Furocoumarins (
psoralens) are
photosensitizers of plant origin, which increase the sensitivity of
biological objects to near ultraviolet (UV-A, 320-400 nm). In combination with UV-A, they are successfully used for treating many dermal and
autoimmune diseases (
PUVA therapy and photophoresis). Along with
therapeutic effects, the
furocoumarin photochemotherapy induces a number of side-effects (
erythema,
edema,
hyperpigmentation, and
premature aging of skin). All photobiological effects of
furocoumarins result from their photochemical reactions. Therefore, in order to advance the
therapy, it is necessary to know the photochemical mechanisms of induction of both side- and
therapeutic effects. The types of photoreactions of
furocoumarins classified with respect to reactive photoproducts interacting with substrate were considered. Primary emphasis was placed on reactions proceeding with the participation of photooxidation products of
furocoumarins. Among these photoproducts, at least two types can be distinguished. Some of them possess membranotoxic properties, others produce the immunosuppressory action in vivo. The photochemical mechanisms of the formation of the photoproducts of
furocoumarins are different. It was found that, by varying the illumination conditions (intensity of UV-A radiation or the concentration of the
photosensitizer), it is possible to obtain the photoproducts of
furocoumarins that have either membranotoxic or immunosuppressory properties. It was found that the mechanisms of the immunosuppressive action of the photooxidation products of
furocoumarins have some features in common with those underlying the
PUVA therapy and photophoresis. It is assumed that the photochemical basis of the therapeutic action of
furocoumarins is the reactions with the involvement of the products of their photooxidation.