Biased estimates of nonrenal clearance.

The goal of the investigation was to critically evaluate published values for oral nonrenal clearance and their postulated dependence on renal function with drugs administered orally to subjects with varying renal function. Derivation of the pertinent equations indicated that the values reported for oral nonrenal clearance tend to systematically overestimate both the true oral and intravenous nonrenal clearances of these drugs. Computations were performed to confirm these findings not only for subjects with normal renal function, but also for patients with renal impairment. The computations evaluated the relevance of bioavailability and renal clearance of a drug for the bias in the estimates of true oral or intravenous nonrenal clearance. The results of the computations showed that the estimates for true oral and intravenous nonrenal clearance derived from oral data exceed systematically the true values in subjects with normal or reduced renal function. Also, a renal function dependent decrease of the true oral or intravenous nonrenal clearance is falsely diagnosed if apparent oral nonrenal clearance values are used for the estimates. The magnitude of bioavailability and renal clearance impact the bias in the estimates derived from oral data. For drugs with predominant renal excretion and small bioavailability the bias is largest. For drugs with predominant nonrenal elimination and large bioavailability the bias is smallest.

Inhibition of thrombin by iopromide in vitro.

Iopromide is a nonionic, iodinated, monomeric, radiographic contrast agent used in various indications, including coronary angiography and visceral and peripheral arteriography. Nonionic contrast media have been postulated to increase thrombogenicity when compared with ionic contrast media. The goal of this study was to characterize the interaction of iopromide with thrombin, specifically to determine the rate, extent, specificity, and reversibility of the thrombin inhibition by iopromide, the integrity of the thrombin-iopromide complex, and the inhibitory potency of iopromide using a validated assay methodology. Iopromide was mixed with purified thrombin or pooled serum from healthy male and female donors. The final concentrations of iopromide in the presence of estimated physiologic concentrations of thrombin (1 nmol/L) were 0-184 mmol/L. After incubation for defined time intervals, the activity of thrombin was determined by adding substrate and measuring the absorbance of the generated chromophores at 405 nm. The possible inhibition of the protease trypsin by iopromide was investigated to evaluate the specificity of thrombin inhibition by iopromide. Iopromide was compared with Thromstop, a known thrombin inhibitor, to assess the relative potency of iopromide. The inhibition of thrombin by iopromide was immediate, rapidly reversible, and proportionate to the iopromide concentrations. The minimum inhibitory concentration of iopromide was 50 mmol/L. At the highest iopromide concentration tested, 184 mmol/L, the mean inhibition of thrombin activity was 44.5%. The mean concentration of iopromide associated with a 50% inhibition was 206 mmol/L. The inhibitory potency of iopromide was 4 x 10(6) times smaller than that of Thromstop. The inhibition of thrombin by iopromide is specific, because trypsin was not inhibited by iopromide. The results indicate that in vitro iopromide at clinically relevant concentrations partially inhibits thrombin activity. However, the in vitro model used does not consider other factors that may be relevant for the overall coagulation response in vivo.

Pharmacokinetics and pharmacodynamics of sotalol in a pediatric population with supraventricular and ventricular tachyarrhythmia.

