Efflux transport of tolbutamide across the blood-brain barrier

In 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 microM, and 4.89 +/- 0.34 microL min(-1) (mg protein)(-1), respectively, for luminal-to-abluminal transport, and 128 +/- 66 pmol min(-1) (mg protein)(-1), 5.59 +/- 4.2 microM, and 4.43 +/- 0.86 microL min(-1) (mg protein)(-1) , respectively, for abluminal-to-luminal transport. At therapeutic plasma concentrations of tolbutamide (1-16.9 microM), 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.