A multifunctional nanoassembly of mesogen-bearing amphiphiles and porphyrins for the simultaneous photodelivery of nitric oxide and singlet oxygen

Article abstract of DOI:10.1039/b713372c

We report a molecular nanoassembly able to supply simultaneously, in the same region of space and under the exclusive control of visible light, nitric oxide and singlet oxygen, two species playing a key role in the therapy of cancer; the considerable fluo

Full text of DOI:10.1039/b713372c

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A multifunctional nanoassembly of mesogen-bearing amphiphiles and
porphyrins for the simultaneous photodelivery of nitric oxide and singlet
oxygen{
Elisa B. Caruso, Enzo Cicciarella and Salvatore Sortino*
Received (in Cambridge, UK) 31st August 2007, Accepted 25th September 2007
First published as an Advance Article on the web 4th October 2007
DOI: 10.1039/b713372c
1
simultaneous photogeneration of NO and O2 (1Dg) in the same
We report a molecular nanoassembly able to supply simulta-
region of space, maximizing the efficiency of the phototreatment.
With this in mind, and stimulated by our recent results on NO
photoreleasing nanosystems,8a–c we report herein a multifunctional
micellar nanoassembly formed by the mesogen-bearing cationic
amphiphilic 1 and the anionic 5,10,15,20-tetrakis(4-sulfonatophe-
nyl)-21H,23H-porphyrin (TPPS), as a fluorescent delivery vehicle
able to supply NO and 1O2 (1Dg) under the control of visible light
(Fig. 1).
neously, in the same region of space and under the exclusive
control of visible light, nitric oxide and singlet oxygen, two
species playing a key role in the therapy of cancer; the
considerable fluorescence of this nanoaggregate and its reduced
size (ca. 40 nm) represent additional advantages that make this
photoactive vehicle an appealing candidate to be tested in
biological systems.
Compound 1 was easily synthesized in two steps (see ESI{) and
incorporates a commercial nitroaniline derivative that we have
recently discovered to be a suitable NO releasing unit under visible
light stimulation.8b,c In particular, the photochemical mechanism
involves a nitro-to-nitrite rearrangement followed by the rupture of
the O–N bond to generate a phenoxyl radical and NO.8a–c
The aggregation behavior of 1, which readily dissolves in water
at room temperature, was investigated by conductivity and
dynamic light scattering (DLS) measurements (Fig. 2). The plots
of the specific conductivity and DLS intensity vs. the surfactant
concentration are in excellent agreement with each other. From the
break points it can be established that the critical aggregation
concentration (cac) of 1 is ca. 0.7 mM. It is noteworthy that such a
value is more than one order of magnitude smaller than that
expected for a cationic surfactant containing both the same alkyl
chain and the polar head and, according to the literature, this can
be explained as a result of a drastic decrease of DGuagg due to the
terminal mesogenic unit.9 The size distribution obtained by DLS
measurements performed above the cac (inset, Fig. 2) is relatively
In recent years, there has been an explosion of interest in nitric
oxide (NO),1 a simple diatomic free radical, for the central role it
occupies in the maintenance and regulation of numerous
physiological processes, encompassing neurotransmission, vasodi-
lation and hormone secretion.2 Besides its involvement in these
vital functions, one of the most exciting properties of NO is its
anticancer activity.3 In this regard, NO releasing systems
exclusively controlled by light inputs are particularly appealing
due to the accurate dosage control of NO they offer with respect to
those based on spontaneous thermolysis. This is a crucial point for
the use of NO in anticancer therapy. In their excellent review on
NO donors with anticancer activity,4 Wang and coworkers have
outlined how NO can act as a double-edged sword either inhibiting
or encouraging tumor proliferation depending on the dose. They
also stressed that for the effective use of NO in anticancer therapy
its combination with common antitumor drugs is highly desirable.4
Porphyrins have extensively proven their activity as anticancer
drugs in photodynamic cancer therapy (PDT).5 In this well-known
non-invasive treatment, porphyrins excited with visible light
transfer the energy of their lowest triplet state to nearby oxygen
molecules to generate reactive singlet oxygen, 1O2 (1Dg), the actual
photodynamically active species inducing cellular death.6
However, incorporation of porphyrins within appropriate biovec-
tors is often necessary in PDT and the preservation of the
photophysical properties of the drug is a key pre-requisite for such
delivery systems.7
On these grounds the achievement of photoactive carriers able
to photogenerate NO themselves and, at the same time, to entrap
porphyrins without alteration of their photodynamic activity,
represents an intriguing objective to pursue. In fact, such
systems can, in principle, produce synergistic effects due to the
Dipartimento di Scienze Chimiche, Universita´ di Catania, Viale Andrea
Doria 8, Catania, I-95125, Italy. E-mail: ssortino@unict.it
{ Electronic supplementary information (ESI) available: Synthetic and
experimental procedures. See DOI: 10.1039/b713372c
Fig. 1 Idealized view of the 1–TPPS multifunctional nanoassembly.
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micelles. This spectroscopic scenario excellently reflects that
reported for the same porphyrin in the presence of optically
transparent cationic micelles11 and is also very similar to that
recently found in the case of cationic amphiphilic drug-carriers.12
Accordingly, our data are consistent with the entangling of TPPS
