Direct Comparison of δ-Aminolevulinic Acid and Methyl-Aminolevulinate-Derived Protoporphyrin IX Accumulations Potentiated by Desferrioxamine or the Novel Hydroxypyridinone Iron Chelator CP94 in Cultured Human Cells

Abstract

Aminolevulinic acid photodynamic therapy (ALA-PDT) is a cancer therapy that combines the selective accumulation of a photosensitizer in tumor tissue with visible light (and tissue oxygen) to produce reactive oxygen species. This results in cellular damage and ablation of tumor tissue. The use of iron chelators in combination with ALA has the potential to increase the accumulation of the photosensitizer protoporphyrin IX (PpIX) by reducing its bioconversion to heme. This study compares directly for the first time the effects of the novel hydroxypyridinone iron chelating agent CP94 and the more clinically established iron chelator desferrioxamine (DFO) on the enhancement of ALA and methyl-aminolevulinate (MAL)-induced PpIX accumulations in cultured human cells. Cultured human cells were incubated with a combination of ALA, MAL, CP94 and DFO concentrations; the resulting PpIX accumulations being quantified fluorometrically. The use of iron chelators in combination with ALA or MAL was shown to significantly increase the amount of PpIX accumulating in the fetal lung fibroblasts and epidermal carcinoma cells; while minimal enhancement was observed in the normal skin cells investigated (fibroblasts and keratinocytes). Where enhancement was observed CP94 was shown to be significantly superior to DFO in the enhancement of PpIX accumulation.

Enhancement of methyl-aminolevulinate photodynamic therapy by iron chelation with CP94: an in vitro investigation and clinical dose-escalating safety study for the treatment of nodular basal cell carcinoma

“…The ability of CP94 to increase PpIX fluorescence particularly in cells where the concentration of MAL is low [24] may explain the increased effect in deeper nodular tumors.…”

“…Several authors have now investigated this technique of enhancement in vitro using ethylenediamine tetraacetic acid (EDTA) [21,11], desferrioxamine (DFO) [21,22] and the novel hydroxypyridinone iron chelator CP94 [23,24]. The ability of EDTA and DFO or alternately, CP94 and DFO to enhance PpIX photosensitizer accumulations have been directly compared in vitro [21,24].…”

“…The ability of EDTA and DFO or alternately, CP94 and DFO to enhance PpIX photosensitizer accumulations have been directly compared in vitro [21,24]. DFO was found to be superior to EDTA in this role [21] and CP94 was found to be superior to DFO for this purpose [24].…”

“…The ability of EDTA and DFO or alternately, CP94 and DFO to enhance PpIX photosensitizer accumulations have been directly compared in vitro [21,24]. DFO was found to be superior to EDTA in this role [21] and CP94 was found to be superior to DFO for this purpose [24]. Therefore the likely order of efficacy of these chelating agents for the enhancement of PDT has been proposed as: CP94>DFO>EDTA (although CP94 and EDTA have not been directly compared) [24].…”

Iron Chelators in Photodynamic Therapy Revisited: Synergistic Effect by Novel Highly Active Thiosemicarbazones

“…The difference in accumulation of PpIX in diseased as compared to healthy tissue results from abnormal activity of enzymes regulating the formation of PpIX (porphobilinogen deaminase) and heme (ferrochelatase) in cancer cells. 15 This phenomenon is the basis of the concept of combination therapy, where iron chelators are used as adjuvants in ALA-PDT. 16 The use of nontoxic iron chelators deprives the cells of Fe 2+ crucial for the last step of heme biosynthesis, thereby increasing the accumulation of PpIX.…”