Restricted suitability of BODIPY for caging in biological applications based on singlet oxygen generation

Article abstract of DOI:10.1039/d0pp00097c

Recent studies report the boron-dipyrromethene (BODIPY) moiety to be interesting for caging applications in photopharmacology based on its response to irradiation with wavelengths in the biooptical window. Thus, in a model study, we investigated the meso-methyl-BODIPY caged CDK2 inhibitor AZD5438 and aimed to assess the usability of BODIPY as a photoremovable protecting group in photoresponsive kinase inhibitor applications. Photochemical analysis and biological characterisation in vitro revealed significant limitations of the BODIPY-caged inhibitor concept regarding solubility and uncaging in aqueous solution. Notably, we provide evidence for BODIPY-caged compounds generating singlet oxygen/radicals upon irradiation, followed by photodegradation of the caged compound system. Consequently, instead of caging, a non-specific induction of necrosis in cells suggests the potential usage of BODIPY derivatives for photodynamic approaches.

Full text of DOI:10.1039/d0pp00097c

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Photochemical & Photobiological Sciences
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DOI: 10.1039/D0PP00097C
ARTICLE
Restricted suitability of BODIPY for caging in biological
applications based on singlet oxygen generation
Received 00th January 20xx,
a
a
b
Theo Rodat, †a Melanie Krebs, †a Alexander Döbber, Björn Jansen, Anja Steffen-Heins, Karin
Schwarz b and Christian Peifer *a
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Recent studies report the boron-dipyrromethene (BODIPY) moiety to be interesting for caging applications in
photopharmacology based on its response to irradiation with wavelengths in the biooptical window. Thus, in a model study
we investigated the meso-methyl-BODIPY caged CDK2 inhibitor AZD5438 and aimed to assess usability of BODIPY as
photoremovable protecting group in photoresponsive kinase inhibitor applications. Photochemical analysis and biological
characterisation in vitro revealed significant limitations of the BODIPY-caged inhibitor concept regarding solubility and
uncaging in aqueous solution. Notably, we provide evidence for BODIPY-caged compounds generating singlet
oxygen/radicals upon irradiation followed by photodegradation of the caged compound system. Consequently, instead of
caging, a non-specific induction of necrosis in cells suggests potential usage of BODIPY derivatives for photodynamic
approaches.
Introduction
The boron-dipyrromethene (BODIPY) moiety is known for
applications in biological chemistry such as dye agent1, fluorescence
label2,3 or photosensitiser2,4–7. More recently, BODIPY has been
investigated in the field of photopharmacology as a photoremovable
protecting group (PPG) towards caging approaches (Table 1). The
BODIPY moiety responds to wavelengths above 500 nm within the
biooptical window, suggesting it to be highly suitable for the caging
of inhibitors in biological chemistry. In line with this notion, Umeda
et al. described a BODIPY-caged histamine that can be photo-
released via irradiation of 500 nm in 2014.8 Herein, the histamine
leaving group was linked to the boron atom. Further studies
connected a meso-methylhydroxy-BODIPY either directly or via a
carbamate linker to a leaving group.9,10 In 2017, Slanina et al.
evaluated the influence of substituents attached to the meso-
methyl-BODIPY group, but without providing further biological
validation.11 Moreover, Sitkowska et al. stated a halide substituted
BODIPY linked to the leaving group via a carbamate linker with fast
uncaging, but some stability restrictions in aqueous solution.12
Similar to this approach, two publications from Kawatani et al.
(boron linked leaving group) and Peterson et al. (red light single
photon PPG) in 2018 were reported.13,14 In 2019, Toupin et al.
published the use of a photocleavable BODIPY moiety combined
with singlet oxygen generation for an increased efficacy in cancer
cell apoptosis.15
a.Institute of Pharmacy, Kiel University, Gutenbergstraße 76, 24118 Kiel
(Germany).
b.Institute of Human Nutrition and Food Science, Division of Food Technology, Kiel
University, Heinrich-Hecht-Platz 10, 24118 Kiel (Germany).
† These authors contributed equally to this work.
Figure 1. Selected cargos caged with BODIPY-moieties in meso position.
I) Goswami et al.10 II) Sitkowska et al.12 III) Rubinstein et al.9 IV) Peterson et al.13
V) Toupin et al.15
Electronic Supplementary Information (ESI) available. DOI: 10.1039/x0xx00000x
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Table 1. Comparison of current publications evaluating BODIPY as PPG for caging.
