Chemical instability of 5-aminolevulinic acid (ALA) in aqueous solution

5-Aminolevulinic acid (ALA) is being used to induce formation and accumulation of endogenous protoporphyrin IX both in preclinical and clinical studies on photodynamic therapy and diagnosis. At the high concentrations needed for clinical application, ALA is not stable in aqueous solutions in the neutral to basic pH range. This could be of critical importance for the clinical use of ALA. The chemical instability of ALA was studied by using UV-Vis absorption spectroscopy. Our results show that ALA undergoes a chemical reaction, which is a nonreversible condensation process yielding two different products: an hydropyrazine and a pyrazine, the latter formed by oxydation of the hydropyrazine. Preliminary kinetic studies of this reaction showed a complex pH-, concentration and temperature-dependency of the reaction rate. On the basis of these studies we optimised conditions for the preparation of ALA solutions to be used in clinical trials, leading to the expected accumulation of protoporphyrin IX in tumour cells.