OBJECTIVE: This pharmacokinetic-pharmacodynamic study was designed to define the steady-state relationship between pharmacologic response and dose or concentration of sotalol in children with cardiac arrhythmias, with an emphasis on neonates and infants. METHODS: The treatment consisted of an upward titration with unit doses of 10, 30, and 70 mg of sotalol per square meter of body surface area. The patients received 3 doses at each dose level. The dosing interval was 8 hours. The Class III and beta-blocking activities of sotalol were derived from the QT and R-R intervals, respectively, of the surface electrocardiogram, which was recorded at 6 scheduled times before and after the third, sixth, and ninth doses. During these three dose intervals, 4 scheduled blood samples were also collected. Drug concentrations were measured with a validated nonstereoselective liquid chromatographic tandem mass spectrometric detection assay. Pharmacokinetic and pharmacodynamic parameters were obtained with standard methods. RESULTS: Twenty-one centers enrolled 25 patients in the study: 7 were neonates, 9 were infants, and 11 were children between the ages of 2 years and 12 years. The area under the drug concentration-time curve increased proportionately with dose. The apparent oral clearance of sotalol was linearly correlated with body surface area and creatinine clearance. The smallest children (body surface area <0.33 m2) displayed greater drug exposure than the larger children. The increase of QTc and R-R intervals was dose dependent. At the 70-mg/m(2) dose level, the mean (+/- standard deviation) maximum increase for the QTc interval was 14% +/- 7% and the average Class III effect during a dose interval was 7% +/- 5%. At the same dose level, the mean maximum increase of the R-R interval was 25% +/- 15% and the average beta-blocking effect during a dose interval was 12% +/- 13%. The effects tended to be larger in the smallest children. The Class III response and the plasma concentrations of sotalol were linearly related. The treatment was well tolerated. CONCLUSIONS: The steady-state pharmacokinetics of sotalol were dose proportionate. Pharmacologically important beta-blocking effects were observed at the 30-mg/m2 and 70-mg/m2 dose levels. Important Class III effects were seen at the 70-mg/m2 dose level. The Class III effect was linearly related to the drug concentration.

Single-dose pharmacokinetics of sotalol in a pediatric population with supraventricular and/or ventricular tachyarrhythmia.

The pharmacokinetics (PK) of the antiarrhythmic sotalol, which elicits Class III and beta-blocking activity, has not been adequately defined in a pediatric population with tachyarrhythmias. The goal of this single-dose study with administration of sotalol HCl at a dose level of 30 mg/m2 body surface area (BSA) was to define the PK of the drug in the following four age groups: neonates (0-30 days), infants (1 month to 2 years), younger children (> 2 to < 7 years), and older children (7-12 years) with tachyarrhythmias of either supraventricular or ventricular origin. The drug was administered in an extemporaneously compounded syrup formulation prepared from the tablets containing sotalol HCl. For safety, vital signs and adverse events were recorded and the QTc interval and heart rate telemetrically monitored. Scheduled blood samples were taken over a 36-hour time interval following dose administration. The drug concentrations in plasma were measured by a sensitive and specific LC/MS/MS assay. Standard compartment model-independent methods were applied to compute the salient PK parameters of sotalol. Twenty-four clinical sites enrolled 34 patients. Thirty-three had analyzable data. Sotalol was rapidly absorbed, with mean peak concentrations occurring 2 to 3 hours after administration. The elimination of sotalol was characterized by an average half-life of between 7.4 and 9.2 hours in the four age groups. There existed statistically significant linear relationships between apparent total clearance (CL/f) or apparent volume of distribution (V lambda z/f) after oral administration and the covariates BSA, creatinine clearance (CLcr), body weight (BW), or age. The best predictors for CL/f were CLcr and BSA, whereas BW best predicted the V lambda z/f. The total area under the drug concentration-time curve in the smallest children with a BSA < 0.33 m2 was significantly greater than that in the larger children. This finding indicated that the BSA-based dose adjustment used in this study led to a larger exposure in the smallest children, whereas the exposure to the drug was similar in the larger children. The dose of 30 mg/m2 was tolerated well. No serious drug-related adverse events were reported. It can be concluded that the PK of sotalol in the pediatric patients depended only on body size, except for the neonates and smallest infants in whom the disposition of sotalol was determined by both body size and maturation of eliminatory processes.

Population pharmacokinetics and pharmacodynamics of sotalol in pediatric patients with supraventricular or ventricular tachyarrhythmia.

