Triplet state characteristics and singlet oxygen generation properties of anthracyclines

Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions

Three homoleptic Al(III) complexes (Al1-Al3) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (1O2), and second, in the isomerization of (E)- to (Z)-stilbene. 1O2 was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using in situ UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO2 to CO, but here Al1 without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, Al1 exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, Al1 provided the highest rate constant (0.043 min-1) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)3]2+. Finally, its superior photostability enabled a long-term test (7 h), in which Al1 was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.

Cationic Phenosafranin Photosensitizers Based on Polyhedral Oligomeric Silsesquioxanes for Inactivation of Gram-Positive and Gram-Negative Bacteria

The high photodynamic effect of the Newman strain of the S. aureus and of clinical strains of S. aureus MRSA 12673 and E. coli 12519 are observed for new cationic light-activated phenosafranin polyhedral oligomeric silsesquioxane (POSS) conjugates in vitro. Killing of bacteria was achieved at low concentrations of silsesquioxanes (0.38 µM) after light irradiation (λ_{em. max} = 522 nm, 10.6 mW/cm²) for 5 min. Water-soluble POSS-photosensitizers are synthesized by chemically coupling a phenosafranin dye (PSF) (3,7-diamino-5-phenylphenazine chloride) to an inorganic silsesquioxane cage activated by attachment of succinic anhydride rings. The chemical structure of conjugates is confirmed by ¹H, ¹³C NMR, HRMS, IR, fluorescence spectroscopy and UV-VIS analyzes. The APDI and daunorubicin (DAU) synergy is investigated for POSSPSFDAU conjugates. Confocal microscopy experiments indicate a site of intracellular accumulation of the POSSPSF, whereas iBuPOSSPSF and POSSPSFDAU accumulate in the cell wall or cell membrane. Results from the TEM study show ruptured S. aureus cells with leaking cytosolic mass and distorted cells of E. coli. Bacterial cells are eradicated by ROS produced upon irradiation of the covalent conjugates that can kill the bacteria by destruction of cellular membranes, intracellular proteins and DNA through the oxidative damage of bacteria.

NO release regulated by doxorubicin as the green light-harvesting antenna

We report for the first time a NO photodonor (NOPD) operating with the widely used chemotherapeutic agent doxorubicin (DOX) as the light-harvesting antenna. This permits NO uncaging from an N-nitroso appendage upon selective excitation of DOX with highly biocompatible green light, without precluding its typical red emission. This NOPD effectively binds DNA and photodelivers NO nearby, representing an intriguing candidate for potential multimodal therapeutic applications based on the combination of DOX and NO.

Singlet oxygen partition between the outer-, inner- and membrane-phases of photo/chemotherapeutic liposomes

Herein, we developed a strategy to quantify the fraction of singlet oxygen lifetime spent in the three distinct local liposomal environments through the combination of direct and indirect singlet oxygen detection approaches.

PCNA damage caused by antineoplastic drugs

Structurally diverse chemotherapeutic and chemopreventive drugs, including camptothecin, doxorubicin, sanguinarine, and others, were found to cause covalent crosslinking of proliferating cell nuclear antigen (PCNA) trimers in mammalian cells exposed to fluorescent light. This PCNA damage was caused by both nuclear and cytoplasmically localizing drugs. For some drugs, the PCNA crosslinking was evident even with very brief exposures to laboratory room lighting. In the absence of drugs, there was no detectable covalent crosslinking of PCNA trimers. Other proteins were photo-crosslinked to PCNA at much lower levels, including crosslinking of additional PCNA to the PCNA trimer. The proteins photo-crosslinked to PCNA did not vary with cell type or drug. PCNA was not crosslinked to itself or to other proteins by superoxide, hydrogen peroxide or hydroxyl radicals, but hydrogen peroxide caused monoubiquitination of PCNA. Quenching of PCNA photo-crosslinking by histidine, and enhancement by deuterium oxide, suggest a role for singlet oxygen in the crosslinking. SV40 large T antigen hexamers were also efficiently covalently photo-crosslinked by drugs and light. Photodynamic crosslinking of nuclear proteins by cytoplasmically localizing drugs, together with other evidence, argues that these drugs may reach the nucleoplasm in amounts sufficient to photodamage important chromosomal enzymes. The covalent crosslinking of PCNA trimers provides an extremely sensitive biomarker for photodynamic damage. The damage to PCNA and large T antigen raises the possibility that DNA damage signaling and repair mechanisms may be compromised when cells treated with antineoplastic drugs are exposed to visible light.

