The Bateman function revisited: a critical reevaluation of the quantitative expressions to characterize concentrations in the one compartment body model as a function of time with first-order invasion and first-order elimination.

The Bateman function, A"(e-k(e)t--e-k(a)t), quantifies the time course of a first-order invasion (rate constant ka) to, and a first-order elimination (rate constant ke) from, a one-compartment body model where A" = (gamma Dose)ka/(ka-ke)V. The rate constants (when ka > 3ke) are frequently determined by the "method of residuals" or "feathering." The rate constant ka is actually the sum of rate constants for the removal of drug from the invading compartment. "Flip-flop," the interchange of the values of the evaluated rate constants, occurs when ke > 3ka. Whether -ka or -ke is estimable from the terminal ln C-t slope can be determined from which apparent volume of distribution, V, derived from the Bateman function is the most reasonable. The Bateman function and "feathering" fail when the rate constants are equal. The time course is then expressed by C = gamma Dtk e-kt. The determination of such equal k values can be obtained by the nonlinear fitting of such C-t data with random error to the Bateman function. Also, rate constant equality can be concluded when 1/tmax and the kmin (value of ke at the minimum value of ek(e)tmax/ke plotted against variable ke values) are synonymous or when kmintmax approximates unity. Simpler methods exist to evaluate C-t data. When a drug has 100% bioavailability, regression of Dose/V/C on AUC/C in the nonabsorption phase gives ke no matter what is the ratio of m = ka/ke. Since k(e)tmax = ln m/(m-1), m can be determined from the given table relating m and k(e)tmax. When gamma is unknown, ke can be estimated from the abscissas of intersections of plots of Cmax ek(e)tmax and keAUC, both plotted vs. arbitrary values of ke, and gamma D/V values are estimable from the ordinate of the intersection. Also, when gamma is unknown, ke can be estimated from the abscissas of intersections (or of closest approaches) of ek(e)tmax/ke and AUC/Cmax, both plotted vs. arbitrary values of ke. The C-t plot of the Modified Bateman function, C = Be-lambda 2t-A e-lambda 1t, does not commence at the origin (i.e., when tc = 0 = 0 and when a lag time does not exist).(ABSTRACT TRUNCATED AT 400 WORDS)

Simplified methods for the evaluation of the parameters of the time course of plasma concentration in the one-compartment body model with first-order invasion and first-order drug elimination including methods for ascertaining when such rate constants are equal.

The many limitations in determining the pharmacokinetic parameters of first-order invasion of, and elimination from, the one-compartment body model by the method of residuals or by "feathering" C-t data can be minimized by applying the simplified methods outlined herein. Comparisons of the apparent volumes of distribution, V, calculated on the premises that the Bateman Function represents ka > ke or its converse, ke > ka, i.e., flip-flop, can permit a proper choice of the correct version. Estimation of ke can be obtained by regression of (A0/V)/C(oncentration) on AUCt/C where A0/V is estimable from knowledge of Cmax and tmax since A0/V = Cmax eketmax. The ratio of the magnitude of the rate constant of invasion to that of elimination, m = ka/ke, is related to ketmax by the expression ketmax = ln m/(m - 1) for all possible values of m. A table for the determination of m from values of ketmax is given. When bioavailability, gamma = Ao/Dose, is known or complete, ke and V can be determined from the respective ordinate and abscissa of the intersection of A0/Cmax eketmax and Cl(clearance)/ke, both plotted against arbitrary ke values. The two functions may not intersect at low values of m due to errored C-t values but the ke value when the two curves are closest (kmin) may approximate ke. The intersections of Cmax eketmax and keAUCT (AUCtrap) plotted against variable ke values (Method A) provide estimates of ke from their abscissa values and A/V from their ordinate values when gamma is unknown. Method B appears to give more reliable estimates of ke at the kmin of the difference eketmax/ke - AUCT/Cmax, plotted against ke. Since kmin of this plot is l/tmax when m = 1, the identity of the m as unity underlying the C-t data is indicated when either kmintmax is approximately unity or kmin is practically synonymous with l/tmax. This was clearly shown when 12 constructed m = 1, C-t cases with 10% random error were evaluated by Method B. Better estimates were effected by all procedures when the raw C-t data were smoothed.

