 Photodynamic Technology in HealthcarePHOTODYNAMIC action refers to the ability of certain compounds, called photosensitisers, to destroy or inhibit the growth of living cells when exposed to light. The photosensitiser is not consumed during the process and may therefore be regarded as a catalyst, or energy transfer agent, where its primary purpose is to capture and channel the energy of light into the biological processes leading to cell death. This light-activated process can be used to destroy any type of living cell selectively, by appropriate targeting of the light, and thus photodynamic technology has numerous applications in healthcare, ranging from the treatment of medical conditions such as cancer, skin diseases and infected wounds to general sterilisation of the environment and infection control. The correct choice of photosensitiser is critical if the photodynamic effect is to be truly beneficial, as it has to be readily deliverable to the relevant cells, either by systemic administration or topical application. As a result, few photosensitisers have to date achieved clinical success. Photopharmica has screened over 100 novel compounds as photosensitisers, and several highly promising different chemical families have been developed and protected. The most advanced are compounds of the phenothiazinium family, and a number of these have proved to have major advantages over existing clinically used photosensitisers. Theory The light-induced cytotoxicity of photosensitisers arises from the photochemical formation of an activated form of molecular oxygen known as singlet oxygen. This species, although it only has a very short lifetime in solution (about one millionth of a second), is so reactive towards biomolecules that it can readily cause cell death if it is generated within the cell. This can be summarised in the following diagram:  1. Healthy bacterial cells 2. Photosensitiser added to the cells, which binds to or is taken up into the cell. 3. Bacterial cells illuminated. The photosensitiser now acts as a catalyst to produce reactive singlet oxygen. 4. Reactive singlet oxygen kills bacterial cells by a multi-targeted mechanism of oxidative damage. The photodynamic effect is based on the following sequence of events: 1. Absorption of light energy by the photosensitiser (S), so promoting it to a highly energetic singlet state, S*.  2. Efficient conversion of the energised S* singlet state of the photosensitiser into an energised triplet state T*.  It is this step that differentiates photosensitisers from conventional dyes, as the latter only form T* states very inefficiently. 3. Transfer of the energy from T* to normal molecular oxygen, O2 , to give highly activated oxygen (singlet oxygen).  In this process the original photosensitiser S is formed and is then available to be used again. 4. Destruction of a living cell by singlet oxygen attack.  ^ top |
