In vitro fluorescence, toxicity and phototoxicity induced by delta-aminolevulinic acid (ALA) or ALA-esters

Synthesis of delta-aminolevulinic acid (ALA) derivatives is a promising way to improve the therapeutic properties of ALA, particularly cell uptake or homogeneity of protoporphyrin IX (PpIX) synthesis. The fluorescence emissio n kinetics and phototoxic properties of ALA-n-pentyl ester (E1) and R,S-ALA -2-(hydroxymethyl) tetrahydrofuranyl ester (E2) were compared with those of ALA and assessed on C6 glioma cells. ALA (100 mu g/mL), E1 and E2 (10 mu g /mL) induced similar PpIX-fluorescence kinetics (maximum between 5 and 7 h incubation), fluorescence being limited to the cytoplasm, The 50% lethal do se occurred after 6 h with 45, 4 and 8 mu g/mL of ALA, E1 and E2, respectiv ely. ALA, E1 and E2 induced no dark toxicity when drugs were removed after 5 min of incubation. However, light (25 J/cm(2)) applied 6 h after 5 min in cubation with 168 mu g/mL of each compound induced 85% survival with ALA, 2 7% with E1 and 41% with E2. Increasing the incubation time with ALA, E1 and E2 before washing increased the phototoxicity, but E1 and E2 remained more efficient than ALA, regardless of incubation time. ALA-esters were more ef ficient than ALA in inducing phototoxicity after short incubation times, pr obably through an increase of the amount of PpIX synthesized by C6 cells.