Effect of renal dysfunction on the individual components of the acyl-glucuronide futile cycle.

Drugs which are cleared predominantly by forming acyl-glucuronides undergo a futile cycle in which plasma drug clearance is a function of the formation, hydrolysis and renal clearance of the glucuronide conjugate. The effect of impaired renal function, induced by uranyl nitrate administration, on each component of this process was studied in rabbits using diphenylacetic acid. Uranyl nitrate administration produced a decrease in creatinine clearance of approximately 70% and diphenylacetic acid plasma clearance of approximately 35%. In healthy rabbits the primary determinant of diphenylacetic acid net clearance was the very large component of glucuronide renal clearance (14.10 ml/min/kg) compared with glucuronide formation (4.79 ml/min/kg) or glucuronide hydrolysis (2.56 ml/min/kg). Uranyl nitrate treatment reduced both creatinine clearance and the glucuronide renal clearance by approximately 70%. Renal dysfunction had little effect on the hydrolysis clearance of the glucuronide, but reduced its formation clearance by 22.8%. The reduction in plasma clearance of diphenylacetic acid was a function of both the decrease in glucuronidation (60% contribution) and the decrease in renal clearance of the glucuronide (40% contribution). This study supports the futile cycle mechanism of reversible glucuronide conjugation for acyl-glucuronides in general, and provides a mechanism for the impairment of the plasma clearance in renal failure of drugs forming acyl-glucuronides.

The disposition of ketoprofen enantiomers in man.

1. The disposition of ketoprofen enantiomers was studied in 21 patients taking racemic ketoprofen (Orudis SR). 2. In each patient the plasma concentrations of the R- and S-enantiomers were similar at all times over a 24 h dosing interval. The mean (+/- s.e. mean) time-averaged plasma ketoprofen concentrations over the dosage interval were 0.76 (+/- 0.06) mg l-1 for R-ketoprofen and 0.78 (+/- 0.06) mg l-1 for S-ketoprofen. 3. Creatinine clearances for the 21 patients ranged from 6-162 ml min-1. There was no correlation between creatinine clearance and time-averaged plasma concentration for either R- or S-ketoprofen. 4. Approximately 30% of the dose was recovered in urine (unconjugated + glucuronide conjugate) and this was made up of 43% R-ketoprofen and 57% S-ketoprofen. Because of incomplete urine recoveries of ketoprofen it was not possible to determine whether inversion from the R- to the S-enantiomer takes place in man. 5. The data suggest that in terms of total (bound + unbound) ketoprofen, half the concentration value derived by a non-enantiospecific analysis would give a reasonable approximation of the pharmacologically active S-enantiomer concentration in plasma.

The stereospecific incorporation of fenoprofen into rat hepatocyte and adipocyte triacylglycerols.

The formation of triacylglycerols containing fenoprofen was studied in rat isolated adipocytes and hepatocytes incubated with [3H]glycerol and R or S fenoprofen. In both hepatocytes and adipocytes there was a high-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen which was stereospecific for the R enantiomer. The apparent Km values for R fenoprofen were 1.0 microM in adipocytes and 2.8 microM in hepatocytes. These results are consistent with the proposed stereospecific formation of R-2-arylpropionyl-CoA thioesters resulting in the stereospecific formation of R-tri-acylglycerol at clinically relevant unbound fenoprofen concentrations. In isolated hepatocytes, but not adipocytes, a second low-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen was also observed. However, this process (Km = 3780 microM) occurred at concentrations much higher than those found in man with usual doses.

High-performance liquid chromatographic quantitation of triacylglycerols containing fenoprofen from biological samples.

A normal-phase high-performance liquid chromatographic (HPLC) method has been developed for the quantitation of radiolabelled triacylglycerols containing fenoprofen, synthesized from [3H]glycerol by isolated hepatocytes and adipocytes. The assay consists of extracting the lipids into diethyl ether, separating triacylglycerols from polar endogenous lipids using silica Sep-Pak cartridges and quantitating endogenous triacylglycerols and triacylglycerols containing fenoprofen by HPLC resolution and scintillation counting. HPLC separation is achieved in less than 10 min. Using [14C]tripalmitin as internal standard the assay has a linear relationship between added triacylglycerol and measured endogenous triacylglycerols and triacylglycerols containing fenoprofen with regression coefficients of 0.997 and 0.998, respectively.

Enantioselective disposition of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs. III. Fenoprofen disposition.

