A Bifunctional Photosensitizer for Enhanced Fractional Photodynamic Therapy: Singlet Oxygen Generation in the Presence and Absence of Light

Article abstract of DOI:10.1002/anie.201511345

The photosensitized generation of singlet oxygen within tumor tissues during photodynamic therapy (PDT) is self-limiting, as the already low oxygen concentrations within tumors is further diminished during the process. In certain applications, to minimize photoinduced hypoxia the light is introduced intermittently (fractional PDT) to allow time for the replenishment of cellular oxygen. This condition extends the time required for effective therapy. Herein, we demonstrated that a photosensitizer with an additional 2-pyridone module for trapping singlet oxygen would be useful in fractional PDT. Thus, in the light cycle, the endoperoxide of 2-pyridone is generated along with singlet oxygen. In the dark cycle, the endoperoxide undergoes thermal cycloreversion to produce singlet oxygen, regenerating the 2-pyridone module. As a result, the photodynamic process can continue in the dark as well as in the light cycles. Cell-culture studies validated this working principle in vitro.

Full text of DOI:10.1002/anie.201511345

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Chemie
International Edition: DOI: 10.1002/anie.201511345
German Edition: DOI: 10.1002/ange.201511345
Photodynamic Therapy
A Bifunctional Photosensitizer for Enhanced Fractional Photodynamic
Therapy: Singlet Oxygen Generation in the Presence and Absence of
Light
Ilke Simsek Turan, Deniz Yildiz, Abdurrahman Turksoy, Gurcan Gunaydin, and
Engin U. Akkaya*
Abstract: The photosensitized generation of singlet oxygen
within tumor tissues during photodynamic therapy (PDT) is
self-limiting, as the already low oxygen concentrations within
tumors is further diminished during the process. In certain
applications, to minimize photoinduced hypoxia the light is
introduced intermittently (fractional PDT) to allow time for
the replenishment of cellular oxygen. This condition extends
the time required for effective therapy. Herein, we demon-
strated that a photosensitizer with an additional 2-pyridone
module for trapping singlet oxygen would be useful in
fractional PDT. Thus, in the light cycle, the endoperoxide of
2-pyridone is generated along with singlet oxygen. In the dark
cycle, the endoperoxide undergoes thermal cycloreversion to
produce singlet oxygen, regenerating the 2-pyridone module.
As a result, the photodynamic process can continue in the dark
as well as in the light cycles. Cell-culture studies validated this
working principle in vitro.
rapidly using intracellular oxygen reserves, creates acute
hypoxia.[5] Many studies suggest that fractional (intermittent)
delivery of light might be a better approach to PDT.[6] Thus,
between irradiation periods, time is allocated for the replen-
ishment of intracellular oxygen.
Considering the fact that singlet oxygen is the ultimate
cytotoxic agent required for effective PDT, we thought of
using chemically generated singlet oxygen for the dark period
of fractional PDT, where photosensitized generation is not
possible. In fact, it would be highly interesting to combine
a photosensitizer and a chemical source of singlet oxygen in
a single molecule.
The endoperoxides of 2-pyridone and derivatives have
been recognized as reliable chemical sources of singlet oxygen
as they undergo clean (no side reactions) cycloreversion
reactions to release singlet oxygen with very high yields.[7–9] In
fact, it has been already reported that singlet oxygen
produced by the thermal decomposition of 2-pyridone
endoperoxides lead to cell death by a process resembling
apoptosis in cancer cell cultures.[10] In addition, phthalocya-
nine[11] and porphyrin endoperoxide[12] derivatives were
reported where the endoperoxides are obtained by self-
photosensitization. Thus, it is evident that a rationally
designed bifunctional photosensitizer/2-pyridone conjugate
may be an optimal agent for a novel approach to fractional
photodynamic therapy. 2,6-Dibromo (and iodo) distyryl
BODIPY derivatives (BODIPY= boron–dipyrromethene)
were shown[13–16] to be promising long-wavelength sensitizers
of molecular oxygen. In addition, the versatility of the
chemistry of BODIPY derivatives has led to the employment
of these molecules more and more frequently in the develop-
ment of novel strategies towards the improvement of PDT.[17]
Based on these considerations, we designed a bifunctional
compound to meet the requirements for enhanced fractional
photodynamic therapy. The operation principle for the
generation of singlet oxygen is shown in Figure 1. When
excited at l = 650 nm, the upper dye molecule in the figure
(Pyr 6) is expected to generate singlet oxygen, and some of it
will be “stored” in the form of a 2-pyridone endoperoxide
(EPO 7). When the irradiation is turned off, as it would be in
fractional PDT (for the replenishment of intracellular
oxygen), the 2-pyridone-endoperoxide (EPO 7) will undergo
thermal cycloreversion producing singlet oxygen in the
absence of light.
