Consequences of Excited State Processes to Adverse Effects in Vivo

11 CONSEQUENCES OF EXCITED STATE PROCESSES TO ADVERSE EFFECTS IN VIVO 

Photoallergy is an acquired altered capacity of the skin to react to light. Thus it is immune mediated while a drug molecule is stimulated by radiation to combine with a protein or other biomacromolecule in the skin to form an antigen. Photoallergy 

It is believed that phototoxicity causes cellular damage by direct modification of certain target molecules, such as DNA, lipids, and/or amino acids and proteins. In principle, this can occur by the photosensitization reactions described earlier, and there is a reasonable correlation between the capacity of a drug to participate in type I and type II processes in vitro and the number of adverse photosensitivity reports registered against them. It is, of course, difficult to compare different drugs in this way because a number of extra variables enter — for example, the dose and pharmacokinetics of the drug s biodistribution. 

The mechanism of photoallergy is postulated in a similar fashion to the nonphoto-dynamic processes. The drug becomes attached to a protein by a free radical process, thereby creating a hapten that can lead to an immune response. It is presumed that the reason for the low incidence of photoallergic reactions is that the body s immune response mechanisms generally deactivate the hapten, or the oxygen present in the blood is able to scavenge the free radical intermediates. 

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