The invention relates to pharmaceutically acceptable salts of amphiphilic photosensitizing agents which have a water solubility of at least 0.5 mg/ml and to their use in methods of photochemical internalization. Such salts may be formed from a pharmaceutically acceptable base, for example an organic amine such as an amino alcohol, or from a pharmaceutically acceptable acid, for example a sulphonic acid or a sulphonic acid derivative. Due to their increased water solubility, such salts are particularly suitable for use in the preparation of parenteral pharmaceutical preparations, e.g. for use as solutions for injection or infusion.
