Protein binding and kinetics of drugs in liver diseases

The pharmacokinetics of antipyrine in patients with graded severity of schistosomiasis

The pharmacokinetics of antipyrine have been studied in patients with schistosomiasis. In comparison to a control group of subjects (n = 6), patients with early (active) schistosomiasis (passing live ova in urine or stools without clinical and laboratory evidence of liver involvement; n = 6) exhibited similar pharmacokinetic parameters. Of seven patients with hepatosplenic schistosomiasis (exhibiting hepatic fibrosis, splenomegaly, at least one episode of haematemesis, ascites), five showed markedly enhanced antipyrine half-life and reduced clearance. Compared to controls, the mean half-life of this group was increased from 10.9 +/- 2.4 to 19.9 +/- 9.5 h (mean +/- s.d.; P less than or equal to 0.05) and clearance reduced from 3.81 +/- 0.74 to 2.18 +/- 0.80 l h-1 (P less than or equal to 0.01). There was no change in the apparent volume of distribution. Liver biopsy was performed on all patients diagnosed as having hepatosplenic schistosomiasis in the 2 weeks prior to the antipyrine study. The results of this study indicate that hepatic microsomal metabolism is impaired in patients with advanced hepatosplenic schistosomiasis.

Clinical pharmacokinetics of the depot antipsychotics

The clinical pharmacokinetics of the 4 depot antipsychotics for which plasma level studies are available (i.e. fluphenazine enanthate and decanoate, haloperidol decanoate, clopenthixol decanoate and flupenthixol decanoate) are reviewed. The proper study of these agents has been handicapped until recently by the necessity of accurately measuring subnanomolar concentrations in plasma. Their kinetic properties, the relationship of plasma concentrations to clinical effects, and conversion from oral to injectable therapy are discussed. The depot antipsychotics are synthesised by esterification of the active drug to a long chain fatty acid and the resultant compound is then dissolved in a vegetable oil. The absorption rate constant is slower than the elimination rate constant and therefore, the depot antipsychotics exhibit 'flip-flop' kinetics where the time to steady-state is a function of the absorption rate, and the concentration at steady-state is a function of the elimination rate. Fluphenazine is available as both an enanthate and decanoate ester (both dissolved in sesame oil), although the decanoate is more commonly used clinically. The enanthate produces peak plasma concentrations on days 2 to 3 and declines with an apparent elimination half-life (i.e. the half-time of the apparent first-order decline of plasma concentrations) of 3.5 to 4 days after a single injection. The decanoate produces an early high peak which occurs during the first day and then declines with an apparent half-life ranging from 6.8 to 9.6 days following a single injection. After multiple injections of fluphenazine decanoate, however, the mean apparent half-life increases to 14.3 days, and the time to reach steady-state is 4 to 6 weeks. Withdrawal studies with fluphenazine decanoate suggest that relapsing patients have a more rapid plasma concentration decline than non-relapsing patients, and that the plasma concentrations do not decline smoothly but may exhibit 'lumps' due to residual release from previous injection sites or multicompartment redistribution. Cigarette smoking has been found to be associated with a 2.33-fold increase in the clearance of fluphenazine decanoate. In 3 different studies, fluphenazine has been proposed to have a therapeutic range from less than 0.15 to 0.5 ng/ml with an upper therapeutic range of 4.0 ng/ml. Plasma concentrations following the decanoate injection are generally lower than, but clinically equivalent to, those attained with the oral form of the drug. Haloperidol decanoate plasma concentrations peak on the seventh day following injection although, in some patients, this peak may occur on the first day.(ABSTRACT TRUNCATED AT 400 WORDS)

Variation in hepatic extraction ratio with unbound drug fraction: discrimination between models of hepatic drug elimination

Systematic examination of model-dependent predictions of changes in the hepatic extraction ratio (E), following alteration in the unbound fraction of drug in plasma (fub), should allow sensitive discrimination between the venous equilibrium model (model I) and the sinusoidal perfusion model (model II) of hepatic sinusoidal function if drugs which show high clearance of free drug are used. Analysis of experimental data from the literature confirmed the utility of this approach. Specifically, data related to diazepam (E = 0.95 at fub = 1) clearly conformed to the predictions of the sinusoidal perfusion model and differed markedly from those of the venous equilibrium model. Conversely, data for phenytoin (E = 0.69 at fub = 1) failed to discriminate between models, as predicted by our analysis. We identify a sensitive, convenient method for discrimination between current models of hepatic sinusoidal function and establish for the first time that a drug substrate (diazepam) conforms closely to the predictions of the sinusoidal perfusion model.

