Unbound Drug In Plasma Research Articles

Phenobarbital Pharmacokinetics in Infants Using Saliva as a Biologic Fluid

Objective: To determine the pharmacokinetic parameters of phenobarbital in infants, using saliva as a biologic fluid, and to correlate the parameters obtained from saliva data with those obtained from plasma data. Design: A prospective, randomized study. Setting: Hospitalized patients at the Medical Center Pereira Rossell, a pediatric hospital in Montevideo, Uruguay. Patients: Sixteen infants with seizure disorders were included in the study. None of them was treated with other medications. Interventions: A direct intravenous loading dose of phenobarbital 10 mg/kg was administered, followed by a maintenance dosage of 5 mg/kg/d (once- or twice-daily dosing) given 12 hours after the loading dose. Main Outcome Measures: Saliva and plasma samples were obtained 6 and 12 hours after the loading dose and 3 days after the initiation of the maintenance dose (trough sample): the samples were analyzed by HPLC and the elimination half-life (t1/2), the volume of distribution (Vd), and the percentage of unbound drug in plasma (UDP) were calculated. Results: The t1/2 obtained from plasma and saliva data was 30 hours; the Vd was 0.73 L/kg from plasma data and 2.4 L/kg from saliva data; and the UDP was 75 percent. Trough concentrations showed no significant difference between treatments. Conclusions: Saliva is a useful biologic fluid to determine phenobarbital pharmacokinetic parameters, mainly in pediatric patients. Moreover, a single daily dose of phenobarbital is sufficient to obtain therapeutic concentrations.

Pharmacokinetics of Diazepam in the Rat: Influence of a Carbon Tetrachloride-Induced Hepatic Injury

The pharmacokinetics of diazepam in normal rats and in rats pretreated with carbon tetrachloride to induce hepatic cirrhosis (cirrhotic rats) was studied after intravenous and oral administration of the drug (4 mg/kg). Animals pretreated with this hepatotoxic agent showed a significant prolongation in the half-life of diazepam in plasma that is due more to an increase in volume of distribution rather than to a decrease in clearance. This study confirmed that diazepam was highly extracted by the rat liver and was not affected by the hepatotoxic agent, although there probably was a saturation of the activity of the cytochrome P450 enzyme when the drug was administered orally. Diazepam binds to plasma proteins to a high degree in both normal and cirrhotic rats; however, in the latter, a significant increase in the fraction of unbound drug in plasma was observed. Pretreatment of rats with carbon tetrachloride did not produce any change either in the distribution of diazepam into erythrocytes or in the disposition of the metabolite desmethyldiazepam.

ヒトにおける薬物脳脊髄液中移行性の評価と予測

The distribution of highly lipophilic drugs into cerebrospinal fluid (CSF) is thought to be predictable from both assumptions of simple diffusion process and pH partition theory. We tried to predict CSF/plasma concentration ratio (CcsF/Cp) from the fraction of unbound drug in plasma (fp), pKa, and pH of blood and CSF and to compare the predicted CCSF/Cp values with the observed CCSF/Cp values collected from literatures.In the case of highly-lipophilic drugs, we found that the penetration into CSF could be predicted from various parameters we above. Since the penetration of more hydrophilic drugs into CSF could not be estimated from those parameters, we tried to predict CCSF/Cp values by using influx (PS1) rate constant from blood to brain and active efflux (kac) rate constant from brain to blood.The PS1 value was calculated from octanol/buffer partition coefficient (PC) and the diffusion constant across water (Dm) based on Sawada's report (Y.Sawada et al., Am.J.Physiol., 258, H1585 (1990)). The kac value was calculated on pharmacokinetic model considering the active efflux mechanisrr from brain to blood.A significant correlation (r=0.96) between kac and PC of each drug was observed.The predicted CCSF/Cp values based on this model wes comparable with the observed values.These findings suggested that the CCSF/Cp value of lipophilic or hydrophilic drugs was predictable from various biochemical, physicochemical and physiological parameters.

