Patupilone reduces survival of MM cell lines either sensitive or resistant to conventional chemotherapy, including multidrug-resistant cells overexpressing MDR1. (A) Cell lines MM.1S (⋄), U266 (▵), and RPMI 8226 (□) were treated for 48 hours with 0, 1, 10, 100, 1000, or 10 000 nM patupilone. IC₅₀ (inhibitory concentration 50%) was less than 10 nM for patupilone, as determined by MTS assay. (B) Patupilone reduces survival of multidrug-resistant cell lines, including cell lines that overexpress the MDR1/Pgp drug efflux pump. RPMI 8226-derived cell lines resistant to MR20, LR5, and Dox40 were treated for 48 hours with 0, 0.01, 0.1, 1, and 10 nM patupilone (left) or 0, 1, 10, 100, 1000, or 10 000 nM paclitaxel (right). Dox40 is shown by ♦; LR5, ▴; and MR20 ▪. Survival was determined by MTS assay. For patupilone, IC₅₀ of Dox40 was less than 10 nM, whereas for paclitaxel, IC₅₀ was not achieved even at 10 000 nM. The IC₅₀s of LR5 and MR20 were 10 nM for patupilone and 100 nM for paclitaxel. (C) Flow cytometry demonstrates expression of MDR1 protein on RPMI 8226 as well as RPMI 8226-derived cell lines resistant to MR20, LR5, and Dox 40. Compared with parental RPMI 8226, almost a 2-log increase in MDR1 protein expression was observed in RPMI 8226-Dox40 cells. White histograms indicate isotype control; black histograms, anti-CD243 (MDR-1)-PE.