Distribution of sodium valproate in normal whole blood and in blood from patients with renal or hepatic disease

Sodium valproate at a concentration of 300 mumol/l in whole blood, partitioned between the red blood cell and plasma to produce a red blood cell/plasma partition ratio of 0.20. Red blood cell uptake was proportional to percent free drug in plasma and uptake was maximal when plasma was replaced by buffer, producing a red blood cell/buffer ratio of 0.87. Reduction of plasma protein binding by plasma dilution, by increasing the total sodium valproate plasma concentration, or by renal or hepatic disease in 24 patients, caused a predictable rise in red blood cell uptake of drug. The red blood cell represented a relatively small compartment for free sodium valproate in blood, however uptake of the drug into this compartment increased considerably in states of reduced plasma protein binding. Because the concentration of drug in the red blood cell reflects free drug concentration in plasma, the red blood cell may serve as an indicator of free drug changes in blood.

Pharmacokinetics of tolfenamic acid in patients with cirrhosis of the liver

Six patients with alcoholic cirrhosis of the liver received 100 mg tolfenamic acid p.o. and i.v. The disposition of tolfenamic acid could be described by a two-compartment open body model, with a mean central compartment volume of 8.71, and a beta-phase volume of 25 l. The elimination rate constant ke averaged 1.13 h-1 and the half-life of the beta-phase was 1.73 h; the mean total plasma clearance was 159 ml/min. These pharmacokinetic parameters differed only slightly from those in two groups of healthy volunteers studied previously; ke was significantly reduced by about 30% in the patients but none of the other parameters differed significantly. There was good correlation between individual elimination rate constants or plasma clearances with the liver function tests, serum albumin and P-coagulation factors. Oral absorption was good and bioavailability of about 100% was shown by comparison of the areas under the plasma concentration - time curves after i.v. and p.o. administration. Metabolism was qualitatively and quantitatively very similar to previous observations in healthy volunteers. There seems no reason to reduce the dose of tolfenamic acid in patients with compensated alcoholic cirrhosis.

Carbon tetrachloride-induced increase in the antitumor activity of cyclophosphamide in mice: a pharmacokinetic study

Carbon tetrachloride is an hepatotoxin that depresses hepatic microsomal cytochrome P-450 and other enzyme activities. Cyclophosphamide is an anticancer drug that is activated by hepatic microsomal cytochrome P-450, while the products of cyclophosphamide metabolism by cytochrome P-450 can be metabolized by other hepatic enzymes. Carbon tetrachloride pretreatment has been found to increase the in vivo antitumor activity of cyclophosphamide against murine leukemia P-388. Carbon tetrachloride did not, however, affect the direct cytotoxicity of cyclophosphamide or 4-hydroxycyclophosphamide to cells in culture. Pharmacokinetic studies in mice revealed a delayed plasma disappearance of cyclophosphamide after carbon-tetrachloride pretreatment with an apparent initial half-time of 20.4 min compared to 9.0 min in non carbon-tetrachloride-pretreated mice. Plasma levels of total alkylating activity and plasma 4-hydroxycyclophosphamide increased more slowly and reached a lower peak, but were maintained for a longer time period in mice pretreated with carbon-tetrachloride than in untreated mice. The half-life for plasma elimination of 4-hydroxycyclophosphamide in untreated mice was 12 min and in carbon-tetrachloride-pretreated mice 27 min. There was, however, no difference in the area under the curve for either plasma total alkylating activity or plasma 4-hydroxycyclophosphamide between the two groups. It is suggested that prolonged exposure of tumor cells to 4-hydroxycyclophosphamide might be responsible for the increased antitumor activity of cyclophosphamide following carbon-tetrachloride pretreatment.

Pethidine binding in whole blood: methodology and clinical significance

A three-compartment equilibrium dialysis method was developed for the simultaneous and direct determination of drug binding in whole blood and in plasma and of the blood to plasma concentration ratio (b/p). The unbound fraction of pethidine in the blood of six healthy volunteers (0.63, s.d. 0.09, n = 23 determinations) was significantly different from that in the blood of six patients (0.72, s.d. 0.08, n = 24 determinations). There was no significant difference in the b/p of six patients (0.84, s.d. 0.09, n = 24 determinations) and six volunteers (0.90, s.d. 0.14, n = 23 determinations). The observed unbound fraction of pethidine in blood (0.6 to 0.7) was considerably lower than previously reported. As this value for the unbound fraction (0.6 to 0.7) is similar to the reported estimated hepatic extraction ratio of the drug in man, it is proposed that pethidine elimination should be described as 'capacity limited, binding sensitive' rather than 'flow-limited'.