Evaluation of saliva as a specimen for monitoring zidovudine therapy in HIV-infected patients

To facilitate studies of the pharmacokinetic properties of zidovudine, the relationship between plasma and salivary concentrations of the drug was studied, after oral dosage, in 10 HIV-infected patients. Zidovudine concentrations were determined in plasma, unstimulated mixed saliva and citric-acid-stimulated mixed saliva over a period of 3 1/2 hours by high-performance liquid chromatography. Correlation coefficients were r = 0.97 (P less than 0.0001) for stimulated saliva compared with plasma and r = 0.89 (P less than 0.0001) for unstimulated saliva, with average values in unstimulated saliva being 113.8 +/- 44.6% in plasma and 67.8 +/- 25.4% in stimulated saliva. Stimulated saliva values found to be 70% of the total reflected the concentration of the unbound drug in plasma. Except for a shorter half-life time (t1/2) in saliva, pharmacokinetic parameters showed a good correlation in the three types of specimen. These findings and the convenience of sample collection suggest that citric-acid-stimulated saliva might be an appropriate specimen for monitoring zidovudine therapy.

Enantioselective steady-state kinetics of unbound disopyramide and its dealkylated metabolite in man

Disopyramide is provided as a racemic mixture of R and S enantiomers, which have different pharmacodynamic and pharmacokinetic characteristics. Five volunteers were given racemic disopyramide 100 mg and 200 mg t.d.s. in a cross-over design. Plasma and urine concentrations of disopyramide and its active metabolite monodesisopropyl-disopyramide (MND) were determined at steady state by an enantioselective HPLC method. Unbound drug in plasma was measured after ultrafiltration. There was enantioselective clearance of unbound disopyramide (0.39 l.h-1.kg-1 for R-disopyramide and 0.58 l.h-1.kg-1 for S-disopyramide after 100 mg t.d.s.). The enantioselectivity was due to differences in the metabolism of disopyramide to MND and in further non-renal clearance, and the renal clearance of disopyramide was not enantioselective. The in vivo protein binding of disopyramide, which was saturable for both enantiomers, was also enantioselective. The difference in binding of the two enantiomers was explained by a difference in apparent binding capacity rather than in apparent binding affinity. The renal clearance of S-MND was significantly higher than R-MND (0.29 and 0.19 l.h-1.kg-1, respectively, after 100 mg t.d.s.). The renal clearance of MND also showed a tendency to saturation at higher concentrations.

Prediction of the pharmacokinetics of cefodizime and cefotetan in humans from pharmacokinetic parameters in animals.

Pharmacokinetic behavior of beta-lactam antibiotics (cefodizime and cefotetan) in humans were predicted from the animal data. Total body clearance (CLp) of these drugs in humans (weighting 65-69 kg) were successfully extrapolated from the allometric relationship between the clearance for the unbound drug in plasma and body weight (r = 0.954-1.000) with a power of 0.948-0.991 for cefodizime and 0.700-0.756 for cefotetan. We also predicted the volume of distribution at steady state (Vdss), the volume of distribution in the central compartment (V1) and the volume of distribution at beta-phase (Vd beta) of these drugs in humans from the observed human plasma unbound fraction, inasmuch as the plasma unbound fraction correlated well (r = 0.913-0.995) with the values of Vdss, V1, and Vd beta among various animal species. Based on these predicted pharmacokinetic parameters, we calculated the plasma concentration profiles of these drugs in humans and found a good agreement between the predicted and observed values. We also report here that the prediction is successful when we consider the plasma protein binding of these drugs.

Evaluation of acidity constants of two-step overlapping equilibria of amphoteric substances from solubility measurements

The therapeutic activity of drugs is related to the concentration of free unbound drug in plasma [l, 21 which is dependent on, among other factors, the ionization and solubility of the drug. In addition, a knowledge of the acidity constants of compounds of pharmaceutical interest is required for the solution of many drug-related problems [3]. However, the choice of method for determining the ionization constants of drugs is severely restricted in a number of cases by poor solubility which does not allow application of normal potentiometric and conductimetric techniques for the determination of the constants in water. In such cases, solubility measurements in combination with any appropriate quantitative analytical technique [4-E] may be used. Although hydroalcoholid solutions may be employed to increase the solubility, studies intended to be of clinical relevance should be conducted in aqueous buffers at physiological temperature and ionic strength. In this report, an equation is proposed that allows the overlapping acidity constants of amphoteric substances to be calculated from solubility measurements.

