Photodynamic Therapy

Abstract

Photodynamic therapy (PDT) is an effective treatment of different types of cancer, virus infections, skin diseases and others using photosensitizers (PSs) which are nontoxic in the dark but become phototoxic following activation by low energy light. The PS is applied topically or systemically and should be selectively accumulated in the target tissue. The molecular mechanism of the therapy is the photodynamic effect: following light absorption energy is transferred from the PS to molecular oxygen generating singlet molecular oxygen ( 1 O 2 ) which causes direct or indirect photodamage of the target tissue. In clinical therapy with first generation PSs like Photofrin ® typically fluences of around 100 J cm −2 of red light are used. The light doses vary in dependence on the treated disease. Using PSs of second generation with very strong absorption in the red region like the very efficient m‐tetrahydroxyphenylchlorin ( m ‐THPC), smaller fluences, typically around 10 J cm −2 , are required. According to the different optical properties and the efficiency of selective accumulation of PS in the target tissue, the drug doses vary from 1–5 mg (first generation PS) to 0.1 mg (second generation PS) per kilogram body weight. PDT is a very efficient method for the treatment of minor diseases, blood cancer, inoperable diseases like some types of bladder, brain or lung cancer, and breast carcinoma metastases. The side effects are less than in the case of chemo‐ or radiotherapy. The most common side effect is skin photosensitization. The strength of this effect depends on the PS used and also on the patients' skin type. Most of the new second generation PSs cause no or only weak skin phototoxicity. The therapeutic radius is limited by the penetration depth of the used light (in the order of some millimeters to centimeters). The method will not be very successful for the treatment of large solid tumors.

Photodynamic inactivation of multi‐resistant bacteria (PIB) – a new approach to treat superficial infections in the 21st century

“…Transferred to bacteria, this means that a PS, that accumulates intracellularly or at least close to the plasma membrane (even outside), after irradiation leads to irreversible damage of the immediate surroundings of the PS due to oxidative modification of important biomolecules. At present it is generally accepted that the production of singlet oxygen plays the key role in photosensitization [13].…”

“Pointsource” Delivery of a Photosensitizer Drug and Singlet Oxygen: Eradication of Glioma Cells In Vitro

“…Sensitizer conjugation to, for example, metal particles or silica have been seen to increase PDT efficiency and singlet oxygen production . Common PDT techniques systemically administer the sensitizer relying on homing to the appropriate site, but attention could be focused on “pointsource” administration techniques as distinct from topical administration.…”

Singlet oxygen in vivo – it is all about intensity

“…Singlet oxygen ( 1 O 2 ), the main mediator of photodynamic therapy (PDT), results from the interaction of a photosensitizer (PS) light-usually within the visible spectrum-and molecular oxygen. These three components are harmless individually, but in combination, they result in the formation of 1 O 2 and other reactive oxygen species [1][2][3]. Excitation of a PS results in a certain percentage in intersystem crossing to long-living triplet excited states with an energy little above that of 1 O 2 .…”