Photophysics and photochemistry of photodynamic therapy: fundamental aspects

Abstract

Photodynamic therapy (PDT) is a treatment modality for cancer and various other diseases. The clinical protocol covers the illumination of target cells (or tissue), which have been loaded with a photoactive drug (photosensitizer). In this review we describe the photophysical and primary photochemical processes that occur during PDT. Interaction of light with tissue results in attenuation of the incident light energy due to reflectance, absorption, scattering, and refraction. Refraction and reflection are reduced by perpendicular light application, whereas absorption can be minimized by the choice of a photosensitizer that absorbs in the far red region of the electromagnetic spectrum. Interaction of light and the photosensitizer can result in degradation, modification or relocalization of the drug, which differently affect the effectiveness of PDT. Photodynamic therapy itself, however, employs the light-induced chemical reactions of the activated photosensitizer (triplet state), resulting in the production of various reactive oxygen species, amongst them singlet oxygen as the primary photochemical product. Based on these considerations, the properties of an ideal photosensitizer for PDT are discussed. According to the clinical experience with PDT, it is proposed that the innovative concept of PDT is most successfully implemented into the mainstream of anticancer therapies by following an application-, i.e. tumor-centered approach with a focus on the actual clinical requirements of the respective tumor type.

Nanodrug applications in photodynamic therapy

“…However, the molecule can also undergo intersystem crossing from the excited singlet state to the more stable, long-lived triplet excited state. Most photosensitizers have a high quantum efficiency for this transition and this is a key characteristic of a good PS [1,5]. Most known photosensitizers used in PDT represent type II photochemical reaction when energy transfer to molecular oxygen ( 3 O 2 ) occurs to yield very reactive singlet oxygen ( 1 O 2 ).…”

“…One of them is high absorption of the PS between 650 nm and 850 nm where the tissue penetration is quite high [5]. It should have high singlet oxygen quantum yield for the photochemical reaction, be a single and pure compound with a stable composition, and be soluble in water and should not form aggregates.…”

Lightening up Ruthenium Complexes to Fight Cancer?

“…Most of the approved PSs act via this second mechanism. [14] However, a known drawback of this technique is its lack of efficiency on cancers characterized by low oxygen tension (i.e. hypoxia).…”

Nanopartículas lipídicas sólidas e vesículas cataniônicas contendo ftalocianina de cloro alumínio aplicadas nos processos fotodinâmicos

“…É importante destacar que oxigênio singlete e as EROs são espécies altamente reativas e com curto tempo de meia-vida, portanto, somente substratos e moléculas muito próximas aos locais de formação dessas espécies reativas sofrem o efeito direto da terapia fotodinâmica (JUARRANZ et al, 2008;PLAETZER et al, 2008).…”

“…A linhagem celular melanocítica B16-F10 também foi utilizada para os experimentos com a terapia fotodinâmica, cujo princípio é gerar o efeito tóxico combinando a concentração do FS e a dose de luz aplicada com o oxigênio do meio tecidual (PLAETZER et al, 2008). A baixa solubilidade da ClAlPc em água, assim como ocorre com a maior parte dos fármacos fotossensibilizantes, representa um grande desafio para sua aplicação na TFD, pois as moléculas de FS hidrofóbicos tendem a se agregar no meio fisiológico, dificultando sua administração in vivo, e causando redução drástica de sua atividade fotodinâmica pela perda das propriedades fotofísicas presentes apenas na forma monomérica da ftalocianina.…”