AIMS: To derive useful pharmacokinetic (PK) and pharmacodynamic (PD) information for guiding the clinical use of sotalol in pediatric patients with supraventricular (SVT) or ventricular tachyarrhythmia (VT). METHODS: Two studies were conducted in-patients with SVT or VT in the age range between birth and 12 years old. Both studies used an extemporaneously compounded formulation prepared from sotalol HCl tablets. In the PK study, following a single dose of 30 mg/m2 sotalol, extensive blood samples (n = 10) were taken. The PK-PD study used a dose escalation design with doses of 10, 30, and 70 mg/m2, each administered three times at 8-hr intervals without a washout. Six ECG recordings for determination of QT and RR were obtained prior to the initial dose of sotalol. Four blood samples were collected six ECG's were determined during the third interval at each dose level. Plasma concentrations of sotalol (C) were assayed by LC/MS/MS. The data analysis used NONMEM to obtain the population PK and PD parameter estimates. The individual PK and PD parameters were estimated with empirical Bayes methodology. RESULTS: A total of 611 C from 58 patients, 477 QTc and 499 RR measurements from 23 and 22 patients, respectively, were available for analysis. The PK of sotalol was best described by a linear two-compartment model. Oral clearance (CL/F) and volume of central compartment (Vc/F) were linearly correlated with body surface area (BSA), body weight or age. CL/F was also linearly correlated with creatinine clearance. The best predictor for both CL/F and Vc/F was BSA. The remaining intersubject coefficients of variation (CV's) in CL/F, and Vc/F were 21.6% and 20.3%, respectively. The relationship of QTc to C was adequately described by a linear model. The intersubject CV's in slope (SL) and intercept (E0) were 56.2 and 4.7%, respectively. The relationship of RR to C was also adequately described by a linear model in which the baseline RR and SL were related to age or BSA. The intersubject CV's for SL and E0 were 86.7 and 14.4%, respectively. CONCLUSIONS: BSA is the best predictor for the PK of sotalol. Both QTc and RR effects are linearly related to C. No covariates are found for the QTc-C relation, while the RR-C relation shows age or BSA dependency.

Pharmacokinetic, pharmacodynamic, and safety evaluation of an accelerated dose titration regimen of sotalol in healthy middle-aged subjects.

BACKGROUND: Current labeling recommends that therapy with sotalol be initiated in a monitored setting at 80 mg every 12 hours for 2 to 3 days, followed by 120 to 160 mg every 12 hours for at least 2 days before safety and efficacy can be ascertained and patients discharged. An accelerated titration regimen that shortens hospital stay without compromising patient safety would improve the usefulness of the drug. Although such regimens have been used by clinicians, they have not been formally evaluated. METHODS: Healthy, middle-aged sedentary men and women received sotalol in a double-blind, two-way crossover study with a 2-week washout phase to evaluate an accelerated titration regimen--placebo every 6 hours for four doses, followed by 80 mg sotalol every 6 hours for four doses, then 160 mg sotalol every 12 hours for nine doses--and compare it with the standard titration--placebo alternating with 80 mg sotalol every 6 hours for eight doses, followed by 160 mg sotalol every 12 hours for nine doses. QT intervals, RR intervals, and sotalol concentrations in plasma were measured at specific times throughout the study and during washout in a similar fashion for both regimens. RESULTS: Thirty-four subjects completed both regimens. The target prolongation of QTc (90% of the value achieved at steady state) was achieved 22 1/2 hours sooner with the accelerated titration regimen (P = .0003). There were no cardiovascular adverse events during either loading phase. At no time during the accelerated titration regimen did the sotalol concentrations in plasma or the QTc or RR interval prolongation exceed the values eventually achieved at steady state. The relationship between sotalol concentration and QTc was linear and independent of the regimen. CONCLUSION: The accelerated titration regimen for sotalol can shorten the time to attain the dosage usually required to effectively control arrhythmias, without excessive QT prolongation and the associated increased risk of torsades de pointes. The hospital stay of patients in whom antiarrhythmic therapy with sotalol is initiated can be shortened by 1 day if this accelerated titration regimen is used.

Comparative serum estradiol profiles from a new once-a-week transdermal estradiol patch and a twice-a-week transdermal estradiol patch.