Naproxen disposition in patients with alcoholic cirrhosis

Chronic liver disease is known to alter the absorption and disposition of many drugs. To assess the influence of chronic alcoholic liver disease on the disposition of naproxen, we administered the drug both as a single dose and to steady state to 10 individuals with alcoholic cirrhosis and to 10 healthy controls. Plasma and serum samples collected after naproxen dosing were assayed for both total and (following equilibrium dialysis) unbound drug concentration. Clearance calculated based on both total and unbound naproxen concentration revealed no change in total plasma clearance of the drug at steady state but a marked reduction of approximately 60% in clearance based on unbound drug. Naproxen volume of distribution changed only minimally. Because clearance based on unbound drug concentration at a given dosing rate determines the plasma or blood free drug concentration, this concentration may increase significantly in patients with alcoholic liver disease given usual doses of naproxen. Unbound drug concentration is thought to determine the pharmacologic effect of a drug. We therefore recommend that naproxen dosing be reduced by at least half in patients with chronic alcoholic liver disease. In the absence of data to the contrary, this recommendation can be extended to individuals with other forms of hepatic disease.

Pharmacokinetics and toxicity testing

The pharmacokinetic basis for the design of toxicity tests is discussed with reference to the absorption and clearance of drugs. The absorption and clearance of a wide range of drugs by laboratory animals and man has been examined and reviewed to provide a firm basis against which new drugs can be compared. Some pitfalls in either the empirical approach to toxicology or the incorrect interpretation of kinetic data are highlighted. An approach is outlined for the rational application of animal pharmacokinetic data in the assessment of the safety in man of a new therapeutic agent.

Cimetidine kinetics and dynamics in patients with severe liver disease

Following cimetidine administration, 60% of the dose is excreted as unchanged drug in the urine, and 40% is eliminated by metabolism. We evaluated the effect of liver disease on cimetidine disposition by comparing its kinetics in 7 healthy subjects and 8 patients with alcoholic cirrhosis. Cirrhotic patients had severe liver disease as evidenced by the presence of ascites, hepatic encephalopathy, jaundice, muscle wasting, and low serum albumin, but serum creatinine and creatinine clearance did not differ significantly between controls and cirrhotics. Following intravenous administration, cimetidine systemic clearance was decreased by 56% in cirrhotics. This reduction was due in major part to an impairment of the renal clearance of unchanged drug. The ratio of cimetidine to creatinine clearance was 3.71 +/- 0.63 in controls, indicating active tubular secretion, and was decreased in cirrhotics (1.22 +/- 0.09, p less than 0.05). The volume of distribution of cimetidine was also decreased by 39% in cirrhotics. To verify whether these findings observed after a single dose could be extended to patients receiving chronic cimetidine treatment, cimetidine trough (predose) plasma levels were measured in an additional group of 56 subjects receiving continuous cimetidine therapy (15 controls and 41 cirrhotics). Trough plasma levels did not differ significantly in controls and patients with compensated liver disease, but were elevated in patients with moderate and severe hepatic dysfunction. It is concluded that cimetidine clearance is decreased in patients with severe liver disease, mostly due to an impairment of the tubular secretion of unchanged drug, and that a reduction of cimetidine dosage is warranted in these patients, even in the presence of a normal creatinine clearance.

Plasma concentrations and central nervous system effects of the new hypnotic agent zopiclone in patients with chronic liver disease

Eight healthy individuals and seven cirrhotic patients received 7.5 mg zopiclone orally. Two further cirrhotics received 3.75 mg. Plasma concentrations of zopiclone and psychometric tests including reaction time and critical flicker fusion threshold and electroencephalographic tracings were performed at regular intervals after drug administration. Peak plasma levels of zopiclone were similar in the two groups but the time to peak was delayed in the cirrhotics. Plasma zopiclone half-life was 8.53 +/- 0.83 h in the cirrhotics and 3.50 +/- 0.33 h in the healthy individuals. In the group of cirrhotics there was a negative correlation between zopiclone half-life and serum albumin concentration (r = -0.87). Zopiclone caused sedation in both groups. Reaction time was prolonged and critical flicker fusion threshold reduced in both groups. Recovery was delayed in the cirrhotics compared to the healthy subjects but was complete by 8 h. The size of the changes was somewhat greater in the cirrhotics but baseline observations differed between the groups. The mean dominant frequency was lower in the cirrhotics and fell slightly in that group after zopiclone administration. The response to zopiclone is delayed and exaggerated in cirrhosis. Precautions are therefore required when using this drug in patients with chronic liver disease.