DOI: 10.1039/D0PP00097C
Activation
wavelength
Reference
Umeda et al.8
Rubinstein et al.9
Leaving group
Histamine
Linker
Biological evaluation
Boron linked
Carbamate
500 nm
Ca2+ rise in HeLa cells
Ca2+ rise in HeLa cells
4-Nitro-aniline
530/540 nm
Goswami et al.10
Slanina et al.11
Acetic acids
Ester
500 nm
507 nm
Compound integration of Drosophilia S2 cells
Different carbon acid
analogues
Mostly ester
Carbamate
none
none
Sitkowska et al.12
Amine cargos
520-560 nm
Mostly carbon acid
analogues
Peterson et al.13
Kawatani et al.14
Mostly ester
Boron linked
635 nm
500 nm
Compound integration of HeLa cells
Intracellular Ca2+ dynamics in CaTM3
Capsaicin analogue
CTSB
Toupin et al.15
present work
CTSB inhibitor
Ester
532 nm
Cell viability and flow cytometry of MDA-MB-
231 and MCF-10A
CDK2/cyclin E1 kinase
Panc89 cell proliferation
AZD5438
Methylamine
519/525 nm
Caspase 3/7-mediated apoptosis
Based on our platform towards photoresponsive kinase
inhibitors16,17 and the above-mentioned findings, we became
interested in investigating whether BODIPY would be suitable for
caging applications using a potent protein kinase inhibitor. Protein
kinases mediate key roles in nearly all important processes in
eukaryote cells like proliferation, migration or apoptosis. Thus,
alterations of kinase activities including gain-of-function in relevant
signal transduction cascades are leading to uncontrolled cell
proliferation or reduced apoptosis, most importantly in the
development of cancer.18,19 As an example for highly critical kinases,
the cyclin-dependent kinases (CDK) are essential for a functional cell
cycle and transcription regulation.20,21 For instance, CDK2 in complex
with cyclin E1 regulates G0 to G1 and G1 to S phase transition.22
Thus, altering the cell cycle activity via kinase inhibitors targeting
CDK2 may be a powerful strategy against cancer.23,24 However,
influencing such essential functions in the cell like proliferation can
cause crucial impact in normal cell physiology, and may thus cause
fatal side effects in patients. Therefore, actually effective CDK2
inhibitor candidates are often not being developed further due to
issues detected in clinical studies.21 Examples include the effective
CDK2 inhibitor AZD5438 of which clinical development was
discontinued based on the appearance of severe side effects.25–27
Regarding kinase inhibitors, several approaches of photochemistry
and photopharmacology have been reported so far to overcome the
lack in specificity of these biologically effective compounds.28–32
Besides caging16,17,33–38 further applications use photoswitchable
moieties including an azobenzene switch introduced in a BRAFV600E
inhibitor to manage the BRAF paradox.39–41 Another approach of
regulating the VEGFR2 activity was achieved by using Axitinib as a
photoresponsive kinase inhibitor.42
cellular apoptosis or DNA damages.43–47 Against this background, the
BODIPY moiety would provide significant advantages towards caging
as it responds to higher wavelengths within the biooptical window,
thus allowing deeper tissue penetration of light.
In the present study, we aimed to use the in vivo actually effective
clinical candidate CDK2 inhibitor AZD5438 for caging with the
BODIPY group. Our hypothesis was that the caged compound would
be biologically inactive, but the potent activity of the leaving group
AZD5438 could be spatially and temporally controlled by irradiation
with light of approximately 500 nm. This approach would probably
provide a highly useful caged CDK2 inhibitor system, which could for
example be employed as photopharmacological tool.
Results and discussion
Design, synthesis and photochemical characterisation
To determine a suitable position at the inhibitor structure for the
functionalisation with BODIPY as PPG, we initially performed
molecular modelling using the x-ray defined ligand complex of
AZD5438 in CDK2 with cyclin E1 (PDB: 4FKO). Herein, the ligand
AZD5438 forms key H-bonds via the cyclic guanidine moiety towards
LEU83, an amino acid residue located in the hinge region of the
kinase (Figure 2). Thus, the binding mode suggested the NH function
to be suitable for caging. In contrast, modelling the BODIPY-caged
AZD5438 into the active site of CDK2 revealed no plausible binding
mode basically due to significant sterical clashes (Figure 2C).
