C-Glucuronidation of the acetylenic moiety of ethchlorvynol in the rabbit.

An acetylenic C-glucuronide of the sedative-hypnotic drug ethchlorvynol was isolated from rabbit urine as the major metabolite. The C-glucuronide represents a novel metabolic pathway for acetylenes and is a rare example of the formation of a carbon-glucuronide bond in mammalian systems.

Salicylate clearance, the resultant of protein binding and metabolism.

Steady-state plasma salicylate concentrations and protein binding were examined in 9 normal subjects to determine relationships among daily dose, total and unbound salicylate concentrations, and total and unbound clearances. Aspirin doses ranging from 0.66 to 4.0 mg/kg/hr were given to steady state. Free and total salicylate concentrations were measured with spectrophotometric, fluorimetric, and equilibrium dialysis techniques. Although unbound clearance decreased over the therapeutic range, total clearance was unchanged. The former is a consequence of saturable metabolism; the latter, of saturable plasma protein binding as well as saturable metabolism. The fraction unbound increased linearly with unbound concentration. Clearance determined at 1.8 mg/kg/hr was used to predict levels obtained at higher aspirin doses. Analysis of residuals was used to ascertain the accuracy of the prediction. The coefficient of variation from prediction among subjects was found to be +/- 14%. It is concluded that, in normal subjects, salicylate clearance changes relatively little over the therapeutic range because the increasing fraction unbound compensates for decreasing clearance of unbound drug.

Hemodialysis of phenytoin in a uremic patient.

Removal of phenytoin by hemodialysis was determined in a uremic patient. The rate of appearance of the drug in dialysate, the plasma concentration with time, and the plasma clearance by dialysis were measured. Plasma protein binding of phenytoin was also determined. In spite of greatly reduced plasma protein binding in the uremic patient, removal rate was observed to be less than 10% of the rate of presentation of the dialyzer. During the 6-hr period of dialysis, the plasma concentration showed little change. The amount collected in the dialyase, 43.6 mg, was only a small fraction of drug in the body. These results indicate that replacement of phenytoin based on the amount of drug removed by dialysis is unnecessary in chronically dialyzed uremic patients. In addition, the utility of hemodialysis in phenytoin overdose is questioned.

Teicoplanin pharmacokinetics and bioavailability during peritoneal dialysis.

The pharmacokinetics of teicoplanin in serum and dialysate were examined in a crossover study after intravenous and intraperitoneal administration of a 3 mg/kg dose to five anuric patients who were undergoing continuous ambulatory peritoneal dialysis (CAPD). Blood and dialysate samples were obtained for 30 and 15 days, respectively, and were assayed microbiologically. The principal pharmacokinetic parameters after intravenous administration were as follows: total body clearance, 2.76 +/- 1.08 ml/min; elimination half-life, 377 +/- 109 hours; volume of distribution at steady state, 1.04 +/- 0.18 L/kg. Only 9% +/- 6% of the intravenous dose was recovered in the dialysate and the net peritoneal clearance was 0.25 +/- 0.21 ml/min. Bioavailability values, which were assessed by use of three methods after intraperitoneal administration and while dialysate was retained in the peritoneal cavity for 5 hours (dwell time), were 0.77 +/- 0.21, 0.78 +/- 0.05, and 0.76 +/- 0.08. Changes in bioavailability with dwell time were also examined. A dosing guide, which accounts for changes in bioavailability with dwell time, is presented.

Pharmacokinetic perspectives on megadoses of ascorbic acid.

Ascorbic acid (vitamin C) is commonly used as a dietary supplement, often in megadoses. However, as the daily oral dose is increased, the concentration of ascorbic acid in the plasma and other body fluids does not increase proportionally, but instead tends to approach an upper limit. For example, when the daily dose is increased from 200 to 2500 mg (from 1.1 to 14.2 mmol) the mean steady state plasma concentration increases only from approximately 12 to 15 mg/L (from 68.1 to 85.2 mumol/L). Published data were reanalyzed with an integrated modeling approach to shed new quantitative light on this phenomenon. This analysis is based on the renal clearance of ascorbic acid, which rises sharply with increasing plasma concentrations as a result of saturable tubular reabsorption. The analysis indicates that both saturable gastrointestinal absorption and nonlinear renal clearance act additively to produce the ceiling effect in plasma concentrations. As a consequence of this ceiling effect, there is no pharmacokinetic justification for the use of megadoses of ascorbic acid.

Renal function and therapeutic concentrations of phenytoin.

