Effects of pro-FTY and other drugs on PDOs. A, PDOs stained with hematoxylin and eosin (H&E), the CTS probe (red), and 4′,6-diamidino-2-phenylindole (DAPI; blue), along with a merged image (merge). Scale bars, 100 µm. B–M, Effects of doxorubicin (B–D), paclitaxel (E and F), FTY720 (H–J), and pro-FTY (K–M) were determined with PDOs 1–3 by performing CellTiter-Glo 3D cell viability assays. PDOs 1 and 2 were generated from patients with treatment-naïve breast cancer, and PDO 3 was generated from a biopsy specimen from a patient acquired multidrug resistance by the end of treatment.