In order to extend knowledge about pharmacophoric features responsible for ABCB1 inhibitory properties of imidazolidin‐2,4‐dione derivatives, 1′‐[4‐(4‐(o‐methoxyphenyl)‐piperazin‐1‐yl)butyl]‐3′‐methyl‐spiro(fluoren‐9,5′‐imidazolidine)‐2′,4′‐dione (3) and its salt (4) with rhodanine‐3‐acetic acid (RA) were prepared and investigated by X‐ray diffraction method, as well as their efflux modulating effects in cancer cells (mouse T‐lymphoma), cytotoxic and antiproliferative activities were evaluated in vitro. The molecular geometry, intermolecular interactions, and crystal packing of base and acid forms of 3 were analyzed to see, if conformational changes influence the biological activities. The geometry of 2‐methoxyphenylpiperazine and 5‐spirofluorenehydantoin moieties was compared with other crystal structures containing these fragments. Our results indicated a very potent inhibitory action on ABCB1 pump, and significant cytotoxic and antiproliferative properties of 3 in T‐lymphoma, even more potent in the case of multidrug resistance cells. Furthermore, the compound 3 converted into the salt 4 of inactive acid (RA) has maintained both, the efflux pump inhibitory and antiproliferative activities, showing strong synergism with doxorubicin. A comparison of geometry of 3 in both crystal structures (3 and 4) shows a significant difference in the arrangement of piperazine ring with respect to the aliphatic linker.
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