A little light relief

8 Nov 2010

Prof David Philips presented a lecture on the subject of light - from sun damage to photo-medicine as part of SCI's Public Lecture Series on 21 October 2010. Prof Phillips' overheads can be downloaded via the link below. You can also listen to an audio version.

'Photomedicine as a 'modern' subject has been around since the late 1880s,' explained Prof Phillips, who is President of the Royal Society of Chemistry, Emeritus Prof of Chemistry and former Dean of Sciences at Imperial College London. 'it currently encompasses the effects of light upon the skin; diagnostic uses of light; therapies using non-laser light; and the use of lasers. The effects of light on the skin include production of Vitamin D, tanning, ageing of the skin, and the skin cancers basal cell and squamous cell carcinomas, and malignant melanoma.' He then went on to outline a brief history of photomedicine, and briefly described some current uses of light in medicine.

Prof Phillips explained luminescence is used for immunoassay; the identification of antigens that may be precursors to disease. Immunometric assay uses two antibodies, one immobilised, which is specific to the antigen being tested for, and the second, the labelled antibody, which binds to a different site on the antigen. The technique is used in testing for pregnancy at early stages by seeking the hormone human chorionic gonadotrophin, or testing for HIV. In terms of the therapeutic uses of light, Prof Phillips presented examples (and some striking demonstrations) of how light is used to treat ailments such as vitiligo, psoriasis, and jaundice.

Prof Phillips explained that, in his view, the main future of photomedicine lies in the development of photodynamic therapy (PDT), which is a minimally invasive procedure used in treating a range of infections and forms of cancer. The photodynamic action relies on the simultaneous interaction between a non-toxic photosensitiser molecule, visible light, and molecular oxygen, offering dual selectivity through preferential uptake of the photosensitiser by diseased cells and the selective application of light. Following activation with visible light of the appropriate wavelength, the photosensitiser generates singlet oxygen, O2(a1Δg).

This leads to localised cell death via irreversible damage to cellular components, such as proteins, lipids and DNA.

A key component of PDT is the photosensitiser, which has to possess a number of key properties including absorption in the red (600-800 nm) allowing photoactivation within deeper tissues; selective uptake by malignant cells; the ability to efficiently generate singlet oxygen; and minimal dark toxicity. A few years ago, Prof Phillips founded Photobiotics, a company that specialises in generating such photosensitisers. New molecules potentially of use in PDT have been identified, and are in clinical trials.

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