Pharmacokinetic and clinical implications of quinidine protein binding

Effect of lipophilicity on drug distribution and elimination: Influence of obesity

AIMS

For a given passively-distributed lipophilic drug, the extent of in vivo distribution (pharmacokinetic volume of distribution, V_d ) in obese individuals increases in relation to the degree of obesity. The present study had the objective of evaluating drug distribution in relation to in vitro lipophilicity, and the relative increase in V_d associated with obesity across a series of drugs.

METHODS

Cohorts of normal-weight control and obese subjects received single doses of drugs ranging from hydrophilic (acetaminophen, salicylate) to lipophilic (imipramine, verapamil). Lipid solubility was measured by the log-transformed values of the high-pressure liquid chromatographic (HPLC) retention index (Log₁₀ (HPLC)), and the octanol-water partition coefficient (LogP).

RESULTS

Among normal-weight controls, V_d normalized for protein binding was highly correlated with Log₁₀ (HPLC) (R² = .65) and with LogP (R² = .78). V_d of all drugs was increased in the obese cohort, but the relative increase (compared to controls) for individual drugs was disproportionately greater as lipid solubility increased. Since clearance was unrelated to lipophilicity, the increased V_d produced a parallel disproportionate increase in elimination half-life in the obese cohort that was associated with Log₁₀ (HPLC) (R² = .62).

CONCLUSION

Lipophilicity is a principal correlate of in vivo V_d , as well as the increased V_d of drugs in obese patients. The consequent prolongation of half-life in obesity has clinical safety implications in terms of delayed drug accumulation and washout during and after chronic dosage. The magnitude and importance of this effect for a given drug depends on the degree of obesity, as well as the lipid-solubility of the specific drug.

Influence of the ORM1 phenotypes on serum unbound concentration and protein binding of quinidine

BACKGROUND

Only unbound or free drug in plasma can be transported to its site of action. The fraction of unbound drug in plasma varies widely for highly bound drugs among individuals. The genetic polymorphism of orosomucoid (ORM) could be related to the interindividual variability in plasma binding of basic drugs, as ORM is the transport protein for these drugs in plasma. The ORM is a major binding protein in plasma for various basic drugs and is coded by two loci, ORM1 and ORM2, which are closely linked on chromosome 9q31-->34.1. ORM1 locus is highly polymorphic and the ORM2 locus is monomorphic in most population.

METHODS

Twenty-eight healthy volunteers were selected with three ORM1 phenotypes, containing homozygotes ORM1 F1 (n=10) and ORM1 S (n=8), and heterozygote ORM1 F1S (n=10), identified by isoelectric focusing on polyacrylamide gels followed immunoblotting after desialylation of sera. After a single oral dose of quinidine 200 mg, serum total (HPLC) and unbound concentrations in ultrafiltrate (ultrafiltration/HPLC) were determined, and the pharmacokinetic parameters and protein binding rate were calculated.

RESULTS

Serum concentrations of ORM (553.8-573.2 mg/l) and albumin proteins (57.5-58.4 mg/l) were similar in the three groups (P>0.05). Unbound quinidine concentration in ORM1 F1 phenotype subjects was higher than that in ORM1 S and ORM1 F1S phenotype; the free drug percentage for the subjects with ORM1 F1 phenotype (19.79%) was twice as high as that with ORM1 S phenotype (10.96%) (P<0.01) at 24 h after administration of oral quinidine when the state of disposition equilibrium occurred. The elimination t(1/2) values and the other pharmacokinetic parameters of quinidine were not affected by the different ORM1 phenotypes.

CONCLUSIONS

Different ORM1 phenotypes may affect the disposition of quinidine, a basic drug, rather than its hepatic metabolism and elimination. The functional heterogeneity of ORM1 could be responsible for the differences in plasma binding of quinidine. Therefore, monitoring of the unbound quinidine concentration would be important for the patients with different ORM1 phenotypes who are treated with quinidine.

Pharmacokinetics of diazepam intramuscularly administered to rhesus monkeys

The purpose of this study was to define the pharmacokinetics of diazepam in monkeys following an im injection of 100 micrograms/kg (the minimum effective dose that prevents nerve agent-induced convulsions in pyridostigmine-pretreated, atropine- and 2-PAM-treated monkeys) in order to predict what im dose in humans is needed to prevent nerve agent-induced convulsions. Six rhesus monkeys were administered diazepam in the hind limb. Blood (3 mL) was collected via an indwelling saphenous catheter immediately prior to and 5, 10, 15, 25, 40, 60, 90, 120, 180, and 240 min after diazepam dosing. A contract laboratory, blind to the labeling code, analyzed diazepam serum concentrations by electron-capture gas chromatography and the percentage of unbound diazepam by equilibrium dialysis. The concentration-time data for total (unbound and bound) diazepam individually determined for each animal was best described by a one-compartment open model with first-order absorption and elimination. The average maximum serum concentration (50 ng/mL) was reached in 29 min. The volume of distribution and systemic clearance, assuming 100% bioavailability, were 1.5 L/kg and 19.4 mL/min/kg, respectively. The percentage of diazepam unbound to serum proteins was 4.6% and, therefore, the maximum concentration of free diazepam was 2.3 ng/mL. These results, when compared with human pharmacokinetic studies, allow a means of extrapolating effective monkey anticonvulsant doses to humans on a pharmacokinetic basis.

