Liposomes for photodynamic therapy

Photodynamic therapy (PDT) is an evolving modality for the treatment of superficial tumors. The technique utilizes photosensitizing agents that are able to photochemically eradicate malignant cells. With the aim of improving the tumoritropic behaviour of photosensitizers, liposomes are presently being used as carrier and delivery systems for PDT. This review covers the different liposomal strategies that are available to target photosensitizers to tumor tissue. In general, conventional liposomes carrying photosensitizers are not able to establish elevated tumor-to-normal tissue ratios, hampering their generalised use as tumoritropic carriers of photosensitizers. Conversely, liposomes with a specifically modified design, i.e. long-circulating and especially actively targeting liposomes, stand a better chance in becoming truly tumoritropic carriers of photosensitizers. Strategies that can be employed to trigger the release of photosensitizer molecules from the liposomes are also discussed. The examined topics are supplemented with examples of the latest developments in the field of photodynamic therapy.