There has been a preliminary report that furifosmin, like the other lipophilic 99mTc cations sestamibi and tetrofosmin, is a substrate for P-glycoprotein, the membrane transporter that is a mechanism of multidrug resistance (MDR) in tumors. This has been further investigated in the rat mammary carcinoma cell line MatB/WT and its doxorubicin-selected resistant variant MatB/AdrR.In vitro studies were performed by adding furifosmin to stirred single-cell suspensions of MatB/WT and MatB/AdrR in the presence or absence of the Pgp-modulating drug PSC833. Dynamic imaging studies over 30 min were performed in rats bearing MatB/WT or MatB/AdrR tumors growing in the leg.Accumulation of furifosmin in MatB/AdrR cells in vitro was much lower than that in MatB/WT cells. The addition of 1 microM PSC833 increased the plateau accumulation in MatB/AdrR cells 2.4-fold, but did not affect accumulation in MatB/WT cells. In rats, furifosmin accumulated rapidly in MatB/WT tumors and washed out with a mean t3 of 78 min. Washout from MAtB/AdrR tumors was more rapid, with a t3 of 46 min (p < 0.025). Following dissection of animals at 30 min, mean tumor-to-muscle ratios were 1.57 and 1.05 in MatB/WT and MatB/ AdrR tumors, respectively (p < 0.025).Furifosmin is suitable for functional imaging of multidrug resistance in tumors.