Fig. 5: Cyclo-(L-1-methyl-Trp-L-1-methyl-Trp) inhibits ABCB1 efflux and sensitises cells to doxorubicin-induced cell death.

a–c Flow cytometry analysis of doxorubicin (Doxo) accumulation. Cells were pre-treated with 5 µM verapamil (Vera), 1 mM Cyclo-(L-Trp-L-Trp), or the indicated dose of Cyclo-(L-1-methyl-Trp-L-1-methyl-Trp) before incubating with 10 µM Doxo. Bar graphs represent the mean ( ± SD of three independent experiments) fluorescent intensity of Doxo relative to the Vera-treated cells (Vera, the second bar in each cell line). a White bars represent parental PaKiT03 cells, while gray bars represent PakiT03 ABCB1 KO cells. b White bars represent HEK293T ABCB1 expressing cells, while gray bars represent HEK293T control cells. c White bars represent HCT-15 cells. d–f Quantification of cell viability by Viakrome 405 staining. PaKiT03 parental and ABCB1 KO cells d, HEK293T control and ABCB1 expressing cells e, and HCT-15 cells f were pre-treated with the indicated amount of Cyclo-(L-1-methyl-Trp-L-1-methyl-Trp) or 5 µM verapamil (Vera) for 30 minutes before adding 1 µM Doxo. The cell viability was measured after 48 hours of Doxo treatment. Bar graphs represent the mean ± SD of seven d, five e, or three f independent experiments. White bar: without Doxo; gray bar: with Doxo.