Singlet oxygen generation by 9-acetoxy-2,7,12,17-tetrakis-(β-methoxyethyl)-porphycene (ATMPn) in solution

Abstract

Despite the approval of Photofrin® in various countries, chemically pure sensitizers for photodynamic therapy (PDT) are still needed. Based on numerous experiments in vitro, singlet oxygen is considered to play the major role in PDT. Therefore, the generation of singlet oxygen by the promising, hydrophobic photosensitizer 9-acetoxy-2,7,12,17-tetrakis-(β-methoxyethyl)-porphycene (ATMPn) in deuterated ethanol (EtOD) has been investigated in detail by time-resolved near-infrared spectroscopy. Using a new, highly sensitive infrared photomultiplier the complete time dependence of singlet oxygen luminescence was measured at 1270 nm. The respective relaxation rates and rate constants of the involved ATMPn triplet T1 state and oxygen 1Δg state were determined, in particular at different oxygen concentrations. Therefore, the oxygen concentration dependence of the proportion PT of the ATMPn triplet T1 state population quenched by oxygen, which is proportional to the singlet oxygen quantum yield ΦΔ, could be calculated. The result shows that PT decreased significantly with decreasing oxygen concentration. The completely different oxygen concentrations in vitro and in vivo lead therefore to a different quantum yield of singlet oxygen generation in both situations. Thus, one has to be careful when transferring results from in vitro experiments to photodynamic therapy in vivo, in particular regarding the role of singlet oxygen (type-II reaction) as compared to the type-I reaction.