5-Aminolevulinic acid and its derivatives: physical chemical properties and protoporphyrin IX formation in cultured cells

Protoporphyrin IX (PpIX) is used as a fluorescence marker and photosensitiz ing agent in photodynamic therapy (PDT). A temporary increase of PpIX in ti ssues can be obtained by administration of 5-aminolevulinic acid (ALA). Lip ophilicity is one of the key parameters defining the bioavailability of a t opically applied drug. In the present work, octanol-water partition coeffic ients of ALA and several of its esters have been determined to obtain a par ameter related to their lipophilicity. The influence of parameters such as lipophilicity, concentration, time, and pH value on PpIX formation induced by ALA and its esters is then investigated in human cell lines originating from the lung and bladder. ALA esters are found to be more lipophilic than the free acid. The optimal concentration (c(opt), precursor concentration a t which maximal PpIX accumulation is observed) is then measured for each pr ecursor. Long-chained ALA eaters are found to decrease the c(opt) value by up to two orders of magnitude as compared to ALA. The reduction of PpIX for mation observed at higher concentrations than c(opt) is correlated to reduc ed cell viability as determined by measuring the mitochondrial activity. Un der optimal conditions, the PpIX formation rate induced by the longer-chain ed esters is higher than that of ALA or the shorter-chained esters. A bipha sic pH dependence on PpIX generation is observed for ALA and its derivative s. Maximal PpIX formation is measured under physiological conditions (pH 7. 0-7.6), indicating that further enhancement of intracellular PpIX content m ay be achieved by adjusting the pharmaceutical formulation of ALA or its de rivatives to these pH levels. (C)2000 Elsevier Science S.A. All rights rese rved.