The plasma protein binding of phenytoin was studied in 40 adult patients with varying degrees of renal function impairment. The patients had stable renal function and no other condition known to alter phenytoin binding. Binding was measured by equilibrium dialysis, and the apparent affinity constant was calculated using the binding data and the measured serum albumin concentration. The apparent affinity of the drug decreased with a decrease in renal function. The decrease was most apparent in patients with creatinine clearances below 25 ml/min. The importance of altered binding on the therapeutic range is discussed, and a method of calculating equivalent therapeutic phenytoin concentrations from serum albumin and renal function is described.

First-pass elimination. Basic concepts and clinical consequences.

First-pass elimination takes place when a drug is metabolised between its site of administration and the site of sampling for measurement of drug concentration. Clinically, first-pass metabolism is important when the fraction of the dose administered that escapes metabolism is small and variable. The liver is usually assumed to be the major site of first-pass metabolism of a drug administered orally, but other potential sites are the gastrointestinal tract, blood, vascular endothelium, lungs, and the arm from which venous samples are taken. Bioavailability, defined as the ratio of the areas under the blood concentration-time curves, after extra- and intravascular drug administration (corrected for dosage if necessary), is often used as a measure of the extent of first-pass metabolism. When several sites of first-pass metabolism are in series, the bioavailability is the product of the fractions of drug entering the tissue that escape loss at each site. The extent of first-pass metabolism in the liver and intestinal wall depends on a number of physiological factors. The major factors are enzyme activity, plasma protein and blood cell binding, and gastrointestinal motility. Models that describe the dependence of bioavailability on changes in these physiological variables have been developed for drugs subject to first-pass metabolism only in the liver. Two that have been applied widely are the 'well-stirred' and 'parallel tube' models. Discrimination between the 2 models may be performed under linear conditions in which all pharmacokinetic parameters are independent of concentration and time. The predictions of the models are similar when bioavailability is large but differ dramatically when bioavailability is small. The 'parallel tube' model always predicts a much greater change in bioavailability than the 'well-stirred' model for a given change in drug-metabolising enzyme activity, blood flow, or fraction of drug unbound. Many clinically important drugs undergo considerable first-pass metabolism after an oral dose. Drugs in this category include alprenolol, amitriptyline, dihydroergotamine, 5-fluorouracil, hydralazine, isoprenaline (isoproterenol), lignocaine (lidocaine), lorcainide, pethidine (meperidine), mercaptopurine, metoprolol, morphine, neostigmine, nifedipine, pentazocine and propranolol. One major therapeutic implication of extensive first-pass metabolism is that much larger oral doses than intravenous doses are required to achieve equivalent plasma concentrations. For some drugs, extensive first-pass metabolism precludes their use as oral agents (e. g. lignocaine, naloxone and glyceryl trinitrate).(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatic binding and Michaelis-Menten metabolism of drugs.

Certain drugs with metabolism that obeys Michaelis-Menten kinetics are extensively bound in the liver. During the initial distribution phase after a single dose, the binding sites act as a "sink" and compete with the metabolizing enzymes for the drug. After this phase is completed, the bound sites act as a source of drug for the enzymes. Computer simulations of a perfused liver system, with well-stirred reservoir and hepatic compartments, were performed to assess whether or not such binding, as measured by the partition coefficient (Kp) between the liver and the emergent venous blood, affects the tendency to saturate metabolism. Metabolism was assumed to follow Michaelis-Menten kinetics and only unbound drug was assumed to have access to the enzymes. The value of Kp was varied to determine the effect of binding (instantaneous equilibrium) on the tendency to saturate metabolism. The effect of binding rate was also determined by adjusting the association and dissociation rate constants while maintaining a constant value of the equilibrium partition coefficient. Input into reservoir and liver were done to simulate "intravenous" and "oral" dosing, respectively. The average clearance (dose divided by the area under the reservoir concentration-time curve) of the "intravenous" dose increased and the bioavailability of the "oral" dose decreased when the value of Kp was increased, indicating that the tendency to saturate metabolism was reduced by hepatic binding. This effect diminished as the binding rate constants were made smaller, but was still substantial, when association was slower than metabolism.

Biliary secretion of methotrexate in rats and its inhibition by probenecid.

The biliary secretion of methotrexate was investigated in rats under steady-state conditions. The transport system involved was saturable and displayed Michaelis-Menten-type kinetics. Values for the maximal rate of transport and a transport constant analogous to the Michaelis constant were 12 mg/hr and 32 mg/liter (7 x 10-5 M), respectively. The inhibition of this transport mechanism by probenecid also was investigated, and the relationship between the plasma concentration of probenecid and the biliary clearance of methotrexate was elucidated. The value of Ki, the dissociation constant for the transport carrier-inhibitor complex, was 23 mug/ml (8 x10-5 M).

