BACKGROUND-Photodynamic therapy (PDT), if given over extended time periods (i.e. hours or days) and at very low irradiance in the μW/cm 2 range, has been shown to be more effective than acute PDT (aPDT) administered over minutes. This has led to the concept of metronomic PDT (mPDT), which consists of ultra-low irradiance light illumination for extended periods of time along with either continuous or repetitive delivery of photosensitizer. Since the drug activating technology photochemical internalization (PCI) is based on PDT it seemed reasonable to expect that ultra-low irradiance, if administered over an extended period of time, could nevertheless result in effective metronomic PCI (mPCI) comparable to or more effective than that obtained with relatively high and short irradiance i.e. acute PCI (aPCI). METHODS-Tumor spheroids consisting of F98 cells were used as in-vitro tumor models. The amphiphilic photosensitizer Al phthalocyanine disulfonate (AlPcS 2a) was used for all PCI experiments. Light treatment was administered from a diode laser at λ= 670 nm at various irradiance exposures of 2 mW/cm 2 for aPCI and 0.05-0.2 mW/cm 2 for mPCI with durations ranging from 3-12 min for aPCI and 120 minutes for mPCI. RESULTS-AlPcS 2a fluorescence was seen throughout the cytosol following short or long light treatment, corresponding to aPCI and mPCI respectively. Spheroid growth was significantly inhibited or completely suppressed at a mPCI radiance of 0.05 or 0.72 J/cm 2 respectively, with all bleomycin (BLM) concentrations used, compared to either BLM alone or aPCI at radiant exposure
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