Role of delivery vehicles for photosensitizers in the photodynamic therapy of tumours.

The use of photosensitizing drugs associated with different types of delivery vehicle has received strong interest within the field of the photodynamic therapy of tumours. Lipid-based delivery vehicles, such as liposomes and oil emulsions, allow the administration of water-insoluble photosensitizers, widening the choice of photosensitizers potentially useful for treating tumours. In some cases, these delivery vehicles increase the selectivity of tumour targeting by favouring photosensitizer uptake in tumour tissue. However, a higher selectivity of tumour targeting could be obtained through the association of photosensitizers with delivery vehicles which can interact preferentially or specifically with tumour cells. With this aim in mind, low-density lipoproteins (LDLs) and monoclonal antibodies, in particular, are regarded as the most promising delivery systems for anticancer drugs. Some pharmacokinetic studies with LDL-associated photosensitizers have demonstrated a higher tumour uptake compared with the same photosensitizers delivered with other formulations. Monoclonal antibody-coupled photosensitizers have been tested mainly in vitro, and have shown a high selectivity towards cells expressing specific antigens. Only a limited number of reports are available on the biodistribution of immunoconjugated photosensitizers and on their selectivity in vivo, so that their importance for the selectivity of tumour targeting has not yet been defined.