New inhibitors of the activity of the multidrug resistance receptor 1 (MDR1), which are highly active to the MDR1 transporter, and showing a medium/ high selectivity to such transporter, described by the structural formula (I): where R is a C1-C4 alkylene, optionally substituted with a C1-C3 alkyl. Said inhibitors are useful in the treatment or prevention of multi-drug resistance.