The present invention relates to the use of copper ions to achieve enhanced retention of a therapeutic agent within a liposome. The invention may be employed to more effectively deliver a liposomally encapsulated therapeutic agent to a target site in vitro and in vivo for anti-cancer or other therapy. The liposome may comprise an interior buffer solution containing the therapeutic agent, the solution having a pH less than 6.5 and most preferably approximating pH 3.5. At least some of the copper ions are retained within the interior solution. In a particular embodiment the therapeutic agent may be a chemotherapeutic drug, such as irinotecan. The invention may also comprise an ionophore to facilitate loading of drug into the liposome. In one particular embodiment the combination of the ionophore A23187 and encapsulated divalent copper (Cu2+) resulted in an irinotecan formulation that exhibited surprisingly improved drug retention attributes.