P
hotodynamic therapy has, for some time, been considered
a promising method for the treatment of various cancers.[1]
The therapeutic procedure involves the photosensitized
generation in tumor tissues of singlet oxygen, which is
cytotoxic and has a short lifetime, increasing the chances of
selective action. There are other aspects of photodynamic
therapy (PDT) which makes it appear truly promising, such as
an enhanced immune response following a PDT session.[2]
Nevertheless, clinical application of PDT seems to limited
mostly to superficial lesions.[3] One reason for the limited
application of the method is the fact that PDT requires
oxygen, but most tumors develop regions of severe hypoxia
where photosensitized singlet oxygen generation would not
be expected.[4] More problematic is the fact that PDT itself, by
[*] Dr. I. S. Turan, Prof. Dr. E. U. Akkaya
UNAM-National Nanotechnology Research Center
Bilkent University
06800 Ankara (Turkey)
E-mail: eua@fen.bilkent.edu.tr
D. Yildiz, A. Turksoy, Prof. Dr. E. U. Akkaya
Department of Chemistry, Bilkent University
06800 Ankara (Turkey)
Dr. G. Gunaydin
Department of Basic Oncology, Hacettepe University
06100 Ankara (Turkey)
Supporting information (including methods, experimental proce-
dures, and additional spectral data) and ORCID(s) from the
author(s) for this article are available on the WWW under http://dx.
doi.org/10.1002/anie.201511345.
The syntheses of the target compounds are shown in
Scheme 1. The key step is the functionalization of 2-pyridone
with an arylaldehyde group. The aldehyde functionality
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with p-methoxybenzaldeyde to form compound 6. The
distyryl–BODIPY compound 7 was formed after irradiation
of 6 in the presence of methylene blue. Compound 7 has an
absorption maximum at l = 667 nm at the red region of the
visible spectrum, with an extinction coefficient of
74200 cmÀ1mÀ1
.
We first set out to demonstrate the reversibility of the
transformation between the pyridone (PYR 6) and the
1
endoperoxide (EPO 7). Fortunately, the H NMR spectra of
the two compounds have well-resolved characteristic signals
1
for both the PYR and EPO forms. Partial H NMR data are
shown in Figure 2. The triplet at d = 6.25 ppm is attributable
to a proton on the pyridone ring and the AB system between
4.6 and 4.9 ppm, an indicator of endoperoxide formation, is
attributable to N-CH2 protons. Spectrum A is the partial
1H NMR spectrum (showing the d = 4.4–6.4 ppm region) of
compound 6, or the PYR form.
Figure 1. Singlet oxygen generation achieved first by irradiation of
bifunctional compound 6 at l=650 nm, and subsequently by thermal
cycloreversion in the dark. The cycles can be repeated indefinitely.
provides easy access to BODIPY structures, and in this case
we were able to obtain compound 4 in good yields.
Bromination with N-bromosuccinimide (NBS) affords com-
pound 5. In 5, the presence of the heavy bromine atoms
facilitated intersystem crossing, which is essential for photo-
sensitized singlet oxygen generation. Next, to shift the
absorption maximum of the BODIPY chromophore to
a region more relevant for PDT, compound 5 was reacted
Figure 2. Top: Partial 1H NMR spectra (CDCl3; 378C) demonstrating
the reversibility of the interconversion between compound 6 (PYR) and
compound 7 (EPO). Spectrum A shows the partial NMR spectrum of
the sample (PYR 6) recorded at 0 min. Spectrum B is that of the
sample after irradiation at l=650 nm for 15 min in the NMR tube,
forming EPO 7 (a very weak stream of oxygen was bubbled through
the sample throughout the experiment). The other spectra were
recorded at 15 min intervals in the dark at 378C. Bottom: Relative
percentages of 6 and 7 present during the conversion between the two
forms. The conversion process was monitored by measuring the
integrated peak areas for the characteristic NMR signals of each form.
