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Rational Design of Modified Oxobacteriochlorins As Potential Photodynamic Therapy Photosensitizers

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Rational Design of Modified Oxobacteriochlorins As Potential Photodynamic Therapy Photosensitizers International Journal of Molecular Sciences Article Rational Design of Modified Oxobacteriochlorins as Potential Photodynamic Therapy Photosensitizers Marta Erminia Alberto , Bruna Clara De Simone * , Emilia Sicilia, Marirosa Toscano and Nino Russo * Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Rende, Italy; [email protected] (M.E.A.); [email protected] (E.S.); [email protected] (M.T.) * Correspondence: [email protected] (B.C.D.S.); [email protected] (N.R.) Received: 4 April 2019; Accepted: 20 April 2019; Published: 24 April 2019 Abstract: The modulation of the photophysical properties of a series of recently synthetized oxobacteriochlorins with the introduction of heavy atoms in the macrocycles, was investigated at density functional level of theory and by means of the time-dependent TDDFT formulation. Absorption frequencies, singlet-triplet energy gaps and spin-orbit coupling (SOC) constants values were computed for all the investigated compounds. Results show how the sulfur- selenium- and iodine-substituted compounds possess improved properties that make them suitable for application in photodynamic therapy (PDT). Keywords: oxobacteriochlorins; PDT; TDDFT; spin-orbit coupling; heavy atom effect 1. Introduction Photodynamic therapy (PDT) is used in cancer treatment due to its mini-invasive nature, limited side effects, easy procedure and short treatment time in respect to surgery or chemotherapy [1–5]. PDT is based upon the so-called photodynamic effect, that can be defined as the tissue destruction or damage by the combined action of a photosensitizer, a visible or near visible light, and molecular 1 oxygen. The key cytotoxic agent is the highly reactive singlet exited oxygen (O2, Dg) produced in a photophysical cycle (type II photoexcitation process) which can be briefly described as it follows [6]: Absorption of light with the appropriate wavelength by a photosensitizer in its ground state (S0) leads to the excitation of one electron into a higher-energy orbital. The excited singlet (S1) that is formed may undergo an ‘intersystem crossing’ (ISC) to generate a more stable excited triplet state (T1). The long lifetime of the triplet state allows to transfer its energy to the molecular oxygen (O2), which 3 1 passes from its triplet ground state ( Sg) to the singlet excited state ( Dg). The efficiency of the ISC process strictly depends on the amplitude of the spin-orbit coupling singlet constants (SOC) [7] and it is possible only if the lowest triplet excited state (T1) lies above the energy needed to produce the molecular singlet oxygen (0.98 eV). In order to better penetrate into the tissue, the photosensitizer absorption wavelength should fall in the so called PDT absorption window that is between 500 to 800 nm. To improve the penetration of the light to the tissue, many scientists work on excitation with longer wavelength at near-IR or even longer wavenumbers [5]. In this spectroscopic region, even if the light deeply penetrates into the tissues it does not produce side effects due to the high frequency vibrations. In addition, the photosensitizer must be non-toxic in the dark, not subjected to intermolecular aggregation phenomena (which would diminish its phototherapeutic efficacy), and soluble in aqueous environment. Although research in this area has increased significantly in recent years [4–6], the number of molecules, that have passed clinical trials to be used in medical protocols, are few and essentially based on porphyrin-like structures. Among these, we mention Foscan® (Temoporfin) [8], Photofrin® Int. J. Mol. Sci. 2019, 20, 2002; doi:10.3390/ijms20082002 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, 2002 2 of 8 Int. J. Mol. Sci. 2019, 20, x 2 of 8 (porfimer sodium) [9], and Laserphyrin® (Talaporfin) [10]. Second generation photosensitizers such as Lutrinas Lutrin® (Lutetium® (Lutetium texaphyrin) texaphyrin) are are in diinff differenterent clinical clinic phasesal phases [11 ].