P-glycoprotein limits oral availability, brain penetration, and toxicity of an anionic drug, the antibiotic salinomycin

Abstract

Salinomycin is a polyether organic anion that is extensively used as a coccidiostatic antibiotic in poultry and commonly fed to ruminant animals to improve feed efficiency. However, salinomycin also causes severe toxicity when accidentally fed to animals in high doses. In addition, humans are highly sensitive to salinomycin and severe toxicity has been reported. Multidrug efflux transporters like P-glycoprotein (P-gp), BCRP, and MRP2 are highly expressed in the intestine and can restrict the oral uptake and tissue penetration of xenobiotics. The purpose of this study was to investigate whether the anionic drug salinomycin is a substrate for one or more of these efflux pumps. Salinomycin was actively transported by human MDR1 P-gp expressed in polarized MDCK-II monolayers but not by the known organic anion transporters human MRP2 and murine Bcrp1. Using P-gp-deficient mice, we found a marked increase in plasma salinomycin concentrations after oral administration and decreased plasma clearance after intravenous administration. Furthermore, absence of P-gp resulted in significantly increased brain penetration. P-gp-deficient mice also displayed clearly increased susceptibility to salinomycin toxicity. Thus far, P-gp was thought to affect mainly hydrophobic, positively charged or neutral drugs in vivo. Our data show that P-gp can also be a major determinant of the pharmacokinetic behavior and toxicity of an organic anionic drug. Variation in P-gp activity might thus directly affect the effective exposure to salinomycin and possibly to other anionic drugs and toxin substrates. Individuals with reduced or absent P-gp activity could therefore be more susceptible to salinomycin toxicity.

FIG. 1.

Structural formula of salinomycin. The molecular mass is 751.02 Da. The predicted pKa value of 4.4 was calculated by MarvinSketch software and by ACD/pKa 8.03 software.

FIG. 2.

Transepithelial transport of salinomycin (5 μM) in MDCK-II cells either nontransduced (A and C) or transduced with human MDR1 (B and D), human MRP2 (E), or murine Bcrp1 (F) cDNA in the absence (A, B, and F) or presence (C, D, and E) of elacridar (5 μM). At t = 0 h, salinomycin was applied in one compartment (apical or basolateral) and the percentage of salinomycin transported to the opposite compartment at t = 2 and 4 h was measured by LC-MS/MS and plotted (n = 3). Translocation from the basolateral to the apical compartment (⧫) and translocation from the apical to the basolateral compartment (▾) are shown. Data represent means ± SD.

FIG. 3.

Plasma concentration-time curves of salinomycin in male FVB WT (▴) and Mdr1a/1b^−/− (▪) mice after oral (A) or i.v. (B) administration of salinomycin at a dose of 1 mg/kg. Data represent mean concentrations ± SD (n = 5 for oral or i.v. administration). The insert in panel B shows a semilog plot of the data.