Response Surface Methodology: An Extensive Potential to Optimize in vivo Photodynamic Therapy Conditions

Purpose: Photodynamic therapy (PDT) is based on the interaction of a photosensitizing (PS) agent, light, and oxygen. Few new PS agents are being developed to the in vivo stage, partly because of the difficulty in finding the right treatment conditions. Response surface methodology, an empirical modeling approach based on data resulting from a set of designed experiments, was suggested as a rational solution with which to select in vivo PDT conditions by using a new peptide-conjugated PS targeting agent, neuropilin-1. Methods and Materials: A Doehlert experimental design was selected to model effects and interactions of the PS dose, fluence, and fluence rate on the growth of U87 human malignant glioma cell xenografts in nude mice, using a fixed drug-light interval. All experimental results were computed by Nemrod-W software and Matlab. Results: Intrinsic diameter growth rate, a tumor growth parameter independent of the initial volume of the tumor, was selected as the response variable and was compared to tumor growth delay and relative tumor volumes. With only 13 experimental conditions tested, an optimal PDT condition was selected (PS agent dose, 2.80 mg/kg; fluence, 120 J/cm2; fluence rate, 85 mW/cm2). Treatment of glioma-bearing mice with the peptide-conjugated PS agent, followed by the optimized PDT condition showed a statistically significant improvement in delaying tumor growth compared with animals who received the PDT with the nonconjugated PS agent. Conclusions: Response surface methodology appears to be a useful experimental approach for rapid testing of different treatment conditions and determination of optimal values of PDT factors for any PS agent.