Influence of Age on the Disposition and Renal Handling of Enprofylline in Rats

Abstract— The effects of ageing on the pharmacokinetics, renal handling and protein binding of enprofylline were investigated in 6-, 13- and 18-month-old male Fischer 344 rats. Concentrations of enprofylline in plasma and urine were determined by HPLC, and pharmacokinetic parameters were estimated by model-independent methods. No significant differences in the volume of distribution, systemic clearance of enprofylline or urinary recovery of unchanged enprofylline (> 85%) were observed among any of the groups of rats. The dissociation constant and free fatty acid concentration in plasma increased with age. Age-dependent decreases in the systemic clearance for unbound drug were observed, and the volume of distribution for unbound drug tended to decrease with age. The ratio of systemic clearance for unbound drug to the glomerular filtration rate (GFR) decreased with ageing. Ageing was associated with decreases in the apparent maximum capacity of transport (V_max) (223·33,160·24 and 142·98 μg min^?1 kg^?1 for 6-, 13- and 18-month-old rats, respectively) and in the tubular secretory intrinsic clearance (V_max/K_m) of enprofylline (75·45, 51·03 and 44·13 mL min^?1 kg^?1, respectively), while a slight change in the Michaelis-Menten constant (K_m) was observed. These results indicate that the mechanism responsible for age-related changes in the disposition and renal handling of enprofylline may be responsible for a decrease in the ability of the tubular anion transport system.     

Effect of folic acid on the pharmacokinetics of acutely administered phenytoin in pregnant and nonpregnant rats

The concentrations of both total and free phenytoin in the plasma of epileptic women tend to decrease during pregnancy, suggestive of a pregnancy-associated increase in the metabolic clearance of the drug. On the other hand, the metabolic clearance of free (unbound) phenytoin decreases during pregnancy in rats. One possible reason for this species difference is the routine dietary supplementation of folic acid in human pregnancy and the apparent ability of folic acid to lower phenytoin plasma concentrations even in nonpregnant humans. The purpose of this investigation was to determine the effect of treatment with folic acid on the pharmacokinetics of phenytoin in pregnant and female nonpregnant rats. In one experiment, the treated animals received folic acid in the drinking water, approximately 100-150 micrograms/kg/d, for 19 d. There was no apparent difference between the treated and untreated rats in the pharmacokinetics of a 10-mg/kg iv dose of phenytoin (which was administered to the pregnant rats on the 20th day of gestation), regardless of pregnancy status, In another experiment, pregnant and female nonpregnant rats received either folic acid, 400 micrograms/kg/d, or an equal volume of the solvent only, by gastric intubation for 19 d. The next day (which was the 20th day of gestation for the pregnant rats), the animals received an intravenous injection of phenytoin, 30 mg/kg. Again, pretreatment with folic acid had no apparent effect on the pharmacokinetics of phenytoin in both pregnant and nonpregnant rats. However, the results of this investigation confirm previous observations of dose-dependent phenytoin pharmacokinetics in rats and of decreased clearance of free phenytoin in late pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dose-dependent pharmacokinetics of enprofylline and its renal handling in rats

The effect of dosage on the pharmacokinetics of the potent bronchodilator enprofylline (3-propylxanthine; PX) and its renal handling were investigated in rats. Enprofylline (PX) was administered iv in dosages of 2.5, 10, 20, and 40 mg/kg, and PX concentration in plasma and urine was determined by HPLC. The pharmacokinetic parameters were estimated by model-independent methods. The disappearance of PX from plasma was delayed as dosage was increased. The corresponding pharmacokinetic parameters also showed dose dependency; increases in the volume of distribution (Vd) and mean residence time (MRT) and a decrease in total body clearance (CLT) were observed as dosage was increased from 2.5 to 40 mg/kg. Approximately 80% of the dose, however, was excreted in urine as unchanged PX. Plasma protein binding studies of PX showed concentration dependency and allowed determination of binding parameters, with an apparent dissociation constant (Kd) of 162.50 microM and a binding capacity (nP) of 565.23 microM. Some pharmacokinetic parameters for unbound PX calculated by total plasma concentration and binding parameters also showed dose-dependent characteristics. However, no significant change in Vd for unbound PX was observed among administered doses, indicating that the distribution of PX into the body tissues is not changed by an increase in dosage. Renal clearance of unbound PX significantly increased as plasma concentration decreased. The maximum transport capacity (Vmax) and the Michaelis-Menten constant (Km) for tubular secretion were 60.53 micrograms/min and 2.27 micrograms/mL, respectively. The aim of the present study is to demonstrate that both saturable tubular secretion and concentration-dependent protein binding are responsible for the dose-dependent pharmacokinetics of PX in rats.     