The disposition of fenoprofen enantiomers has been studied in nine healthy rabbits. A mean (S.E.M.) of 0.73 (0.07) of R-fenoprofen was inverted to S-fenoprofen and the distribution volumes for bound plus unbound R-fenoprofen and S-fenoprofen were 50.3 (4.5) and 98.5 (5.9) ml/kg, respectively. A model was developed which predicted the area under the S-fenoprofen plasma concentration-time curve after bolus administration of racemic fenoprofen. The mean (S.E.M.) predicted area, 2.1 (0.2) mg X min/ml/kg, was within 94% of the observed area 2.2 (0.2) mg X min/ml/kg. The effect of phenobarbital on the disposition of fenoprofen enantiomers was examined in an additional eight rabbits. During the control study the glucuronidation of R-fenoprofen exceeded the corresponding clearance term for the S-enantiomer by 2.1-fold. The clearances of individual enantiomers to their respective glucuronides increased after phenobarbital pretreatment by a mean 1.6-fold for R- and 2.3-fold for S-fenoprofen. The clearance of S-fenoprofen by processes other than glucuronidation and elimination of unchanged drug in urine was increased by a mean of 2.1-fold after phenobarbital pretreatment but the fractional inversion and the inversion clearance of R- to S-fenoprofen were not affected. These data indicate that on racemic fenoprofen administration the area under the curve for the pharmacologically active S-enantiomer would be reduced by phenobarbital pretreatment.

Enantioselective disposition of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs. IV. Ketoprofen disposition.

The disposition of ketoprofen enantiomers has been studied in 12 rabbits with normal renal function (control) and in 6 of these rabbits with renal dysfunction. In control animals a mean (S.E.M.) of 0.09 (0.01) of R-ketoprofen was inverted to its S-enantiomer. The mean distribution volumes for R- and S-ketoprofen were 114 (7.4) and 294 (76) ml/kg, respectively. The mean clearance of the R-enantiomer from plasma was 292 (40) ml/kg/hr compared to 36 (3.2) ml/kg/hr for the S-enantiomer. This high degree of enantioselectivity was not due to inversion, renal clearance or clearance by glucuronidation, but was the result of an unknown clearance process(es). Comparing the mean parameters of ketoprofen disposition in renal dysfunction with those in the same animals during the control study, the mean fraction of the R-dose inverted increased by 290%, but this effect was not due to decreased clearance of R-ketoprofen renally or by glucuronidation. The distribution volumes of R- and S-ketoprofen were increased by 200 and 183% by renal dysfunction, consistent with an implied decrease in plasma protein binding. The clearances of bound plus unbound R- and S-ketoprofen from plasma remained unaltered by renal dysfunction.

Enantioselective disposition of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs. I. 2-Phenylpropionic acid disposition.

The disposition of 2-phenylpropionic acid (2PPA) enantiomers has been studied in 12 rabbits with normal renal function (control) and in 6 rabbits with renal dysfunction. In control animals a mean (S.E.M.) of 0.23 (0.02) of R-2PPA was inverted to S-2PPA, and the distribution volumes for unbound S- and R-2PPA were 0.30 (0.02) and 0.62 (0.07) liters/kg, respectively, but there was no evidence of stereoselectivity in the glucuronidation of R- or S-2PPA. A model was developed that predicted the fraction of the 2PPA in plasma present as unbound S-2PPA upon infusion of racemic 2PPA to steady state. The mean (S.E.M.) observed fraction was 0.69 (0.01) and the predicted fraction was 0.66 (0.03). Comparing the mean parameters of 2PPA disposition in renal dysfunction with those in the same animals during a control study, the fraction of R-2PPA undergoing inversion increased by 350%, the clearance of unbound S-2PPA decreased to 25% and the unbound clearance of R-2PPA by processes other than inversion decreased to 9% of the control means. These changes are consistent with the effects of renal dysfunction on acyl-glucuronide futile cycles for both R- and S-2PPA. Using the above data it can be calculated that, in an animal with severe renal dysfunction, upon an infusion of racemic 2PPA to steady state the concentration of the unbound S-enantiomer would be 5.6 times that in a control animal.

Enantioselective disposition of 2-arylpropionic acid nonsteroidal anti-inflammatory drugs. II. 2-Phenylpropionic acid protein binding.