[11]. The The success success of the of the porphyrin-like porphyrin- systemslike systems as PDT as PDT agents agents is dueis due to to the the fact fact that, that, as as itit is required, they they are are not not toxic, toxic, easily easily soluble soluble in inphysiological physiological solutions solutions and and able able to toabsorb absorb into into the the spectral spectral region region of of the the therapeutic therapeutic window. window. Furthermore, their long-wavelength absorption Q band can be tuned easily introducing didifferentfferent chemical groups at distinct sites around the perime perimeterter of the hydroporphyrin skeleton. In In particular, particular, the presence of auxochromes positioned along the bisectionbisection axis of pyrrole rings causes batochromic shifts with respect to thethe unsubstitutedunsubstituted chorineschorines oror bacteriochlorins.bacteriochlorins. Recently, aa series series of of oxo- oxo- and and thioxo- thioxo- substituted substitute bacteriochlorinsd bacteriochlorins [12 ,[12,13]13] have have been been synthetized synthetized and theirand their photophysical photophysical properties properties determined. determined. Moreover, Moreover, other photosensitizers other photosensitizers with diff erentwith chemicaldifferent structures,chemical structures, squaraine squaraine [14], BODIPY [14], [15 BODIPY], aza-BODIPY [15], aza-BODIPY [16] and BOIMPY [16] and [17 BOIMPY] have been [17] proposed have been for theirproposed use infor PDT their on use the in basis PDT of on cytotoxicity the basis of tests. cytotoxicity Some of tests. these Some works of underline these works the underline possibility the to redshiftpossibility the to absorption redshift Qthe band absorption as a function Q band of the as substituentsa function positionof the substituents in the macrocyclic position skeleton, in the beingmacrocyclic this, one skeleton, of the mostbeing important this, one of requirements the most important to satisfy requirements in designing to new satisfy PDT in drugs. designing new PDT Indrugs. this scenario, modern methods of theoretical and computational chemistry can offer significant aids forIn this a reliable scenario, and rationalmodern designmethods of targetedof theoretical drugs byand shortening computational the usually chemistry long chain can fromoffer chemicalsignificant synthesis aids for toa clinicalreliable approval.and rational In particular, design of densitytargeted functional drugs by based shortening methods the (DFT) usually are long able tochain predict from the chemical structural, synthesis energetic to andclinical spectroscopic approval. propertiesIn particular, of a density great number functional of complex based methods chemical systems(DFT) are conjugating able to predict reasonable the structural, computational energetic eff ortand with spectroscopic chemical accuracy. properties of a great number of complexIn this chemical work we systems will show conjugating how, by usingreasonable DFT, it computational is possible not effort only to with correctly chemical reproduce accuracy. a series of photophysicalIn this work properties, we will show but alsohow, predict by using their DFT, change it is as possible a result not of a only targeted to correctly substitution reproduce of atoms a orseries functional of photophysical groups. Starting properties, from somebut also recently predict synthesized their change oxobacteriochlorins as a result of a targeted [12], the substitution calculation of theiratoms absorption or functional spectra, groups. singlet-triplet Starting from energy some gaps recently and spin-orbitsynthesized couplings oxobacteriochlorins constants allowed [12], the to suggestcalculation chemical of their modifications absorption spectra, that fulfill singlet-tr the criteriaiplet necessaryenergy gaps to proposeand spin-orbit them as couplings photosensitizers constants in photodynamicallowed to suggest therapy. chemical modifications that fulfill the criteria necessary to propose them as photosensitizers in photodynamic therapy. 2. Results and Discussion 2. Results and Discussion The ground and excited singlet and triplet states optimized structures (see Table S1 for Cartesian coordinates)The ground present and aexcited planar singlet conformation and triplet of the states hydroporphyrin optimized structures macrocycle (see inTable all studiedS1 for Cartesian systems (seecoordinates) Scheme1 present). a planar conformation of the hydroporphyrin macrocycle in all studied systems (see Scheme 1). Scheme 1. Schematic representation of compounds investigated in this work. In orderorder to to evaluate evaluate the the effects effects of the of heavy the heavy atom onatom the photophysicalon the photophysical properties properties of the considered of the systems,considered vertical systems, excitation vertical energies excitation at TDDFTenergies level at TDDFT of theory level have of theory been calculated have been (see calculated Table1 and(see FigureTable 1 S1). and From Figure the obtainedS1). From values the itobtained results avalues batochromic it results shift a in batochromic going from compounds shift in going1, with from an compounds 1, with an oxo group (599 nm), to 2 (623 nm) and 3 (678 nm) in which S and a Se atoms, respectively substitute the oxygen one. Int. J. Mol. Sci. 2019, 20, 2002 3 of 8 oxo group (599 nm), to 2 (623 nm) and 3 (678 nm) in which S and a Se atoms, respectively substitute the oxygen one. Table 1. B3LYP/6-31+G* vertical excitation energies, DE in eV and (nm), oscillator
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