Bioavailability of racemic ibuprofen and its lysinate from suppositories in rabbits.

Plasma concentration (C)-time (t) plots of ibuprofen from suppositories in rabbits are characterized by the sum of two exponentials, C = Be-lambda 2t-Ae-lambda 1t. When A exceeds B, there is a lag time (tlag = tc = 0) before the start of absorption. Evidence for dose-dependent area under the curve (AUC) of C versus t values is presented with the implication that elimination rate constants (ke) decrease and the AUC/dose ratio increase with dose. The lack of significant differences between the AUC values from suppository and the intravenous studies with similar doses implies complete absorption of ibuprofen from all the suppositories studied. The absorption rate constants (ka) were estimated on the presumption of complete absorption and dose-dependent elimination. Ibuprofen lysinate was absorbed significantly more readily than the free acid from suppositories. The lysinate suppository with a lipophilic surfactant had a higher absorption rate constant than that with a hydrophilic surfactant. The ka values did not significantly differ for a twofold difference in dose. Equations were developed to calculate true AUC and area under the moment curve (AUMC) values when A exceeds B, and to transform C versus t plots to the origin with A' = A = B. Times of maximum peak heights, mean residence (MRT), and mean absorption times (MAT = 1/ka) of suppositories when A > B are shown to differ by the lag time (tc = 0) from the plots of C versus t transposed to the origin. Although corrected AUC values when A > B are equal to the AUC values of C versus t plots transposed to the origin (A' = A = B), the corrected AUMC values when A > B significantly differ from the AUMC values for the C versus t plots transposed to the origin.

Pharmacokinetics and bioavailabilities of hymecromone in human volunteers.

Specific and ultrasensitive reverse-phase HPLC assays of the choleretic and biliary antispasmodic hymecromone (down to 0.05 ng ml-1) and its glucuronide, using fluorimetric detection, and sulfate metabolites using UV detection, were developed. Sodium salt solutions of 400 mg (over 3 min) and 800 mg (over 5 min) were infused i.v. into 6-8 normal human volunteers. The half-life of the major rate constant averaged 28 +/- 2 min (SE). Subsequently, less than 0.8 per cent of the dose was eliminated with terminal half-lives of 70-359 min. The apparent volume of distribution of hymecromone, referenced to the total plasma concentration, averaged 20.8 +/- 1.41 (Vc, central compartment volume) and 36.4 +/- 2.11 (Vss steady state volume). Hymecromone's total body clearance averaged 1413 +/- 89 ml min-1. The pharmacokinetics of hymecromone were dose-independent. Only 0.3 +/- 0.3 per cent unchanged hymecromone was renally excreted. Mostly dose-independent glucuronidated drug (93 +/- 4 per cent of the dose) was excreted in the urine; a smaller amount was renally excreted as the sulfate (1.4 +/- 0.3 per cent of the dose). The oral bioavailability estimated from the relative areas under the hymecromone plasma concentration-time curves following oral and i.v. administration of hymecromone to six volunteer subjects showed no dose-dependence and was 1.8 +/- 0.6 per cent. However, an anomalous c. 200 per cent of the glucuronide produced by i.v. hymecromone was produced from orally administered hymecromone as determined from the ratio of the AUC values of glucuronide obtained after peroral and i.v. administration of the same dose of hymecromone to demonstrate a high first-pass effect and implicate renal glucuronidation.

High performance liquid chromatographic assays of the illicit designer drug "Ecstasy", a modified amphetamine, with applications to stability, partitioning and plasma protein binding.

Specific, sensitive, reverse-phase high-performance liquid chromatographic (HPLC) assays of 3,4-methylenedioxymethamphetamine (MDMA) and 3,4-methylenedioxyamphetamine (MDA) have been devised with analytical sensitivities as low as 2.7 ng/ml of plasma for MDMA and 1.6 ng/ml for MDA, using spectrophotometric detection at 280 nm. The assays were used to determine some properties of MDMA and MDA. Both drugs were stable in aqueous 1 M HCl, and 1 M NaOH solutions at room temperature. The half-life for MDMA was 6.6 h and for MDA was 7.1 h under the extreme conditions of 90 degrees C and 6 M HCl. MDMA and MDA were highly stable for 28 h in plasma at 25 degrees and 39 degrees C. The concentrations of the drugs were unchanged in frozen plasma after 47 days. The apparent red blood cell-plasma partition coefficient determined from assayed concentrations of the drugs in plasma and erythrocytes was 1.45 for both MDMA and MDA. An equation is presented to correct drug concentration in erythrocytes for the trapped equilibrated plasma/buffer in the packed red blood cells. The fraction of MDMA and MDA bound to dog plasma proteins was determined by several methods and it is 0.34-0.40 for both drugs. The extent of protein binding was independent of the drugs' concentration.

