Photodegradation and phototransformation of 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (m-THPBC) in solution.

The kinetics of photobleaching and formation of photoproducts upon irradiation (735 nm) of 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (m-THPBC) in phosphate buffer saline (PBS) supplemented with human serum albumin (HSA) were studied by means of absorption and steady-state fluorescence spectroscopy. Measurements were performed either immediately after the dye was dissolved in the HSA solution (0 h) or after six hours incubation in the HSA solution (6 h). Spectroscopic studies indicated that the dye was mainly present as aggregates in freshly prepared solutions, whereas incubation favored monomerisation. Irrespective to incubation time, the rates of photobleaching obtained by fluorescence measurements were higher than those obtained from absorbance measurements. Photobleaching of freshly prepared m-THPBC can be described by a single exponential decay, while the absorbance and fluorescence decays of the incubated dye solutions better fit a bi-exponential decay. Two photobleaching rates probably reflect differences in the photosensitivity of monomer (bound to proteins) and aggregated (non-bound) forms. Irradiation of the freshly prepared m-THPBC solution led to phototransformation of 50% of the bleached m-THPBC into 5,10,15,20-tetrakis(m-hydroxyphenyl)chlorin (m-THPC), a clinically used second generation photosensitizer. For irradiation 6 h after dissolving m-THPBC, different kinetics of m-THPC formation were found. A rapid decrease in concentration of m-THPBC was accompanied by a slow formation of m-THPC. The quantum yield of this process was small since only 5% of m-THPBC was transformed to m-THPC. The kinetics characteristics of m-THPBC photobleaching reported in the present study, together with the different kinetics of photoproduct formation during m-THPBC photobleaching, may provide important indications in the m-THPBC-based PDT dosimetry.