Alterations in Renal Uptake Kinetics of the Xanthine Derivative Enprofylline in Endotoxaemic Mice

The pharmacokinetics and renal uptake of enprofylline, which is primarily excreted into the urine by an active tubular secretion mechanism, were investigated in endotoxaemic mice by lipopolysaccharide isolated from Klebsiella pneumoniae. Lipopolysaccharide (1 mg kg^?1) was infused 2 h before starting the examination, thereby inducing a decrease in the systemic clearance and an increase in the steady-state volume of distribution of enprofylline while inducing no changes in the urinary recovery (> 90%). The protein binding of enprofylline significantly decreased in the presence of lipopolysaccharide. Both the systemic clearance for unbound enprofylline and glomerular filtration rate decreased in the treated mice. A nonlinear relationship was found in both groups between the steady-state unbound plasma concentration and renal uptake of enprofylline after constant infusion for 1 h. The renal uptake rate of enprofylline decreased in the treated mice. Lipopolysaccaharide caused increases in the apparent maximum capacity for renal uptake (V_max) from 17.3 to 32.2 μg h^?1 g^?1 of kidney and in the Michaelis–-Menten constant (K_m) from 2.7 to 21.7 μg mL^?1 and decrease in the nonsaturable uptake rate constant (K_d) from 0.87 to 0.43 mL h^?1 g^?1 of kidney. These results indicate that lipopolysaccharide decreases the renal tubular secretion of enprofylline by inducing a decrease in the renal uptake ability.     

The Effects of N-Benzoyl-β-alanine,a New Nephroprotective Drug,on the Distribution and Renal Excretion of Enprofylline in Rats

Abstract— The effects of the new nephroprotective drug N-benzoyl-β-alanine (BA) on the disposition and renal excretion of the bronchodilator enprofylline, which is actively secreted in urine, were investigated in rats. Enprofylline was administered intravenously at a dosage of 2·5 mg kg^?1 under three different steady-state plasma BA concentrations (100,200 and 400 μg mL^?1) which were achieved by constant infusion rates. Pharmacokinetic parameters for both total and unbound enprofylline were estimated by model-independent methods. The presence of BA (400 μg mL^?1) increased the systemic clearance by 25% and the volume of distribution at steady-state by 90%. A significant increase in the dissociation constant, which is the protein binding parameter of enprofylline was observed in the presence of BA (400 μg mL^?1), indicating that BA competitively inhibits the protein binding of enprofylline. However, BA significantly decreased the systemic clearance and volume of distribution for unbound enprofylline. These results suggest that BA, the organic anion transport inhibitor, inhibits renal excretion of enprofylline with a high affinity for renal tubular secretion, although the unbound concentration of enprofylline increases with administration of BA. We conclude that BA decreases the renal tubular secretion of enprofylline probably by reducing the affinity of the tubular transport system, and that these changes have marked effects on the pharmacokinetic behaviour of enprofylline.     

Influence of pregnancy on the pharmacokinetic disposition of two aromatic retinoids (etretinate and acitretin) in the rat. I. Intravenous studies

The influence of pregnancy on the disposition of two related aromatic retinoids (etretinate and its metabolite, acitretin) was evaluated in a rodent model. The plasma concentrations of etretinate and acitretin were monitored by a specific HPLC method following iv bolus doses to 17-day pregnant and nonpregnant Sprague-Dawley rats. The systemic clearance of etretinate was significantly lower in the pregnant rats compared to nonpregnant controls (129 vs. 185 ml/hr, respectively; p less than 0.05). This decrease was entirely due to a lower formation clearance of acitretin (acid) from etretinate (ester) in the pregnant animals (96 vs. 146 ml/hr; p less than 0.05). The in vitro plasma hydrolysis rate of etretinate was also lower in the pregnant animals. By contrast, the systemic clearance of acitretin was greater in the pregnant compared to the nonpregnant control animals (184 vs. 145 ml/hr, respectively; p less than 0.05). The apparent volumes of distribution for both retinoids were comparable in the pregnant and nonpregnant animals. Etretinate infusions in nonpregnant animals yielded systemic clearances (mean = 164 ml/hr) which were similar to those obtained for bolus dose experiments. Acitretin clearance increased (plasma levels decreased) following acitretin infusion to nonpregnant rats over the time course of the infusion. The results illustrate the marked effect of pregnancy on the disposition of these retinoids and suggest that acitretin may pose less of a teratogenic hazard than the parent compound etretinate.     

