A compound of formula I: ##STR00001## wherein X.sub.1, X.sub.2, X.sub.3, X.sub.4, Y.sub.1, Y.sub.2, Y.sub.3, Y.sub.4 and Z have meanings given in the description, and metallated forms of such compounds, which are useful in the treatment of medical conditions for which a photodynamic compound is indicated. Pharmaceutical formulations and methods of treatment of a medical condition for which a photodynamic agent is indicated are also disclosed. Sterilizing solutions comprising a compound of the invention, and the use thereof, are also disclosed.