The binding of 2-phenylpropionic acid (2PPA) enantiomers to rabbit albumin has been studied using fatty-acid-free albumin, with and without oleic acid, and using plasma from control rabbits and from rabbits rendered uremic with uranyl nitrate. The models of binding examined included specific binding at one and at two species of binding site, nonspecific binding and with inhibition between enantiomers with competitive or noncompetitive kinetics. Although any one aspect of the data is adequately modeled by nonspecific binding together with a single species of specific site, the simplest physical model consistent with the whole data requires two species of specific site together with nonspecific binding. Oleic acid in vitro, or other modifiers in vivo, inhibit the binding at both specific sites and reduce nonspecific binding. The inhibition at one site is sufficient that the situation in whole plasma simplifies to one site plus nonspecific binding. Competition between enantiomers occurs at this remaining site, at which R-2PPA binds more avidly than S-2PPA. Both specific and non-specific binding are reduced further in uranyl nitrate-induced renal failure. In the light of these findings, we discuss the implications of enantioselective binding and of competition between enantiomers for binding sites on the interpretation of drug disposition studies.

Enantiospecific high-performance liquid chromatographic analysis of 2-phenylpropionic acid, ketoprofen and fenoprofen.

A high-performance liquid chromatographic method has been developed for the quantitation of the R- and S-enantiomers of 2-phenylpropionic acid, ketoprofen and fenoprofen. The assay consists of extracting the arylpropionic acid with an internal standard and measuring the total (R + S) concentration of enantiomers by reversed-phase chromatography, derivatising the chromatographic fraction corresponding to the enantiomers to form R- and S, R-2-phenylethylamide distereoisomers which are resolved by normal-phase chromatography in order to calculate the fraction of each enantiomer. The limits of sensitivity of the assay for 2-phenylpropionic acid, ketoprofen and fenoprofen are 6, 0.2 and 2.5 mg/l, respectively.

Cortisol does not inhibit prednisolone clearance.

The disposition of prednisolone has been studied in eight male subjects with and without the concomitant administration of cortisol which produced plasma cortisol concentrations 10-fold higher than endogenous levels. The clearance and steady-state distribution volume of total prednisolone were increased as was the prednisolone free fraction but the clearance of unbound prednisolone was unaltered by cortisol co-administration.

Diisopropylfluorophosphate increases clofibric acid clearance: supporting evidence for a futile cycle.

Clofibric acid has been shown previously to undergo a futile cycle in which its net clearance is dependent upon conjugation to form an acyl (ester) glucuronide and a combination of hydrolysis of the conjugate and its renal clearance. If hydrolysis is mediated by esterases then inhibition of these enzymes should increase clofibric acid clearance. Plasma clofibric acid clearance was measured in a group of six rabbits both while conscious and then while anesthetized and treated with diisopropylfluorophosphate (DFP). Clofibric acid clearance also was determined in a control group of six rabbits both while conscious and subsequently while anesthetized. Plasma clearances of unbound and total clofibric acid (protein bound plus unbound) were similar in both groups while conscious but increased 2-fold in DFP-treated compared with conscious animals (P less than .001) and were 3-fold greater in DFP-treated animals than in control, anesthetized animals (P less than .001). These data support the futile cycle for clofibric and implicate esterases in the in vivo hydrolysis of clofibric acid glucuronide.

Alterations in prednisolone disposition as a result of oral contraceptive use and dose.

The disposition of total and free prednisolone has been studied in eight female subjects who used combined oestrogen-progestogen oral contraceptives and in eight female subjects who did not, each of whom received separate intravenous doses of 0.1 mg/kg (low) and 1.0 mg/kg (high) of prednisolone. Mean free prednisolone clearance was reduced congruent to 30% in oral contraceptive users compared to control subjects (P less than 0.001), the difference being greater for the low dose (39%) than for the high dose (24%). Pre-dose plasma cortisol concentrations were elevated two-fold (P less than 0.001) in oral contraceptive users compared to control subjects. These effects are consistent with a mechanism in which the competitive inhibition of free prednisolone clearance by cortisol contributes to the reduction of free prednisolone clearance by oral contraceptive use. Mean total prednisolone clearance and steady state distribution volume showed an approximate two-fold dose dependent increase consistent with a similar increase in plasma prednisolone free fraction (P less than 0.001). Free prednisolone clearance showed an 18% dose dependent decrease (P less than 0.001) but free steady-state distribution volume did not change with dose. At plasma prednisolone concentrations less than 400 ng/ml, prednisolone free fractions at any prednisolone concentration were greater after the low, than after the high dose. This effect is consistent with the displacement of prednisolone by cortisol from transcortin but not from albumin.

