Photochemical Oxygen Consumption Sensitized by a Porphyrin Phosphorescent Probe in Two Model Systems

Abstract

Phosphorescence quenching of certain metalloporphyrins is used to measure tissue and microvascular pO(2). Oxygen quenching of metalloporphyrin triplet states creates singlet oxygen, which is highly reactive in biological systems, and these oxygen-consuming reactions are capable of perturbing tissue oxygenation. Kinetics of photochemical oxygen consumption were measured for a Pd-porphyrin in two model systems in vitro over a range of irradiances (1.34-134 mW cm(-2)). For a given irradiance, and, after correction for differing porphyrin concentrations, rates of oxygen consumption were similar when the Pd-porphyrin was bound to bovine serum albumin and when it was taken up by tumor cells in spheroids. At irradiances comparable to those used in imaging superficial anatomy, rates of oxygen consumption were sufficiently low (2.5 microM s(-1)) that tissue oxygenation would be reduced by a maximum of 6%. An irradiance of 20 mW cm(-2), however, initiated a rate of oxygen consumption capable of reducing tissue pO(2) by at least 20-40%. These measured rates of consumption impose limitations on the use of phosphorescence quenching in thick tissues. The irreversible photobleaching of the Pd-porphyrin was also measured indirectly. The bleaching branching ratio, 23 M(-1), is significantly lower than that of porphyrin photodynamic agents.

Time-Resolved Fluorescence in Photodynamic Therapy

“…Large tumor masses usually manifest an oxygen level of nearly zero in tissue far away from the capillary supplies. Studies have been done to show the oxygen diffusion capability into the tissue and the kinetics between oxygen pressure and photosensitizer quenching using Photofrin [28,111], Pd-porphyrins [112], and mTHPC [113]. However, the oxygen pressure and the extent of oxygen depletion were not solely based on the blood perfusion, but also on the local photosensitizer concentration and light dose.…”

Triplet Imaging of Oxygen Consumption During the Contraction of a Single Smooth Muscle Cell (A7r5)

“…Because it induces transition back to the ground state, it helps control the essentially dark triplet states by allowing reexcitation and emission of the molecule. However, in doing so, it photosynthesizes singlet-oxygen-which is a highly reactive radical and a prominent source for photobleaching and cell phototoxicity (31)(32)(33)(34)(35).…”

Using Pd-porphyrin phosphorescence and photodynamic oxygen consumption to study oxygen diffusion in cells

“…Outside of the depletion radius the concentration of oxygen is assumed to be constant. This approximation has been used in several reports studying diffusion of oxygen in cell spheroids [6,8] and in cells [1] .…”

“…Quenching of phosphorescence from the triplet state of Pd-porphyrin by molecular oxygen is widely used to measure oxygen concentration [5] . The quenching also results in photodynamic consumption of oxygen and can efficiently deplete intracellular oxygen [6] . If the photodynamic consumption is suddenly switched on, the Pd-porphyrin phosphorescence signal will increase as cells in the suspension are deoxygenated.…”