A photosensitiser and red light

The first generation PS were porphyrins whose selectivity for malignant cells was pretty low. To limit the risk of causing lesions in the healthy tissues around the cancerous tissues being treated, researchers developed PS which had a greater affinity for the malignant cells, whist being non-toxic and rapidly eliminated by normal cells. They are for the main part derivatives of poryphyrins, chlorines, bacteriochlorines or phthalocyanines.

But it is also possible to bring about a significant accumulation of endogenous porphyrin within the malignant cells through the aid of 5-aminovulinic acid (5-ALA). ‘In a normal cell,’ explains Professor Piette, ‘during the different enzymatic processes, this amino acid is converted into protoporphyrin IX and then, thanks to heme synthase, into a heme which will be incorporated into haemoglobin. Yet this enzyme is inhibited or very weakly expressed in the majority of malignant cells. Thus if the latter are placed in contact with 5-ALA they will accumulate protoporphyrin IX, which is photosensitive, enabling the malignant cells to be killed through illumination with red light.’

Why red light? Because it is the only one capable of penetrating tissues deeply. In effect the majority of UVCs and UVBs are absorbed by the keratinous layer of the skin. UV-A radiation and visible light pass through the epidermis, but are in part stopped by the haemoglobin of the blood cells and the melanin responsible for skin pigmentation. In the end, only red and infrared cross the epidermis and the dermis to reach the hypodermis, but infrared is to a great extent absorbed by water. The spectral range enabling sick tissues to be treated by light is thus situated in the red.