mainly as a monomer within the micellar network. DLS
measurements were also performed in the presence of TPPS and
showed only negligible changes in the size of the micellar
aggregates (see ESI{).
The most convenient method for testing the suitability of the
1
1–TPPS nanoassembly to photogenerate NO and O2 (1Dg) is the
direct and real-time monitoring of these transient species. To this
end, we used an ultrasensitive NO electrode, which directly detects
NO concentration by an amperometric technique and a time-
resolved luminescence apparatus to monitor the typical infrared
luminescence of 1O2 (1Dg) at 1270 nm13 (ESI{). Fig. 4 shows
unambiguous evidence for the light-controlled generation of NO
and 1O2 (1Dg) from the nanoassembly. In fact, we observed linear
photogeneration of NO which promptly stopped when the light
was turned off, besides the characteristic infrared luminescence of
1O2 (1Dg) decaying with first-order kinetics in ca. 4 ms.
Fig. 2 Specific conductivity (#) and DLS intensity (&) of aqueous
solutions of 1 as a function of surfactant concentration. The inset shows
the size distribution of 1 micellar aggregates.
narrow with a mean diameter = 41.5 nm and a s = 14.1 nm.10 This
value is larger than that expected for regular spherical micelles of 1
but is perfectly in line with the results reported for similar
amphiphiles of the type cationic group–decyl spacer–aromatic tail,
which are shown to form elongated micellar aggregates in aqueous
solution.9
A further, remarkable point of interest of this work is the
photochemical independence of the two photoactive units of the
nanoaggregate. Comparative photochemical experiments demon-
strated that the photogeneration efficiency of NO (WNO y 0.01)
1
and O2 (1Dg) (WD y 0.6) from the nanoassembly is virtually the
The self-assembly of TPPS and micelles of 1 is strongly
encouraged by favorable electrostatic interactions in view of their
opposite charges, and was demonstrated by UV–Vis absorption
and emission spectroscopy. As shown in Fig. 3A, the absorption
spectrum of TPPS in the presence of micelles (spectrum c) does not
reproduce the sum of the single components (spectrum d),
accounting for an interaction of the porphyrin with the micellar
system in the ground state. Clearer evidence for such an interaction
is provided by a comparison of the spectra (b) and (e) where it can
be readily noted that TPPS undergoes hypochromism and red-shift
of the Soret absorption upon binding with micelles. Besides, a red-
shift was also observed in the Q(0,0)-band peaks (see inset) as well
as in the typical double band emission of TPPS (Fig. 3B). The
fluorescence quantum yield was only slightly affected by the
micelles, going from 0.16 (for the free TPPS) to 0.11, and did not
exhibit any dependence on the excitation wavelength, suggesting
the presence of a single population of fluorophores within the
same as that of the single components (Fig. 5), ruling out any
Fig. 4 A: NO released upon 450 nm light irradiation of 1–TPPS
nanoassembly in aqueous solution. (B) Representative decay kinetics of
1O2 (1Dg) and related first-order fitting observed upon 532 nm laser
excitation of 1–TPPS nanoassembly in aqueous solution. [1] = 2 mM,
[TPPS] = 20 mM.
Fig. 5 A: photochemical NO release from micellar solution of 1 (2 mM)
Fig. 3 A: absorption spectra in the Soret band, for (a) 1 (2 mM), (b)
TPPS (20 mM) and (c) their mixture in aqueous solution. Cell path: 1 mm.
The sum (d) = (a) + (b) and the difference (e) = (c) 2 (a) spectra are also
shown, for the sake of comparison. The magnified (b) and (e) spectra in
the Q-bands region are also shown in the inset. B: Fluorescence spectra
(lexc = 580 nm) of TPPS (f) in the absence and (g) in the presence of 1
(2 mM).
in the absence (%) and in the presence ($) of TPPS (20 mM). lexc
=
450 nm. B: Luminescence intensity of 1O2 (1Dg) at initial time (LD at t = 0)
as a function of the laser intensity in the case of aqueous solution of TPPS
(20 mM) in the absence (%) and in the presence ($) of 1 (2 mM). lexc
=
532 nm. The data are corrected for the slight difference of photons
absorbed by the different samples at the excitation wavelengths.
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7 See, for example: S. Sortino, A. Mazzaglia, L. Monsu` Scolaro,
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10 We emphasize that the present work focuses on the simultaneous
photogeneration of NO and 1O2 (1Dg) from a molecular nanoassembly.
Detailed morphological investigations on the present nanoaggregate are
outside the scope of the present study and will be the object of a
forthcoming investigation.
quenching effect due to potential photoinduced energy and/or
electron transfer involving the two chromophores.
In summary, we report a photoactivable micellar nanoassembly
that, to our knowledge, represents the first molecular system able to
deliver simultaneously, in the same region of space, and under the
1
exclusive control of visible light, NO and O2 (1Dg), two species
playing a key role in the therapy of cancer. The considerable
fluorescence of the nanoaggregate, a useful tool for mapping its
localization in cells, and its reduced sizes, represent additional
advantages that make this photoactive vehicle an appealing
candidate to be tested in biological research. Studies concerning
this are currently in progress. Finally, we believe that the extension
of the present approach to a variety of ad-hoc chosen NO
photodonors and porphyrin derivatives may pave the way for the
development of novel classes of nanoscaled, photoactivable
systems in the emerging field of nanomedicine, for multimodal
therapy.
This work was supported by MIUR, Rome, Italy (PRIN 2006,
n. 2006031909).
Notes and references
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