Antimicrobial Agents and Chemotherapy 2007; 51(8):2748–2757 Modulation of the Cellular Accumulation and Intracellular Activity of Daptomycin towards phagocytized Staphylococcus aureus by the P-glycoprotein (MDR1) Efflux Transporter in human THP-1 macrophages and Madin-Darby canine kidney cells. Lemaire S, Van Bambeke F, Mingeot-Leclercq MP, Tulkens PM. Unite de Pharmacologie cellulaire et moleculaire, Université catholique de Louvain, B-1200 Brussels, Belgium. P-glycoprotein (P-gp; MDR1), a major efflux transporter, recognizes various antibiotics and is present in macrophages. We have examined its effect on the modulation of the intracellular accumulation and activity of daptomycin towards phagocytized S. aureus (ATCC 25923) in human THP-1 macrophages, in comparison with MDCK epithelial cells (wild type and MDCK-MDR1 overexpressing P-gp; the bulk of the protein was immunodetected at the surface of all three cell types). Daptomycin displayed concentration-dependent intracellular activity (Hill-equation pattern) in THP-1 and MDCK cells with (i) EC50 (50 % effective drug extracellular concentration; relative potency) and static concentrations at 9-10 x the MIC, and (ii) Emax (CFU decrease at infinite extracellular drug concentration) at 1.6-2 log compared to the post-phagocytosis inoculum. Verapamil (100 microM) and elacridar (GF 120918; 0.5 microM), two known inhibitors of P-gp, decreased daptomycin EC50 (about 3-fold) in THP-1 and MDCK cells without affecting Emax. Daptomycin EC50 was about 3-4-fold higher and accumulation in MDCK-MDR1 commensurately lower than in wild-type cells. In THP-1 macrophages, (i) verapamil and ATP depletion increased, and ouabain (an inducer of mdr-1 expression) decreased the accumulation of daptomycin in parallel with that of DiOC2 (a known substrate of P-gp); (ii) silencing mdr1 (the gene encoding P-gp) with duplex human mdr-1 siRNAs reduced the cell content in immunoreactive P-gp to 15-30 % of controls, and caused a 8-13-fold increase in daptomycin accumulation. We conclude that that daptomycin is subject to efflux from THP-1 macrophages and MDCK-cells by P-gp, which reduces its intracellular activity against phagocytized S. aureus. PMID: 17548493 [PubMed - as supplied by publisher]