Pharmacokinetics in the rat and the dog of a nonionic nitroxide contrast agent for magnetic resonance imaging.

Nitroxides are paramagnetic stable free radicals that have demonstrated effectiveness as contrast agents in proton magnetic resonance imaging (MRI). The pharmacokinetics and metabolic fate, determinants of the time course of MRI contrast enhancement, of a new nonionic pyrrolidine nitroxide derivative, TAP (2,2,5,5-tetramethylpyrrolidine-1-oxyl-3-carbonic acid-(2,3-dihydroxy-1-hydroxymethyl)-amide), were evaluated in the rat and the dog. A biexponential decline of the blood concentration of TAP was observed in both species after intravenous administration of 0.1- and 2.5-mmol/kg doses. The clearances in the rat, estimated after the low and high doses (15.4 +/- 2.0 and 15.3 +/- 1.4 mL/min.kg, respectively), were about twofold higher than those observed in two beagles (7.4 and 7.1 mL/min.kg for Dog #1 and 6.3 and 6.0 mL/min.kg for Dog #2). The hydroxylamine of TAP, formed by a one-electron reduction of the nitroxide moiety, was the only metabolite observed. This bioreduction of TAP has implications for its use as a MRI contrast agent because the diamagnetic hydroxylamine lacks contrast enhancing activity. In both animal species, the urinary recoveries of the dose as unchanged TAP and its hydroxylamine were essentially complete for the 24-h urine collections (92 to 98% and 83 to 95% for the rats and the dogs, respectively). Anticipated conjugative metabolism of the hydroxyl-containing side chain of TAP was not observed. Renal excretion of unchanged TAP was the predominant route of elimination, as renal clearance was estimated to be between 47 and 89% of total clearance in the dogs. Bioreduction in vivo was slower than that expected from the observed reduction of TAP in vitro in ascorbic acid solution and in rat liver and kidney homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Dependence of renal clearance on urine flow: a mathematical model and its application.

A mathematical model is developed to explain the dependence of renal clearance on urine flow rate. The model is tested using human data from the literature on compounds that are neither secreted nor reabsorbed by active or pH-sensitive mechanisms. The physiologically derived model explains and predicts the relationship between renal clearance and urine flow for a broad spectrum of compounds (i.e., butabarbital, chloramphenicol, creatinine, ethanol, theophylline, and urea) for which appropriate data are available.

Glucocorticoid-dextran conjugates as potential prodrugs for colon-specific delivery: hydrolysis in rat gastrointestinal tract contents.

Chronic colitis, e.g., ulcerative colitis and Crohn's disease, is presently treated with glucocorticoids and other antiinflammatory agents. Side effects limit chronic glucocorticoid therapy. The dose, and consequently the side effects, may be reduced by using prodrugs that selectively deliver drug to the colon. We previously synthesized glucocorticoid-dextran conjugates in which dexamethasone and methylprednisolone were attached to dextran (weight-average molecular weight = 72,600) using dicarboxylic acid linkers (succinate and glutarate). In the present study, the hydrolysis kinetics of the hemiesters (hemiester = glucocorticoid+linker) and dextran conjugates were determined after incubation at 37 degrees C in diluted luminal contents of the gastrointestinal (GI) trace of male Sprague-Dawley rats. The hemiesters were rapidly hydrolyzed in the proximal small intestine (e.g., dexamethasone-hemiglutarate t1/2 = 0.5 h). This rate decreased progressively down the GI tract (t1/2 = 4.8, 54, and 68 h in distal small intestine, cecum, and colon, respectively). The enzyme responsible for hemiester hydrolysis, apparently a type-A alkaline carboxylesterase, is probably of host origin because its activity is highest in the small intestine where bacterial count is low. The dextran conjugates resisted hydrolysis in upper GI tract contents but were rapidly degraded in cecal and colonic contents where the bacterial count is high. The dextran conjugate tested, methylprednisolone-succinate-dextran, was easily hydrolyzed by an endodextranase, indicating that substrate specificity is not lost upon the attachment of glucocorticoid. The results of this study indicate that dextran conjugates may be useful in selectively delivering glucocorticoids to the large intestine for the treatment of colitis.

Nonliner tissue disposition: salicylic acid in rat brain.