Pharmacokinetics of morphine and its surrogates. XI: Effect of simultaneously administered naltrexone and morphine on the pharmacokinetics and pharmacodynamics of each in the dog.

There were no dramatic modifications of the pharmacokinetics in the dog of i.v. bolus doses of 0.5, 2.7 and 5 mg kg-1 morphine by coadministering i.v. 5 mg kg-1 naltrexone as bolus injections over 15-20s and 12.3 mg kg-1 by continuous infusion. Morphine's terminal half-life, clearances, apparent volumes of distribution (except for that of the central compartment), percentages of drug and conjugated metabolite excreted in urine and bile did not differ significantly by paired t-test (probability (p) greater than 0.05 for rejection of the null hypothesis of no difference) when naltrexone was coadministered. There were no statistically significant (by t-test) modifications of the plasma pharmacokinetics in the dog of i.v. bolus doses of 5 mg kg-1 naltrexone with and without morphine coadministration except for the coefficient of the second (or terminal) exponential of the sum that fitted the plasma concentration-time data of naltrexone. Although morphine coadministration did not significantly affect the terminal half-life of naltrexone, its clearances or apparent volumes of distribution by t-test of the differences between averages (with each dog equally weighted), drug coadministration did significantly (by t-test) affect the fraction of naltrexone dose secreted into bile as conjugate (fB), the fraction of the dose excreted as conjugate in urine, and the fraction excreted elsewhere (f'B). Although naltrexone reversed the central action of morphine in affecting monitored pupil diameters, it did not antagonize the peripheral effects of morphine in perturbing renal and biliary flow rates. This led to a larger fraction of the naltrexone dose being metabolized to conjugate on morphine coadministration. Since less naltrexone conjugate was renally and biliary excreted initially, due to morphine inhibition of the initial renal and biliary processes, naltrexone conjugate plasma concentrations were higher when morphine was coadministered.

Pharmacokinetics of morphine and its surrogates. X: Analyses and pharmacokinetics of buprenorphine in dogs.

Specific and sensitive reverse-phase HPLC assays of buprenorphine and its metabolite in biological fluids were developed with sensitivities of 2-6 ng ml-1 using fluorimetric detection. Pharmacokinetics were monitored on acute bolus administration of buprenorphine in 6 dogs within the 0.7-2.6 mg kg-1 dose range. Toxicity was circumvented when terminal plasma concentrations were increased by infusing 3.7-4.8 mg kg-1 doses of buprenorphine over 3 h in six studies in 6 dogs. The terminal rate constants of the IV infusion studies from the triexponential fits of plasma concentration-time data averaged 41.6 +/- 7.5 h with an averaged total body clearance of 191 +/- 19 ml min-1. This terminal rate constant was in contrast to the less than 100 min half-life of the second exponential fitting of the less lipophilic morphine, naloxone, and naltrexone. The apparent volumes of distribution of buprenorphine, referenced to the total plasma concentration, were 33 +/- 61 (Vc, central compartment volume) and 663 +/- 891 (Vd, total body volume), indicative of a highly bound, sequestered or lipophilic drug. Unchanged buprenorphine was insignificantly renally (less than 0.2 per cent of the dose) and biliary (less than 0.6 per cent) excreted. The major route of buprenorphine disposition was by hepatic conjugation to glucuronide which was eliminated into the bile (about 92 per cent) with only small amounts appearing in urine (less than 1 per cent as metabolite). Minor metabolites excreted in the bile accounted for about 3 per cent of the administered dose. Direct IV administration of the metabolite, buprenorphine glucuronide, gave a terminal half-life of 6 h and more than 90 per cent of the systemically circulating metabolite was excreted in bile; only 10 per cent in urine. The oral bioavailability, estimated from the areas under the buprenorphine plasma concentration-time curve following IV and oral administration of buprenorphine in the dogs, was 3-6 per cent. There were no apparent correlations of the buprenorphine time course with cardiovascular parameters such as heart rate, ECG, and blood pressure. Miotic effect was significant. Respiratory depression was observed during the first 4 h after IV bolus injection, but not during the infusion studies.