Improved procedure for the determination of protein binding by conventional equilibrium dialysis

The binding of drugs to plasma proteins has been studied extensively using a variety of methods, including equilibrium dialysis. Published information on controls used in these studies is frequently inadequate; in other cases, there are deficiencies in the experimental design for the controls. A method is described that eliminates many of the problems associated with artifactual errors in dialysis studies. Multiple replicated controls are performed at the same time as the test, under identical conditions. The controls are used to correct for concentration-dependent binding of drug to the membrane or other equipment. The method was used to determine the binding of sulfadimethoxine to CF-IV-1 alpha-globulin at therapeutic concentrations. The level of binding was low (9-13%), but the stringent control technique permitted statistical analysis which showed each mean test value to be significantly different from its corresponding control. Furthermore, there was a linear relationship between the control-corrected percentage binding values and total drug concentration, whereas there was no correlation between total drug concentration and the uncorrected percentage binding values.

Pharmacokinetics of aspirin and salicylate in elderly subjects and in patients with alcoholic liver disease

Plasma aspirin, salicylate and salicyluric acid concentrations were monitored in young, elderly and alcoholic subjects after ingestion of a single 1.2 g dose of soluble aspirin. The plasma aspirin, salicylate and unbound salicylate concentration-time profiles varied considerably between individual subjects. Most of the pharmacokinetic parameters derived from these profiles were not significantly different between young subjects, elderly subjects and subjects with alcoholic liver disease. Individual plasma albumin concentrations provided a better index of the unbound plasma salicylate clearances and salicylate plasma protein binding than the age of the subject or the presence of alcoholic liver disease. Highest unbound plasma salicylate concentrations were found in subjects with the lowest plasma albumin concentrations.

An intensive drug monitoring study suggesting possible clinical irrelevance of impaired drug disposition in liver disease

1 Liver disease can alter the disposition and clinical effects of drugs. However, even though altered drug disposition occurs, there is no clinical evidence relating it to an increased susceptibility to adverse drug reactions (ADRs). 2 An intensive prospective drug monitoring study of 2,582 hospitalized patients was conducted. The adverse drug reactions probability scale (APS) was used to assess ADRs. Only non-mild, definite or probable ADRs (APS greater than or equal to 5) were included. Severity of liver dysfunction was assessed by a composite clinical and laboratory index (CCLI). 3 The frequency of ADRs was higher in 402 patients with cirrhosis (27.4%) than in 661 with renal dysfunction (22.8%) and in 249 with other parenchymatous liver diseases (13.7%) or in 1,270 patients with neither liver diseases nor renal dysfunction (10.9%) (chi 2 3 = 85.53, P less than 0.001). The frequency of ADRs in cirrhotics was highly correlated with the severity of the liver dysfunction measured by CCLI (r = 0.82, P less than 0.001). 4 Drugs predominantly eliminated by liver metabolism were not among those most commonly inducing ADRs or those causing severe reactions in cirrhotics. Thus, frusemide caused the most common and the most severe ADRs, whereas reactions induced by sedatives were uncommon. Drug-induced hepatic encephalopathy was more common in cirrhotics receiving diuretics (13.3%) than in those receiving sedatives (1.8%) (chi 2 y.c. = 5.29, P less than 0.025). Patients with alcoholic liver disease had more drug-induced hepatic encephalopathy (7.7%) than those with non-alcoholic liver disease (1.2%) (chi 2 y.c. = 11.86, P less than 0.001). 5 These results indicate that susceptibility to ADRs is increased only in severe cirrhosis and that the most common and severe ADRs seem more likely related to enhanced pharmacodynamic action than to impaired drug disposition.

Variations in drug free fraction during alcohol withdrawal

1 Free fractions of diazepam, propranolol and warfarin were determined in 15 male chronic alcoholics in alcohol withdrawal. 2 On admission the mean free fraction of diazepam was 25% above and propranolol 44% below the limits of normal range, while the mean warfarin free fraction was in high normal range. One week later mean free fraction of diazepam declined by 20% while propranolol and warfarin increased by 24% and 19% respectively (P less than 0.05). 3 Propranolol free fraction and alpha 1-AGP concentrations were highly correlated (linear r = -0.83, P less than 0.001). In contrast the sources of variation in diazepam and warfarin free fraction were more complex and less certain. 4 Statistically significant changes of drug free fractions in serum of chronic alcoholics were observed during alcohol withdrawal. The extent and direction of these changes differed for various classes of drugs and their potential causes appear to be quite different. 5 Clinically important changes in drug effect may be present acutely, within the dosing interval, as a result of altered drug binding. These are more likely when the clinical response is closely related to drug concentration and will occur within the dosing interval due to larger fluctuations in free drug concentration, even though the average free drug concentration will remain unchanged. 6 Total drug level changes will be observed during alcohol withdrawal even in absence of detectable changes of drug metabolism.

Host factors influencing the response to antimicrobial agents

When the desired clinical response to an antibiotic therapeutic regimen is not achieved, despite appropriate antibiotic selection and organism sensitivity, the clinician must be aware that several host factors exist that may influence the outcome. Examples of the influence of host-related factors on drug disposition have been briefly reviewed in this article. It should be noted, however, that further investigation is needed to determine whether these factors truly exert a significant influence on the outcome of antibiotic utilization.