Furthermore, over the course of our studies towards caged kinase
inhibitors, the NH moiety of the AZD5438 pyrimidine-anilino core
structure was found to be a feasible leaving group.48
However, the wavelengths used to uncage PPGs is frequently not
within the biooptical window.35,36 UV light only has a penetration
range of millimetres in biological tissue and can additionally cause
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Herein, the starting material 3-ethyl-2,4-dimethylpyrrole as an
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electron-rich aromatic compound waDsOI: r1e0a.1c0t3e9d/D0PwPi0th00972C-
chloracetylchloride in dry dichloromethane. After addition of dry
triethylamine and borontrifluoriddiethyletherate as Lewis acid the
reaction was left stirring for 1 h at 50°C. A Finkelstein reaction
exchanged chloride for iodide, in which sodium iodide was added to
a solution of 1 in dry THF and the mixture was refluxed for 45 min to
yield 2, which was used in the following step providing a better
Figure 2. Molecular modelling of CDK2 inhibitor AZD5438 and the respectively
caged compound 3 in the ATP binding pocket of CDK2/cyclin E1 (PDB: 4FKO). (A)
Overview of CDK2 kinase structure with AZD5438 as ligand bound within the ATP
pocket. (B) Binding mode of AZD5438 addressing H-bonds (yellow dotted lines) to
the hinge region LEU83 within the ATP pocket. (C) Superposition of BODIPY-caged
AZD5438 (compound 3) in the CDK2 ATP binding pocket analogous to the original
binding mode of the ligand. Caged compound 3 cannot bind into the binding
pocket due to significant sterical clashes (red dotted lines).
Taken together, our modelling data suggested the BODIPY caged
AZD5438 to be biologically inactive, whereas uncaging by irradiation
with light of ca. 500 nm wavelength could cleave the BODIPY cage
restoring the potent CDK2 inhibition of AZD5438. To prove this
hypothesis, we set out to synthesise and subsequently perform
photochemical characterisation of the caged compound. Synthesis
of the key BODIPY caging group 1 was achieved according to
literature49,50 based on a Friedel-Crafts-acylation (Scheme 1).
Figure 3. Uncaging of 3 under irradiation with 525 nm in DMSO solution. 25 µM of
3 were irradiated with 6.5 mW/cm² per well in a white 96-well plate. Concentrations
of reaction products were quantified by HPLC. Error bars indicate SD (N = 3).
leaving group. In turn, using a SN reaction in dry DMF substitution of
the benzylic BODIPY position by the deprotonated NH position of
AZD5438 at -35 °C yielded key compound 3 (Scheme 1). Notably, all
reactions were performed under argon atmosphere and were
protected from light.
Next, we performed photochemical characterisation in terms of the
uncaging process of compound 3. Determination of UV spectra
revealed significant absorption of 3 at a wavelength of 525 nm (S12).
Thus, we initially irradiated a 25 µM solution of 5 in DMSO with light
at this wavelength and monitored the uncaging reaction progress by
HPLC analysis (Figure 3).
The BODIPY uncaging reaction was considered to proceed via a
cationic transition state, analogously to the coumarin caging group,
although the detailed mechanism is still under discussion.9,12
However, in our experiments, by irradiation of 3 in aprotic DMSO
solution, the decline of the BODIPY moiety in compound 3 could be
clearly detected via adsorption at 544 nm (to avoid artefacts, a
higher wavelength was chosen, see S13). While under the DMSO
conditions, uncaging of 3 yielded AZD5438 as determined by LC/MS
analysis, we could, in contrast, not detect a main BODIPY derivative,
neither by UV adsorption nor by LC/MS analysis (S17). Interestingly,
the situation turned out to be different under aqueous uncaging
conditions and HPLC quantification could not be realised. Compared
to the situation in DMSO, an aqueous solution of caged compound 3
was not quite stable even without irradiation and immediately after
irradiation, the caged compound declined under the detection limit
while the free inhibitor AZD5438 was released, but not in an
equimolar manner compared to 3. Instead, irradiation resulted in a
complex product mixture indicating nonspecific degradative
reactions.