A model was developed to detect nonlinear disposition of a drug in a tissue. The model was experimentally tested relating to salicylic acid disposition in the brain. Experimental data obtained in rats are reported for doses of 25 and 40 mg/kg ip. The parameters measured for each dose were the ratio of the area under the brain concentration-time curve to the area under the plasma concentration-time curve and the ratio of the maximum brain concentration of salicylic acid to the plasma concentration at that point in time. The ratios increased with dose; furthermore, ratios calculated using plasma concentrations corrected for plasma protein binding were dose dependent. Calculations performed on literature data for salicylic acid disposition in mouse brain corroborated the results of this sutdy. The existence of a saturable transport system for the elimination of salicylic acid from the brain is supported by the data presented. The rationale necessary to apply the model to any tissue is discussed.

Human erythrocyte membrane permeability and nitroxyl spin-label reduction.

Nitroxyl spin labels are paramagnetic compounds that have demonstrated utility as contrast enhancing agents in proton magnetic resonance imaging. The time-course of contrast enhancement depends on distribution and elimination of these agents. Reduction, resulting in formation of the diamagnetic hydroxylamine, is the major metabolic pathway observed in vivo. This bioreduction has implications for the design of contrast agents and for understanding their imaging behavior. Bioreduction has been shown to occur, at least in part, intracellularly. As such, cell membrane permeability to nitroxyl spin labels may influence their bioreduction. In this study, this influence was examined using eight nitroxyl derivatives and the human erythrocyte suspension as a model biomembrane system. Ionizable weak acids and bases were found to equilibrate rapidly across the erythrocyte membrane with half-times of equilibration ranging from less than 10 s to 1.6 min. These derivatives had low octanol:buffer distribution coefficients and were extensively ionized at the pH of the system (7.0). A strong acid, a phosphate ester, and a quaternary amine derivative were excluded by the cell membrane. Reduction of nitroxyl spin labels by the erythrocyte was shown to occur intracellularly. Except for the impermeable probes, the reduction rate was slow in comparison with the membrane penetration rate. The structural dependence of reduction rate was unrelated to penetration rate but correlated well with that observed in other reducing systems, namely, ascorbic acid solution and rat tissue homogenates.

Pharmacokinetic concepts in assessing intake of pentachlorophenol by rats after exposure through drinking water.

The objective of this study was to predict concentrations of a toxicant in plasma after exposure to the toxicant through drinking water using basic pharmacokinetic principles. As an example, we studied pentachlorophenol (PCP), a widely used wood preservative of public health concern as an environmental pollutant. We added PCP to the drinking water (30 micrograms/mL) of five rats for 3 days. Blood was sampled, and water consumption was monitored every 12 h on the days 1 and 2 and every 3 h on day 3. After a 4-day washout, a PCP dose of 2.5 mg/kg was given intravenously, and blood was withdrawn at selected times for 2 days. PCP concentrations in plasma were measured by capillary gas chromatography. A one-compartment model with zero-order input and kinetic parameters (clearance, volume of distribution, and bioavailability) estimated after intravenous administration adequately predicted PCP concentrations in plasma during exposure to PCP. The average steady-state concentration (Css), which reflects the overall exposure, was predicted using the clearance (CL) concept [i.e., Css = (bioavailability.rate of intake)/CL] and compared with the observed value. The data for PCP demonstrate the potential utility of CL and other kinetic concepts in assessing exposure to a toxicant in drinking water, food, or air.

Volume shifts and protein binding estimates using equilibrium dialysis: application to prednisolone binding in humans.

Sizable volume shifts can occur during equilibrium dialysis. This net movement of water, presumably caused by the osmotic effect of plasma proteins, reduces the concentration of binding proteins. In this paper the theory of protein binding estimation is extended, equations are developed for calculating the unbound and bound drug concentrations at dialysis equilibrium by correcting for the dilution of the proteins, and the equations are applied to a study of prednisolone. To demonstrate the importance of correcting for the volume shift, the parameters of a model in which prednisolone binds to corticosteroid-binding globulin, a protein with a limited capacity, and albumin were estimated. Unbound and bound concentrations were determined by correcting for both volume shifts (average 31%) and loss of drug to the buffer side, by correcting only for loss of drug to buffer side, and by making no correction at all (the usual method of treating equilibrium dialysis data). The error introduced by neglecting volume shifts was analyzed by comparing the parameter values obtained using the three methods. The results confirm the need to adjust for volume shifts and imply that reported binding constants obtained by equilibrium dialysis may be in error for many substances.

Concurrent determination of hepatic bioavailability of salicylamide by three techniques in the dog.

