Figure 1.

General schematic representation of PDT procedure. The photosensitizer (PS) is a photoactivatable theranostic agent that upon light activation can serve as both an imaging agent and a therapeutic agent. The PS can be injected unconjugated or in various formulations to enhance specificity to the tumor (conjugated to an antibody or peptide, conjugated to plasmonic nanoparticles or encapsulated in multi-compartmental liposomes). After administration of the PS, light is locally delivered to the tumor after a given PS-light interval, i.e., at this time point, the PS has preferentially localized to the tumor. The localized light delivery adds a second layer of tumor selectivity even if the PS is present in non-tumor sites. The photochemistry resulting from the absorption of photon energy (photoexcitation) by the PS causes tumor destruction (upper right inset). Upon photoexcitation, the PS is promoted to an electronic excited state. The excited PS reacts with the surrounding environment (such as ground state molecular oxygen) to generate cytotoxic reactive species (such as singlet oxygen) leading to cell death. The bottom panel lists various structural and functional information that can be obtained using optical imaging techniques to guide PDT dosimetry; i.e., key parameters required for pretreatment planning, therapy monitoring and outcome assessment. StO₂ indicates blood oxygen saturation and pO₂ indicates oxygen partial pressure.