Comparison of disposition values obtained by two assay methods for quinidine gluconate in patients with ventricular tachycardia

Eight patients with previously untreated ventricular tachycardia, age 48.54 +/- 28.02 years (mean +/- SD), were enrolled in a protocol evaluating the disposition of quinidine gluconate as determined by two assay methods. Patients received two infusions of 5 mg/kg over 30 minutes separated by 20-30 (24.9 +/- 4.0) minutes of electrophysiologic testing. Blood samples were obtained at 0.17 hours and just prior to the second infusion, and then at 0.17, 0.25, 0.33, 1.0, 6.0, 12.0, and 24.0 hours after the second infusion. Paired serum samples were assayed for quinidine concentrations by fluorescence polarization immunoassay and high-performance liquid chromatography. The two assays compared well, with a linear regression equation of Y = 0.927X + 0.247 with a correlation coefficient of 0.985. With the exception of the beta elimination rate constant and beta distribution volume, t test comparison of disposition values demonstrated no significant difference. Differences in the estimates of the beta elimination rate constant reflected differences in the two methods and indicated that even though both assays were comparable, subtle differences in specificity could be reflected in significant differences in this variable.

Disposition of 3-hydroxyquinidine in patients receiving initial intravenous quinidine gluconate for electrophysiology testing of ventricular tachycardia

The formation rate constant and elimination rate constant for 3-hydroxyquinidine were determined in eight patients with ventricular tachycardia. These two parameters (mean +/- SD) were found to be 0.784 +/- 0.202 and 0.042 +/- 0.058 h-1, respectively. Coefficients of determination for the computer-generated line of best fit for serum concentration-time data were 0.986 +/- 0.008. Patients received two infusions of quinidine gluconate 5 mg/kg over 30 minutes separated by a 20-30 minute electrophysiologic testing period. Unbound and total 3-hydroxyquinidine concentrations were also determined. Among the eight patients, 3-hydroxyquinidine was 61.9 percent bound. Studies in healthy volunteers had shown 50 percent binding. Linear regression of unbound and total 3-hydroxyquinidine was described by the equation Y = 0.3814X-1.448, r = 0.813. Although half-lives of 3.5-12.4 hours had been reported in healthy volunteers, prolonged half-lives were observed in all but two of our arrhythmia patients.

Human serum and plasma protein binding of enoximone and its sulfoxide metabolite

Experimental factors and determinants of the protein binding of enoximone (a new cardiotonic agent) were investigated in human serum from healthy, drug-free subjects using a rapid ultrafiltration method; these factors and determinants included nonspecific binding to the apparatus, ultrafiltrate volume, temperature, serum pH, enoximone serum concentration, and enoximone sulfoxide (metabolite) concentration. It was demonstrated from mass balance experiments that nonspecific binding to the apparatus did not occur. Within the range investigated, ultrafiltrate volume did not affect the binding result. However, serum pH and temperature were critical variables. At pH 7.4 and 37 degrees C, enoximone serum binding occurred to the extent of approximately 70%; over the therapeutic serum concentration range, this binding was concentration independent. Experiments with purified albumin solutions indicated that much of the serum binding could be accounted for by albumin. At concentrations exceeding those observed clinically, enoximone sulfoxide did not affect enoximone serum binding. In another experiment, enoximone binding to serum was compared with that from plasma containing either heparin or disodium EDTA. There were essentially no differences. Enoximone sulfoxide serum protein binding was also investigated in serum from healthy, drug-free human subjects; binding occurred to the extent of approximately 5%.

Pharmacokinetics of the trimethoprim-sulphamethoxazole combination in the elderly

The pharmacokinetics of a co-trimoxazole preparation (Bactrim Forte) containing trimethoprim (TMP) 160 mg and sulphamethoxazole (SMZ) 800 mg were determined in six young adults (29.3 +/- 4.4 s.d. years) and six elderly people (78.6 +/- 6.6 s.d. years). Following oral administration of a single dose, the pharmacokinetic parameters of SMZ and its N4-acetylated metabolite (N4SMZ) were similar in both groups. However Cmax of TMP was greater (2.06 +/- 0.29 s.d. vs 1.57 +/- 0.32 s.d. mg l-1; P less than 0.01) and its area under the curve was larger (34.30 +/- 6.98 s.d. vs 23.87 +/- 3.82 s.d. mg l-1 h; P less than 0.001) in elderly people than in younger subjects. Total clearance (CL/F) of TMP normalized to body weight was not significantly different in the two groups. There was no significant difference in serum protein binding of TMP and SMZ between the two groups. Urinary excretion of TMP, SMZ and N4SMZ was reduced by about 50% in the elderly compared to the young subjects. Renal clearance of TMP was significantly lower in the elderly group (19 +/- 10 s.d. vs 55 +/- 14 s.d. ml h-1 kg-1; P less than 0.001). Renal clearance of SMZ was not significantly different in the two groups. A study of plasma concentrations of TMP, SMZ and N4SMZ during continuous dosing in seven elderly patients treated for urinary or respiratory infections showed that steady state was reached after 3 days of treatment and that plasma drug concentrations were about two to three times higher than those observed after a single dose.

