Utilization and mechanism of fluence and fluence rate effects in photodynamic therapy

Using the photosensitizer HPPH (2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a), it has been determined that the in vivo biological responses, such as vascular permeability and tumor response, to Photodynamic Therapy (PDT) are highly dependent on fluence rate (the power per unit area of the irradiating light source). The increased vascular permeability is maximized by using a low fluence, 48 J/cm2, delivered at a low fluence rate, 28 mW/cm2. It was hypothesized that this novel use of PDT would facilitate the delivery of Doxil® to tumors and thus enhance its therapeutic index. PDT (48 J/cm2, 14 mW/cm 2) followed immediately by the systemic administration of Doxil ® results in a 2–3 fold increase in the uptake of the macromolecular chemotherapeutic agent in Colon-26 tumors 3 hours after injection compared to untreated controls. One injection of a low dose of Doxil® (5 mg/kg) immediately after PDT optimized for macromolecular delivery results in cures for 80% of treated mice, while treatment with the same dose of either Doxil® or PDT alone resulted in 0% or 18% cures, respectively.; As the true photodynamic dose is the quantity of singlet oxygen generated during treatment, it can be hypothesized that the mechanism underlying the observed fluence rate effects is an enhanced deposition of this reactive oxygen species at low fluence rates. It has been shown in the literature that in vivo photosensitizer photobleaching can be used as an indirect measure of singlet oxygen deposition. Unlike other photosensitizers studied (e.g. PPIX or mTHPC) it has been found that in vivo HPPH photobleaching is more efficient at high fluence rates than low fluence rates. This result suggests that an anoxic pathway, possibly due to Type I photochemistry, is responsible for in vivo HPPH photobleaching. It was further found that HPPH photobleaching in vivo can be described as a function of fluence and fluence rate. Using this mathematical description of photobleaching, an expression for photodynamic dose which accurately predicts tumor response at given fluence and fluence rates is derived.