Interspecies differences and scaling for the pharmacokinetics of xanthine derivatives.

Pharmacokinetic characteristics of the new xanthine bronchodilators, enprofylline and 1-methyl-3-propylxanthine (MPX), were investigated in mice, rats, guinea-pigs, rabbits and dogs. The possibility of an interspecies pharmacokinetic scale was also evaluated. The concentration of these two drugs in plasma and urine was determined by HPLC. Pharmacokinetic parameters were calculated using model-independent methods. The disappearance curves of the two drugs from plasma varied markedly among animal species. Interspecies differences in the plasma protein binding of each drug were observed for all animals in the study. Differences in the biotransformation of enprofylline and MPX were also confirmed among the various animal species: enprofylline is mainly excreted in an unchanged form in urine while MPX follows a non-renal route of elimination. In all animals, the renal clearance for enprofylline was greater than the glomerular filtration rate, indicating active tubular secretion. Significant allometric relationships were seen between the values of total body clearance and steady state volume of distribution for both total and unbound enprofylline and species body weight, but similar correlations could not be recognized for MPX. Renal clearance of enprofylline was also closely correlated with species body weight, suggesting no interspecies difference with relation to affinity and/or capacity for the active tubular secretion mechanism of enprofylline. Our findings suggest that xanthine derivatives, including enprofylline, are mainly eliminated via the kidney, and an estimate of the basic pharmacokinetics in man can be obtained from data in experimental animals.     

The effect of pregnancy on renal clearance of boron in rats given boric acid orally.

Boric acid (H(3)BO(3)) has been shown to cause developmental abnormalities in the offspring of pregnant rats. Comparative data on the renal clearance of boron (B) in rats and humans, both pregnant and nonpregnant, exposed to boric acid (BA) would reduce uncertainty in interspecies extrapolation from rats to humans. The purpose of this study was to evaluate the effect of pregnancy on the plasma half-life and renal clearance of boron in Sprague-Dawley rats given a single oral dose of boric acid. For the half-life study, nonpregnant and pregnant (gestation day 16) rats were given a single dose of 30 mg/kg of boric acid by gavage, and plasma samples were collected at 2-3 h intervals. The plasma half-life of boron was determined to be 2.9 +/- 0.2 and 3.2 +/- 0.3 h in nonpregnant and pregnant rats, respectively. In the clearance study, nonpregnant and pregnant (GD 16) rats were given a single gavage dose of 0.3, 3, or 30 mg/kg of boric acid. Boron clearance was slightly higher in pregnant rats (3.3 +/- 0.6, 3.2 +/- 0.5, and 3.4 +/- 0.5 ml/min/kg, respectively) compared to nonpregnant rats (3.1 +/- 0.8, 3.0 +/- 0.6, and 3.2 +/- 0.5 ml/min/kg, respectively), but the difference was not statistically significant and not dose-related. Boron clearance was less than creatinine clearance, suggesting tubular reabsorption in both groups. In conclusion, pregnancy did not appear to significantly alter the renal clearance or the plasma half-life of boron in Sprague-Dawley rats under the conditions of this study.     

The pharmacokinetics of salicylate in the pregnant Wistar rat

Sodium salicylate, in a single dose of 50 mg/kg, was administered by iv injection to nonpregnant female and 20-day pregnant Wistar rats. Blood samples (for serum) and urine were collected and analyzed for salicylate, gentisic acid, salicyluric acid, and salicyl glucuronides by HPLC. Pregnant rats showed a significant decrease in body weight-normalized total clearance but no change in absolute total clearance of salicylate. On a body weight-adjusted basis there was a slight increase in the apparent volume of salicylate distribution in pregnancy but this increase becomes highly significant if expressed in absolute terms. The biological half-life of salicylate was significantly increased in late pregnancy. Serum protein binding of salicylate is decreased in pregnancy relative to nonpregnant females but in both groups binding shows a concentration dependence. The partial clearances of both salicyluric and gentisic acids were reduced by pregnancy in the rat whereas that of the salicyl glucuronides appeared unchanged. This latter result in intact pregnant animals contrasts with previously reported decreases in glucuronyltransferase activity in isolated liver preparations from pregnant rats.     