Use of benzodiazepines during pregnancy, labour and lactation, with particular reference to pharmacokinetic considerations

Knowledge of the pharmacokinetic properties of the benzodiazepines is playing an increasingly important role in their use during pregnancy, labour and lactation. All of the benzodiazepine derivatives are lipophilic, undissociated agents which readily penetrate membranes. Thus, they exhibit rapid placental transfer with significant fetal uptake of the drug. In the first trimester of pregnancy there is seldom a clear indication for the use of benzodiazepines. In late pregnancy and at parturition there may be more clear indications for their use. During delivery, the lowest effective dose should be used, since after high doses the so-called 'floppy infant syndrome' may occur, and the slow elimination of these agents by the newborn should be considered. Oxazepam, lorazepam, nitrazepam and, especially, flunitrazepam, appear to penetrate the human placenta more slowly than diazepam, but the clinical significance of this phenomenon remains uncertain. All of these derivatives appear in human milk, but only high clinical doses might be expected to exert a possible effect on the nursing newborn.

Effect of cardiopulmonary bypass on the pharmacokinetics of drugs

The cardiopulmonary bypass apparatus must temporarily substitute for the cardiac and pulmonary function of the patient undergoing heart surgery. In order to meet the metabolic needs of the patient and the technical demands of the surgeon, within the limits of engineering technology, a number of major alterations are made in normal physiology. The patient is typically cooled to 27 degrees C and perfused with a non-pulsatile flow of blood which has been diluted with saline to a haematocrit in the mid-20s. Blood flow and pressure are often considerably less than normal. Blood coagulation is prevented by administration of a massive dose of heparin. Central redistribution of blood flow, elaboration of stress-reactant hormones, and fluid and electrolyte shifts occur in response to these changes. In the postoperative period, these alterations are reversed, and normal physiology is restored. Effects upon the pharmacokinetics of drugs are anticipated. The clearance of many drugs may be reduced. Protein binding is diminished by haemodilution, but may rise above normal in the postoperative period for basic drugs which bind to alpha 1-acid glycoprotein. Changes in volume of distribution depend upon the opposing influences of protein binding and reduced peripheral perfusion. Previous studies on the pharmacokinetics of drugs during and after cardiopulmonary bypass illustrate many of these effects. The clearance of digoxin, fentanyl, and the cephalosporins is reduced after cardiopulmonary bypass, and the volume of distribution of cefazolin is increased during cardiopulmonary bypass. Studies of digitoxin and propranolol are also reviewed. Many of the investigations in this area of study have been limited by logistical and methodological factors. Thus, the effects of cardiopulmonary bypass on the pharmacokinetics of drugs are incompletely understood, and the subject merits further attention.

Drugs as indicators of hepatic function

It is well recognized that liver disease may influence the disposition of many drugs. Conversely, it has been suggested that knowledge of the disposition of a model drug might provide an index of certain aspects of hepatic function. This review discusses the physiology of drug disposition and indicates how recent progress in understanding the determinants of drug disposition has provided useful indices of individual aspects of hepatic function. Topics which are discussed are the interpretation of pharmacokinetic parameters as indices of hepatic function, including half-life clearance, and intrinsic clearance. Utilizing the "intact hepatocyte hypothesis" as an operational model, an approach is described that uses the pharmacokinetic disposition of high and low intrinsic clearance drugs following p.o. and i.v. administration to provide quantitative estimates of hepatic function, flow to functioning hepatocytes, and the extent of portasystemic shunting through the liver. Thus, the theoretical basis for quantitation of certain aspects of hepatic function are available. It remains to be determined whether these indices will provide clinically useful measures to follow the natural history of hepatic disease.

Clinical pharmacokinetics of high dose mebendazole in patients treated for cystic hydatid disease

The plasma concentrations of mebendazole and its metabolites have been monitored in twelve patients after receiving a 10 mg/kg dose for cystic hydatid disease. The mebendazole plasma concentration-time profiles differed considerably between patients; elimination half-lives ranged from 2.8-9.0 h, time to peak plasma concentration after dosing ranged from 1.5-7.25 h and peak plasma concentrations ranged from 17.5 to 500 ng/ml. The mean peak plasma concentration of mebendazole after an initial dose (69.5 ng/ml) was lower than found in patients during chronic therapy (137.4 ng/ml). The plasma AUCTS for the major metabolites of mebendazole (methyl 5-(alpha-hydroxybenzyl)-2-benzimidazole carbamate and 2-amino-5 benzoylbenzimidazole) were about five times the plasma AUCT found for mebendazole in patients on chronic therapy. It is suggested that the slower clearance of these polar metabolites relative to mebendazole results from enterohepatic recycling. Since mebendazole is also highly plasma protein bound, caution should be observed in administering mebendazole to patients with liver disease. Concentrations of mebendazole found in the tissue and cyst material collected from two patients during surgery ranged from 59.5 to 206.6 ng/g wet weight.