Scheme 1. Convergent synthesis of caged compound 3. The BODIPY precursor 1 was
synthesized by a Friedel-Crafts-acylation.49,50 The 8-iodomethyl BODIPY 2 was achieved
based on a Finkelstein reaction.50
Biological evaluation
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DOI: 10.1039/D0PP00097C
Figure 4. Kinase assay and cell proliferation assay of CDK2 inhibitor AZD5438,
caged 3, and BODIPY derivative 5. (A) Dose-response curve of 3 without
irradiation (squares, green dotted line, IC50 = 398 nM), 3 with 519 nm irradiation
(rhomb, green line, IC50 = 258 nM) and AZD5438 as positive control (circles, black
line, IC50 = 1.9 nM) on a CDK2/cyclin E1 system. (B) Dose-response curve of 5
without irradiation (purple squares), 5 with 519 nm irradiation (purple rhombs)
and AZD5438 as positive control (circles, black line) on a CDK2/cyclin E1 system.
Compound 5 as well as 5 upon irradiation followed no dose-dependent inhibition.
Kinase assay was performed with a final concentration of 10 µM ATP and
irradiated for ten minutes, 6.5 mW/cm² per Well. (C) Dose-response curve of 3
without irradiation (squares, green dotted line, IC50 > 10 µM), 3 with 525 nm
irradiation (rhombs, green line, IC50 = 0.03 µM) and AZD5438 as positive control
(circles, black line, IC50 = 0.9 µM) in a Panc89 cell proliferation assay. (D) Dose-
response curve of 5 without irradiation (squares, purple dotted line, IC50 > 10 µM),
5 with 525 nm irradiation (rhombs, purple line, IC50 = 0.02 µM) and AZD5438 as
positive control (circles, black line, IC50 = 0.9 µM) in a Panc89 cell proliferation
assay. Well plate was irradiated for two minutes, 8.9 mW/cm² per Well.
Figure 5. Biological evaluation of 3 and 5 on Panc89 cells. (A) Caspase 3/7 assay of
3 under irradiation (squares, green line, no caspase activity) and 5 (rhombs, purple
line, no caspase activity or necrosis) and AZD5438 as positive control (circles, black
line, dose-dependent caspase activity). (B) Fluorescence microscopy of Panc89
treated with 5. Nucleus was stained with Hoechst 33342 (blue) and compound
distribution of 5 is coloured in green. Scale bar represents a length of 20 µm (C)
Fluorescence microscopy of Panc89 treated with DMSO, AZD5438, 3 or 5 under
controlled light conditions. Scale bar at the lower right represents a total length of
70 µM.
(IC50 = 0.91 µM, Figure 4C). Thus, our results indicate a bioactive role
of the BODIPY moiety upon irradiation, which is in line with further
recent reports.6,52,53 This substantial increase in potency could not
be explained with additive effects of light or temperature increase
due to irradiation (control experiments S21). To further investigate
this unexpected effect, we synthesised the proposed hydroxylated
BODIPY cleavage product 5 without any inhibitor attached (Scheme
2).
Having compound 5 in hand, we next tested its biological activity in
the CDK2 kinase assay (no inhibition, Figure 4B) and cellular Panc89
proliferation assay with and without irradiation by light of 525 nm
(Figure 4D). Interestingly, the inhibition of the Panc89 cells following
treatment with 5 and irradiation with light (IC50 = 0.022 µM) is similar
to the results obtained by performing the assay using compound 3
(IC50 = 0.026 µM). These results suggest the irradiated BODIPY
moiety and not the potent inhibitor AZD5438 to be mainly
responsible for the strong inhibition of PANC89 cell proliferation in
the assay.
Since obviously the BODIPY moiety mediated a strong bioactivity in
combination with the irradiation of light with 525 nm in cells, we
next set out to evaluate this effect in greater detail. First, based on
the fluorescence of BODIPY, we used fluorescence microscopy to
detect cellular localisation of the compound (Figure 5B). Herein,
decent cellular bioavailability was proven for 5, which is in
accordance with further reports from BODIPY derivatives detecting
accumulation probably in mitochondria.54–56
However, since AZD5438 is partly released under irradiation with
525 nm, we decided on further biological in vitro evaluation of 3 in
an enzymatic CDK2/cyclin E1 assay and towards CDK2 dependent
Panc89 cells51 (Figure 4). Accordingly to our initial hypothesis, when
tested without irradiation in the enzymatic CDK2/cyclin E1 assay,
caged 3 proved to be significantly less active compared to AZD5438.