Two identical open-label, randomized crossover studies were conducted to compare serum estradiol profiles from the new 12.5- and 25-cm2 once-a-week adhesive patches with those from the 10- and 20-cm2 commercially available twice-a-week Estraderm patches when applied as directed during a 1-week patch-wear period. Both studies were conducted in healthy postmenopausal women; serum estradiol levels were determined by gas chromatography/mass spectroscopy (GC/MS). Although both sizes of both patch treatments produced mean serum estradiol levels in the therapeutic range, the once-a-week patch provided more constant mean levels, avoiding large peak-to-trough fluctuations. As expected, the differences in mean serum estradiol concentrations between the two patch treatments occurred during the second application of the twice-a-week patch. Based on these results, the once-a-week drug in adhesive patch appears to be an acceptable means of hormone replacement therapy.

Pharmacokinetics of sematilide in renal failure.

A randomized, two-period, two-treatment study was conducted to investigate the effect of renal impairment on the pharmacokinetics of the Class III antiarrhythmic sematilide HCl. The pharmacokinetic-pharmacologic effect relationship and tolerability of sematilide HCl were also studied. The study included 22 subjects: 6 healthy volunteers and 16 patients with various degrees of renal impairment, including functionally anephric patients on intermittent hemodialysis. Separated by a 14-day washout period, the subjects received a constant rate intravenous infusion of 40 mg sematilide HCl over 30 minutes and a tablet containing 100 mg of the drug. The functionally anephric patients were studied during and off dialysis after intravenous and oral administration of the drug, respectively. Blood and urine samples were collected at defined times up to 48 hours and 72 hours, respectively, after administration. Sematilide concentrations in plasma, urine, and dialysate were measured by a validated high-performance liquid chromatography (HPLC) method with ultraviolet detection. The pharmacokinetic data analysis used a compartment model independent approach. The heart rate-corrected Lead II QT interval was recorded as a pharmacologic endpoint. Subjective symptoms, cardiovascular parameters, routine serum chemistry, and hematology and urinalysis parameters were measured to assess tolerability. Mean renal clearance after intravenous and oral administration was reduced in patients with severe renal impairment. Statistically significant linear correlations existed between total clearance of sematilide and creatinine clearance for all subjects who could be evaluated after both intravenous and oral administration. Steady-state volume of distribution, absolute bioavailability, and nonrenal clearance of sematilide were independent of renal function. The mean dialysis clearance was 98 mL/min, indicating effective removal of the drug by hemodialysis. In accord with the drug's Class III pharmacologic activity, the heart rate corrected Lead II QT intervals were prolonged in all subjects after intravenous and oral administration of the drug. The pharmacologic effect to plasma concentration relationship in renal patients and in healthy subjects was comparable. Based on the experimentally determined linear relationship between total clearance of sematilide and creatinine clearance, modified dose regimens for sematilide HCl in patients with renal impairment and functionally anephric patients off hemodialysis were developed.

Kinetics and dynamics of sematilide.

Sematilide HCl is a novel class III antiarrhythmic drug. The goals of this study in volunteers were to determine the pharmacokinetics, effect (QTc interval), and tolerability after intravenous and oral administration of 25 mg of the drug. Plasma and urine concentrations were measured by a specific high-performance liquid chromatography method. Pharmacokinetic data analysis used a compartment model independent approach. An effect on QTc was observed only after intravenous administration, and its relationship to the plasma concentration showed a counterclockwise hysteresis. A semiparametric approach was used to collapse the hysteresis and then evaluate the effect-site-concentration-to-effect relationship. After intravenous and oral administration, 75.1 (6.5)% (mean +/- SD) and 36.0 (11.5)% of the dose was excreted unchanged in urine, respectively. The respective renal clearances were 250 (41) ml.min-1 and 222 (44) ml.min-1. The bioavailability of sematilide was 0.47 (0.15). A maximum percent effect on QTc of 12 (1)% occurred with a delay of 14 min after termination of an intravenous infusion of 10 min. After collapsing the hysteresis, the pharmacokinetic-pharmacodynamic data could be fitted appropriately by a linear model in four subjects and by an Emax model in two subjects. Sematilide HCl was well tolerated.

