By combining the application of pharmacologically active agents (such as cytotoxins in the case of tumor therapy, e. g. Ontak™) carrying target-cell specific ligands with a saponin, the uptake of the agent can be enhanced more than ten thousand fold. Due to this specific enhancement, the total concentration of the agent can be reduced by a comparable factor still achieving the desired effects towards e. g. tumor cells. Severe side-effects, however, such as cytotoxicity towards benign cells, are thereby suppressed. Target cell specific ligands can be covalently coupled or non-covalently associated to the pharmacologically active agent or can be expressed as a fusion protein with the agent. Beside the application in tumor therapy, a more general approach can be anticipated regarding any therapy in which difficulties in transferring an agent into a specific
compartment exist (e. g. treatment of inflammatory diseases by cell-specific inhibition of inflammatory mediators).