Pharmacokinetics of morphine and its surrogates. IX: Discrepancies among infused buprenorphine plasma concentrations sampled from different veins.

When blood was drawn from the brachial vein of the dog upon infusion of buprenorphine hydrochloride in saline through a plastic catheter into the jugular vein, the post-infusion jugular vein plasma concentrations observed during the first 15 min of the post-infusion distributive phase were significantly higher than the highest observed brachial vein plasma concentrations at the time of cessation of infusion. When blood was drawn from the jugular vein following infusion of buprenorphine hydrochloride into the left brachial vein, the post-infusion left brachial plasma concentrations during the first 15 min of the post-infusion distributive phase were significantly higher than the highest jugular and contralateral brachial vein concentrations observed just before and after the cessation of infusion. Dependent on whether the plasma concentrations sampled from the infused catheter or another catheter were used, the apparent calculated total body clearances differed by 25-39%. These results demonstrated that the observed differences in the post-infusion buprenorphine concentrations in plasma obtained from different veins were not due to any drug-induced changes in the circulatory physiology of the dog. Evidence is presented to show that the discrepancies were due to the repartitioning of the catheter-bound drug into the blood drawn through the catheter for assay, which significantly increased the apparent blood concentration of drug, not-withstanding the fact that only an extremely small fraction (0.004-0.009) of a simulated 1-3-h infused dose partitioned into the plastic catheter. When buprenorphine hydrochloride was administered by a bolus injection, there was no significant partitioning of drug into the infusion catheter.(ABSTRACT TRUNCATED AT 250 WORDS)

Superiority of perfusion over bolus administration in cancer chemotherapy: proposition of a compartmental model.

Recent studies indicate that, for cancer chemotherapy, prolongation of drug injection time yields lower toxicity and better efficacy. We propose a compartmental pharmacological model to explain these observations. The model has been simulated on an analog computer. The curves obtained give a credible explanation for the clinical data. The superiority of perfusion over bolus injection may be attributed to plasma peak suppression.

Pharmacokinetics of morphine and its surrogates. VIII: Naloxone and naloxone conjugate pharmacokinetics in dogs as a function of dose and as affected by simultaneously administered morphine.

Reversed-phase HPLC assays with electrochemical detection, developed to quantify naloxone, 6 beta-naloxol, and their hydrolyzed conjugates in biological fluids provided assay sensitivities of 10 to 20 ng/mL in plasma, urine, and bile. These fluids were monitored in dogs after iv bolus administrations of 0.47 and 4.7 mg/kg of naloxone. Plasma concentration-time data were well fitted by the sums of two exponentials with two sequential half-lives of 11 +/- 1 (SEM) and 56 +/- 3 min. Pharmacokinetics were dose-independent; total and renal clearances were 1334 +/- 133 mL/min and 42 +/- 9 mL/min, respectively, with a renal clearance of 65 +/- 5 mL/min for the conjugate. The percentage of the dose excreted in the urine as naloxone was 4.4 +/- 1.0%. There was a possible dose-dependent excretion of conjugate with 46 +/- 1 and 22 +/- 5% of the dose renally excreted at the high and low doses, respectively. In incomplete biliary cannulation, 13 and 18% were collected as conjugate in the bile of two bile-cannulated dogs. There was negligible biliary secretion of unchanged naloxone. Neither 6 beta-naloxol nor its conjugates were metabolites of naloxone in dogs. The simultaneous administration of naloxone does not reverse the dose-dependent pharmacokinetic perturbations of morphine. Morphine significantly lessened its own body, renal, and biliary clearances, as well as those of naloxone, and also lowered their apparent overall volumes of distribution. Plasma levels of naloxone and its conjugate were elevated with simultaneous morphine administration. Urinary flow rates were also greatly lessened and initial renal shut-down was implied at the higher morphine dose. Thus, the established competitive antagonistic action of naloxone on morphine does not extend to the reversal of the biological feedback effects of morphine on the metabolism and excretion of itself and simultaneously administered naloxone.

