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Light against cancer
1/27/12

Photodynamic therapy (PDT) uses a photosensitizing agent, light and oxygen to destroy malignant cancer cells. Professor Jacques Piette has been working for a number of years on photosensitizing molecules and the cellular mechanisms  implicated during PDT. Together with an international consortium he has written a recent article (1) which summarizes the main driving forces of this treatment in the field of oncology.

Lumière rougeIn what ways can light interact with our bodies? One example of interaction could be provided by certain medications and chemical substances, called photosensitizing or photosensitizers (PS)  that  become active in the presence of light and cause cell damage whose classical manifestation  are skin rashes, burns or even necrosis. To limit these side effects it is necessary to protect oneself from light during  therapy with photosensitive drugs.

Another example: some molecules produced through cellular metabolism are also sensitive to certain wavelengths of light. That is true for  porphyrins, molecules found in haemoglobin which ensure the transport of oxygen and CO2, or cytochromes involve in cellular respiration. The absorption of certain wavelengths of light can excite the porphyrins in triggering a series of biological reactions. Fortunately the cells possess different defence and repair mechanisms which serve to limit  damage.

For various reasons, connected to amongst other things their abnormal proliferation and metabolism, malignant cells accumulate porphyrins whilst healthy cells rapidly eliminate them. ‘From that came the idea that in administrating a photosensitizing agent which has procured a certain selectivity as far as malignant cells are concerned, it would subsequently be possible, by lighting them up with an appropriate wavelength, to destroy them without harming the healthy tissues: that is the principle of photodynamic therapy!’ summarises Professor Jacques Piette, FRS-FNRS Research Director and the Director of the GIGA-Research Unit at the University of Liège. It was  in the 1970s that researchers showed for the first time  a total tumor eradication  in mice, without any side effects, through the action of hematoporphyrin derivatives and red light. Since these early times photodynamic therapy has been the subject of numerous research programmes and clinical trials which have confirmed its value in the fight against cancer.

The treatment protocol is not very aggressive and relatively simple. A photosensitizer, inactive whilst it is not illuminated, is administered either through direct injection into the tumour in the case of superficial cancers or into the bloodstream for deeper cancers, or applied locally, as it is done in the case of skin cancers. After a certain period of time, called the drug-light interval, the tumour has accumulated a large quantity of the PS – it has become photosensitive – whilst the healthy tissues have eliminated it for the greater part. The doctor then applies a red light to the tumour by lighting it up for a few minutes through direct illumination in the case of superficial cancers or with the help of a laser beam by endoscopy for deeper cancers. The side effects are limited: at the very most a localised pain or a residual photosensitivity during which patients have to avoid being exposed to sunlight.

(1) Photodynamic therapy of cancer: an update.  A Cancer Journal for Clinicians, Vol 61, 2011

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