Unbound Steady-state Concentrations Research Articles

In Vivo Blood-Brain Barrier Transport of Oxycodone in the Rat: Indications for Active Influx and Implications for Pharmacokinetics/Pharmacodynamics

The blood-brain barrier (BBB) transport of oxycodone was studied in rats. Microdialysis probes were inserted into the striatum and vena jugularis. Ten animals were given a bolus dose followed by a 120-min constant rate infusion to study the steady-state concepts of oxycodone BBB equilibration. Another 10 animals were given a 60-min constant rate infusion to study the rate of equilibration across the BBB. Oxycodone-D3 was used as a calibrator for the microdialysis experiments. The samples were analyzed with a liquid chromatography-tandem mass spectrometry method and a population pharmacokinetic model was used to simultaneously fit all the data using NONMEM. A two-compartment model which allowed for a delay between the venous and arterial compartments best described the pharmacokinetics for oxycodone in blood and plasma, whereas a one-compartment model was sufficient to describe the pharmacokinetics in the brain. The BBB transport of oxycodone was parameterized as CL(in) and K(p,uu). CL(in) describes the clearance of oxycodone across the BBB into the brain, whereas K(p,uu) describes the extent of drug equilibration across the BBB. CL(in) across the BBB was estimated to 1910 microl/min x g brain. K(p,uu) was estimated to 3.0, meaning that the unbound concentration of oxycodone in brain was 3 times higher than in blood, which is an indication of active influx of oxycodone at the BBB. This is the first evidence of an opioid having an unbound steady-state concentration in brain that is higher than unity, which can explain potency discrepancies between oxycodone and other opioids.

Validation of different microdialysis methods for the determination of unbound steady-state concentrations of theophylline in blood and brain tissue.

Three microdialysis methods, the "tritium" method, the "point-of-no-net-flux" method, and a method using the low perfusion rate of 0.1 microliter/min, were compared with respect to their ability to generate estimates of unbound steady-state concentrations (Cu(ss)) of the antiasthmatic drug theophylline in blood and brain tissue in anesthetized rats. Concomitantly, the influence of the perfusion flow rate on the estimated extracellular Cu(ss) obtained with the point-of-no-net-flux method was investigated. Theophylline was administered as a rapid intravenous bolus dose followed by constant intravenous infusion. Changes in perfusion flow rate from 2.0 to 0.75 microliter/min and, finally, to 0.25 microliter/min, using the point-of-no-net-flux method, had no significant effect on the estimated Cu(ss) of theophylline in blood and striatum. This observation, particularly in the case of brain tissue, is not consistent with the theory that the process of dialysis drains a significant amount of substance from the immediate vicinity of the dialysis probe. Similar estimates of Cu(ss) in blood as well as in brain tissue were obtained with all three methods. Their accuracy in estimating Cu(ss) in blood was further strengthened by observations of unbound fractions similar to those reported in the literature. Furthermore, all three methods gave striatum/blood ratios at steady state of approximately 0.5, indicating that there is active transport of theophylline from brain tissue. It is concluded that the tritium method, when validated, can be used to study the time course of unbound drug concentrations in blood and tissues.

Effect of a protein binding change on unbound and total plasma concentrations for drugs of intermediate hepatic extraction.

For substances eliminated from blood by the liver, the effect of a change in unbound fraction of drug (fu(b)) on steady state total (Cb) and unbound (Cu(b)) blood concentrations has hitherto only been considered for the two limiting cases, i.e., at the upper and lower extremes of hepatic intrinsic clearance (CL(int)). For a substance of very low CL(int), if fu(b) changes, Cb will change and Cub will remain constant, whereas if CL(int) is very high, Cub will change and Cb will remain constant. The present study defines the effects of a change in fu(b) on Cb and Cub over the whole CL(int) range. Computer simulations were undertaken which predicted that, for a given change in fu(b), absolute and relative changes in Cb would decrease nonlinearly with increasing CL(int), while the relative change in Cub would increase with CL(int). The absolute change in Cub would be independent of CL(int). Significant changes in Cb and Cub would be observed at intermediate values of CL(int) not just at the high and low extremes. These theoretical predictions were investigated experimentally in the isolated perfused rat liver by examining the effects of a change in fu(b) of sodium taurocholate a substance with intermediate CL(int) (such that at fu(b) = 0.27, hepatic extraction ratio = 0.71) induced by concurrent administration of sodium oleate. Sodium 24-14 C-taurocholate (specific activity 52 microCi/mmol) was infused into the reservoir in a recycling system at 30 mumol/hr for 105 min (n = 6). At 45 min a bolus dose of sodium oleate (50 mmol) was administered to the reservoir, followed by a constant infusion of 143 mmol/hr for 1 hr. Following the administration of oleate, taurocholate fu(b) fell promptly by 55% (0.27-0.12). There was a relative increase of taurocholate Cb of 22.7% and a relative decrease in Cub of 45.4%, in accordance with the simulations (p less than 0.05). We conclude that important changes in unbound steady-state concentration, the pharmacologically active moiety, can occur upon changes in unbound fraction with compounds of intermediate hepatic intrinsic clearance.