Kinetics of Phenytoin in Pregnant and Non-pregnant Rats

Abstract The plasma kinetics of phenytoin were studied in pregnant and nonpregnant rats after a single intravenous dose. The apparent volume of distribution was greater in the pregnant rats resulting in lower plasma concentrations in these rats within the first hour of injection. The beta half-life in pregnant rats (5.0 hours) was prolonged compared to the non-pregnant rats (2.2 hours). The plasma clearance values which take the altered volume of distribution into account, showed a smaller but still significant difference between the two groups of rats. This indicates a lower capacity of the pregnant rats to metabolize the drug.     

Effect of pregnancy on tissue distribution of salicylate in rats

The effect of pregnancy on tissue distribution of salicylate was studied by comparing both pharmacokinetic and protein-binding parameters between 20-day-pregnant rats and nonpregnant (control) rats. In the pregnant rats, the volume of distribution increased significantly (p less than 0.05) from 164 ml/kg of the control to 225 ml/kg, and the total body clearance also increased significantly (p less than 0.05) from 12.1 ml/hr/kg of the control to 19.8 ml/hr/kg. But these changes did not affect the plasma disappearance half-life of salicylate in the pregnant rats. The serum unbound fraction (fs) of the pregnant rats at 8 hr after iv administration of salicylate increased remarkably from 0.14 of the control to 0.67. The fs in the fetal serum (0.41) was lower than that in the maternal serum in spite of the lower albumin concentration in the fetal serum. A nonlinear serum protein binding was observed both in the control and in the fetal rats, but not observed in the pregnant rats. In the pregnant rats, the tissue-to-serum concentration ratios (Kp) of all tissues studied were larger than those in the control rats, and the values of Kp in the fetal were larger than those in the maternal. To elucidate these difference in Kp values between the pregnant and control rats, a mathematical model was proposed, where salicylate was distributed in the interstitial fluid, bound to the interstitial albumin, and translocated into the intracellular fluid according to the pH partition theory. The Kp values of most tissues in the control and pregnant rats were predicted successfully by using this model.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of ampicillin on the elimination of enprofylline in man

The effect of intravenously administered ampicillin on plasma levels and renal clearance of enprofylline was investigated in seven young and six elderly volunteers. On one occasion, each subject received 2 g of ampicillin intravenously, and serum concentrations of ampicillin were followed for 4 hours. On a separate occasion, a loading infusion of enprofylline was administered over 60 min., aiming at a plasma level of 2 micrograms/ml, and followed by a maintenance infusion. During this an infusion of 2 g of ampicillin was given for 10 min., and subsequently, renal clearance and plasma levels of enprofylline were followed for 4 hours. Plasma enprofylline levels increased significantly in the subjects after ampicillin infusion, but the effects on renal clearance of enprofylline were not statistically significant. The magnitude of the effects on enprofylline plasma levels and renal clearance from high doses of ampicillin do not suggest that interaction with beta-lactam antibiotics will be a serious problem for patients on enprofylline treatment.     

Pharmacokinetics of enprofylline in patients with impaired renal function after a single intravenous dose

Summary Enprofylline, a new bronchodilating drug, was given i.v. at 1.0 mg/kg to 7 healthy subjects and to 14 patients with differing degrees of chronic renal insufficiency. Plasma and urine concentrations of unchanged drug were followed by HPLC. In the patients the plasma half-life was prolonged and the total and renal clearances were reduced in direct proportion to the degree of renal insufficiency as determined by creatinine clearance. The unbound fraction of enprofylline in plasma increased from 55% in the healthy subjects to 66% in the group of patients with the highest degree of renal impairment. The volume of distribution terms, V and V_ss, both tended to decrease with decreasing creatinine clearance. When the volume term calculations were based on the unbound drug level in plasma, this tendency was enhanced. Side-effects were noted in 4 subjects, and to some extent were related to the plasma level of the drug.     

Pharmacokinetics of sulfisoxazole in rabbits with experimental renal failure after single and multiple dosing

In rabbits injected intravenously with 0.75 mg/kg uranyl nitrate to precipitate acute renal failure, the renal elimination of sulfisoxazole was essentially non-existent 3 days after the injection. In addition a significant (40%) reduction in the metabolic elimination (unbound clearance) was observed in the renal failure animals. Whether the decrease in metabolic clearance is due to a decrease in renal or non-renal metabolism is not clear from the available data. The protein binding dramatically decreased in renal failure animals, with the lowest binding occurring during the multiple dosing therapy. The half-life is greater (2–3 times) and the apparent volume of distribution (unbound) is significantly decreased due to decreased binding in plasma and tissues.     