Pharmacokinetics of drug overdose

Pharmacokinetics of drugs taken in overdose may differ from those observed following therapeutic doses. Differences are due both to dose-dependent changes and to effects of drugs or pathophysiological consequences of the overdose on kinetics. Dose-dependent changes in rate and extent of absorption, bioavailability (saturation of first-pass metabolism), distribution (saturation of protein binding sites) and metabolism are discussed. Gastrointestinal motility is affected both by specific drug actions, such as delayed gastric emptying by anticholinergic drugs, and by general nervous system depression caused by many drugs. Drug-induced circulatory insufficiency may retard tissue distribution and reduce clearance. Disturbances in blood and urine pH may alter distribution and clearance of weak acids and bases. Drug-induced renal or hepatic failure can significantly decrease clearance. Hypothermia is a common complication of drug overdose and might retard distribution and also reduce clearance. The data concerning pharmacokinetics during overdose are usually incomplete and difficult to interpret. Doses and times of ingestion are uncertain, duration of blood and urine sampling is often inadequate to distinguish absorption from distribution and elimination phases, active metabolites are not measured, protein binding is not determined and clinical features of patients not adequately described. We have, however, reviewed available data for salicylate, paracetamol (acetaminophen), barbiturates, ethchlorvynol, glutethimide, chloral hydrate, tricyclic antidepressants, lithium, phenytoin, ethanol, theophylline, digoxin, amphetamine and phencyclidine.

Plasma protein binding of azapropazone in patients with kidney and liver disease

1 The free fraction of azapropazone in the plasma of 37 healthy volunteers ranged from 0.0027 to 0.0070 (0.0044 +/- 0.0009, mean +/- s.d.). The principal binding protein was found to be albumin. 2 In 27 patients with various degrees of renal failure the free fraction values of azapropazone were markedly enhanced (0.0260 +/- 0.0239, mean +/- s.d.) and increased more than tenfold in some patients. There was a weak correlation (r = 0.46, P less than 0.05) between the free fraction and the clearance of endogenous creatinine. Such correlation was not found for serum creatinine, serum albumin, serum uric acid and serum urea nitrogen. 3 In 32 patients with chronic liver disease the free fraction values of azapropazone were also markedly higher (0.0210 +/- 0.0242, mean +/- s.d.) than in healthy subjects. There were statistical significant correlation between free fraction values and the prothrombin complex activity in the plasma (r = 0.40, P less than 0.05) and the total bilirubin concentration in the plasma (r = 0.90, P less than 0.001), respectively. Such correlation was not found for serum albumin, serum glutamic oxalacetic transaminase, serum gamma-glutamyl transpeptidase and serum alkaline phosphatase. 4 In patients with kidney and liver disease the free fraction values of azapropazone correlated well with those of the anticoagulant drug phenprocoumon (r = 0.93, P less than 0.001). However, the binding of the latter drug was less impaired. Bilirubin, when added in vitro, displaced both drugs from plasma proteins but this displacing effect was much smaller than the binding changes observed in patients with liver disease. 5 Kidney and liver disease caused a marked impairment of the plasma protein binding of azapropazone. In patients with kidney disease the degree of impairment of azapropazone binding cannot or only poorly (creatinine clearance) be predicted from the biochemical parameters of kidney function whereas in patients with chronic liver disease the total bilirubin concentration in the plasma may serve as an index of the binding defect.

Effects of liver disease on urinary excretion of methadone and metabolites in maintenance patients: quantitation by direct probe chemical ionization mass spectrometry

This study was performed to define the amounts of methadone and metabolites excreted in urine in otherwise healthy maintenance patients, and to determine whether the metabolism and elimination of methadone, as assessed by analyses of urines, is altered in patients with liver disease. A method was developed for the simultaneous quantitation of methadone and six of its major and minor metabolites using chemical ionization mass spectrometry with direct probe introduction to increase sensitivity for analyses of the minor metabolites. Analyses of urine from unmedicated volunteers showed that the interferences at the mass range of interest (264-326) were usually small and therefore would not introduce significant error into analysis. Nineteen patients well-stabilized in chronic long-term methadone treatment were studied, five otherwise healthy males and fourteen patients with chronic liver disease (nine males and four females). Twenty-four hour urine collections were made and analyzed following extraction procedures. The concentrations of methadone and the major pyrrolidine metabolite exceeded 1 microgram ml-1 in all cases; the concentration (listed in descending order) of pyrrolidone, pyrroline, hydroxymethadone, hydroxypyrroline, methadol and hydroxypyrrolidine were all less than 1 microgram ml-1. The total 24 hour urinary excretion of methadone and its metabolites was 48.3% (+/- 1.71 SEM) in otherwise healthy patients but was significantly lower, 32.6% (+/- 3.19 SEM) in patients with liver disease (p less than 0.05). The total 24 hour excretion of the pyrrolidone metabolite, the end product of two pathways of methadone metabolism, was also significantly reduced in patients with liver disease (p less than 0.05). Females with liver disease had significantly higher ratios of pyrrolidine to methadone than did males with liver disease (p less than 0.05).

