A conjugate of an active agent and a targeting moiety having affinity for a target cell, in which the active agent has been modified by attachment of a cell membrane-impermeabilizing group so that, if the active agent so modified is cleaved from the conjugate in the blood plasma instead of inside the target cell, the cell membrane-impermeabilizing group prevents or limits entry of the modified active agent into cells, thus reducing its systemic or non-specific adverse effects, including toxicity.