Respiration in sprague-dawley rats during pregnancy

Minute ventilation and tidal volume increase in humans during pregnancy. Little data exists, however, on the respiration in pregnant rats, despite their widespread use as an animal model. Since respiration will affect the pharmacokinetics of volatile compounds and ultimately the dose to the fetus, we conducted a study to evaluate respiration in rats during pregnancy. Whole-body plethysmography was used to measure the breathing frequency and tidal volume approximately every other day from gestation day (GD) 1 to 21 in 16 timed pregnant and 16 nonpregnant, female, Sprague-Dawley rats. Minute ventilation was calculated as a product of the breathing frequency and tidal volume, and the body weight of each rat was used to determine the scaled ventilation. Multivariate analysis of variance methods for a repeated-measures design were used to analyze the respiratory data. Breathing frequency was not affected by pregnancy; however, tidal volume was somewhat greater in pregnant versus nonpregnant rats. The increase in tidal volume resulted in significantly increased minute ventilation in pregnant rats compared to nonpregnant rats during the latter period of gestation. Due to the increased body weight of the pregnant rats, the scaled ventilation at the end of gestation was significantly lower in pregnant rats compared to nonpregnant rats. This study provides important reference values that can be used in pharmacokinetic models during pregnancy.     

Effect of acute renal failure on the disposition of cefoperazone.

The effect of acute renal failure on the disposition of cefoperazone was investigated. Rats, 3 days after uranyl nitrate treatment, were used to model acute renal failure. Although plasma-protein binding of cefoperazone decreased significantly in acute renal failure compared with control rats, the plasma clearance of total (bound plus unbound) drug after intravenous administration (50 mg kg(-1)) did not differ significantly between the two groups (5.61+/-2.37 mL min(-1) for control and 4.75+/-2.82 mL min(-1) for acute renal failure). Consequently the plasma clearance of the unbound drug in acute renal failure (6.14+/-1.16 mL min(-1)) was significantly lower than in control rats (15.6+/-3.7 mL min(-1), P < 0.025). Plasma clearance of the drug (both total and unbound) was also dependent on bile flow, and clearance of the unbound drug in acute renal failure rats was lower than in control rats with identical bile flow rates. To examine the mechanism of reduced unbound cefoperazone clearance, an in-vitro experiment using a simultaneous perfusion system of rat liver and kidney was performed. By changing perfusate plasma protein from bovine serum albumin to human serum albumin, the plasma clearance of the total cefoperazone changed to one-sixth in proportion to the unbound cefoperazone in the perfusate plasma. On the other hand, the plasma clearance of the total and unbound drug in acute renal failure rats decreased significantly compared with controls. These results demonstrate that the plasma clearance of unbound cefoperazone, which is mainly eliminated by the liver, decreased in acute renal failure in rats, probably due to changes in hepatic transport.     

The effect of pregnancy on renal clearance of boron in humans: a study based on normal dietary intake of boron.

Boron occurs most frequently in nature as borates and boric acid, never as the free element. Its largest uses are in glass, detergents, and agriculture. Essential for higher plants, there is growing evidence for essentiality in vertebrates. Humans consume daily about a milligram of boron, mostly from fruit and vegetables. At high doses, boron is a developmental and reproductive toxin in animals. Pregnant rats were the most sensitive. An oral NOAEL of 9.6 mg B/kg/day was established for developmental toxicity in Sprague-Dawley rats fed boric acid. To extrapolate from the large, animal boron toxicity database to humans, especially to pregnant women, information on renal clearance of boron was needed. This study's purpose was to measure renal clearance of boron in pregnant and nonpregnant woman. In 16 second trimester women and 15 nonpregnant age-matched referents, dietary boron provided the blood and urine boron concentrations used for calculating boron clearance. The pregnant and nonpregnant boron intake was 1.35 and 1.31 mg boron/24 h, respectively. Blood for boron, creatinine, and urea was collected at the start, at 2 h, and at 24 h. Urine was collected during the first 2 h in the Clinical Research Center and during a 22-h period outside the center for measurement of volume, boron, and creatinine. Renal boron clearance measured over the initial 2 h, the most complete urine collection period, was 68.30ml/min/1.73 m(2) for pregnant subjects and 54.31ml/min/1.73 m(2) for nonpregnant subjects. Comparison of renal boron clearance with creatinine clearance indicated that tubular reabsorption of boron occurred in both pregnant and nonpregnant women.     

