Evaluation of singlet oxygen explicit dosimetry for predicting treatment outcomes of benzoporphyrin derivative monoacid ring A-mediated photodynamic therapy

As a nonionizing radiation treatment, photodynamic therapy (PDT) has been used effectively for the treatment of various easily accessible lesions, such as head and neck cancers, esophageal cancers, microinvasive lung cancer, and skin lesions such as premalignant actinic keratosis.1–4 PDT requires the administration of a photosensitizer that localizes in tumor tissue and is excited by the appropriate treatment wavelength of light. With the absorption of light, the photosensizer transitions from its ground state to a short-lived excited singlet state. The singlet state photosensitizer transitions to a longer-lived electronically excited triplet state via intersystem crossing. In typical type II PDT, which is the focus of this study, the triplet state transfers energy to molecular ground state oxygen (O23) that is present in the treatment environment to produce singlet-state oxygen (O21). The production of O21 and reactions with the surrounding biological molecules are thought to be the major cause of cytotoxicity.5 Due to the high reactivity and short lifetime of O21, only cells that are proximal to the area of O21 production are directly affected by PDT.6 For that reason, PDT causes significantly less harm to healthy tissue than chemotherapy or radiation.6,7 A well-defined dosimetric metric for PDT that is able to predict clinical outcomes that can also be implemented in a clinical setting would benefit the advance of the PDT field. Benzoporphyrin derivative monoacid ring A (BPD, trademark Visudyne®) is a commonly used photosensitizer that was approved by the US Food and Drug Administration in 2000 for the treatment of wet age-related macular degeneration.8 Utilizing a macroscopic model, reacted singlet oxygen ([O21]rx) can be calculated to evaluate its effectiveness as a dosimetric predictor for BPD-mediated PDT outcome. In addition, several other dose metrics were evaluated in this study, including total light fluence, photobleaching ratio, and PDT dose, to predict the PDT outcome. Radiation-induced fibrosarcoma (RIF) tumors on mice were treated with BPD-PDT and a range of in-air fluences (30 to 350 J/cm2) and in-air fluence rates (50 to 150 mW/cm2). For each PDT treatment group, explicit measurements of BPD concentration in tumor and tissue optical properties were performed pre- and posttreatment. For a subset of mice, real-time in-vivo measurement of BPD concentration and tissue oxygenation level ([O23]) throughout PDT were taken to optimize the photosensitizer-specific PDT photochemical parameters (ξ, σ, and g), reduce their uncertainty from a previous study, and calculate [O21]rx. These photochemical parameters were used to calculate [O21]rx for each PDT treatment group. Other dose metrics, such as photobleaching ratio and PDT dose, were determined either directly using explicit measurements pre- and post-PDT or calculated using the time dependence of BPD concentration based on the macroscopic model and the definition of PDT dose. This study, to our knowledge, is the first to investigate the threshold value of [O21]rx and the relationship between various dose metrics (fluence, PDT dose, and [O21]rx) and the cure index (CI) at 14 days in an in-vivo mouse model for BPD-mediated PDT. The results of our study with additional real-time measurements of BPD concentration and [O23] provide reduced uncertainties for the photochemical parameters determined for BPD-mediated PDT, as well as a validation that our macroscopic model can accurately predict the oxygen consumption for BPD-mediated PDT, making it feasible to determine [O21]rx without oxygen measurements.