Relative percentages of both forms (corresponding to the upper NMR
spectra) are given in the table under the plot.
Scheme 1. Synthesis of the target pyridone (6) and endoperoxide (7)
derivatives. DCM=dichloromethane; TFA=trifluoroacetic acid; TEA=
triethylamine; p-chloroanil=tetrachloro-p-benzoquinone.
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1
The H NMR spectrum of EPO 7 in the same region is
presented in Figure 2 (top) as the other reference spectrum
(spectrum B). This spectrum is obtained by irradiation of
compound 6. When the irradiation was stopped and the
solution kept in the dark at 378C, the spectra gradually
evolved from spectrum B to F. At that point, the NMR tube
was irradiated again for a short time, which clearly regen-
erated unmistakable resonance signals for the endoperoxide
(such as the AB system around d = 4.75 ppm). Thus, it is clear
that interconversion between the two forms occurred as long
as there was dissolved oxygen present during the irradiation.
The formation of singlet oxygen was further studied using
the selective singlet-oxygen trap molecule 1,3-diphenyliso-
benzofuran (DPBF). Under irradiation with an LED array at
l = 650 nm, halogenated distyryl–Bodipy sensitizes ground-
state molecular oxygen to generate singlet oxygen. The trap
molecule reacts with the singlet oxygen, resulting in a decrease
in the absorbance of DPBF at l = 416 nm. As mentioned
earlier, during this light cycle the molecule is transformed
from the PYR form into the EPO form. When the irradiation
at l = 650 nm is switched off, the EPO form gradually
cycloreverts to the PYR form, generating simultaneously
singlet oxygen in the dark cycle. The results of this experiment
and that of the control, which is the trap compound DPBF (in
DMSO) subjected to the dark and light cycles, are shown in
Figure 3.
structure. Micelles were prepared following literature proce-
dures.[17c] The size distribution of the micelles was studied by
using dynamic light scattering, which showed that the average
size of the micelles carrying embedded 7 or 8 is about 50 nm.
HeLa cells were incubated with complete Dulbeccoꢀs modi-
fied Eagleꢀs medium (DMEM) under environmental condi-
tions of 378C, 5% CO2, and 60% humidity. Cells were
exposed to varying concentrations of the compounds in
cremophor EL micelles (9 nm–2.5 mm) and illuminated with
a red light source (l = 655 nm LED array, 324 mmolmÀ2 sÀ1
photon flux) for 10 min in every one hour, which was repeated
24 times. Thus, the total illumination time was exactly 4 h.
This 24 h period of light–dark cycles was then followed by
a 24 h incubation period in the dark (total 48 h). The control
group of cells were incubated in the dark for 48 h under
identical environmental conditions. MTT assays were per-
formed to assess cell viability and cytotoxicity (Figure 4). It
The performance of the bifunctional photosensitizer was
also studied in HeLa (human cervical cancer cell line) cell
cultures. Our expectation was that EPO 7 would be a more
effective cytotoxic agent compared to a simple photosensi-
tizer in when employed in the dark–light cycle regime (as is
done in fractional PDT). Since the apoptotic response is time-
dependent, a more fair comparison would be to compare the
effects of PYR and EPO. Since both compounds had very
limited water solubility, the non-ionic surfactant cremo-
phor EL was used to deliver the agents within a micellar
Figure 4. Cell viabilities as determined by MTT assays. Data labeled as
“light” refers to results obtained when indicated concentrations of
micelle-delivered agents PYR (6) and EPO (7) were exposed to cycles
of light (l=655 nm, 10 min) and dark (50 min) with a total of 4 h of
light exposure, followed by 24 h of incubation in the dark. The label
“dark” indicates compounds which had been incubated in the dark
only for 48 h.
