A number of tricyclic carboxamides have been synthesized and tested to evaluate their ability to reverse multidrug resistance in the CHRC/5 cell line. Among them the acridone derivatives were the most potent. A key feature is the presence of a dimethoxybenzyl or phenethylamine cationic site, separated from the tricyclic lipophilic part by a carbamoylphenyl chain. Optimization led to compounds 2 orders of magnitude more active than the prototype inhibitors verapamil and amiodarone. On the basis of in vitro and in vivo activities, 9,10-dihydro-5-methoxy-9-oxo-N-[4-[2-(1,2,3,4-tetrahydro-6,7- dimethoxyisoquinol-2-yl)ethyl]phenyl]-4-acridinecarboxamide (84) has been selected for further development.