Effect of tolrestat on red blood cell sorbitol levels in patients with diabetes

The effect of the aldose reductase inhibitor, tolrestat, on red blood cell (RBC) sorbitol levels was studied in 23 patients with diabetes after oral dosing with tolrestat, 25 or 100 mg b.i.d. The mean (+/- SE) RBC sorbitol levels (measured 12 hours after the preceding dose) after 3, 7, and 13 days of dosing decreased after both dose levels. After 25 mg tolrestat the RBC sorbitol levels fell from 25.1 +/- 4.0 to 20.0 +/- 5.7 nmol/gm hemoglobin (21%) and after 100 mg tolrestat the level fell from 26.7 +/- 3.7 to 11.4 +/- 1.7 nmol/gm hemoglobin (57%; P less than 0.001). This latter RBC sorbitol concentration is similar to levels in individuals without diabetes. At both dosage levels the maximum decrease in RBC sorbitol levels occurred after only 3 days of dosing. Tolrestat had no effect on plasma glucose or hemoglobin A1 concentrations. The overall mean plasma unbound drug concentration measured 12 hours after 100 mg tolrestat (11.7 +/- 3.0 ng/ml; 3.3 X 10(-8) mol/L) was similar to the median inhibitory level (3 X 10(-8) mol/L) of tolrestat for sorbitol accumulation in human RBCs incubated in a high-glucose medium. Our results demonstrate the systemic bioavailability of tolrestat and its aldose reductase inhibitory activity in erythrocytes of patients with diabetes.

Use of plasma unbound drug concentrations in adjusting phenytoin doses.

The application of a Bayesian feedback algorithm for phenytoin (PHT) dosing adjustments was modified empirically to take advantage of measurements of plasma unbound PHT concentrations in the estimation of Km values. Doses were calculated to achieve steady-state unbound PHT nadirs of 1.2 mg/L with an allowable range of 0.9-1.5 mg/L. The success of this approach was compared with that of a routine strategy encompassing the use of monitored total plasma PHT concentrations and no estimates of kinetic parameters. Allowing up to a double feedback of monitored values, the Bayesian approach was better than the routine procedure in predicting doses that led to PHT concentration within the targeted range (p less than 0.005). The approach to optimizing PHT dosing described herein offers the advantage of rapidly converging on near-optimal therapy for those patients who are already on PHT but whose seizures are not suitably controlled, and may be useful even for patients who exhibit abnormal PHT plasma protein binding, such as those who also must take valproic acid.

A novel method to predict the elimination half-lives and the renal excretion mechanisms of cephalosporins.

A novel method was proposed to predict the elimination half-lives of cephalosporins from plasma protein binding (unbound fraction, f) and fraction of the dose excreted into urine (f*) on the basis of the following four assumptions. 1) The drug is only distributed to the extracellular fluid, 2) the bound fraction of the drug in plasma is independent of the plasma drug concentration, 3) the binding protein of the drug is albumin, 4) the unbound drug in plasma is excreted by the glomerular filtration and the contribution of active secretion and reabsorption is negligible. The VSS's and t1/2 beta's of MT-141, one of cephalosporins, in rabbits, dogs and healthy human subjects were well predicted, whereas in rats, the prediction of the both values was failed. The t1/2 beta's of various cephalosporins in healthy subjects were calculated from f and f*, in reasonably good agreement with the observed ones, except for some cephalosporins which have been reported to be secreted actively in the renal tubules. Thus, the comparison of the calculated t1/2 beta's with the observed ones makes it possible to presume the renal excretion mechanism. Moreover, this method will be applicable to other drugs which satisfy the above four assumptions.

Appearance of pethidine and norpethidine in cerebrospinal fluid of man following intramuscular injection of pethidine.

The plasma and cerebrospinal fluid (CSF) concentrations of pethidine and its main metabolite in plasma, norpethidine, were determined in 20 patients undergoing minor surgery who had received pethidine chloride as premedication in a standard dose of 100 mg intramuscularly. The disposition of pethidine and norpethidine in plasma was followed for 3-8 h after administration. The rate of transfer of the drug and its metabolite from plasma to CSF was assessed on the basis of a single sample of CSF taken from each patient. Pethidine appeared within less than 18 min in the CSF, reaching a maximum after about 90 min. After that, the pethidine concentration ratio CSF/plasma was relatively stable at 0.4-0.5. This is in agreement with the concept that the concentration of a drug in CSF is correlated with the concentration of unbound drug in plasma at equilibrium. Norpethidine which was present in rapidly increasing concentrations in plasma after a delay of 30 min, appeared in CSF in a slower and more erratic fashion as compared to the parent compound. However, after 240 min, the CSF/plasma concentration ratio was similar for pethidine and norpethidine. Thus, transfer from plasma to CSF occurs relatively rapidly. There is little evidence for a functionally significant blood-brain barrier for pethidine and norpethidine.