PIP: The disposition of total and free prednisolone has been studied in 8 females who used combined estrogen-progestogen oral contraceptives (OCs) and in 8 females who did not, each of whom received separate intravenous doses of 0.1 mg/kg (low) and 1.0 mg/kg (high) of prednisolone. Mean free prednisolone clearance was reduced about 30% in OC users compared to controls (P0.001), the difference being greater for the low dose (39%) than for the high dose (24%). Predose plasma cortisol concentrations were elevated 2 fold (P0.001) in OC users compared to controls. These effects are consistent with a mechanism in which the competitive inhibition of free prednisolone clearance by cortisol contributes to the reduction of free prednisolone clearance by OC use. Mean total prednisolone and steady state distribution volume showed an approximate 2-fold dose dependent increase consistent with a similar increase in plasma prednisolne free fraction (P0.001). Free prednisolone clearance showed an 18% dose dependent decrease (P0.001) but free steady state distribution volume did not change with dose. At plasma prednisolone concentrations 400 ng/ml, prednisolone free fractions at any prednisolone concentration were greater after the low, than after the high dose. This effect is consistent with the displacement of prednisolone by cortisol from transcortin but not from albumin.

Alterations in prednisolone disposition as a result of time of administration, gender and dose.

The disposition of total and free prednisolone has been studied in four male and four female volunteers, each of whom received an intravenous dose of 0.075 mg/kg (low) and 1.5 mg/kg (high) of prednisolone at both 06.00 h and 18.00 h. For the low dose, free prednisolone clearance was 14% lower (P = 0.012) and time-averaged prednisolone free fraction was 22% higher (P less than 0.001) in the morning, there being no circadian difference in total prednisolone clearance. There was no circadian differences in prednisolone disposition at the high dose. These findings are consistent with a mechanism in which cortisol causes a simultaneous competitive inhibition of prednisolone clearance and plasma protein binding at low, but not at high prednisolone doses. Prednisolone clearance was higher in female than in male subjects, the mean increase being 18% (P = 0.022) for total prednisolone and 21% (P = 0.036) for free prednisolone. Mean total prednisolone clearance and steady-state distribution volume were two-fold higher at the high vs the low dose (P less than 0.001), but free prednisolone clearance showed a dose dependent decrease of 11% (P = 0.019). There was no change in free prednisolone steady-state distribution volume.

A comparison of plasma methylprednisolone concentrations following intra-articular injection in patients with rheumatoid arthritis and osteoarthritis.

Plasma concentrations of methylprednisolone following intra-articular injection were measured in rheumatoid arthritis and osteoarthritis patients. While substantial plasma concentrations were seen in both groups of patients there was no significant difference in the rate or extent of absorption of methylprednisolone from osteoarthritic or rheumatoid knees. This study suggests that it is the dissolution rate of the steroid formulation rather than the characteristics of the synovial membrane which determine rate and extent of systemic absorption of methylprednisolone after intra-articular injection.

Ranitidine disposition in patients with renal impairment.

Ranitidine disposition has been studied in 12 patients with renal impairment following 50 mg given intravenously and 150 mg given by mouth on separate occasions. The clearance of ranitidine from plasma (y) was correlated with creatinine clearance (x): y = 10.47 + 0.289x,r2 = 0.751, but there was no significant correlation of creatinine clearance with distribution volume or bioavailability. The mean (s.e. mean) distribution volume was 1.62 (0.08) 1/kg and the mean bioavailability 0.81 (0.05). These data suggest that in order to obtain similar ranitidine plasma concentrations in anephric patients and patients with normal renal function, the maintenance dose in the anephric patients should be 25-30% of that for patients with normal renal function.

Reduced clofibric acid clearance in renal dysfunction is due to a futile cycle.

Although clofibric acid is cleared predominantly by formation of an ester glucuronide, its plasma clearance is reduced in renal dysfunction. The mechanism of this phenomenon has been investigated in rabbits in which uranyl nitrate administration (0.0625-0.5 mg/kg) produced a dose-dependent decrease in creatinine clearance and clofibric acid plasma clearance. Uranyl nitrate doses of 0.3 mg/kg reduced creatinine clearance by approximately 90%, clofibric acid plasma clearance by approximately 60%, clofibric acid glucuronide renal clearance by approximately 85% and increased the time averaged plasma concentration of clofibric acid glucuronide approximately by 6-fold, compared to control values. Fifty percent of administered clofibric acid glucuronide is hydrolyzed to clofibric acid in rabbits with normal renal function. These studies support a mechanism for the renal function dependent plasma clearance of clofibric acid involving a futile cycle in which the net plasma clearance of clofibric acid is determined by the conjugation of clofibric acid and competition between renal clearance and hydrolysis of the ester glucuronide.