Heterocycles in Medicine 179The process involves administering (systemically or topically) a photoactive substance, then irradiating (with visible
or ultraviolet light) to activate the compound so as to generate cytotoxic substances – particularly singlet oxygen,
although direct interaction of the drug with DNA may also occur. Selectivity can be achieved by, for example, applica-
tion of the drug to a defi ned area of the skin or by selective area irradiation. In some cases lasers may be used. These are
not burning lasers, but just high intensity sources of the appropriate activating wavelength.
Two signifi cant forms of photochemotherapy are PUVA (Psoralen-UltraViolet A) for skin diseases such as psoriasis,
and PDT (PhotoDynamic Therapy), which can be used to treat tumours and macular degeneration. For the PDT,
porphins are often used but in an ingenious modifi cation, 5-aminolevulinic acid (a natural porphin biosynthetic pre-
cursor) can be applied to skin lesions so as to allow the body to make the required local high concentration of the
sensitiser! (Psoralen is the name given to a class of furocoumarins, such as methoxsalen.)