Photodynamic Therapy (PDT) and Clinical Applications
PDT requires a drug (photosensitizer), light corresponding to an absorption band of the photosensitizer, and endogenous oxygen. The absence of any of these agents precludes the photodynamic effect.
The putative cytotoxic agent is singlet oxygen,
an electronically excited state of ground-state triplet oxygen formed according to the Type II photochemical process, as follows:
P + h
P* (S) Absorption P* (S)
P* (T) Intersystem crossing P* (T) + O
+ P Energy transfer
where P = photosensitizer, P*(S) = excited singlet state, P*(T) = excited triplet state, and
= singlet excited state of oxygen.