The quenching of 1O2(1Δg) by anthracyclines in aqueous solution. Evidence for a charge-transfer mechanism

Photon- and Singlet-Oxygen-Induced Cis-Trans Isomerization of the Water-Soluble Carotenoid Crocin

Studying the cis-trans isomerization process in crocin (CR), one of the few water-soluble carotenoids extracted from saffron, is important to better understand the physiological role of cis-carotenoids in vivo and their potential as antioxidants in therapeutic applications. For that, cis-trans isomerization of both methanol- and water-dissolved CR was induced by light or thermally generated singlet oxygen (¹O₂). The kinetics of molecular concentrations were monitored by both high-performance liquid chromatography (HPLC) and non-destructive spectrophotometric methods. These last made it possible to simultaneously follow the cis-trans isomerization, the possible bleaching of compounds and the amount of thermally generated ¹O₂. Our results were in accordance with a comprehensive model where the cis-trans isomerization occurs as relaxation from the triplet state of all-trans- or 13-cis-CR, whatever is the way to populate the CR triplet state, either by photon or ¹O₂ energy transfer. The process is much more (1.9 to 10-fold) efficient from cis to trans than vice versa. In H₂O, a ¹O₂-induced bleaching effect on the starting CR was not negligible. However, the CR "flip-flop" isomerization reaction could still occur, suggesting that this process can represent an efficient mechanism for quenching of reactive oxygen species (ROS) in vivo, with a limited need of carotenoid regeneration.

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.