Oxaprozin pharmacokinetics in the elderly

A series of 42 healthy male and female volunteers aged 21 to 89 years received a single 1200 mg oral dose of oxaprozin. Kinetics were determined from multiple plasma oxaprozin concentrations measured by h.p.l.c. during 14 days after the dose. Peak plasma oxaprozin concentrations were reached between 3 and 6 h after dosage the majority of subjects, probably reflecting slow absorption from the gastrointestinal tract. Elimination also was slow with a mean half-life of 59 h (range 36 to 92 h). Owing in part to extensive protein binding (mean free fraction 0.0023%), oxaprozin distribution was limited, with apparent volume of distribution averaging 0.25 l/kg. Apparent volume of distribution declined with increasing age, probably reflecting the reduction in lean mass relative to total weight that occurs in the elderly. Total apparent oxaprozin clearance declined with age in men (r = -0.58, P less than 0.01), but was not significantly related to age in women (r = -0.25, NS). This is consistent with the previously described gender-specific reduction in hepatic oxidizing capacity association with increasing age. Thus oxaprozin is a slowly eliminated nonsteroidal anti-inflammatory agent that should be suitable for once daily or every other day administration.

Alpha 1-acid glycoprotein concentrations and propranolol binding in elderly patients with acute illness

Alpha 1-acid glycoprotein (AAG) concentrations and propranolol binding were investigated in the serum of elderly hospitalized patients with acute illness, and healthy elderly and young subjects. Significantly greater AAG concentrations and reduced unbound propranolol fraction were observed in the elderly with acute disease compared to the elderly controls. The greatest changes (up to five-fold) occurred with cancer, with lesser changes associated with myocardial infarction and ischaemic heart disease, acute infection, heart failure, chronic obstructive respiratory disease, and cerebrovascular accident. Various miscellaneous conditions were also associated with high AAG concentrations and enhanced propranolol binding. The healthy elderly had higher AAG concentrations and lower unbound propranolol fractions than the healthy young group. Overall there was a highly significant correlation between the propranolol binding ratio (bound/free) and the serum AAG concentration. These results suggest that the elderly population may be particularly susceptible to changes in AAG concentrations, and that during acute illness interpretation of serum concentrations of drugs which bind mainly to AAG, may require knowledge of their free fractions.

Factors affecting quinidine protein binding in humans

The free (unbound) concentration of drug in plasma is often an important determinant of pharmacological and toxicological effects. Unfortunately, studies examining the factors influencing the free fraction of quinidine in plasma have yielded inconsistent results. It is probable that differences in the type of blood collection tubes utilized and the analytical procedure employed biased some of these estimates of quinidine binding. The present study was executed in a manner free of factors now known to introduce artifacts into estimates of the free fraction of quinidine. In healthy volunteers, the free fraction of quinidine (1.0 microgram/mL) was 0.129 +/- 0.019 (mean +/- SD) and was constant throughout the therapeutic range. A high-affinity, low-capacity binding site (K = 1.17 X 10(5) M-1; nP = 3.49 X 10(-5) M) and a low-affinity, high-capacity binding site (K = 1.33 X 10(3) M-1; nP = 3.14 X 10(-3) M) were identified. The characteristics of quinidine binding in a 4.5-g/dL solution of human serum albumin (K = 3.05 X 10(3) M-1; nP = 1.36 X 10(-3) M) suggested that the low-affinity, high-capacity binding site was on this quinidine free fraction increased from 0.114 to 0.231. A lidocaine concentration of 250 micrograms/mL caused a similar increase. Patients suffering traumatic injury had a significant increase in alpha 1-acid glycoprotein concentration (197 mg/dL) and a decreased quinidine free fraction (0.075 +/- 0.019). Patients with hyperlipidemia had free fractions similar to those observed in healthy individuals (0.118 +/- 0.019).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of pentobarbital, quinidine, lidocaine, and theophylline in the thermally injured rat

Previous studies have shown that rats with 15% third-degree burns show a severe depression in various in vitro hepatic drug-metabolizing enzymes. This effect was assessed in vivo by measuring the disposition of four liver-metabolized drugs in 16% third-degree burned rats at 7 d postburn. Compared with pair-fed control rats, pentobarbital demonstrated a significantly prolonged clearance and elimination half-life without a change in volume of distribution. Quinidine demonstrated a significantly increased volume of distribution and a significantly decreased clearance without a change in elimination half-life. Lidocaine showed a significantly increased volume of distribution. Theophylline, which is only 50% metabolized in the rat, showed no changes in any pharmacokinetic parameters. The free drug fractions of quinidine and lidocaine were found to be significantly decreased at 1 d postburn and normal at 7 d postburn. These results warrant pharmacokinetic studies in human burn patients.

