Aminolevulinic acid: from its unique biological function to its star role in photodynamic therapy.

Porphyrins are molecules essential for life. They are involved in the key processes of photosynthesis and respiration. The biosynthesis of tetrapyrroles in all living cells occurs through several steps where the formation of aminolevulinic acid (ALA) is the first committed intermediate. Two alternative routes for the formation of ALA have been proposed: one involves the condensation of Succinyl CoA and glycine catalyzed by ALA synthetase taking place in the mitochondria, and the second one is the so called 5-carbon route, occurring in the stroma of plastids. Eight molecules of ALA are used in the formation of protoporphyrin IX. Specific deficiencies in one of the enzymes of the heme pathway produce the porphyrias. In the acute porphyrias, the pathogenesis of the neurological dysfunction is attributed to the accumulation of ALA. Fluorescent and photosensitizing properties of protoporphyrin accumulated after the exogenous administration of ALA, can be used to visualize and destroy malignant cells in the so-called photodynamic diagnosis (PDD) and photodynamic therapy (PDT) of cancer. Many clinical ALA-PDT applications to malignant and non-malignant pathologies are currently in use. Different approaches to enhance ALA penetration in cells are under investigation, including the use of more lipophilic ALA derivatives and studies of the transport mechanisms of ALA. ALA has also been proposed to be used as a biodegradable herbicide, as an insecticide and as a plant growth regulator.