Factors Influencing Tumor Response to Photodynamic Therapy Sensitized by Intratumor Administration of Methylene Blue

Background and Objectives: We examined tumor response to methylene blue (MB)-mediated photodynamic therapy (PDT) in a murine tumor model. The goal was to investigate the effects of drug light interval (DLI), injection vehicle, and fluence on tumor destruction. Fluorescence and reflectance spectroscopy informed our understanding. Materials and Methods: EMT6 tumor cells were implanted intradermally on the backs of female BALB/c mice and grown to similar to 4-mm diameter. Mice were given a 35 mu l, single site, intratumor injection of 500 mu g/ml MB administered in either a water or a 5% ethanol-5% Cremophor-90% saline vehicle. PDT was begun either immediately or after a 1-hour DLI with a fluence rate of 60 mW/cm(2). Each animal received a fluence of 240 or 480 J/cm(2). Fluorescence and reflectance spectra were captured before and during irradiation. Results: A protocol consisting of the Cremophor-based vehicle, 0 DLI, and a fluence of 480 J/cm(2) was the most effective, with a 55% cure rate as measured by no evidence of tumor 90 days after PDT. Use of the water vehicle with this fluence and DLI reduced the cure rate to 20%. Reducing the fluence to 240 J/cm(2) similarly reduced treatment efficacy with 0 and 1-hour DLIs. Univariate Cox proportional hazards analysis identified increased fluence, 0 versus 1-hour DLI, and the Cremophor versus water vehicle as highly significant independent predictors of long term tumor control (P<0.01 in each case). Multivariate analysis with model selection revealed fluence and injection vehicle as the best predictors of survival hazards. Fluorescence spectroscopy in vivo showed that MB fluorescence decreased monotonically during a 2-hour dark interval but was restored by irradiation. Reflectance spectroscopy revealed that MB at this injected concentration attenuates the treatment beam significantly. Conclusion: Sensitizer delivery vehicle, drug light interval, and fluence contribute significantly to the tumor response to MB-mediated PDT. Lasers Surg. Med. 42:728-735, 2010. (C) 2010 Wiley-Liss, Inc.