Low-dose Pimecrolimus, an FDA-approved Calcineurin Inhibitor, Sensitizes Drug-resistant Cancer Cells via Strong P-gp Inhibition

Background/Aim: Co-treatment with calcineurin inhibitors, such as tacrolimus and cyclosporin A, can sensitize chemotherapy-resistant cancer cells with P-glycoprotein (P-gp)-over-expression. Pimecrolimus (PIME) is a clinically available calcineurin inhibitor with a structure similar to that of tacrolimus. Whether PIME can sensitize P-gp-over-expressing resistant cancer cells remains unclear. Materials and Methods: Cell viability assay, annexin V analyses, cellular morphology and density observation with a microscope, western-blotting, fluorescence-activated cell sorting (FACS), and analysis for P-gp inhibitory activity were performed to investigate the mechanism of action. Results: PIME exhibited strong cytotoxicity to vincristine (VIC)-treated drug-resistant cell lines (KBV20C and MCF-7/ADR) over-expressing P-gp. Co-treatment with VIC and PIME increased apoptosis and down-regulated the ERK signaling pathway, resulting in G₂ arrest. PIME could be co-administered with vinorelbine or eribulin to sensitize resistant KBV20C or MCF-7/ADR cancer cells. Moreover, PIME strongly inhibited the efflux of both rhodamine 123 and calcein-AM substrates through P-gp after 4 h of treatment, indicating that VIC+PIME sensitized cancer cells by inhibiting VIC efflux via direct PIME binding to P-gp. Low doses of PIME, tacrolimus, and cyclosporin A showed similar sensitizing efficiencies in resistant KBV20C cells. These drugs showed similar P-gp inhibitory activities using both rhodamine 123 and calcein-AM substrates, suggesting that calcineurin inhibitors generally have strong P-gp inhibitory activities that sensitize drug-resistant cancer cells with P-gp over-expression. Conclusion: PIME, currently used in clinics, can be repositioned for treating patients with P-gp-over-expressing resistant cancer (stem) cells.