Influence of age and smoking on drug kinetics in man: studies using model compounds

The effect of advanced age on drug disposition and response is the subject of an increasing number of research studies. Cigarette smoking and other environmental factors have been shown to influence the metabolism of some drugs. Studies of the effect of age on the pharmacokinetics of antipyrine, indocyanine green and propranolol indicate that the association of increased drug metabolism and smoking occurs predominantly in the young and that apparent liver blood flow declines with age regardless of smoking habits. This age related effect of smoking on drug metabolism may reflect a reduced capacity for hepatic enzyme induction in the elderly. The relative importance of age related alterations in the intrinsic drug metabolising ability of the liver and in liver blood flow depend on the pharmacokinetic characteristics of the drug being investigated.

Disease-induced changes in the plasma binding of basic drugs

The plasma binding of basic (cationic) drugs differs from that of the more completely studied acidic drugs. Basic drugs associate with a number of plasma constituents. alpha 1-Acid glycoprotein, lipoprotein, and albumin all appear to play an important role in the binding of most of these drugs. Acidic drugs bind largely to albumin. The variation in plasma albumin is relatively narrow and is almost always in the direction of decreased concentrations. alpha 1-Acid glycoprotein and lipoproteins show large fluctuations due both to physiological and pathological conditions. Decreases and increases in concentration have been observed. Associated with these changes in binding proteins, both decreases and increases in plasma binding of basic drugs have been recorded. Increased binding with disease appears to be virtually unique to basic drugs. The implications of these newly described disease-induced increases in plasma binding have yet to be explored. With the limited information in hand the following consequences are predicted. Increased binding will tend to decrease the volume of distribution of total (bound plus free) drug. The clearance will be unchanged or decreased depending upon the initial clearance of the drug and the avidity of the protein binding. As the half-life depends upon both clearance and volume of distribution, changes in it will be variable, depending upon changes in these two parameters. It is predicted that the area under the free drug plasma concentration-time curve will decrease with increasing binding after an intravenous dose while it will be unchanged after an oral dose. The relationship of total drug plasma concentration to free drug concentration will change with changes in binding. Thus plasma concentration monitoring of drug therapy by use of total drug concentrations will be inaccurate in situations in which large variations in binding occur. Misinterpretations of both therapeutic monitoring and pharmacokinetics studies in disease states with altered binding are likely unless these changes are appreciated.

Prazosin protein binding in health and disease

1 Experiments have been performed using equilibrium dialysis to determine the binding of [¹⁴C]-prazosin to albumin, to α₁-acid glycoprotein and to the plasma proteins of normal subjects and of patients with cirrhosis, chronic renal failure or chronic heart failure.

2 The influence of propranolol on prazosin binding has been studied. In addition, red blood cell to plasma partitioning of prazosin has been quantified.

3 The dissociation constant for prazosin binding to albumin was 3 × 10^-5 M and to α₁-acid glycoprotein was 1.9 × 10^-6 M.

4 In fourteen normal subjects the free fraction of prazosin was 0.051 ± 0.007.

5 In seven patients with cirrhosis free fraction of prazosin was 0.064 ± 0.017 (P < 0.05 compared to normal). In nine patients with chronic renal failure the free fraction was 0.077 ± 0.033 (P < 0.05) and in eight congestive heart failure patients the value was 0.064 ± 0.027 (P > 0.05).

6 The range of prazosin free fraction was substantially greater in the patients than in normal subjects. In cirrhotic patients free fraction of prazosin correlated significantly (r = -0.92) with albumin concentration.

7 Propranolol did not influence prazosin protein binding. In blood 20% of prazosin is associated with red cells.

8 The considerably greater range of prazosin free fraction in the patients suggests that caution should be used when prescribing the drug for subjects with these conditions. Both albumin and the acute phase reactant α₁-acid glycoprotein bind prazosin in vitro.

