The high photoactivity of m-THPC in photodynamic therapy. Unusually strongretention of m-THPC by RIF-1 cells in culture

5,10,15,20-Tetra(m-hydroxyphenyl)chlorin (m-THPC, Foscan) is an extremely p owerful photosensitizer showing up to 200 times the photodynamic activity o f Photofrin in patients, in terms of drug/light dose. The influence of trea tment conditions on the photodynamic efficacy of m-THPC has been compared t o polyhematoporphyrin (PI-IP), a Photofrin equivalent, and a cationic pyrid inium zinc II phthalocyanine (PPC), using the RIF-1 cell Line. As predicted , the presence of serum during sensitizer incubation reduced the photodynam ic efficacy of all three sensitizers. However, the presence of serum during the illumination period only had an inhibitory effect with PHP and PPC but not m-THPC, Quantification of the intracellular levels of sensitizer revea led that this was due to the efflux of PPC and PHP but not m-THPC into the medium, suggesting that m-THPC is tightly sequestered on entering the cell. This may partially explain the high efficacy of m-THPC in clinical photody namic therapy and also highlights the importance not only of incubation con ditions but also illumination conditions when in vitro comparisons are perf ormed.