Laser photochemotherapy with anthracyclines on cultured human squamous carcinoma cells

A new treatment for cancer has been tested in vitro using light-sensitive anthracyclines followed by laser photoactivation, as described by several investigators. We previously reported 10-fold enhanced laser killing after 2 hours of incubation with daunomycin by cultured human carcinoma cells. This short-term uptake leads to drug localization in cytoplasmic and membrane sites prior to nuclear accumulation and topoisomerase inhibition. In the present study, daunomycin was incubated for 2 or 24 hours with P3 squamous carcinoma cells to directly compare cytoplasmic vs. nuclear drug targeting before and after KTP-532 laser activation. Monolayer cultures of the P3 cells sensitized with daunomycin for 2 hours, then chilled (4 degree C), and exposed to the KTP laser (532 nm, 94.2 J/cm2) had a 2- to 10-fold increased therapeutic response compared with drug or laser alone when measured by MTT tetrazolium assays. After 24 hours of incubation with daunomycin, the chemotherapeutic response of P3 tumor cells was amplified 2-fold by laser exposure. The results suggest that daunomycin and laser treatment can be combined for improved therapy of human cancer.

Laser photochemotherapy: a less invasive approach for treatment of cancer

The effectiveness of combining surgery with chemo- and radiation therapy in treatment of human cancer provides a useful model for further development of new multimodality approaches including laser photochemotherapy. Laser endoscopy often is a useful treatment for obstructive tumors in airways, but interstitial laser fiberoptics is becoming a more precise, minimally invasive alternative for ablation of unresectable or recurrent neoplasms. Combining intratumor chemotherapy with laser energy delivery via interstitial fiberoptics should be most effective using drugs activated by photothermal energy. A number of investigators have shown that anthracyclines and cis-platinum are likely candidates for light or heat activation in cancer cells. An advantage of anthracyclines is their dual role as antitumor drugs and as photosensitizers. Because they are effective chemotherapy agents without photoactivation, two approaches are possible to increase tumor responses. Maximum tolerated dose followed by photoillumination via laser fiberoptics can be used to obtain better tumor palliation. Improved treatment response to lower intratumor drug levels after laser activation also should reduce systemic toxicity. Preclinical studies and recent case reports from several groups suggest photochemotherapy with currently approved drugs and lasers may soon become an attractive alternative for treatment of recurrent tumors in cancer patients.

Phototherapeutic potential of alternative photosensitizers to porphyrins

Because of promising clinical results obtained with photodynamic therapy, more and more photosensitizers continue to be isolated (from natural sources), synthesized and evaluated, the development of which is considered to be a key factor for the successful clinical application of photodynamic therapy. Porphyrins and their analogs (as classical types of phototherapeutic agents) have been extensively reviewed. In this review, we have attempted to summarize the phototherapeutic potential (in particular, anticancer and antiviral aspects) of nonporphyrin photosensitizers (as a new generation of phototherapeutic agents) in more detail, which have been relatively much less reviewed hitherto. They include anthraquinones, anthrapyrazoles, perylenequinones, xanthenes, cyanines, acridines, phenoxazines and phenothiazines. They have shown certain phototherapeutic advantages over the presently used porphyrins. Some anthraquinones, perylenequinones, cyanines, phenoxazines and phenothiazines exhibit strong light absorption in the 'phototherapeutic window' (600-1000 nm), high photosensitizing efficacy and low delayed skin photosensitivity. Some of the nonporphyrin photosensitizers (such as rhodamine 123, merocyanine 540 and some cyanine cationic dyes) demonstrate higher selectivity for tumor cells. They can also be explored in connection with selective carcinoma photolysis strategy based on mitochondrion-, lysosome- or DNA-directed localization mode.

Enhanced response to daunomycin of normal, tumor and metastatic cell lines via drug photoactivation

We have compared the cytotoxicity of daunomycin in vitro to highly differentiated normal epithelial cells (Fisher rat thyroid cells, FRTL-5) and to two neoplastic cell lines, a thyroid carcinoma (TK-6) and its lung metastasis (MPTK-6). Whereas the cell lines are equally sensitive to the drug in the dark, if irradiated during incubation with daunomycin (86 J/cm2 at 488 nm), they become more and differently sensitive. Namely, the drug doses producing 50% mortality decrease by factors of about 22, 28 and 16 for FRTL-5, TK-6 and MPTK-6 cell lines, respectively. This result correlates with differences in drug uptake and resistance observed in the normal and neoplastic cell lines.

