Photodynamic therapy (PDT) has drawn great interest in recent years mainly due to its low side effects and few drug resistances. Nevertheless, one of the issues of PDT is the need for oxygen to induce a photodynamic effect. Tumours often have low oxygen concentrations, related to the abnormal structure of the microvessels leading to an ineffective blood distribution. Moreover, PDT consumes O2. In order to improve the oxygenation of tumour or decrease hypoxia, different strategies are developed and are described in this review: 1) The use of O2 vehicle; 2) the modification of the tumour microenvironment (TME); 3) combining other therapies with PDT; 4) hypoxia-independent PDT; 5) hypoxia-dependent PDT and 6) fractional PDT.

Indocyanine green (ICG) is a water-soluble anionic tricarbocyanine dye developed during the Second World War that was first approved for clinical use in humans in 1956. The main features of ICG that make it suitable for bioimaging applications are its near infrared absorption and its fluorescence. Although ICG is mainly used for its fluorescence emission properties, it has also been hypothesized that it can serve as a photosensitizer for photodynamic therapy applications, eliciting cytotoxic effects both in vitro and in vivo when used in combination with light at wavelengths in the region of 800-830 nm. Moreover, ICG can be used for hyperthermia of enhanced-photocoagulation of blood vessels treatment. In this paper we have gathered all the available data concerning the use of ICG for different treatments.

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