ABC transporters play an important role in the disposition of
avermectins in several animal species. In this study the interactions of three key
avermectins,
abamectin,
emamectin and
ivermectin, with human and mouse homologues of MDR1 (ABCB1/Abcb1a) and MRP (ABCC/Abcc), transporters endogenously expressed by human SH-SY5Y and mouse N2a
neuroblastoma cells were investigated. In both cell lines, retention of the
fluorescent dye H33342 was found to be significantly increased in the presence of
avermectins and
cyclosporin A. These effects were shown to be unresponsive to the BCRP inhibitor
Ko-143 and therefore MDR1/Mdr1-dependent.
Avermectins inhibited MDR1/Mdr1a-mediated
H33342 dye efflux, with apparent Ki values of 0.24±0.08 and 0.18±0.02μM (
ivermectin); 0.60±0.07 and 0.56±0.02μM (
emamectin) and 0.95±0.08 and 0.77±0.25μM (
abamectin) in SH-SY5Y and N2a cells, respectively. There were some apparent affinity differences for MDR1 and Mdr1a within each cell line (affinity for
ivermectin>emamectin≥abamectin, P<0.05 by One-Way ANOVA), but importantly, the Ki values for individual
avermectins for human MDR1 or mouse Mdr1a were not significantly different. MK571-sensitive retention of GSMF confirmed the expression of MRP/Mrp efflux transporters in both cell lines.
Avermectins inhibited MRP/Mrp-mediated
dye efflux with IC50 values of 1.58±0.51 and 1.94±0.72μM (
ivermectin); 1.87±0.57 and 2.74±1.01μM (
emamectin) and 2.25±0.01 and 1.68±0.63μM (
abamectin) in SH-SY5Y and N2a cells, respectively. There were no significant differences in IC50 values between individual
avermectins or between human MRP and mouse Mrp. Kinetic data for endogenous human MDR1/MRP
isoforms in SH-SY5Y cells and mouse Mdr1a/b/Mrp
isoforms in N2a cells are comparable for the selected
avermectins. All are effluxed at concentrations well above 0.05-0.1μM
ivermectin detected in plasma (Ottesen and Campbell, 1994; Ottesen and Campbell, 1994) This is an important finding in the light of toxicity seen in the Mdr1-deficient animal models CF-1 mice, Mdr1ab (-/-) double knockout mice and Collie dogs. We also confirm MRP/Mrp-mediated
avermectin transport in both N2a and SH-SY5Y cell lines.