Pharmacokinetics of morphine and its surrogates. VII: High-performance liquid chromatographic analyses and pharmacokinetics of methadone and its derived metabolites in dogs.

Reversed-phase HPLC assays with on-column UV detection and post-column fluorescent detection of ion pair-extracted material were developed and used for the quantitative assay of methadone, its presumed metabolites, and acid- and alkali-hydrolyzable conjugates of these metabolites in biological fluids with assay sensitivities of 10-20 ng/mL. Plasma, urine, and bile were monitored in dogs after intravenous bolus administration of 0.8, 1.0, 2.0, and 2.2 mg/kg methadone hydrochloride. Plasma-time data showed two sequential half-lives of 8.3 +/- 3.4 (SEM) and 128 +/- 37 min, with apparent dose-independent pharmacokinetics in the studied dose range. Total body clearances were 899 +/- 103 (SEM) mL/min. Renal clearances (6-82 mL/min) of methadone were highly variable within and among studies but showed no significant variation with urinary pH or flow rate. The percentages of the dose excreted in the urine as methadone and (+)-2-ethyl-1,5-dimethyl-3,3-diphenylpyrroline (2) were 3.6 +/- 0.5% (SEM) and 4.1 +/- 0.4% (SEM), respectively, but there were no significant concentrations of 2 in plasma. The presumed metabolites 2-ethyl-5-methyl-3,3-diphenyl-1-pyrroline (3), 1,5-dimethyl-3,3-diphenyl-2-pyrrolidone (4), (-)-alpha-N-normethadol (7), 4-dimethylamino-2,2-diphenylvaleric acid (8), p-hydroxymethadone (9), and (-)-alpha-methadol (10) were not observed in the plasma of dogs given methadone. Quantities of presumed metabolites 3, 4, 7, 8, 9, and 10 were negligible in urine (less than 0.03% of dose). No acid-hydrolyzable conjugates, or generators on acidification, of 3, 4, 7, 8, or 10 were detectable in urine. No alkali-hydrolyzable conjugates, or generators on alkalinization, of 3, 4, 8, or 10 were detectable in urine. There was no significant biliary secretion of unchanged methadone; 2 in bile amounted to only 2% of the dose. In bile and urine, 50% and 17-27%, respectively, of the radiolabeled dose was not extractable into hexane. In a non-bile-cannulated dog, 35% of the total radiolabeled intravenous dose was present in the feces. As much as 88% of an intravenous radiolabeled dose could be accounted for, even though only small amount of methadone was disposed through the metabolic routes claimed in the literature. The intravenous administration of 2 resulted in two sequential half-lives of 3 and 270 min and no apparent pharmacokinetic dose dependency. Amounts of 2 excreted unchanged in urine and bile were 23% and 5-16% of the dose, respectively. Renal and total body clearances were 170 and 1150 mL/min.(ABSTRACT TRUNCATED AT 400 WORDS)

Rho Chi Lecture Award. Is it possible to teach scientific creativity in the pharmaceutical sciences? Anecdotes and memorabilia.

Finally, and what may be a fitting end for this recounting, there is a quotation of Francis Bacon's that gives a poetic interpretation of creativity in the scientist: "A scientist is neither an 'ant,' storing what it finds lying about ready-made, nor a 'spider,' spinning a web out of what its entrails secrete. He is a bee, visiting innumerable flowers and collecting the nectar it finds in them; but storing not this nectar in its crude state but the honey into which it turns."

Pharmacokinetics of morphine and its surrogates VI: Bioanalysis, solvolysis kinetics, solubility, pK'a values, and protein binding of buprenorphine.