Drug Metabolism: Renal Excretion of Rhodamine 123, a P-Glycoprotein Substrate,in Rats with Glycerol-induced Acute Renal Failure

To clarify renal handling of rhodamine 123, a substrate for P-glycoprotein, in normal and diseased states, in-vivo clearance studies were performed with normal rats and rats with glycerol-induced acute renal failure. For normal rats the excretion ratio of unbound rhodamine 123-to-inulin was 3.25, indicating the presence of the renal tubular secretion of rhodamine 123. Co-administration of cyclosporin, a P-glycoprotein inhibitor, significantly reduced tubular secretion of rhodamine 123. Administration of glycerol induced both an increase in blood urea nitrogen and a reduction in the glomerular filtration rate, confirming the induction of acute renal failure. Total plasma, renal, and tubular secretory clearances of rhodamine 123 were significantly lower for rats with acute renal failure than for control rats. There was no difference between the ATP content of the renal cortex in control rats and those with acute renal failure. In addition to the decrease in renal clearance, a decrease in the biliary clearance of rhodamine 123 was also observed in rats with acute renal failure. These results imply that rhodamine 123 is secreted via P-glycoprotein in renal tubules and that the renal secretory clearance of rhodamine 123 was reduced after acute renal failure, probably because of impairment of P-glycoprotein.     

Comparative in vivo and in vitro studies of phenytoin protein binding and in vitro lipolysis in plasma of pregnant and nonpregnant rats

This investigation was designed to determine the cause of the changes in drug protein binding that occur in rat plasma, particularly in plasma from pregnant animals, during in vitro drug-protein binding measurements. In vivo estimates of phenytoin binding in plasma were obtained from steady-state CSF-plasma concentration ratios in pregnant and nonpregnant rats. Immediate ultrafiltration of heparin- or EDTA-anticoagulated plasma yielded phenytoin free fraction values that were in good agreement with in vivo estimates for nonpregnant rats but that were about one-third higher than in vivo estimates for pregnant animals. In vitro free fraction values tended to increase during incubation of plasma and/or during equilibrium dialysis. The concentrations of the four major endogenous free fatty acids were similar in plasma of pregnant and nonpregnant rats if determined immediately after blood collection. Six hours of incubation at 37 degrees C caused fatty acid concentrations to increase about fivefold and twofold in heparin-anticoagulated plasma from pregnant and nonpregnant animals, respectively. The corresponding increases in EDTA-anticoagulated plasma were only about twofold and 1.14-fold, respectively. These changes were associated with decreased plasma protein binding of phenytoin. The in vivo differences between pregnant and nonpregnant rats with respect to phenytoin binding in plasma are not due to differences in fatty acid concentrations, but the in vitro differences are due primarily to corresponding differences in free fatty acid concentrations if extensive in vitro lipolysis occurs.     

Relationship between the ocular and systemic disposition of flurbiprofen: the effect of altered protein dynamics at steady state

The differences in flurbiprofen disposition in the aqueous humor and the plasma were examined after systemic doses. Steady state plasma concentrations of flurbiprofen (20-60 micrograms/mL) were achieved via intravenous infusion to albino rabbits. Flurbiprofen demonstrated linear systemic kinetics throughout the dosing range, with constant body clearance and unbound fraction in plasma. At steady state, aqueous humor drug concentrations depended on the corresponding plasma drug concentration. Two clearance terms--CLS----O, the systemic clearance to ocular tissues, and CLO----S, the ocular clearance to systemic circulation--were used. After systemic doses, the drug concentration in the aqueous humor was related to that in the plasma as well as to the ratio of these two clearances. Flurbiprofen was extensively bound to plasma proteins and showed limited ocular distribution; its CLS----O to CLO----S ratio was very small. Thus, the concentration of flurbiprofen in the aqueous humor after systemic doses was lower than that obtained after ophthalmic doses. A plasmapheresis technique was utilized to lower the plasma protein concentrations to 60% of normal levels. As a consequence, flurbiprofen demonstrated reduced aqueous humor protein concentrations, increased unbound fractions in the plasma and the aqueous humor, elevated aqueous humor drug concentrations, and elevated total body clearance. The unbound body clearance stayed unchanged. Our study indicated that a drug should present a significant CLS----O/CLO----S ratio in order to achieve therapeutic concentrations in the eye via systemic doses. The drug-protein binding kinetics can be different between the plasma and the aqueous humor circulations. Because the ocular compartment is very small compared to the overall systemic distribution of flurbiprofen, it has little effect on the steady state systemic concentrations.