structural diversity by direct modification of the
fluorophore core, we applied two readily available dye
components with either asymmetric or symmetric backbones
and a central chlorine substituent to various C-C-coupling and
other reactions. All derivatives are highly water soluble and
show good chemical stability in aqueous media, and the
modifications dictate the photophysical properties: absorbance
and emission wavelengths cover a range of almost 100 nm in
the near-infrared region, and markedly different photostabilities
were observed. Selected derivatives were coupled to folic acid
via a carboxylic function. While the absorbance and emission
maxima remained virtually unaffected, a remarkable increase in
photostability was observed for the folate conjugates.
Furthermore, the folate derivatives enable highly selective
staining of live cancer cells over normal cells, as documented by
fluorescence microscopy and flow cytometry. Remarkably, the
brightness of alkyne modified probes, although much lower
compared to the other probes in PBS, is in the cellular
microenvironment sufficient for selective labelling with good
contrast. Affinity of the probes to folate receptors on the cell
To evaluate the cell labeling properties of the folate-modified
probes, we chose HeLa cells (human cervical carcinoma) as
representative cell line since these cells have higher expression
[
44]
levels of the folate receptor FRα than healthy cells.
The
obviously most important property of folate-targeted fluorescent
probes is their selectivity for cancer cells over normal cells. To
elucidate whether our new probes show this selectivity, both
HeLa cells and healthy cells (human dermal fibroblasts, HDFa)
were treated with 20-24 (100 nM in PBS) for 15 minutes at 37 °C
(
optimal incubation conditions were chosen based on
investigations regarding labeling kinetics and saturation
concentrations for two model compounds, see Fig. S8 and S9,
ESI for details). In all cases, we observed selective labeling of
the cancer cells (Fig. 4, upper panel) while no NIR fluorescence
was detected from the healthy HDFa cells (Fig. 4, lower panel;
nuclei were co-stained with Hoechst 33342 (blue)). Pleasingly,
NIR fluorescence emission was also clearly sufficient for probes
23 and 24 in the cellular environment, although their
fluorescence brightness values in PBS were significantly lower
than it is the case for 20-22.
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Chemistry - A European Journal
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surface is also affected by the structural modifications; notably,
affinity in the in the low nanomolar range was observed for an
aryl derivative obtained by C-C-coupling with an easily
accessible, symmetric dye precursor. In summary, our study
highlights the impact of structural modifications at the
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[
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Bolzati, N. Salvarese, D. Carpanese, A. Rosato, J. Med. Chem. 2015,
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fluorophore
core
on
the
in
vitro
properties
of
[
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This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
10.1002/chem.201700026
FULL PAPER
FULL PAPER
Sandra G. König, Roland Krämer*
Page No. – Page No.
Accessing Structurally Diverse Near-
Infrared Cyanine Dyes for Folate
Receptor-Targeted Cancer Cell
Staining
Tuneable wavelengths and selective cancer cell staining: A series of structurally
diverse NIR cyanine derivatives has been prepared using e.g. Suzuki and Sonogashira
cross-couplings. These derivatives cover a wavelength range of almost 100 nm.
Selected modified cyanines were conjugated to folic acid for selective cancer cell
targeting and were found to display a favourable combination of high fluorescence
brightness and high affinity to folate receptor positive cancer cells.
This article is protected by copyright. All rights reserved.