Antiarrhythmic therapy: clinical pharmacology update

Currently available antiarrhythmic agents are limited by side effects and the potential for increasing arrhythmias. In addition, drug interactions, altered disposition of drug as a result of changes in protein binding or concomitant disease processes, active metabolites, and poorly defined therapeutic ranges with great interpatient variability are some of the factors which complicate therapy. An awareness of the possible contribution of these factors in the use of antiarrhythmics is invaluable in both the choice of agent and the establishment of an optimum benefit-to-risk ratio for the patient.

Noninteraction of temazepam and cimetidine

The possible kinetic interaction of the hypnotic temazepam and the H2-receptor antagonist cimetidine was evaluated. Nine healthy male and female volunteers received a 30-mg oral dose of temazepam on two occasions in random sequence, separated by at least 1 week. On one occasion, temazepam was given in the otherwise drug-free state; on the other, temazepam was given with concurrent administration of cimetidine, 300 mg every 6 h. Mean pharmacokinetic parameters for temazepam in control versus cimetidine trials were: peak plasma concentration, 560 versus 498 ng/mL; time of peak concentration, 2.0 versus 2.1 h after the dose; volume of distribution, 1.30 versus 1.39 L/kg; elimination half-life, 9.9 versus 11.4 h; total clearance, 1.59 versus 1.60 mL/min/kg; free fraction of temazepam in plasma, 4.1 versus 3.8% unbound. Cimetidine has been shown to reduce the metabolic clearance of the benzodiazepines that are biotransformed by oxidative mechanisms. Temazepam, transformed by conjugation, appears unaffected by the coadministration of cimetidine.

Propranolol binding in serum: comparison of methods and investigation of effects of drug concentration, pH, and temperature

Three methods, equilibrium dialysis, ultrafiltration through dialysis bags, and ultrafiltration through ultrafree filters were compared for the determination of propranolol binding in serum. The method of choice was equilibrium dialysis, which gave values for the free fraction which were not significantly different from those obtained by ultrafiltration through the filters. Of the three methods, equilibrium dialysis also had the best precision and required the least sample volume (1 ml). Similar values for the free fraction were also obtained using either 14C- or 3H-radiolabeled propranolol by this method. Investigation of the effect of temperature and pH of serum indicated that small changes in either parameter could result in up to twofold alterations in the free fraction, but changes in propranolol concentrations over the range 10-500 ng/ml had little effect on the free fraction. Using a standardized equilibrium dialysis method (i.e., 4 hr dialysis against isotonic phosphate buffer, at 37 degrees C, pH 7.4), a mean free fraction of 14.7% (range 8.4-23.3) was observed in 34 samples from 21 healthy subjects. This was significantly greater (p less than 0.001) than that (6.8%, range 3.1-12.3) found in nine elderly hospitalized patients with acute disease. The patients also had significantly raised levels of alpha 1-acid glycoprotein (AAG), which is a major binding protein for propranolol in serum. A significant linear correlation (r = 0.884, p less than 0.02) was found between the binding ratio for propranolol and serum AAG concentrations in these subjects.

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.

Interaction of cimetidine with the triazolobenzodiazepines alprazolam and triazolam

The influence of cimetidine on the pharmacokinetics of alprazolam and triazolam, two triazolobenzodiazepines metabolized by hepatic microsomal oxidation, was evaluated in a series of healthy volunteers. Subjects ingested single 1.0 mg dose of alprazolam or 0.5 mg doses of triazolam on two occasions, with and without concurrent administration of cimetidine (300 mg) every 6 h. For alprazolam, which has a low hepatic clearance and low extraction ratio, cimetidine significantly impaired total metabolic clearance (1.05 versus 1.66 ml/min/kg, P less than 0.005), resulting in significantly prolonged elimination half-life (16.6 versus 12.4 h, P less than 0.005). For triazolam, which has higher hepatic clearance and an intermediate extraction ratio, total clearance was reduced by cimetidine (3.9 versus 5.9 ml/min/kg), causing a significant increase in total area under the plasma concentration curve (25 versus 38 ng/ml X h, P less than 0.02). However, elimination half-life of triazolam was not influenced by cimetidine (3.3 versus 3.2 h), indicating that the reduction in clearance was manifested as increased systemic availability. Thus, cimetidine impairs the clearance of both alprazolam and triazolam, but the consequences of the kinetic change are different because of the differing hepatic extraction profiles of the two drugs.