Excited singlet state properties of anthracenedione photosensitizers

The absorption, fluorescence and S1 state kinetics of anthracycline antitumour drugs (e.g. daunomycin, adriamycin) and several imino- and/or amino-substituted derivatives are investigated. The study, which includes all anthracyclines which possess photocytocidal activity, is extended to the disubstituted aminoanthracenedione, mitoxantrone, a red-light-absorbing antitumour drug whose activity, both in vitro and in vivo, is enhanced by photoactivation. The S1 state of the anthracycline imino and amino derivatives, in aqueous buffer at pH 7.4, is characterized by bi-exponential decay kinetics which indicates the presence of two ground state populations differing in the extent of hydrogen bonding. The ammonium group of the sugar moiety of anthracyclines contributes to the quenching of the S1 state population through a prototropic mechanism.

Enhancement of antitumor drug cytotoxicity via laser photoactivation

We investigate the efficacy of daunomycin, some imino- and amino-substituted daunomycin analogues and the disubstituted aminoanthracenedione, mitoxantrone, in photosensitizing short-term cell kill upon irradiation in the long wavelength visible range, during incubation of Fisher rat thyroid cells with the drugs. While all compounds exhibit similar cytocidal effects on our cell line, in the absence of irradiation, administering 86 J/cm2 at wavelengths either coincident or close to drug absorption peaks causes greater enhancement in cell mortality for the 4-demethoxydaunomycin analogues than either the parent drug or its 5-imino-derivative. A lower enhancement is observed with mitoxantrone. In particular, C50 doses (i.e. concentrations that would kill 50% cells) as low as approximately 10(-9) M are found for both 6- and 11-amino 4-demethoxydaunomycin, compared with the values obtained in the absence of light, which are 2.59 x 10(-4) and 0.43 x 10(-4) M, respectively. Our previous studies of the photophysical and photochemical properties of the excited states of these drugs, and ESR and spin trapping studies of photosensitized generation of singlet oxygen, which were extended in this work to include mitoxantrone, indicate that the cytocidal effects proceed via type I rather than type II mechanisms.

Time-resolved luminescence spectroscopy of photosensitizers of biomedical interest

Studying the fluorescence decay of chromophores, either used as fluorescent labels to stain specific biomolecules or as photosensitizers to produce irreversible chemical or physico-chemical modifications on biological substrates, is being demonstrated to be a valuable method of investigating the interactions underlying a variety of phenomena. In fact, all possible primary steps in a photosensitized biological system are phenomena that may occur during the chromophore S1 lifetime and act as quenching mechanisms of the S1 state. Thus they can be identified, and the relative importance of the corresponding transient species quantitatively determined, with suitable techniques of time-resolved fluorescence spectroscopy. The examples discussed in this paper concern both tumor photosensitizing drugs, such as anthracyclines and porphyrins, and skin sensitizers (e.g. furocoumarins).

Photosensitization by selected anticancer agents

Novel anticancer anthrapyrazoles and anthracenediones are available as alternatives to the cardiotoxic clinical agents, doxorubicin and daunorubicin. Certain representatives of these new classes of compounds possess photosensitizing properties. The structural features influencing the photophysical parameters of these agents are discussed. Photosensitizing reactions involving singlet oxygen production, free radical formation, decomposition of hydrogen peroxide and organic hydroperoxides, oxidation of certain biochemical electron donors, DNA damage and killing of human leukemic cells in vitro in the presence of photoactive anthrapyrazoles, anthracenediones and anthracyclines are described.

Cell photosensitization by 5-iminodaunomycin activated with red light

5-Iminodaunomycin, an anthracycline antitumor drug exhibiting an absorption peak at 595 nm, is shown to photosensitize in vitro cell kill. The photoactivation is performed irradiating the culture dishes during the incubation with the drug for 2 h with 34 mW/cm2 intensity, that is with light doses of up to 245 J/cm2. Long-term effects of administering 50 ng/ml and light for 2 h are studied in terms of growth curves. We show that photoactivation enhances the dark toxicity by a factor of about 10. Immediate cell death is produced by irradiating the cells in the presence of higher drug concentrations (e.g., 1000 ng/ml) which, however, are not toxic in the short term if administered in the dark. The viable cell percentage decreases at increasing light doses, being about 0.6% at the maximum dosage. Administering lower light doses, such as 30 J/cm2, which corresponds to an exposure duration of 15 min, has a short-term effect on the cell survival that strongly depends on the timing of the exposures within the incubation period.