In contrast, the potent CDK2 inhibitor AZD5438 was determined
with an IC50 value of 1.9 nM, which is in accordance to data from
literature25 (Figure 4A). A similar situation was detected in our
cellular assay, where caged 3 was initially less active without
irradiation compared to AZD5438 (Figure 4C).
Unexpectedly, when we irradiated caged compound 3 within the
enzymatic assay, the results showed only minor inhibition of CDK2
activity. The biological activity of the irradiated 3 was actually
comparable to the situation without irradiation and in sharp contrast
to the potency of the native AZD5438 (Figure 4A). Potential
explanations may again include degradation under the aqueous
assay conditions (in contrast to uncaging determined in DMSO, see
Figure 3). However, to our great astonishment, the irradiation of 3
in the cellular Panc89 assay with 525 nm resulted in a very strong
biological activity (IC50 = 0.026 µM). This efficacy is significantly
higher compared to the potent CDK2 inhibitor AZD5438 itself
O
Cl
O
O
OH
O
TEA
O
N
BF3-OEt2
LiOH
Additionally, induction of apoptosis has been described related to
incubation of cells with BODIPY derivatives under irradiation,
N
N
DCM
THF/H2O 1:1
RT, 30 min
N
H
B
N
0-50 °C
B
F
F
F
F
5
suggesting the BODIPY moiety to show
a potential for
4
92%
photodynamic approaches.57–59 In line with this notion, apoptosis
and respectively necrosis has been reported to be a common cell
Scheme 2 Convergent synthesis of the hydroxylated BODIPY (5). Synthesis was
adapted from literature.49
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death path for photodynamic agents.60 Thus, we treated Panc89
cells with compounds 3, 5, and AZD5438 with and without
irradiation and determined apoptosis rates via a bioluminescence
signal of caspase 3/7 activation respectively (Figure 5A). Herein,
AZD5438-treated cells showed a distinct apoptosis according to the
mode of action as CDK2 inhibitor. In contrast, when irradiated with
light of 525 nm, Panc89 cells treated with both 3 and 5 resulted in
significant necrosis. Notably, the BODIPY bearing compounds 3 and
5 do not show detectable cell death without irradiation. Images from
the fluorescence microscopy (Figure 5C) further support these cell
proliferation results. Cell density of Panc89 cells is highly decreased
after treatment with 3 (-93 %) and 5 (-90 %) under irradiation with
light of 525 nm whereas cells treated with DMSO (-14 %) reacted less
light-dependent or AZD5438 (+1 %) light-independent, respectively
(S21).
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DOI: 10.1039/D0PP00097C
However, compounds 3 and 5 required only one minute of
irradiation for a substantial singlet oxygen response in contrast to 15
minutes for uncaging at similar settings. Thus, it seemed that singlet
oxygen generation appears preferably during the uncaging process
of the BODIPY moiety. By correlating this effect to the observed
cytotoxicity in Panc89 cells, we conclude that 3 and 5 produce singlet
oxygen upon irradiation, thus severely damaging cells, thereby
exceeding the effect of the potent CDK2 inhibitor AZD5438.61,62
Furthermore, the proposed radical mechanism provides a possible
explanation for the observation of non-successful uncaging for 3
upon irradiation in the enzymatic assay (Figure 3A). According to the
production of short-lived radicals detected by ESR, we assume that
the molecule decomposes, yielding a mixture of fragments instead
of providing the actual CDK2 inhibitor AZD5438. Due to solubility
issues of 3 the photochemical characterisation was performed in
DMSO yielding uncaging of 3 without difficulty (Figure 3). However,
biological evaluation takes place in aqueous solutions at µM
compound concentrations. Thus, we performed respective
experiments in aqueous solutions for 3 and 5. Indeed, under
Photodegradation
Having clearly established a significant photodynamic effect of
BODIPY derivatives 3 and 5, we aimed to elucidate mechanistical
aspects of their supposed mode of action. Further evidence showed
BODIPY derivatives are also known for creating reactive oxygen
species under irradiation.5–7,56 We could demonstrate the
considerable formation of short-lived radicals by ESR using the cyclic
hydroxylamine TMTH after irradiation of compound 3 at 525 nm
(6A). In another test system, both 3 and 5 decreased the absorption
maximum of read out signal from 1,3-diphenylisobenzofuran under
irradiation, providing further evidence for generating singlet oxygen
(Figure 6B). In DMSO, singlet oxygen generation as well as
photouncaging appeared instantly upon irradiation with +500 nm
(Figure 6B vs. Figure 3).
irradiation we observed
a strong photodegradation of the
compounds yielding non-defined product mixtures providing further
evidence for radical reaction processes (5C, S16).
