The influence of different illumination parameters on protoporphyrin IX induced cell death in squamous cell carcinoma...

Topical photodynamic therapy (PDT) is a highly effective therapy especially for extended cancerized fields of the skin. Whenever extended fields are treated pain management is advisable. Light source mediated pain management can be performed by reducing fluence rates, as long as this does not compromise efficacy.

Two squamous cell carcinoma cell lines (A431 and SCC-13) were subjected to in vitro PDT using two different ALA concentrations and synthesis intervals and protoporphyrin IX (PpIX) synthesis was assessed. Two total light doses (6 J/cm² and 37 J/cm²) were applied at three different fluence rates and cell viability was measured using the MTS-test.

Both cell lines synthetized PpIX at different kinetics. A431 cells produced a maximum 28.6 nmol/l PpIX, while SCC-13 reached only a production of 8.7 nmol/l. Illumination reduced cell viability depending on PpIX content and light dose. When a lower light dose (6 J/cm²) was applied, only the combination with the highest PpIX content was effective in A431 cells and no effect could be detected in SCC-13 cells. With a light dose of 37 J/cm², lower PpIX amounts became effective in A431 and cell death could be induced in SCC-13 cells. Light fluence rate had no differential effect in this setup.

In both, A431 and SCC-13 cells, total light dose is a key factor for photodynamic efficacy. Additionally, our results hint towards a threshold concentration of PpIX upon which a drastic loss of viability occurs. Light fluence rate in the analyzed range is not a limiting factor of photodynamic cytotoxicity. This may allow for the clinical implementation of low fluence rate protocols for pain management without compromising efficacy.