Single-dose interaction study of diprafenone HCl and propranolol HCl in healthy volunteers.

Using a 3 x 3 Latin Square design, a possible interaction between diprafenone HCl a class IC antiarrhythmic drug with nonspecific beta-antagonist activity and propranolol HCl was investigated in nine young, healthy, caucasian, male volunteers. The volunteers randomly received 3 single-dose treatments: (A) 200 mg DHCl, (B) 80 mg PHCl, and (C) 200 mg DHCl and 80 mg PHCl. Scheduled blood samples were taken and plasma concentrations of both diprafenone and propranolol were measured by sensitive and specific assay methods. Lead II electrocardiogram intervals at rest, heart rate during erect bicycle ergometry, and echocardiographic variables at rest and shortly after exercise were recorded. The data analysis used compartment model independent methods. There was no evidence for a pharmacokinetic interaction between the two drugs. With DHCl, two of the nine subjects showed greatly increased areas under the plasma concentration-time curves and apparent disposition half-lives in the presence and absence of PHCl, indicating that metabolism of diprafenone may be subject to pharmacogenetic polymorphism. There was evidence for a pharmacodynamic interaction between DHCl and PHCl regarding the negative chronotropic effect at rest and during exercise. There was no difference in the pharmacodynamics and tolerability of the three treatments in suspected "poor" and "extensive metabolizers" of DHCl.

Comparative kinetics of sematilide in four species.

The goals of this retrospective study with the novel class III antiarrhythmic sematilide HCI were to investigate: a) whether there existed interspecies correlations and b) whether reliable animal-to-human predictions were possible for the main pharmacokinetics parameters. Information on plasma concentrations after intravenous administration was available in rats, rabbits, dogs, and humans. Except for the rabbit, data on the urinary amounts of drug excreted were also available in these species. The drug concentrations in plasma and urine were assayed by a specific HPLC method with UV or electrochemical detection or liquid scintillation spectrometry. In the interspecies correlations and in the animal-to-human predictions, an allometric approach was used with log-log linear regressions of the pharmacokinetic parameters steady-state volume of distribution (Vss), total clearance (CL), mean residence time (t), and terminal disposition half life (t1/2 lambda z), on body weight. The results showed that significant interspecies correlations exist for the tested in vivo pharmacokinetic parameters for sematilide. Reliable animal-to-human predictions with errors < 60% were found for Vss, CL, t1/2 lambda z, and t. In rats, dogs, and humans, sematilide's renal elimination includes glomerular filtration and net tubular secretion. The relative contributions of the renal mechanisms are similar in all three species studied. There were no important species differences observed in the in vitro determined parameters, fraction unbound in plasma, and blood-to-plasma concentration ratio between rats, dogs, and humans.

Pharmacokinetics of digoxin and main metabolites/derivatives in healthy humans.