Importance of protein binding for the interpretation of serum or plasma drug concentrations.

The extent of drug binding to plasma protein does not influence the concentration of unbound drug in plasma at steady state but does influence the interpretation of total drug concentrations. An increase in free fraction (reduction in the extent of binding) decreases the therapeutic and toxic ranges for total drug concentrations. Conversely, a reduction in free fraction (increase in the extent of binding) increases these ranges. Laboratories generally measure total rather than unbound drug concentrations, and clinicians must use caution in interpreting these total drug concentrations in clinical situations where the extent of binding of extensively bound drugs may be altered. Free drug serum or plasma concentrations are inherently more reliable indices of the intensity of drug action than are total concentrations. Methodology for routine measurement of free concentration is becoming available and should ultimately be utilized for therapeutic monitoring of drugs that are highly bound to protein.

Clinical pharmacokinetics of valproic acid.

Valproic acid is rapidly absorbed from the gastrointestinal tract, peak concentrations being attained 1 to 2 hours after administration of the conventional tablet, but later with the enteric-coated tablets. The bioavailability of valproic acid is complete and independent of the preparation used. The apparent volume of distribution is relatively small (0.1 to 0.4 L/kg), due to high plasma protein binding. Protein binding is decreased in patients with renal insufficiency, in patients with chronic liver disease, and possibly in the presence of other displacing agents. The total plasma clearance of valproic acid is in the range of 5 to 10ml/min. Plasma elimination half-life is between 10 and 16 hours, and does not change after continued treatment with valproic acid alone. In combination therapy with other antiepileptic drugs, the half- life can be as short as 6 to 8 hours due to liver enzyme induction. Renal excretion of unchanged valproic acid accounts for only 1 to 3% of the total dose. Valproic acid is present in cerebrospinal fluid in concentrations equal to the unbound drug in plasma (around 10% of the total concentration). Valproic acid concentration in saliva is less than and unrelated to the free drug concentration in plasma. The drug is excreted into breast milk and evidence suggests that it also crosses the placenta. Four independent metabolic pathways — glucuronidation, β-oxidation and ω-oxidation (ω1, and ω2) have been demonstrated in man. Analytical difficulties caused by the similarity of the metabolites with many normal endogenous compounds and by chemical lability of several metabolites impede the isolation, identification and especially the quantification of valproic acid metabolites. Quantitative aspects of metabolism are essential for the understanding of drug effects in patients. The main metabolite 3-oxo-valproic acid shows comparable pharmacological activity to valproic acid itself in mice; unsaturated metabolites also show some activity. In young infants under 2 months of age valproic acid elimination half-life can be 60 hours, but in older children, plasma elimination appears to be identical to the adult situation. Valproic acid elimination is impaired in acute viral hepatitis and in liver cirrhosis. No information is available on valproic acid kinetics in renal insufficiency. Phenobarbitone plasma concentrations rise under combination therapy with valproic acid, because phenobarbitone elimination is impaired. Valproic acid lowers total plasma concentrations of phenytoin by displacing it from its plasma albumin binding site, but free phenytoin concentrations remain unchanged and phenytoin doses need not be modified. Valproic acid elimination is enhanced in the presence of inducing drugs like phenobarbitone, primidone, phenytoin and carbamazepine. The valproic acid dose-plasma concentration relationship is curvilinear, since valproic acid protein binding decreases with higher concentrations. Therefore, the increase in the plasma concentration is smaller than expected from the dose increase. The lower value of the therapeutic plasma concentration range of valproic acid is around 50μg/ml. Because of low clinical toxicity the upper value of the therapeutic range has yet to be determined.

Distribution of antiepileptic drugs between plasma, plasma water, cerebrospinal fluid, saliva and brain.

