Efflux Transport of Tolbutamide Across the Blood-brain Barrier

AbstractIn an attempt to determine the reason for the low brain distribution of tolbutamide, we have demonstrated the transport of tolbutamide from the brain to the blood via a non-P-glycoprotein efflux transport system which is inhibited by sulphonamides. We evaluated the directional transport of tolbutamide across the blood-brain barrier by means of an in-vivo brain-tissue distribution study and experiments on in-vitro transcellular transport and uptake in cultured mouse-brain capillary endothelial cells (MBEC4).The brain-to-unbound-plasma concentration ratio of [14C]tolbutamide increased in the presence of high concentrations of unlabelled tolbutamide or sulphonamide at steady-state in-vivo. The brain-to-blood concentration ratios of [14C]tolbutamide were very low compared with that of [3H]propranolol obtained by in-vivo integration plot analysis. From the in-vitro transcellular transport study using a monolayer of MBEC4 cells, we found that the abluminal-to-luminal flux of [14C] tolbutamide was higher than the reverse flux. Both luminal-to-abluminal and abluminal-to-luminal transport of tolbutamide were saturable. The maximum transport rate (Jmax), the half-saturation concentration (Kt), and the first-order rate constant (kd) were 65.9 ± 29 pmol min−1 (mg protein)−1, 7.54 ± 4.4 μM, and 4.89 ± 0.34 μL min−1 (mg protein)−1, respectively, for luminal-to-abluminal transport, and 128 ± 66 pmol min−1 (mg protein)−1, 5.59 ± 4.2 μM, and 4.43 ± 0.86 μL min−1 (mg protein)−1, respectively, for abluminal-to-luminal transport. At therapeutic plasma concentrations of tolbutamide (1–16.9 μM), the efflux rate would be faster than the influx rate. The estimated net efflux was consistent with the very low in-vivo brain distribution of tolbutamide. The efflux process observed in MBEC4 cells was inhibited by sulphonamides such as sulphaphenazole, sulphamethoxazole and sulpha-dimethoxine whereas the steady-state uptake of [14C]tolbutamide was not affected by either cyclosporin or verapamil, specific inhibitors of P-glycoprotein.These findings suggest that tolbutamide is partly transported from the brain via the non-P-glycoprotein-efflux transport system, which is inhibited by sulphonamides.