Protoporphyrin IX-sensitized photoinactivation of 5-aminolevulinate-treated leukemia cells: Effects of exogenous iron

Photodynamic therapy with 5-aminolevulinic acid (ALA) is based on metabolis m of ALA to a photosensitizing agent, protoporphyrin IX (PpIX), in tumor ce lls. Photosensitivity of target cells may be influenced by mitochondrial ir on levels because ferrochelatase-catalyzed insertion of Fe2+ into PpIX conv erts it to heme, a nonsensitizer, To investigate this prospect, we exposed L1210 cells (similar to 10(6)/mL in 1% serum-containing medium) to a lipoph ilic iron chelate, ferric-8-hydroxyquinoline (Fe[HQ](2), 0.5 mu M), prior t o treating with ALA (0.2 mM, 4 h) and irradiating with broadband visible li ght. When Fe(HQ), was added to cells immediately or 1 h before ALA, the ini tial rate of photokilling, as measured by thiazolyl blue (mitochondrial deh ydrogenase) assay, was markedly less than that of non-iron controls. The HP LC analysis of cell extracts indicated that ALA-induced PpIX was at least 5 0% lower after this Fe(HQ)(2) treatment, presumably explaining the drop in photolethality. By contrast, cells treated with ALA and light 20 h after be ing exposed to Fe(HQ)(2) contained the same amount of PpIX as non-iron cont rols and were photoinactivated at nearly the same rate. The 20 h delayed ce lls contained similar to 12 times more immunodetectable ferritin heavy subu nit than controls or 1 h counterparts, which could account for the disappea rance of iron's antisensitization effects in the former. Consistent with th is idea, the short-term effects of Fe(HQ), on ALA-induced sensitization wer e found to be blunted significantly in ferritin-enriched cells. The Fe(HQ), produced strikingly different results when cells were sensitized with exog enous PpIX, stimulating photokilling after short-term contact but inhibitin g it after long-term contact while having no significant effect on the leve l of cell-associated PpIX in either case. Thus, iron can have diverse effec ts on PpIX-mediated photokilling, depending on contact time with cells and whether the porphyrin is metabolically derived or applied as such.