was found that even low doses of compound 7 (EPO) results
in a significant decrease of the cell viability. The CC50 values
(50% cytotoxic concentration) of the compounds both in the
dark and after irradiation were estimated by fitting a model
with nonlinear regression. The CC50 value of compound 7
(EPO) was found to be significantly less than that of 6 (PYR)
after intermittent illumination of both (approximately 8.6 nm
versus 49.0 nm). This result validates our design as it suggests
that initial EPO cycloreversion results in a large difference in
singlet oxygen generation when followed by the application of
Figure 3. Decrease in the absorbance (at l=416 nm) of the trap
compound DPBF in the presence (blue line) and the absence (green
line; i.e. trap compound DPBF alone) of the EPO/PYR agent (6/7). The
reaction was carried out at 378C in DMSO. White bars correspond to
the periods of irradiation (L, light cycles) which was carried out using
a red LED array irradiating at l=650 nm for 30 s. Black bars (D, dark
cycles) correspond to the dark periods, where both solutions were kept
in the dark for 30 min. A gentle stream of oxygen was bubbled through
the reaction and control samples. There is a slight decrease in the
absorbance of the control in the light cycle as expected, but no
significant change takes place in the dark.
a
light–dark cycle. Additionally, when considering the
reported CC50 values, EPO 7 is, as expected, much more
active as a photosensitizing cytotoxic agent compared to
previously reported Bodipy-based long-wavelength photo-
sensitizers which do not have reversible singlet-oxygen
storage modules.[1f]
In this proof-of-principle study, we have prepared a bifunc-
tional photosensitizer to improve fractional PDT. We dem-
onstrated that continuous release of singlet oxygen from the
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Skarlatos, L. Corti, S. Blandamura, M. Piazza, K. C. Gatter,
A. L. Harris, Cancer Res. 2001, 61, 1830 – 1832.
photosensitizer can be achieved in both light and dark cycles,
leading to significant differences in cytotoxic activity in cell
culture studies compared to conventional PDT agents. We
propose that when the system is applied in vivo, while the
oxygen levels in the tumor tissues are replenished for the next
round of photosensitized singlet oxygen generation, thermal
cycloreversion will continue to produce singlet oxygen in
dark, without consuming molecular oxygen. Considering the
fact that tumor hypoxia and PDT-induced hypoxia are major
problems hindering the broader applicability of PDT, we are
confident that bifunctional PDT agents will have a promising
future.
[5] a) C. Robertson, D. H. Evans, H. Abrahamse, J. Photochem.
Photobiol. B 2009, 96, 1 – 8; b) I. Van Geel, H. Oppelaar, P.
Rijken, H. Bernsen, N. Hagemeier, A. Van der Kogel, R.
Hodgkiss, F. Stewart, Br. J. Cancer 1996, 73, 288 – 293; c) T.
Sitnik, J. Hampton, B. Henderson, Br. J. Cancer 1998, 77, 1386 –
1394; d) T. M. Busch, S. M. Hahn, S. M. Evans, C. J. Koch,
Cancer Res. 2000, 60, 2636 – 2642.
[6] a) Z. Xiao, S. Halls, D. Dickey, J. Tulip, R. B. Moore, Clin.
Cancer Res. 2007, 13, 7496 – 7505; b) L. Yang, Q. Chen, Y. Wei,
D. Xing, Lasers Surg. Med. 2010, 42, 671 – 679.
[7] a) J. Poully, J. Schermann, N. Nieuwjaer, F. Lecomte, G.
GrØgoire, C. DesfranÅois, G. Garcia, L. Nahon, D. Nandi, L.
Poisson, Phys. Chem. Chem. Phys. 2010, 12, 3566 – 3572; b) M.
Matsumoto, M. Yamada, N. Watanabe, Chem. Commun. 2005,
483 – 485.
[8] a) C. Wiegand, E. Herdtweck, T. Bach, Chem. Commun. 2012,
48, 10195 – 10197; b) J.-M. Aubry, C. Pierlot, J. Rigaudy, R.
Schmidt, Acc. Chem. Res. 2003, 36, 668 – 675.
Acknowledgements
I.S.T. gratefully acknowledges support from TUBITAK in the
form of a postdoctoral scholarship. A.T. is grateful to
TUBITAK for a graduate student scholarship (2210-E).
