Photopharmica discovering and developing photosensitisers and light sources for medical use.

Photosensitisers

The primary requirements of a photosensitiser are (a) it should absorb light efficiently and (b) in the presence of oxygen it should be able to use the light energy to generate singlet oxygen efficiently.

To be of any therapeutic value, there are equally important secondary requirements, namely:

The photosensitiser should have an absorption band in the 600-750 nm range, in order to ensure that the absorbed light (red or near-infrared) will readily penetrate cells, tissue and blood.

It should have little or no host toxicity in the absence of light.

It should rapidly clear from normal tissue (especially skin) so that there is no prolonged sensitivity to light after treatment.

It should show some degree of preferential uptake by the target cells relative to the host tissue.

There are also such commercial implications as cost of manufacture and ease of formulation (water solubility) to consider.

Current clinically used photosensitisers belong to the porphyrin-related molecular classes (porphyrins, chlorins, bacteriochlorins, phthalocyanines), whereas Photopharmica's lead photosensitisers are phenothiazinium compounds, generalised by the following structure:

Diagramme

These compounds satisfy all the requirements listed above, and in addition can be manufactured in a two step process and are easily formulated for topical or systemic administration. They are blue compounds with an intense absorption band in the 650-680 nm range. Their molar absorption coefficients (~ 120,000 dm³ mol-¹ cm-¹) are much higher than those of porphyrin, chlorin and bacteriochlorin photosensitisers.