High-performance liquid chromatographic determination of plasma propylthiouracil

A reversed-phase high-performance liquid chromatographic analysis was developed for propylthiouracil in plasma (1 ml). After protein precipitation with acetonitrile, the solution was diluted with water and injected into a liquid chromatograph equipped with C-18 and C-8 columns in series. The peak area was linear over the 0.25-10-mg/liter range, and the recovery was 101 +/- 4.5%. This assay has the advantages of specificity, simplicity, and speed over previously published methods and requires smaller sample volumes. None of 19 drugs tested interfered with the assay.

Pharmacokinetics of dihydroquinidine in congestive heart failure patients after intravenous quinidine administration

The pharmacokinetics of dihydroquinidine were studied in 8 patients with congestive heart failure following a 22 min intravenous infusion of a quinidine preparation that contained 5.9% dihydroquinidine as an impurity. Using a thin layer chromatography-fluorometric assay procedure for dihydroquinidine, the post-infusion plasma dihydroquinidine concentrations declined biexponentially. The half-life of the fast and slow dispositional processes was 4.42 +/- 1.81 min and 6.52 +/- 2.40 h, respectively. The central compartment volume for dihydroquinidine in these patients was 0.44 +/- 0.11 l/kg with an overall apparent volume of distribution of 1.14 +/- 0.38 l/kg. The computed values of total body plasma clearance of dihydroquinidine ranged from 1.29 to 2.69 ml/min/kg with a mean value of 1.94 +/- 0.60 ml/min/kg. In these patients, approximately 16% of the administered dihydroquinidine dose was excreted intact into the urine in 48 h. The estimated value of renal clearance was 0.314 +/- 0.129 ml/min/kg. When compared to control cardiac patients, the data showed that the apparent volume of distribution for dihydroquinidine is smaller in patients with congestive heart failure and as a result of this diminished volume, the clearance rate of dihydroquinidine was slower. The net effect of these differences was the production of higher plasma concentrations of dihydroquinidine in the heart failure group.

[Decreased serum protein binding of glymidine in icteric liver disease and its partial correction with charcoal (author's transl]

In patients with hyperbilirubinemic liver disease serum protein binding of the antidiabetic agent glymidine is reduced considerably. After treatment of the serum with charcoal the reduced glymidine binding can be assimilated partly, but not completely, to the glymidine binding in healthy subjects. It may be concluded there from that the reduced glymidine binding in patients with hyperbilirubinemic liver disease has to be partly caused by substances which are retained in the blood of these patients. Besides bilirubin, other substances must be responsible herefore. Bilirubin, when added in vitro to the serum of healthy subjects, is not able to decrease glymidine binding to a degree similar to the results obtained in the patients with liver disease.

The influence of Bacillus piliformis (Tyzzer) infections on the reliability of pharmacokinetic experiments in mice

The half-lives of warfarin and trimethoprim were significantly longer in mice acutely infected with Bacillus piloformis and in mice which ad clinically recovered from previous experimental infection with the organism. The volume of distribution of trimethoprim but not of warfarin was significantly greater in infected mice than in controls. Body clearances of warfarin was significantly reduced in both disease states. For trimethoprim this parameter was only reduced in the acute state of the disease. The importance of careful control of Tyzzer's disease in laboratory animals for use in pharmacological research is stressed.

Disposition of synethetic glucocorticoids. I. Pharmacokinetics of dexamethasone in healthy adults

The pharmacokinetics of dexamethasone alcohol is described in six male and six female healthy adult volunteers who each received 8 mg of dexamethasone phosphate by bolus intravenous injection. Quantitation of the alcohol was done using a high-performance liquid chromatographic method with improved specificity. Statistical evaluation of the results generated by nonlinear least-squares regression analysis of the plasma concentration-time data shows that the phosphate ester is very rapidly hydrolyzed to the alcohol and a biexponential equation is the simplest polyexponential equation that is consistent with the data. The terminal phase half-life t1/2 beta was significantly greater (P less than 0.05) in males (mean 201.5 min) than in females (mean 142.3 min). They prolonged t1/2 beta in males did not appear to be caused by an impaired capacity to eliminate dexamethasone since the total plasma clearance did not differ between males (mean 24.5 ml/min) and females (mean 242.9 ml/min). There was, however, a high positive correlation between t1/2 beta and Vdss among the 12 adults (r = 0.92, p less than 0.001). There were also significant correlation between Vdss and body weight (r = 0.67, p less than 0.05) and t1/2 beta and body weight (r = 0.80, p less than 0.01). The difference in body weight between the sexes seems to be the main factor contributing to the difference observed in t1/2 beta. An average of only 2.6% of the dose was found unchanged in a 24-hr urine sample, and hence it appears that dexamethasone is primarily eliminated by extrarenal, probably hepatic, mechanisms.