In Vitro Fluorescence, Toxicity and Phototoxicity Induced by δ-Aminolevulinic Acid (ALA) or ALA-Esters

Abstract

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 emission 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 micrograms/mL), E1 and E2 (10 micrograms/mL) induced similar PpIX-fluorescence kinetics (maximum between 5 and 7 h incubation), fluorescence being limited to the cytoplasm. The 50% lethal dose occurred after 6 h with 45, 4 and 8 micrograms/mL of ALA, E1 and E2, respectively. ALA, E1 and E2 induced no dark toxicity when drugs were removed after 5 min of incubation. However, light (25 J/cm2) applied 6 h after 5 min incubation with 168 micrograms/mL of each compound induced 85% survival with ALA, 27% 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 efficient than ALA in inducing phototoxicity after short incubation times, probably through an increase of the amount of PpIX synthesized by C6 cells.

A quantitative assessment of protoporphyrin IX metabolism and phototoxicity in human skin following dose-controlled delivery of the prodrugs 5-aminolaevulinic acid and 5-aminolaevulinic acid-n-pentylester

“…Several in vitro studies have previously compared the PpIX fluorescence levels induced by ALA and ALA esters, but have not examined the relative phototoxicity 6,13 , 14 . Now, in agreement with a very recently published in vitro study, 15 we have shown that phototoxicity after PDT, as assessed by resultant skin erythema levels and theoretical TDs for erythema, is measurably higher following the use of ALA‐ n ‐pentylester than of ALA. Although the differences were relatively small, this suggests that the use of this compound may potentially enhance the efficacy of PDT treatment clinically.…”

New 5-Aminolevulinic Acid Esters-Efficient Protoporphyrin Precursors for Photodetection and Photodynamic Therapy¶

“…However, ALA-induced PPIX accumulation is limited by the high hydrophilicity of ALA, and therefore fairly high doses are needed to reach clinically relevant concentrations of PPIX. Hydrophilic compounds such as ALA poorly cross biological barriers like cellular membranes (3,4). To overcome this problem, ALA esters have been used as prodrugs instead of ALA itself.…”

The Effects of 5-Aminolevulinic Acid Esters on Protoporphyrin IX Production in Human Adenocarcinoma Cell Lines¶

“…However, PPIX accumulation is limited by the high hydrophilicity of ALA, and therefore fairly high doses are needed to reach clinically relevant concentrations of PPIX. Such hydrophilic compounds poorly cross biological barriers like cellular membranes with lipophilic properties (4,5). The improvement in the bioavailability of ALA by the use of ALA derivatives is determined by two processes: the rate of diffusion of the derivative through biological barriers and its rate of enzymatic conversion into ALA (6).…”