Prolongation of drug half-life due to obesity: studies of desmethyldiazepam (clorazepate)

Desmethyldiazepam pharmacokinetics were determined after oral administration of its precursor, clorazepate, to 12 obese subjects (mean weight: 105.4 kg; mean percent ideal body weight: 170%) who were matched for age, sex, and smoking habits with 12 normal controls (66.5 kg; percent ideal body weight: 103.3%). After an overnight fast, a single 15-mg clorazepate capsule, equivalent to 10.3 mg of desmethyldiazepam, was administered. Multiple plasma samples drawn 10-42 days postdose were analyzed for desmethyldiazepam by electron-capture GLC. Obese subjects compared to controls had a prolonged desmethyldiazepam elimination half-life (t1/2) (154.1 hr versus 57.1 hr; p less than 0.005). Assuming quantitative conversion of clorazepate to desmethyldiazepam and 100% systemic availability, volume of distribution (Vd) was greatly increased in the obese (158.8 liters versus 63.3 liters; p less than 0.001). The value of Vd remained greater even after correction for body weight (1.52 liter/kg versus 0.94 liter/kg; p less than 0.005). However, clearance of desmethyldiazepam was not different between groups (13.2 ml/min in obese versus 13.4 ml/min in controls). The percent ideal body weight was highly correlated with Vd (r = 0.82), as was total body weight (r = 0.86). The value of t1/2 was correlated highly with Vd (r = 0.89) but only weakly with clearance (r = -0.38). Therefore, the large increase in the desmethyldiazepam t1/2 value seen in obese subjects is predominantly due to the disproportionate distribution of this lipid-soluble drug into body fat as opposed to lean tissue. The contribution of clearance to desmethyldiazepam t1/2 was of much less importance than was Vd in this obese study population.

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

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

Pharmacokinetics of drugs in obesity

Pharmacokinetic data in obesity are only available for a limited number of drugs. The rate or extent of drug absorption is not known to be altered by obesity which is not complicated by other medical disease. In contrast, drug distribution is in some instances significantly altered in obesity. Digoxin and bromsulphalein, both hydrophilic drugs, do not distribute into excess bodyweight over ideal bodyweight. However, antipyrine, paracetamol, aminoglycoside antibiotics and theophylline, all of intermediate lipid solubility, distribute to a limited extent into excess bodyweight over ideal bodyweight, although not as extensively as into ideal bodyweight. Benzodiazepines, halothane and enflurane, highly lipophilic agents, may distribute much more extensively into excess bodyweight over ideal bodyweight than into ideal bodyweight. This suggests some relationship between drug distribution and drug lipid-solubility in obese subjects. Whether these distributional changes in obesity will hold for other drugs is not currently known. Drug protein binding has not been demonstrated to be changed in obesity, data being presently available only for the basic drugs diazepam and desmethyldiazepam. Drug biotransformation in obesity is unchanged for oxidatively metabolised drugs studied to date. Paracetamol, the only metabolised drug which is conjugated for which pharmacokinetic parameters have been accurately determined in obesity, undergoes increased clearance in obese subjects.

Factors influencing the apparent protein binding of quinidine

Various factors influencing the apparent protein binding of quinidine were examined. Different binding values in rabbit plasma were obtained by equilibrium dialysis techniques employing three commonly used buffers. Binding values comparable to those found by ultrafiltration were achieved after dialysis against isotonic phosphate buffer for approximately 4 hr. Dialysis beyond 8 hr gave an increased free fraction with time. The reported effect of in vitro added heparin on plasma protein binding could be prevented by reducing the final concentration in blood from 20 to 5 U/ml, a concentration still sufficient to prevent clotting of the blood sample. Daily freezing and thawing of plasma samples over 1 week did not alter the binding of quinidine. The samples were stable for at least 2 months at -20 degrees.

Diazepam kinetics in patients with renal insufficiency or hyperthyroidism

1 Eight patients with end-stage renal insufficiency on maintenance haemodialysis, and seven patients with newly diagnosed hyperthyroidism, received a single intravenous dose of diazepam, followed by blood sampling over the next 7 days. Fifteen healthy volunteer controls, matched with patients for age and sex, were similarly studied. 2 Diazepam half-life in renal failure patients (mean 37 h) was greatly reduced compared to controls (mean 92 h, P less than 0.05) and clearance of total (free plus bound) diazepam correspondingly increased (0.94 v 0.34 ml min-1 kg-1, P less than 0.01). 3 However, differences were largely related to disease-related changes in drug binding and distribution. Mean unbound fraction of diazepam in plasma of renal patients (7.0%) was greatly increased over controls (1.4%, P less than 0.01) and Vd of unbound diazepam greatly reduced (57 v 157 l/kg, P less than 0.01). 4 Clearance of pharmacologically active unbound diazepam (intrinsic clearance) was not significantly different between renal patients and controls (23 vs 30 ml min-1 kg-1). 5 None of the kinetic variables for total or unbound diazepam in thyrotoxic patients differed significantly from those in controls matched for age and sex. 6 End-stage renal failure (or its associated drug therapy) alters diazepam protein binding and distribution, but does not significantly change clearance of unbound drug. Thyrotoxicosis does not influence diazepam kinetics.