Three methods of measuring hepatic first-pass metabolism of salicylamide in dogs that had undergone portacaval transposition were compared. The drug in both its radiolabeled (0.74 MBq) and unlabeled (20 mg/kg) forms was infused concurrently into forelimb and hindlimb veins, respectively. Because of the transposition, drug from the hindlimb is subject to first-pass metabolism in the liver. Bioavailability is a complementary measure of the extent of this metabolism. The three methods of determining bioavailability were continuous withdrawal of blood to determine the ratio of the areas under the plasma concentration versus time curves, ratio of specific activities in plasma after all the drug had been administered, and the conventional method, measurement of the ratio of areas determined from sequential plasma concentrations. The three techniques were found to give virtually identical values for bioavailability. Each method has its own advantages, limitations, and possible applications. The continuous withdrawal technique is potentially most applicable for drugs with short half-lives. The ratio of specific activities may be the preferred method for drugs with long half-lives. The conventional method is limited by the number of samples needed, but is potentially useful under those conditions in which data following test and intravenous routes of administration are available.

Factors affecting nitroxide reduction in ascorbate solution and tissue homogenates.

Because of their paramagnetic properties, nitroxides are potentially useful as contrast agents in magnetic resonance imaging (MRI). They are reduced in vivo to their corresponding hydroxylamines which are nonparamagnetic and have no contrast enhancing property. Nitroxides with high resistance to reduction would be advantageous as pharmaceutical contrast enhancing agents. We show that in the presence of ascorbic acid and in tissue homogenates, the reduction is faster for piperidine than for pyrrolidine nitroxides and for positively-charged than for negatively-charged derivatives. The data also suggest that nitroxide reduction in tissue homogenates is mainly due to sulfhydryl groups on proteins and that endogenous ascorbic acid plays a relatively minor role.

Enhanced MRI of tumors utilizing a new nitroxyl spin label contrast agent.

Nitroxyl spin labels have been shown to be effective in vivo contrast agents for magnetic resonance imaging (MRI) of the central nervous system, myocardium, and urinary tract. A new pyrrolidine nitroxyl contrast agent (PCA) with better resistance to in vivo metabolic inactivation than previously tested agents was studied for its potential to enhance subcutaneous neoplasms in an animal model. Twenty-two contrast enhancement trials were performed on a total of 15 animals 4-6 weeks after implantation with human renal adenocarcinoma. Spin echo imaging was performed using a .35 T animal imager before and after intravenous administration of PCA in doses ranging from 0.5 to 3mM/kg. The intensity of tumor tissue in the images increased an average of 35% in animals receiving a dose of 3 mM/kg. The average enhancement with smaller doses was proportionately less. Tumor intensity reached a maximum within 15 min of injection. The average intensity difference between tumor and adjacent skeletal muscle more than doubled following administration of 3 mM/kg of PCA. Well-perfused tumor tissue was more intensely enhanced than adjacent poorly perfused and necrotic tissue.

Transperitoneal movement and pharmacokinetics of cefotiam and cefsulodin in patients on continuous ambulatory peritoneal dialysis.

The kinetics of cefotiam and cefsulodin were studied in plasma and dialysate after intravenous and intraperitoneal administration of 1 g to patients undergoing continuous ambulatory peritoneal dialysis. Instillation of autologous hemoglobin as a marker permitted calculation of the cavity volume and, hence, the rate of transfer to and from the peritoneal cavity with time. The patients were divided into 4 groups. Groups 1 and 2 were intravenously given cefotiam (5 patients) and cefsulodin (4 patients), respectively. Groups 3 and 4 (5 patients each) were given cefsulodin intraperitoneally. Group 3 did not have peritonitis, while the patients in Group 4 were studied during peritonitis. Blood and dialysate samples were obtained at selected times during the 5-hour dwell and, for plasma, until 24 hours after drug administration. Pharmacokinetic analysis of the data showed that only 6.0 and 8.7% of the intravenous doses of cefotiam and cefsulodin, respectively, were recovered in the dialysate at the end of the dwell. The net amounts of cefsulodin lost from the dialysate after intraperitoneal administration were 81 and 84%, in Groups 3 and 4 respectively. The peritoneal transfer clearances (using a unidirectional clearance model), calculated after intravenous (17 +/- 10 ml/min, Group 2) and intraperitoneal (17 +/- 5 ml/min, Group 3) administrations were the same. Mass balance of cefsulodin in the body and in the dialysate after intraperitoneal administration indicated that a significant amount (40%, Group 3) of the dose is unaccounted for. One explanation for this imbalance is retention of the drug in the peritoneal lining. This hypothesis is supported by the retention being lower in the peritonitis patients (less than 20%, Group 4), for whom the linings are expected to be partially eroded.