Photodynamic therapy (PDT) is a clinical modality used to treat cancer and infectious diseases. The main agent is the photosensitizer (PS), which is excited by light and converted to a triplet excited state. This latter species leads to the formation of singlet oxygen and radicals that oxidize biomolecules. The main motivation for this review is to suggest alternatives for achieving high-efficiency PDT protocols, by taking advantage of knowledge on the chemical and biological processes taking place during and after photosensitization. We defend that in order to obtain specific mechanisms of cell death and maximize PDT efficiency, PSes should oxidize specific molecular targets. We consider the role of subcellular localization, how PS photochemistry and photophysics can change according to its nanoenvironment, and how can all these trigger specific cell death mechanisms. We propose that in order to develop PSes that will cause a breakthrough enhancement in the efficiency of PDT, researchers should first consider tissue and intracellular localization, instead of trying to maximize singlet oxygen quantum yields in in vitro tests. In addition to this, we also indicate many open questions and challenges remaining in this field, hoping to encourage future research.

Methylene Blue (MB⁺) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB⁺ properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB⁺ form complexes, changing the properties of both the micelles and the MB⁺ solutions. Surface tension measurements show that the c.m.c. of SDS decreases from ∼7 mM to ∼70 μM when the MB⁺ concentration increases from 0 to 45 μM. Above the c.m.c., binding of MB⁺ in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB⁺ and SDS. In the presence of excess of SDS, MB⁺ is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB⁺ molecules partition to the micelle pseudo-phase which favors or disfavors dimers as a function of its volume. MB⁺ transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB⁺ radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB⁺ monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB⁺ can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB⁺).

Photosensitization reactions have been demonstrated to be largely responsible for the deleterious biological effects of UV and visible radiation, as well as for the curative actions of photomedicine. A large number of endogenous and exogenous photosensitizers, biological targets and mechanisms have been reported in the past few decades. Evolving from the original definitions of the type I and type II photosensitized oxidations, we now provide physicochemical frameworks, classifications and key examples of these mechanisms in order to organize, interpret and understand the vast information available in the literature and the new reports, which are in vigorous growth. This review surveys in an extended manner all identified photosensitization mechanisms of the major biomolecule groups such as nucleic acids, proteins, lipids bridging the gap with the subsequent biological processes. Also described are the effects of photosensitization in cells in which UVA and UVB irradiation triggers enzyme activation with the subsequent delayed generation of superoxide anion radical and nitric oxide. Definitions of photosensitized reactions are identified in biomolecules with key insights into cells and tissues.