Quinidine in falciparum malaria

Fourteen patients with falciparum malaria were successfully treated with oral quinidine. Twelve of these patients were followed for 35 days without recrudescence. In six patients the infection had already recrudesced after antimalarial treatment, which in two cases had included a full course of quinine. Quinidine caused no cardiotoxicity, although the electrocardiogram QTc interval was prolonged by more than 25% in four patients. In-vitro cultures from nine of these patients and a further seven patients with falciparum malaria showed that the minimum inhibitory concentration was consistently lower for quinidine than for quinine. Quinidine is an effective antimalarial drug for Plasmodium falciparum infections and may be more potent than quinine.

Clobazam kinetics in the elderly

1 The effects of age and sex on the disposition of clobazam (CBZ), a 1.5-benzodiazepine derivative, were evaluated in a series of 29 healthy volunteers aged 18 to 72 years, who ingested single 20 mg oral doses. CBZ kinetics were determined from multiple plasma concentrations measured during 7 days after the dose. 2 CBZ was rapidly absorbed, with peak levels reached an average of 1.5 h after dosing (range 0.5--2.5 h). Mean absorption half-life was 19.7 min. Absorption kinetics were not influenced by age of sex. 3 Elimination half-life ranged from 11 to 77 h, and was significantly longer in elderly v young males (48 v 17 h, P less than 0.01). In women, half-life also increased with age, but differences between young and elderly women were less striking (31 v 49 h, P less than 0.05). 4 Volume of distribution (Vd) was influenced by age and sex. Vd became larger with age regardless of sex, and within each age group was larger in women than in men. Total clearance was unrelated to age in women, but declined significantly with age in men (P less than 0.01). 5 The mean free fraction for CBZ in plasma was 11.5% (range 8.6--15.0%), and tended to increase with age, partly due to a significant age-related decline in plasma albumin concentration (r = -0.68, P less than 0.001). Correction of Vd and clearance for individual differences in binding did not alter their relation to age and sex. 6 As in the case of other benzodiazepines biotransformed by oxidative pathways, the capacity for N-demethylation of CBZ declines with age in men, but age has a minimal effect on CBZ clearance in women.

Effect of age and gender on disposition of temazepam

Thirty-two male and female volunteers, 24-84 years of age, ingested single 30-mg doses of temazepam, a 3-hydroxy-1,4-benzodiazepine derivative used as a hypnotic agent. Kinetics of total and unbound temazepam were determined from multiple plasma temazepam concentrations measured during 48 hr after the dose. The temazepam elimination half-life ranged from 8 to 38 hr and was longer in women than in men (16.8 versus 12.3 hr, p less than 0.05). Likewise, clearance of total temazepam (assuming complete absorption) was higher in men than in women (1.35 versus 1.02 ml/min/kg, p less than 0.025). Neither half-life nor clearance was significantly related to age. The volume of distribution of total temazepam (mean 1.40 liters/kg) was unrelated to age or gender. Temazepam was extensively protein bound, with a mean free fraction of 2.6% (range 12.7-3.4%). The free fraction increased with age (r = 0.45, p = 0.01), partly due to the inverse relation of the free fraction to plasma albumin concentration (r = -0.34, p = 0.06) and the age-related decline in plasma albumin (r = -0.49, p less than 0.005). After correction for individual differences in binding, clearance of unbound temazepam in men was higher than in women (50.5 versus 39.7 ml/min/kg, 0.05 less than p less than 0.01), and it tended to decline with age in both sexes (r = -0.44 and -0.43, respectively, p = 0.1).

Human blood preservation: effect on in vitro protein binding

In vitro plasma protein binding for phenytoin, meperidine, and bretylium tosylate was affected by the type of preserved human blood used for its estimation. Fresh heparinized plasma and serum gave equivalent fractions bound at the concentrations studied for all three drugs. However, the in vitro plasma binding of phenytoin and meperidine decreased 9-50% when estimated in fresh citrated plasma or commercially available lyophylized human serum at the concentration levels investigated. The fraction of bretylium tosylate bound to plasma protein decreased 30-40% when estimated in fresh citrated plasma but was unchanged when estimated in the lyophylized human serum.