[9] S. Benz, S. Nçtzli, J. S. Siegel, D. Eberli, H. J. Jessen, J. Med.
Chem. 2013, 56, 10171 – 10182.
[10] a) K. Otsu, K. Sato, Y. Ikeda, H. Imai, Y. Nakagawa, Y. Ohba, J.
Fujii, Biochem. J. 2005, 389, 197 – 206; b) K. Otsu, K. Sato, M.
Sato, H. Ono, Y. Ohba, Y. Katagata, Cell Biol. Int. 2008, 32,
1380 – 1387; c) D. Posavec, M. Zabel, U. Bogner, G. Bernhardt,
G. Knçr, Org. Biomol. Chem. 2012, 10, 7062 – 7069.
[11] T. Stuchinskaya, M. Moreno, M. J. Cook, D. R. Edwards, D. A.
Russell, Photochem. Photobiol. Sci. 2011, 10, 822 – 831.
[12] a) W. Freyer, H. Stiel, M. Hild, K. Teuchner, D. Leupold,
Photochem. Photobiol. 1997, 66, 596 – 604; b) C. Changtong,
D. W. Carney, L. Luo, C. A. Zoto, J. L. Lombardi, R. E. Connors,
J. Photochem. Photobiol. A 2013, 260, 9 – 13; c) M. A. Filatov, E.
Heinrich, D. Busko, I. Z. Ilieva, K. Landfester, S. Baluschev,
Phys. Chem. Chem. Phys. 2015, 17, 6501 – 6510.
[13] a) I. S. Turan, F. P. Cakmak, D. C. Yildirim, R. Cetin-Atalay,
E. U. Akkaya, Chem. Eur. J. 2014, 20, 16088 – 16092; b) S. H.
Lim, C. Thivierge, P. Nowak-Sliwinska, J. Han, H. van den
Bergh, G. Wagnieres, K. Burgess, H. B. Lee, J. Med. Chem. 2010,
53, 2865 – 2874; c) S. G. Awuah, J. Polreis, V. Biradar, Y. You,
Org. Lett. 2011, 13, 3884 – 3887.
Keywords: BODIPY · peroxides · photochemistry ·
photodynamic therapy · singlet oxygen
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Angew. Chem. 2016, 128, 2925–2928
[1] a) R. Bonnett, Chem. Soc. Rev. 1995, 24, 19 – 33; b) J. P. Celli,
B. Q. Spring, I. Rizvi, C. L. Evans, K. S. Samkoe, S. Verma, B. W.
Pogue, T. Hasan, Chem. Rev. 2010, 110, 2795 – 2838; c) S. B.
Brown, E. A. Brown, I. Walker, Lancet Oncol. 2004, 5, 497 – 508;
d) T. J. Dougherty, J. E. Kaufman, A. Goldfarb, K. R. Weish-
aupt, D. Boyle, A. Mittleman, Cancer Res. 1978, 38, 2628 – 2635;
e) D. E. Dolmans, D. Fukumura, R. K. Jain, Nat. Rev. Cancer
2003, 3, 380 – 387; f) A. Kamkaew, S. H. Lim, H. B. Lee, L. V.
Kiew, L. Y. Chung, K. Burgess, Chem. Soc. Rev. 2013, 42, 77 – 88;
g) T. Yogo, Y. Urano, Y. Ishitsuka, F. Maniwa, T. Nagano, J. Am.
Chem. Soc. 2005, 127, 12162 – 12163.
[14] A. Loudet, K. Burgess, Chem. Rev. 2007, 107, 4891 – 4932.
[15] a) S. G. Awuah, Y. You, RSC Adv. 2012, 2, 11169 – 11183; b) W.
Wu, Y. Geng, W. Fan, Z. Li, L. Zhan, X. Wu, J. Zheng, J. Zhao,
M. Wu, RSC Adv. 2014, 4, 51349 – 51352; c) A. Kamkaew, K.
Burgess, J. Med. Chem. 2013, 56, 7608 – 7614.
