Role of blood-brain barrier P-glycoprotein in limiting brain accumulation and sedative side-effects of asimadoline, a peripherally acting analgaesic drug

Abstract

Studies with knockout mice lacking mdr1a P-glycoprotein (P-gp) have previously shown that blood-brain barrier P-gp is important in preventing the accumulation of several drugs in the brain. Asimadoline (EMD 61753) is a peripherally selective kappa-opioid receptor agonist which is under development as a therapeutic analgaesic. From the structural characteristics of this drug and its peripheral selectivity, we hypothesized that it is transported by P-gp. Using a pig-kidney polarized epithelial cell line transfected with mdr cDNAs, we demonstrate that asimadoline is transported by the mouse mdr1a P-gp and the human MDR1 P-gp. Furthermore, we show that in mdr1a/1b double knockout mice, the absence of P-gp leads to a 9 fold increased accumulation of asimadoline in the brain. In line with this accumulation difference, mdr1a/1b (-/-) mice are at least 8 fold more sensitive to the sedative effect of asimadoline than wild-type mice. Interestingly, the oral uptake of asimadoline was not substantially altered in mdr1a/1b (-/-) mice. Our results demonstrate that for some drugs, P-gp in the blood-brain barrier can have a therapeutically beneficial effect by limiting brain penetration, whereas at the same time intestinal P-gp is not a significant impediment to oral uptake of the drug.

Figure 1

Transepithelial transport of [¹⁴C-]-asimadoline (10 μM) in LLC-PK1, L-mdr1a, L-mdr1b and L-MDR1 monolayers. At t=0, the radioactive drug was applied in one compartment (basal or apical), and the percentage of radioactivity appearing in the opposite compartment at t=1, 2, 3 and 4 h was measured and plotted. Data show a representative experiment (with n=3) of three independent experiments (each experiment performed at least in duplicate). Results are expressed as mean values, with bars indicating the s.d. (for some values the range is smaller than the size of the symbols used). One per cent translocation per hour corresponds to a permeability coefficient of about 0.042 mm h⁻¹.

Figure 2

Sedative effects of asimadoline in mdr1a/1b (−/−) mice (open columns) and mdr1a/1b (+/+) mice (solid columns). Results are expressed as sedation times (means±s.d.) with n=4. Asimadoline was administered i.v. at doses of 1.25, 2.5, 5, 10 and 20 mg kg⁻¹ for both mdr1a/1b (−/−) and mdr1a/1b (+/+). Doses below 1.25 mg kg⁻¹ were given only to mdr1a/1b (−/−) mice.