The following conclusions can be drawn from our results. The concentrations in cerebrospinal fluid correspond to the concentrations of the unbound part and saliva at a high lipid solubility. Cerebrospinal fluid and brain were interpreted as a uniform compartment. The unbound concentrations of low lipid soluble drugs in plasma do not reflect the concentrations of drugs in cerebrospinal fluid. The correlation between the unbound drug in plasma, CSF and brain must be determined. The concentrations of lipid soluble drugs in plasma, water or saliva is representative for the concentration of antiepileptic drugs in central nervous system.

Influence of acute viral hepatitis on disposition and plasma binding of tolbutamide.

To study the influence of acute hepatic disease on the disposition of tolbutamide, we measured tolbutamide plasma protein binding and pharmacokinetic parameters after intravenous administration of the drug to 5 subjects during and after apparent recovery from acute viral hepatitis. Although during the acute phase of illness protein binding of the drug decreased in all, volume of distribution of tolbutamide (0.15 +/- 0.03 L/kg) did not change. Clearance based on total concentration of tolbutamide in plasma increased in all subjects during the acute phase of study (26 +/- 5.4 ml/hr/kg) in comparison to the recovery phase (18 +/- 2.8 ml/hr/kg, p less than 0.02). Protein binding decreased after unconjugated bilirubin was added to plasma from the recovery phase, but not to the extent observed during the acute phase of illness at comparable levels of bilirubin. Clearance based on unbound drug concentration, calculated by dividing the observed plasma clearance by the fraction of unbound drug in plasma, did not differ significantly between the 2 study phases (300 +/- 47 and 260 +/- 39 ml/hr/kg). These observations suggest that the increase in clearance based on total drug concentration in plasma during hepatitis can be attributed solely to decreased plasma binding. This decrease in binding may be attributed in part, but not entirely, to increased combination of bilirubin during illness. The concentration of unbound drug in plasma at steady-state is determined by the rate of drug administration and the clearance based on unbound drug. If this clearance does not change during hepatic disease, no dosage alterations for tolbutamide and other comparable drugs are necessary to maintain a constant concentration of unbound drug.

Disposition of valproic acid in man

The pharmacokinetics of valproic acid (VPA) have been studied in 6 healthy subjects following a single 600 mg dose, and after multiple doses over 12 days (1200 mg daily) of enteric-coated sodium valproate. A time lag before absorption of 1 to 2 h was observed in each subject, and then absorption was rapid, peak concentrations being recorded 3 to 4 h after administration of the dose. The plasma level decline was biphasic with a terminal half-life of 15.9 +/- 2.6 h in the single dose and 17.3 +/- 3.0 h in the multiple dose experiments. There was no evidence of dose dependent kinetics or autoinduction. Total plasma clearance was 0.0064 +/- 0.0011 l/kg X h. The apparent volume of distribution was small at 0.15 +/- 0.2 l/kg. The mean steady state plasma concentration (Css) reached after 4 days was 81.3 +/- 13.0 microgram/ml. Css observed was lower than Css predicted (99.2 +/- 14.7 microgram/ml) from single dose kinetics (p less than 0.001). The difference was probably due to a reduction in plasma protein binding at higher concentrations. VPA concentration in saliva was between 0.4 and 4.5% of the total plasma concentration and was not equal to the concentration of unbound drug in plasma (6.7 +/- 0.8% unbound). 3.2% of the dose was excreted in urine as the parent drug and 21.2% as conjugated metabolites.

Pharmacokinetics of the antiarrhythmic disopyramide in healthy humans.

The pharmacokinetics of the antiarrhythmic disopyramide, 4-diisopropylamino-2-phenyl-2-(2-pyridyl)butyramide phosphate, and its monodealkylated metabolite were investigated in seven volunteers after intravenous (1 and 2 mg/kg) and oral (3 and 6 mg/kg) administration. Unchanged drug (52%) and the monodealkylated metabolite (25%) were renally excreted on intravenous administration. The pharmacokinetics of disopyramide were first order and dose independent only when referenced to the drug not bound to plasma proteins since this binding was dose dependent. The apparent half-lives of the α and β phases on intravenous administration were 2 min and 4.5 hr, respectively. The apparent volumes of distribution of the central and peripheral compartments, referenced to unbound disopyramide in the plasma, were 9 and 80 liters, respectively. The half-life of absorption of oral aqueous disopyramide phosphate was 30 min with a lag time of 16 min and an apparent first-pass metabolism of 16% of the absorbed dose, consistent with the hepatic efficiency of 14%. The renal and metabolic clearances were 125 and 111 ml/min, respectively. Graphical and computer analysis of the plasma and urine data showed dose-independent first-order pharmacokinetics of plasma unbound drug in a two-compartment-body model to give two metabolites and a first-pass transformation of a fraction of the oral dose. The absorption efficiency of unchanged drug was 83%.

