Summary Currently, the clinical use of 5-aminolaevulinic acid (ALA)-induced protoporphyrin IX (PPIX) for photodynamic therapy (PDT) is limited by the maximum tolerated oral ALA dose (60 mg kg-'). This study investigates whether hydroxypyridinone iron-chelating agents can be used to enhance the tissue levels of PPIX, without increasing the administered dose of ALA. Quantitative charge-coupled device (CCD) fluorescence microscopy was employed to study PPIX fluorescence pharmacokinetics in the colon of normal Wistar rats. The iron chelator, CP94, when administered with ALA was found to produce double the PPIX fluorescence in the colonic mucosa, compared with the same dose of ALA given alone and to be more effective than the other iron chelator studied. CP20. Microspectrofluorimetric studies demonstrated that PPIX was the predominant porphyrin species present. PDT studies conducted on the colonic mucosa showed that the simultaneous administration of 100 mg kg-' CP94 i.v. and 50 mg kg-' ALA i.v. produced an area of necrosis three times larger than similar parameters without the iron-chelating agent with the same light dose. It is possible, therefore, to increase the amount of necrosis produced by ALAinduced PDT substantialty, without increasing the administered dose of ALA, through the simultaneous administration of the iron-chelating agent, CP94.Keywords: 5-aminolaevulinic acid; photodynamic therapy: iron chelators; protoporphynn IX; hydroxypyridinones Photodynamic therapy (PDT) is a non-thermal technique in which a preadministered photosensitizer is activated w-ith light of a specific wavelength. so that a cytotoxic species can be formed from molecular oxygen. thus. producing localized tissue necrosis (Bow-n. 1989 dose. are therefore beinc inv-estigated. 'ith the effects of ironchelating agents being studied in this paper. The hydroxypy ridinones are a relativels newa series of ironchelatincg agents. They can be administered orally and enter the intracellular iron pools rapidlN. being both neutrally charged and of lowa molecular weig-ht (Hoy es and Porter. 1993). Oririnallv developed to supersede desferrioxamine for the treatment of thalassaemia and other disorders of iron overload. the hydroxvpyridinones are now beincg investicated to enhance ALA-induced PDT. They do this by chelatincg iron. thus reducincg the conversion of PPIX to haem. resultincg in an even greater accumulation of PPIX and. thus. a greater photodynamic effect (Chanc et al. 1997).Tw o hVdroxypyridinones are studied in this paper: the 1.2-dimethyl derixative (CP2O) and the l.2-diethxl derivative (CP94). Both of these compounds has-e been given to patients with iron overload v ithout sicnificant toxicity and produced rapid and effective iron mobilization (Brittenham. 1992). MATERIALS AND METHODS ChemicalsALA powder (ALA.HC1. 99% purity. DUSA Pharmaceuticals. NY. USA) was dissolved in physiological strength. phosphatebuffered saline (PBS. pH 2.8) and administered intravenously ('aith a concentration of 50 mc, ml and a maximum xolume of 0.2 ml). T...