[2] a) A. P. Castano, P. Mroz, M. R. Hamblin, Nat. Rev. Cancer 2006,
6, 535 – 545; b) M. Korbelik, B. Stott, J. Sun, Br. J. Cancer 2007,
97, 1381 – 1387; c) S. Gollnick, S. Evans, H. Baumann, B.
Owczarczak, P. Maier, L. Vaughan, W. Wang, E. Unger, B.
Henderson, Br. J. Cancer 2003, 88, 1772 – 1779.
[16] a) S. Ozlem, E. U. Akkaya, J. Am. Chem. Soc. 2008, 130, 48 – 49;
b) S. Erbas, A. Gorgulu, M. Kocakusakogullari, E. U. Akkaya,
Chem. Commun. 2009, 4956 – 4958; c) G. Ulrich, R. Ziessel, A.
Harriman, Angew. Chem. Int. Ed. 2008, 47, 1184 – 1201; Angew.
Chem. 2008, 120, 1202 – 1219.
[17] a) S. Kolemen, M. Is¸ık, G. M. Kim, D. Kim, H. Geng, M.
Buyuktemiz, T. Karatas, X. F. Zhang, Y. Dede, J. Yoon, Angew.
Chem. Int. Ed. 2015, 54, 5340 – 3344; Angew. Chem. 2015, 127,
5430 – 5434; b) S. Erbas-Cakmak, E. U. Akkaya, Angew. Chem.
Int. Ed. 2013, 52, 11364 – 11368; Angew. Chem. 2013, 125, 11574 –
11578; c) Y. Cakmak, S. Kolemen, S. Duman, Y. Dede, Y. Dolen,
B. Kilic, Z. Kostereli, L. T. Yildirim, A. L. Dogan, D. Guc,
Angew. Chem. Int. Ed. 2011, 50, 11937 – 11941; Angew. Chem.
2011, 123, 12143 – 12147; d) S. G. Awuah, S. K. Das, F. D’Souza,
Y. You, Chem. Asian J. 2013, 8, 3123 – 3132; e) J. F. Lovell, T. W.
Liu, J. Chen, G. Zheng, Chem. Rev. 2010, 110, 2839 – 2857.
[3] a) Z. Huang, Technol. Cancer Res. Treat. 2005, 4, 283 – 293;
b) M. G. Bredell, E. Besic, C. Maake, H. Walt, J. Photochem.
Photobiol. B 2010, 101, 185 – 190; c) N. C. Zeitouni, A. R.
Oseroff, S. Shieh, Mol. Immunol. 2003, 39, 1133 – 1136.
[4] a) Y. Liu, Y. Liu, W. Bu, C. Cheng, C. Zuo, Q. Xiao, Y. Sun, D.
Ni, C. Zhang, J. Liu, Angew. Chem. Int. Ed. 2015, 54, 8105 – 8109;
Angew. Chem. 2015, 127, 8223 – 8227; b) J. Xu, S. Sun, Q. Li, Y.
Yue, Y. Li, S. Shao, Analyst 2015, 140, 574 – 581; c) S. Wang, H.
Liu, J. Mack, J. Tian, B. Zou, H. Lu, Z. Li, J. Jiang, Z. Shen,
Chem. Commun. 2015, 13389 – 13392; d) K. Kiyose, K. Hanaoka,
D. Oushiki, T. Nakamura, M. Kajimura, M. Suematsu, H.
Nishimatsu, T. Yamane, T. Terai, Y. Hirata, J. Am. Chem. Soc.
2010, 132, 15846 – 15848; e) G. Zhang, G. M. Palmer, M. W.
Dewhirst, C. L. Fraser, Nat. Mater. 2009, 8, 747 – 751; f) J.
PouyssØgur, F. Dayan, N. M. Mazure, Nature 2006, 441, 437 –
443; g) X. Zheng, X. Wang, H. Mao, W. Wu, B. Liu, X. Jiang, Nat.
Commun. 2015, 6, 5834; h) W. Gallagher, L. Allen, C. OꢀShea, T.
Kenna, M. Hall, A. Gorman, J. Killoran, D. OꢀShea, Br. J. Cancer
2005, 92, 1702 – 1710; i) M. I. Koukourakis, A. Giatromanolaki, J.
Received: December 7, 2015
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