Pharmacokinetics of Trichloroethanol and Metabolites and Interconversions Among Variously Referenced Pharmacokinetic Parameters

The pharmacokinetics of intravenously administered trichloroethanol, chloral hydrate, sodium trichloroacetate, and trichloroethanol glucuronide were studied in the dog with developed GC analyses of blood, urine, and feces, and analyzed with aid of analog and digital computation. Trichloroethanol pharmacokinetics were evaluated at multiple-dosage levels and showed distribution from a central compartment assignable to extracellular body water into readily available and less available tissues assignable to total body water and lipoidal and other tissues. The major route of removal of trichloroethanol in the dog was by conjugation to the glucuronide, and the first-order rate constants of this metabolic route were dose dependent but did not appear to be a consequence of enzymic saturation or change in apparent volumes of distribution with dose. A possible explanation is that the hepatic lipid-water ratio may vary with the dose of trichloroethanol. The formed trichloroethanol glucuronide, after release from the liver, was rapidly distributed in the extracellular body water of the dog and rapidly eliminated in the urine by glomerular filtration and tubular secretion. Biliary elimination of the glucuronide accounted for about 5% of an intravenous dose of trichloroethanol. Saturation of the bile secretory mechanism was demonstrated at higher trichloroethanol doses, e.g., 100 mg./kg., but the absence of trichloroethanol and its glucuronide in the feces may be explained by hydrolysis of the biliary glucuronide in the GI tract with subsequent reabsorption of the formed trichloroethanol, i.e., an entero-hepatic shunt. No assayable trichloroacetic acid was observed in the blood or urine of dogs dosed with trichloroethanol. A long apparent disposition half-life of 75hr. was observed after intravenous administration of sodium trichloroacetate; this can be explained by the extensive tissue binding of trichloroacetate which gave high apparent volumes of distribution. Intravenously administered chloral hydrate was rapidly and quantitatively converted to trichloroethanol in the dog, with an apparent half-life of 3min. Protein binding of trichloroethanol and its glucuronide (about 35%) was determined. Although the glucuronide did not partition into the red blood cells, the trichloroethanol did so instantaneously, with a partition coefficient of 2.1 in favor of the red blood cells. Since apparent volumes of distribution and microscopic pharmacokinetic rate constants have quantitative values that vary widely, depending on whether they are referenced to total blood, total plasma, or unbound drug in plasma concentrations, a detailed analysis of the conversions among these variously referenced constants was made and applied to the data of these studies. Estimates of the microscopic rate constants and apparent volumes of distribution referenced to unbound drug in plasma have the most valid physiological significances. These are complex functions of the hematocrit, the degree of protein binding, the true volume of plasma or blood in the animal, and the red blood cell/plasma partition coefficient when the drug is assayed per millilitcr of whole blood or per milliliter of plasma. The awareness of these facts will necessitate extensive recalculation of many pharmacokinetic constants now given in the literature.

Carbonic anhydrase inhibitors, parathyroid hormone and calcium metabolism.

The ability of a series of carbonic anhydrase inhibitors to inhibit parathyroid hormone (PTH) has been studied in nephrectomized rats. Three enzyme inhibitors (acetazolamide, methazolamide and ethoxzolamide) are also inhibitors of parathyroid hormone. When the concentration of unbound drug in plasma is considered, ethoxzolamide is 41 times as potent as methazolamide with respect to PTH inhibition. An acetazolamide analogue (CL 13,850) devoid of carbonic anhydrase inhibitory properties has been shown to be inactive as a PTH inhibitor when given at a dose of 9 mg/100 g. The same dose of acetazolamide results in PTH inhibition. (Endocrinology 91: 1160, 1972)