Microscopic analysis of the localization of two chlorin-based photosensitizers in OSC19 tumors in the mouse oral cavity

Background and Objective The effect of photodynamic therapy (PDT) is dependent on the localization of photosensitizer in the treatment volume at the time of illumination. Investigation of photosensitizer pharmacokinetics in and around the treatment volume aids in determining the optimal drug light interval for PDT. Materials and Methods In this paper we have investigated the distribution of the photosensitizers chlorin e6 and Bremachlorin in the oral squamous cell carcinoma cell-line OSC19-Luc-Gfp in a tongue tumor, tumor boundary, invasive tumor boundary, and normal tongue tissue by the use of confocal microscopy of frozen sections. Tongues were harvested at t = [3, 4.5, 6, 24, 48] hours after injection. Results Both photosensitizers showed a decreasing fluorescence with increasing incubation time, and at all time points higher fluorescence was measured in tumor boundary than in tumor itself. For short incubation times, a higher fluorescence intensity was observed in the invasive tumor border and normal tissue compared to tumor tissue. Bremachlorin showed a small increase in tumor to normal ratio at 24 and 48 hours incubation time. Ce6 was undetectable at 48 hours. We did not find a correlation between photosensitizer localization and the presence of vasculature. Conclusion The modest tumor/tumor boundary to normal selectivity of between 1.2 and 2.5 exhibited by Bremachlorin 24 and 48 hours after administration may allow selective targeting of tongue tumors. Further studies investigating the relationship between Bremachlorin concentration and therapeutic efficacy PDT with long incubation times are warranted.

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