5-Aminolevulinic acid (5-ALA) is a natural biochemical precursor of heme that is metabolised in a series of enzymatic reactions to fluorescent porphyrins, particularly PPIX. 5-ALA synthesis is regulated by an intracellular pool of free heme via a negative feedback mechanism. Administration of excess exogenous 5-ALA avoids the negative feedback control, and accumulation of PPIX occurs in target tissue. In the presence of visible light, fluorescence of PPIX (photodynamic effect) in certain target tissues can be used for photodynamic diagnosis.
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