Bioavailability of 11 quinidine formulations adn pharmacokinetic variation in humans

The bioavailabilities of eight quinidine sulfate, two gluconate, and one polygalacturonate formulations were compared, with one of the sulfate formulations as a reference (R) in a panel of 24 volunteers, according to a design comprising duplicate 6 x 6 Latin squares in two subject groups. Only one gluconate formulation (H) gave a significantly lower (p less than 0.05) area under the curve from 0 to 30 hr (AUC30), 90% or R, which was not as significant as AUC infinity (94% of R). Formulation H also gave a significantly lower peak concentration (Cmax) and a longer time to peak concentration (tmax) and generally exhibited some characteristics of sustained-release product. In addition, one product (F) gave a significantly higher Cmax while another formulation (D) gave a longer tmax. The wide range of dissolution times obtained with these products with three test conditions was not reflected in the AUC, Cmax, or tmax values obtained, except the Formulation H was consistently the slowest to dissolve. The terminal rate constants, expressed as t 1/2, of the 24 subjects gave an overall mean of 7.49 +/- 0.77 hr and ranged from 6.24 +/- 0.28 to 0.49 +/- 0.90 hr in individuals. The estimated total body clearance, with the assumption that the oral bioavailability was 70%, gave an overall mean of 4.22 +/- 1.05 and ranged from 2.49 +/- 0.28 to 6.42 +/- 0.70 mg/min/kg in individuals, demonstrating the wide range of quinidine disposition even in healthy subjects; this finding is in agreement with recently published results.

Comparative protein binding of diazepam and desmethyldiazepam

The extent of plasma protein binding of diazepam (DZ) and its major metabolite, desmethyldiazepam (DMDZ), was determined by equilibrium dialysis in plasma samples drawn from 62 nonfasting unheparinized volunteers aged 20 to 85 years. The free fraction for diazepam averaged 1.48 per cent (range 0.85 to 2.30 per cent) and increased with age (r = 0.33). Desmethyldiazepam also was extensively bound. The mean free fraction was 2.97 per cent (range 1.78 to 5.28 per cent) and increased with age (r = 0.27). Free fractions for both diazepam and desmethyldiazepam were negatively correlated with plasma albumin concentration (r = --0.17 and --0.39). However, age, sex, and albumin explained only a small proportion of variability in free fraction for either compound. Free fraction for desmethyldiazepam always exceeded that for diazepam, and the two were correlated (r = 0.32). Thus, at any given total plasma concentration, the unbound concentration of desmethyldiazepam will exceed that of diazepam.

Protein binding of methotrexate in sera from normal human beings: effect of drug concentration, pH, temperature, and storage

Ultrafiltration through dialysis tubing was used to assess factors influencing methotrexate (MTX) binding to serum proteins. Determinations of percentage bound by this method either using addition of radioactive tracer or measuring absolute concentrations by radioimmunoassay were not significantly different. Likewise, similar values were obtained using equilibrium dialysis and ultracentrifugation for assessment of percentage bound. Reduction in the pH of serum from 7.9 to 7.2 resulted in a significant decrease in binding, whereas reduction in temperature from 37 to 21 degrees C resulted in a significant increase. Binding was also observed to decrease significantly in serum samples containing MTX that had been stored frozen at -20 degrees C for a month. No significant reduction in the percentage bound was observed on increasing MTX concentration from 10(-10) M until it had risen to 10(-4)--10(-3) M. Analysis at 37 degrees C of fresh sera with 10(-6) M MTX added and pH 7.4 from 19 healthy volunteers gave a mean percentage bound 46.5 +/- 2.7 (SD) with range 41.5-51.0%. No significant difference was found between males and females, and no significant linear correlations were observed between the extent of binding with serum concentrations of total proteins, albumin, total bilirubin or triglycerides, or with age.

Intravenous quinidine in congestive cardiomyopathy

Eight male patients with compensated congestive cardiomyopathy received single 300-mg doses of intravenous quinidine by 15-min infusion. Left ventricular (LV) performance was evaluated by echocardiography at multiple points in time during the next 24 h. Quinidine kinetics and protein binding were determined from multiple serum samples drawn for up to 36 h after dosage. LV function was not impaired. Instead, quinidine transiently increased ejection fraction (mean: +39%) and rate of circumferential shortening (mean: +46%). Endsystolic and end-diastolic LV internal diameter likewise were decreased (means: -13% and -7%). Blood pressure and ventricular rate were not significantly altered. Compared to 8 healthy controls matched for age, sex, and weight, quinidine volume of distribution among patients was smaller (means: 2.27 vs 1.90 l/kg), as was total quinidine clearance (3.49 vs 2.84 ml/min/kg); however, differences were not statistically significant. Well-controlled, slow intravenous infusion of quinidine does not impair LV performance and is safe for patients with compensated congestive cardiomyopathy. However, such patients may have reduced quinidine clearance and hence require lower doses than expected based on age and weight.

Binding of diazepam and desmethyldiazepam to plasma protein: concentration-dependence and interactions

The binding of diazepam (DZ) and its major metabolite desmethyldiazepam (DMDZ) to plasma protein was evaluated in a series of controlled in vitro studies using equilibrium dialysis. Free fraction (FF) of both drugs alone changed significantly with total plasma drug concentration, but the increased FF (reduction in binding) did not occur until concentrations considerably exceeded those encountered during typical therapeutic use. Increasing concentrations of one drug at a time tended to increase FF for the other, although the effects were, at most, of borderline significance. Simultaneously increasing concentrations of both drugs led to significantly increased FF for both. Thus, DZ and DMDZ appear to bind to the same site or sites on plasma albumin. Binding is concentration-independent within and considerably above the usual therapeutic range.