Photodegradation has already been reported for photosensitisers,
e.g. porphyrines and semi-porphyrines.63 Compared to features of
these photosensitisers, solutions of compound 3 and 5 decoloured
upon irradiation suggesting
a
type
Interestingly,
proper
2
photobleaching (true
not only
photochemical
photobleaching)
photodegradation
behaviour.
prevents
a
characterisation in aqueous solution, but also precipitation occurred
in the samples upon irradiation probably as the result of radical
polymerisation reactions. Similar to our observations, Descalzo et al.
also reported about aggregation and dimerisation of BODIPY
moieties64 in aqueous solutions, which are, however, indispensable
for biological applications. We also observed precipitation of 3 and
5 when water was added to a DMSO stock solution (S14).
Accordingly, aggregation of BODIPY moieties has been reported
elsewhere to be responsible for precipitation when water was
added.65,66 Furthermore, NMR experiments employing D2O to
investigate these effects in more detail proved to be non-successful
again due to precipitation.
Conclusions
Uncaging of BODIPY-caged compounds is well established in organic
solvents and could be also achieved in this work.9,11 However, with
regard to biological evaluation in aqueous solutions, the situation
turned out to be different. In line with recent reports from Sitkowska
et al. and Takeda et al., we observed unstable storage conditions for
BODIPY derivatives.12,66 Although the BODIPY moiety is tempting for
caging approaches based on its response to green light with
wavelength of > 500 nm, we provide evidence for the BODIPY-caging
concept being limited in an aqueous environment due to the
production of singlet oxygen upon irradiation. Thus, the generated
Figure 6. Photochemical degradation of compound 3 and 5. (A) Electron spin
resonance spectrum of the spin probe TMT in the presence of 3 without irradiation
(black line) and 3 with 525 nm irradiation (green line) in DMSO, 30 s and intensity of
90 mW/cm² per well. (B) Singlet oxygen assay. 1,3-Diphenylisobenzofuran (DPBF)
and compounds were dissolved in DMSO. Absorption of DPBF decreases due to
singlet oxygen production of 3 under irradiation (green rhombs) and 5 under
irradiation (purple rhombs). 3 without irradiation (green circles) and 5 without
irradiation (purple circles) have not generated singlet oxygen. Singlet oxygen
quantum yield of 3 (Φ = 0.38) and 5 (Φ = 0.74) upon irradiation was determined with
methylene blue (Φ = 0.52).(C) Absorption spectra of an aqueous solution of 0.1 mM
3 over time and under controlled light conditions. In aqueous solution maxima
decrease over time. Full deletion at 500 nm absorption was achieved irradiating with
519 nm for 10 minutes, 6.5 mW/cm² per well.
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Journal of the American Chemical Society, 2017, 139(42),
radical species mediate non-selective degradation effects.
Accordingly, biological activities of BODIPY containing compounds 3
and 5 in a Panc89 cell proliferation assay turned out to be highly
effective. In this assay, due to singlet oxygen generation, 3 as a caged
compound is 30 times more active upon irradiation compared to the
actually potent CDK2 inhibitor AZD5438. Hence, in order to avoid
misinterpretation of the results for BODIPY caged compounds in
biological applications we strongly recommend to investigate
BODIPY moieties case-by-case towards their potential to produce
ROS upon irradiation. Whether BODIPY derivatives will be useful in
photodynamic approaches remains to be seen.
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DOI: 10.1039/D0PP00097C
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We would like to thank Martin Schütt and Dr. Ulrich Girreser from
the Pharmaceutical Institute, Kiel, for excellent technical and
analytical assistance. We also kindly acknowledge Till Priegann for
technical support during fluorescence microscopy. At last, we
gratefully acknowledge the Deutsche Forschungsgesellschaft for
financial support (grant PE1605_2_2).
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Photochemical & Photobiological Sciences
Page 8 of 8
View Article Online
DOI: 10.1039/D0PP00097C
Photochemical analysis and biological characterization in vitro revealed significant limitations of the
BODIPY-caged inhibitor concept regarding uncaging in aqueous solution but also promising cancer
cell treatment through photodynamic effects.