In the PCI technology, photosensitising molecules (photosensitisers) are used to exploit the energy in visible light for delivering drugs.

Photosensitisers (S) are molecules that upon absorption of light become excited to an excited state (S*), initiating further photochemical reactions. The photochemical reactions of the photosensitisers used in PCI proceed mainly via a pathway where the energy from the excited photosensitiser molecule is transferred to oxygen, inducing formation of so-called singlet oxygen. Schematically:

* Singlet oxygen is a highly reactive form of oxygen that can oxidize various biomolecules, inducing damage in various cellular structures.

* Some photosensitisers localise in the membranes of endosomes and lysosomes, and light activation of such sensitisers destroys these membranes and releases the organelle contents into the cytosol. Thus, photochemical treatment may be used to translocate endocytosed therapeutic macromolecules from endocytic vesicles and into the cytosol, from where they have access to other intracellular compartments.

* In cells, singlet oxygen has a very short short range of action (10-20 nm). Therefore, only targets very close to the generated singlet oxygen will be affected by the photochemical reactions, and the therapeutic molecules to be delivered will as a rule be left unaffected.

* PCI Biotech is developing a proprietary photosensitiser specially designed for in vivo use with the PCI technology.