Three healthy, young male volunteers received doses of 0.6 and 1.2 mg of specifically labelled [3H]digoxin each by intravenous (i.v.) bolus injection and oral (p.o.) administration in accordance with a randomized four-way crossover design. Plasma, urine, and feces samples were taken over an interval of 144 h after drug administration. Total radioactivity and individual radioactivity assignable to digoxin and its metabolites were measured. After i.v. administration, the mean +/- SD recovery of total radioactivity, as percent of dose, was complete, urine 81.3 +/- 2.0% and feces 17.1 +/- 2.8%. The mean recovery of digoxin and that of its metabolites in urine was digoxin 75.6 +/- 3.0%, dihydrodigoxin 2.8 +/- 1.6%, digoxigenin bisdigitoxoside 1.6 +/- 0.1%, and additional metabolites 1.5 +/- 0.3%. Judging from the metabolite data in urine and considering the 5% impurity of the administered dose, metabolism of digoxin appeared to be insignificant after i.v. administration. The total and renal clearances of digoxin were, on average, 193 +/- 25 ml min-1 and 152 +/- 24 ml min-1. The mean steady state volume of distribution was 489 +/- 73 L and the mean residence time 41 +/- 5 h. For the metabolites dihydrodigoxin and digoxigenin bisdigitoxoside the mean residence times were on average 35 +/- 9 h and 53 +/- 11 h; the renal clearances were 79 +/- 13 ml min-1 and 100 +/- 26 ml min-1. After p.o. administration, the mean recovery of total radioactivity, as percent of the dose, was also complete, urine 65.7 +/- 1.98% and feces 31.6 +/- 7.6%. The mean recovery of digoxin and that of its metabolites, as percent of dose, in urine was digoxin 51.5 +/- 11.4%, dihydrodigoxin 4.5 +/- 3.9%, digoxigenin bisdigitoxoside 1.9 +/- 0.1%, polar metabolites 5.5 +/- 3.8%, and additional metabolites 1.3 +/- 0.6%. After p.o., as compared to i.v. administration, larger amounts of all the metabolites were formed in accordance with first pass metabolism/degradation. Maximum mean plasma concentrations of 4.3 +/- 2.5 ng ml-1 and 9.5 +/- 1.1 ng ml-1 for digoxin were observed at 40 +/- 10 min after p.o. administration of 0.6 and 1.2 mg of the drug. The mean absolute bioavailability of digoxin from an aqueous solution was 0.67 +/- 0.14. Renal clearance and mean oral residence time for digoxin were on average 176 +/- 28 ml min-1 and 37 +/- 4 h after p.o. administration.(ABSTRACT TRUNCATED AT 400 WORDS)

[Developments in drugs and populations at risk]. / Arzneimittelentwicklung und Risikopopulationen.

No therapeutic effectiveness without unwanted side-effects. This well-known problem persists in spite of modern methods for development of new drugs. Therefore no drug-therapy should be initiated without exact risk-benefit-analysis. Especially for drugs introduced recently, effectiveness is better defined than tolerance. In the USA annual costs of 3 million $ due to drug-side-effects have been estimated, indicating an immense socio-economic relevance next to medical or personal aspects. Therefore effective means and strategies should be accepted and used: General reduction of drug consumption, avoidance of polytherapy with respect to known and unknown interaction of side-effects, strict observation of proper indications, amelioration of surveillance for recently introduced drugs and better consideration of special populations at risk.

Integrated plasma and synovial fluid pharmacokinetics of tenoxicam in patients with rheumatoid arthritis and osteoarthritis: factors determining the synovial fluid/plasma distribution ratio.

Single oral doses of 40 mg of the nonsteroidal antiinflammatory drug, tenoxicam, were given to four patients (three with rheumatoid arthritis, one with osteoarthritis). The concentrations of the drug in synovial fluid and plasma were measured by a specific high-performance liquid chromatography method. The unbound fractions of the drug in both fluids were determined at pH 7.4 and 37 degrees C by equilibrium dialysis. The possible influence of the pH on the protein binding was also assessed. The total concentration time curves in plasma and synovial fluid were fitted to linear oral 1 and 2 compartment body models with an additional synovial fluid compartment connected to the central compartment. The unbound fractions of drug in synovial fluid and plasma were on average 0.015 and 0.011, respectively: not significantly different from each other. The protein binding of tenoxicam was pH dependent with increased free fractions at pH values less than 7.4. The average peak concentrations of tenoxicam in plasma and synovial fluid were 4.3 and 1.4 micrograms/ml, respectively. The mean ratio of the areas under the total concentration time curves in synovial fluid and plasma was 0.42, which corresponded to the steady state of equilibrium ratio of the total drug concentrations in the two body fluids. Two hypotheses were tested: hypothesis I assuming that equilibration across the synovial tissue takes place between the unbound, unionized tenoxicam molecules; hypothesis II assuming that equilibration across the synovial tissue is established between the unbound (unionized + ionized) tenoxicam molecules. Based on the available evidence hypothesis II was rejected.

Detection of populations at risk and problem drugs during drug development and in pharmacotherapy.