The 10-fold greater sensitivity of buprenorphine to fluorescence compared with morphine provides excellent detection for HPLC assay of buprenorphine in biological fluids with a 5-ng/mL sensitivity. Buprenorphine yields a stoichiometric final acid degradation product, a fluorescent-detectable, rearranged demethoxy analogue of buprenorphine, which serves as an excellent bioassay internal standard. Buprenorphine solvolysis is specific-acid and specific-base catalyzed. Alkaline hydrolysis produces no fluorescent products. Acid hydrolysis also produces a fluorescent-detectable, transient dehydro intermediate that is also completely transformed to the demethoxy analogue. The rate constants and Arrhenius parameters for these transformations have been determined. Estimated buprenorphine pK'a values are 8.24 and 10 for the ammonium and phenol groups, respectively. The intrinsic aqueous solubility of neutral buprenorphine is 12.7 +/- 1.2 micrograms/mL at 23 degrees C. The red blood cell-plasma water partition coefficients of buprenorphine ranged between 6 and 15. Ultracentrifugation and the red blood cell partition methods led to an estimated 95-98% plasma protein binding. Ultrafiltration and equilibrium dialysis methods were inappropriate because of the high membrane binding of neutral buprenorphine.

Disposition of etofibrate, clofibric and nicotinic acid esters, and their products in dogs.

Etofibrate, the ethylene glycol diester of clofibric and nicotinic acids, on intravenous infusion into dogs, has a terminal half-life of 2 min. The intermediate half-esters, the nicotinate and the clofibrate, have respective terminal half-lives of 4.6 and 1.7 min and appear fleetingly when etofibrate is administered. In contrast to the 42-h terminal half-life of clofibric acid, the other final transformation product, nicotinic acid, shows saturable or dose-dependent pharmacokinetics in dogs that conform to the Michaelis-Menten equation with a terminal half-life of 4.4 min at low concentrations (less than 6.9 microM/kg). Three distinct metabolites of nicotinic acid can be identified and assayed chromatographically in the urine. The partition properties were similar to nicotinic acid. Nicotinic acid is excreted 30% unchanged into urine with a renal clearance of 70 mL/min in 27-kg dogs.

Pharmacokinetics of morphine and its surrogates V: Naltrexone and naltrexone conjugate pharmacokinetics in the dog as a function of dose.

Sensitive reversed-phase HPLC assays with electrochemical detection, developed to quantify naltrexone, 6 beta-naltrexol, and their conjugates in biological fluids, provided assay sensitivities of 2-14 ng/mL in plasma, urine, and bile. Plasma, urine, and bile were monitored in dogs after bolus administrations of 0.5 and 5.0 mg/kg iv naltrexone hydrochloride. Plasma-time data showed two sequential half-lives of 5 +/- 1 (SEM) and 47 +/- 5 min. Pharmacokinetics were dose-independent; total and renal clearances were 1043 +/- 98 mL/min and 72 +/- 11 mL/min, respectively, with a similar renal clearance (85 +/- 12) for the conjugate. The percentages of the dose excreted in the urine as naltrexone and its conjugate were 7 +/- 1% and 58 +/- 3%, respectively, with the remainder being excreted in the bile as conjugates. As much as 36% was collected as conjugate in the total bile of the bile-cannulated dog. There was no biliary secretion of unchanged naltrexone. The conjugate was apparently enterohepatically recirculated. 6 beta-Naltrexol is not a metabolite of naltrexone in dogs. Within the limits of analytical detection (2 ng/mL) neither 6 beta-naltrexol nor its conjugates appeared in any monitored biological fluids when such fluids were assayed quickly after sampling.

Electrochemical chromatographic determinations of morphine antagonists in biological fluids, with applications.

The morphine antagonists naltrexone and naloxone were extracted from plasma and urine, separated on a chromatographic column, and assayed by electrochemical detection. Optimum oxidation potentials were 0.65 V for morphine and 0.75 V for naloxone and naltrexone. Assay sensitivities were 2-5 ng/mL for plasma and 10 ng/mL for urine. The assays were applied to determine red blood cell partition coefficients of 1.83 +/- 0.15 (SD) for naltrexone and 1.49 +/- 0.27 (SD) for naloxone in a concentration range of 10-3500 ng/mL. No significant time dependence for the partitioning could be observed. Plasma protein binding in the same concentration range, determined by ultracentrifugation, was 27.7% +/- 2.5% (SD) for naltrexone and 30.1% +/- 5.1% (SD) for naloxone. The degree of protein binding did not change in the presence of morphine for morphine-antagonist ratios between 1:10 and 10:1. No concentration dependencies of red blood cell partitioning or protein binding were observed.

