Contrasting a pro-drug, a soft-drug is active per se when given locally but is rapidly inactivated when it reaches the general circulation. Thus, a long duration of action at a minimum of systemic side effects may be achieved after inhalation of a h2-agonist designed in this way. One such example is ZK-90055, an indol derivative with a hydrolysable ester bond on the ring system. The development of ZK-90055 is stopped before clinical trials are commenced^[2].