Experimental findings utilising a new iron chelating ALA prodrug to enhance protoporphyrin IX-induced photodynamic therapy

Administration of a separate iron chelating agent during protoporphyrin IX (PpIX)-PDT has previously been demonstrated to increase the temporary accumulation of PpIX (by reducing its iron dependent bioconversion to haem by ferrochelatase), resulting in increased efficacy on irradiation. A novel ester between aminolaevulinic acid (ALA) and the hydroxypyridinone iron chelating agent CP94 was therefore synthesised (AP2-18) and experimentally evaluated to determine if PpIX-induced PDT effectiveness could be improved by this new combinational agent. A variety of cultured human primary cells were investigated with both PpIX fluorescence and cell viability being assessed in comparison to the PpIX prodrugs normally utilised in clinical practice (aminolaevulinic acid (ALA) or its methyl ester (MAL)) either administered alone or concurrently with the comparator iron chelator, CP94. Iron chelation achieved via CP94 or AP2- 18 administration consistently increased PpIX accumulation but the benefits of enhancement on PpIX-PDT cell kill were most pronounced when lower doses of ALA or MAL were utilised (i.e. where PpIX accumulation was observed to be most limited without this intervention). Importantly, AP2-18 was observed to be as least as effective as CP94 + ALA/MAL co-administration throughout and produced no significant dark toxicity in initial experimentation undertaken in lung fibroblasts. Additionally, statistically significant enhanced effects in terms of both PpIX accumulation and PDT cytotoxicity were observed experimentally with AP2-18 in both skin cancer and glioma cells. Newly synthesised AP2- 18 is therefore concluded to be an efficacious combined PpIX prodrug and iron chelating agent for the enhancement of PpIX-induced PDT that warrants further investigation.