Bioanalyses and pharmacokinetics of nafronyl in the dog.

Improved specific and sensitive reverse-phase HPLC assays of nafronyl (I) and its acidic metabolite and hydrolysis product (II) in biological fluids were developed with sensitivities of 3-6 ng/mL using fluorometric detection with 225 nm excitation and 330 nm emission wavelengths. There were no significant differences in the stabilities and assays of I and II in plasma obtained using heparin, citrate phosphate dextrose solution, EDTA, citrate, or oxalate as anticoagulant. Inordinately high membrane binding did not permit the quantification of the high plasma protein binding of I by ultrafiltration; its instability precluded the use of equilibrium dialysis. Plasma protein binding of II by ultrafiltration was 76.4% and was not concentration dependent. The apparent red blood cell-plasma partition coefficients for I and II were 2.00 and 0.49, respectively, with almost all anticoagulants; the red blood cell-plasma water partition coefficient for II was 2.08 when corrected for plasma protein binding. Thus, both I and II had erythrocyte binding sites in addition to simple volume partitioning. Only heparin-treated blood gave anomalously low erythrocyte-plasma partition coefficients, indicating that heparin inhibited the partitioning of I and II into red blood cells from plasma water. The total body clearance of nafronyl (I) referenced to total plasma concentration [1295 +/- 65 (SEM) mL/min] was dose independent (35-70-mg range) and showed biphasic plasma half-lives (intravenous) of 12 and 100 min. Only 34% of the nafronyl appears as systemically circulating II in the plasma. Apparent volumes of distribution similarly referenced were 39.8 and 163 L for the central compartment and total body, respectively. Renal clearances referenced to total plasma concentration were 8.3 and 0.18 mL/m for I and II, respectively. The respective total urinary excretions of I, II, and the glucuronide of II (III) were 0.48, 0.021, and 0.32% of the administered intravenous doses. The respective total urinary excretions of I, II, and III for a bile-cannulated dog were 0.005, 0.16, and 0.40%. The total body clearance of intravenously administered II was 225 mL/min, with a renal clearance of 0.057 mL/min referenced to total plasma concentration. The respective total urinary excretions of II and III were 0.027 and 0.44% of the intravenous dose of II. Respective plasma half-lives of II (intravenous) were 2.5, 10.9, and 225 min. The apparent volume of distribution referenced to total plasma concentration was 2.2 L (9.1 L referenced to plasma water concentration). The apparent overall volume of distribution referenced to plasma concentration was 73 L.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasmolysis, red blood cell partitioning, and plasma protein binding of etofibrate, clofibrate, and their degradation products.

Etofibrate (I), the ethylene glycol diester of clofibric and nicotinic acids, degrades almost equally through both half-esters with half-lives of approximately 10 and 1 min in fresh dog and human plasma, respectively. The nicotinate V degrades with half-lives of approximately 12 hr and 50 min in fresh dog and human plasma, respectively. Ester III and clofibrate VI degrade by saturable Michaelis-Menten kinetics in fresh human plasma, with similar maximum initial rates and respective terminal first-order half-lives of 12 and 26 min. Tetraethyl pyrophosphate at 100 micrograms/ml inhibited human plasma and red blood cell esterases permitting plasma protein binding and red blood cell partitioning studies. The red blood cell-plasma water partition coefficient was 5.4 for 0.2-80 micrograms/ml of I. Clofibrate (VI) showed a saturable erythrocyte partitioning that decreased from 7.8 (10 micrograms/ml) to 1 (50 micrograms/ml). The strong binding of I and VI to ultrafiltration membranes necessitated the determination of their plasma protein binding by the method of variable plasma concentrations of erythrocyte suspensions to give 96.6% (0.2-80 micrograms/ml) and 98.2% (13.6-108.4 micrograms/ml) binding, respectively. Methods for the determination of the parameters of saturable and nonsaturable plasma protein binding for unstable and membrane-binding drugs by the method of variable plasma concentrations in partitioning erythrocyte suspensions are presented.