Rational drug therapy requires knowledge about the ratio of risk (adverse drug reaction) to benefit (therapeutic efficacy) for all drugs to be used in humans. However, with newly marketed drugs, the risk/benefit ratio is usually not sufficiently known. Safety is often less well defined than efficacy. This is the result of the present mode of drug development. Premarketing studies are conducted in comparatively small, homogenous populations over relatively short time intervals and under standardized conditions. Only after marketing are larger, more diversified populations exposed over prolonged times, often under uncontrolled conditions. Adverse drug reactions (ADRs) are the result of either overdosage, or allergic or idiosyncratic reactions. They can be life-threatening or mild. Some of the ADRs are common (greater than 1:10); others are very rare (less than 1:1000). The overall rate of ADR occurrence in ambulatory and hospitalized patients is high enough to have significant socioeconomic consequences. Some of the risk populations can be suspected a priori: elderly, multimorbid patients and patients with compromised drug elimination who may be overdosed if the regimens are not appropriately modified. Some problem drugs may be recognized if they display one or more of the following characteristics: narrow therapeutic index, steep dose-effect relationship, nonlinear kinetics, variable bioavailability, and pharmacogenetically determined kinetics. Other individuals at risk, however, may not be readily identifiable. They develop allergic and idiosyncratic reactions after drug exposure without exhibiting easily recognizable predisposing factors. In order to determine the number of individuals so affected, and the associated drugs as quickly as possible during the developmental process, specific ADR surveillance measures are taken.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative evaluation of equilibrium dialysis methods employing biological and artificial membranes for the determination of protein binding of drugs.

The goal of the study was to investigate comparatively the performance of the conventional equilibrium dialysis method using artificial membranes (AED) and an alternative equilibrium dialysis method employing biological membranes of red blood cells (BED). The following criteria were employed for an assessment of the two methods: (a) mean estimate of the fraction of drug unbound in plasma, (b) precision, and (c) time required for establishing equilibrium dialysis. For this purpose, plasma protein binding data by AED and BED obtained for several compounds in our laboratory were employed. In addition, suitable results of further compounds on the plasma protein binding by AED and on the partitioning in red cell buffer and plasma systems were collected from the literature, allowing a calculation of the plasma protein binding by BED. Plasma protein binding values by AED and BED were available for a total of 22 nonelectrolytic and electrolytic compounds, including the entire possible range of binding values. Plots of the mean plasma unbound fractions as obtained by AED and BED for the compounds studied could be fitted by a straight line with slope and intercept not significantly different from unity and zero, respectively. Also, the precision of the two methods appeared to be similar. However, the times required to reach equilibrium dialysis were significantly different: With BED and AED, this time span ranged between 2 and 45 and 180 and 960 min, respectively. These results indicate that overall the BED method offers a significant advantage over the AED procedure: It is less time consuming and hence possibly more reliable.

Comparative in vivo evaluation of a radioimmunoassay and a chromatographic assay for the measurement of digoxin in biological fluids.

The concentrations of digoxin in plasma and urine samples obtained from three healthy male volunteers, who received 1.2 mg of labeled digoxin perorally and intravenously, were simultaneously measured by a commercially available radioimmunoassay (RIA) and by a combined column thin-layer chromatographic assay (CA). The CA method, previously shown to assay digoxin specifically, was also used to monitor the individual digoxin metabolites. The results of this investigation showed that digoxin was significantly metabolized, particularly after peroral administration. The lower level of sensitivity of the RIA in plasma was 0.4 ng/mL. There were highly significant positive linear correlations between the values of the following parameters of digoxin as obtained by the RIA and CA methods: the concentrations in plasma and urine, the AUCs, and the cumulatively excreted amounts in urine. The two assays did not give completely identical results either with plasma or urine; the slopes of the regression lines deviated from unity in a significant number of cases. However, there was no relationship between the magnitude of the slopes of the regression lines and the extent of metabolism. It was concluded that the commercially available RIA evaluated was specific for digoxin and that the presence of digoxin metabolites did not affect the determinations.