Effect of age and sex on disposition of desmethyldiazepam formed from its precursor clorazepate

Desmethyldiazepam (DMDZ) disposition was evaluated in 32 healthy male and female volunteers who ingested single 15-mg doses of the precursor compound, clorazepate dipotassium. DMDZ concentrations were measured in multiple plasma samples obtained between 7 and 9 days after dosage. Appearance of DMDZ in blood was rapid, with peak concentrations attained on average 1.5 h after dosage. Absorption half-life (t1/2 a) averaged 24 min. Neither peak time nor t1/2 a were influenced by age or sex. After a rapid phase of distribution, DMDZ elimination was slow, with a mean elimination half-life (t1/2 beta) of 82 h (range 27-219 h). t1/2 beta became prolonged with age in men but not in women. Likewise, clearance of total (free bound) DMDZ declined with age in male subjects (r=- 0.47, P less than 0.1), but was unrelated to age in women. DMDZ was extensively bound to protein in all subjects. The mean free fraction (FF) was 3.1% (range 2.0-4.3%), and increased significantly with declining plasma albumin concentrations (r=-0.57, P less than 0.001). Partly due to a decline in plasma albumin with age (r=-0.47, P less than 0.01), FF tended to increase with age (r=0.23). After correction for individual differences in FF, clearance of pharmacologically active unbound DMDZ declined significantly with age in men (r=-0.65, P less than 0.01), but actually was slightly higher in elderly as opposed to young women. Thus, the age-related decline in the capacity for hepatic hydroxylation of DMDZ is highly sex-specific.

Entry of quinidine into cerebrospinal fluid

Some of the unwanted effects of quinidine commonly occurring in clinical practice involve the central nervous system. We therefore assessed the rate and extent of quinidine passage into cerebrospinal fluid (CSF) in humans and dogs. In eight human subjects receiving oral quinidine therapy, lumbar CSF quinidine concentrations averaged 16% of unbound serum concentrations (range: 4% to 37%). The findings were confirmed when simultaneous serum (total and unbound) and CSF quinidine concentrations were followed for up to 8 hours after a single intravenous dose of quinidine in anesthetized dogs. Quinidine appeared promptly in CSF of all animals, but CSF concentrations averaged only 37% to 46% of unbound serum levels. The in vitro octanol:water partition coefficient for quinidine at physiologic pH was greater than 100, indicating that unbound quinidine should readily traverse the blood-brain barrier. Thus, passage of quinidine into CSF appears not to be governed by passive diffusion alone. Quinidine may participate in an active transport system such as that which removes certain other basic substances from CSF.

Clinical pharmacokinetics of quinidine

The elimination of quinidine is accomplished by a combination of renal excretion of the intact drug (15 to 40% of total clearance) and hepatic biotransformation to a variety of metabolites (60 to 85% of total clearance). Many of the metabolites appear to be pharmacologically active. Typical ranges for kinetic properites of quinidine in healthy persons are: apparent volume of distribution 2.0 to 3.5 litres/kg; elimination half-life 5 to 12 hours; clearance, 2.5 to 5.0 ml/min/kg. Quinidine clearance is reduced in the elderly, in patients with cirrhosis, and in those with congestive heart failure. Oral quinidine is available either as relatively rapidly absorbed conventional tablets (usually quinidine sulphate) or as a variety of slowly absorbed sustained release preparations. Absolute systemic availability generally is 70% or greater. Quinidine is 70 to 95% bound to plasma protein, primarily to albumin but also to a number of other plasma constituents. Binding is reduced in patients with cirrhosis, partly because of hypoalbuminaemia, but is not influenced by renal insufficiency. Clinical interpretation of total serum or plasma quinidine concentrations must be altered in patients with reduced or increased binding, since it is the unbound fraction which is pharmacologically active.

Entry of diazepam and its major matabolite into cerebrospinal fluid

Five dogs received a single 1.0 mg/kg dose of diazepam (DZ) IV. Concentrations of DZ and its major metabolite desmethyldiazepam (DMDZ) were simultaneously measured in plasma and cisternal cerebrospinal fluid (CSF) for up to 8 h after the dose by electron-capture gas-liquid chromatography. DZ was rapidly eliminated from plasma (half-life 0.3--1.3 h); DZ disappearance was mirrored by formation of DMDZ, which in turn was eliminated slowly, Both DZ and DMDZ rapidly penetrated CSF and concentrations in CSF declined parallel with those in plasma. Despite rapid uptake, the extent of CSF transfer of DZ and DMDZ was limited by plasma protein binding. Mean CSF:plasma concentrtion ratios for DZ (range 0.023--0.137) and DMDZ (range 0.047--0.119) were highly correlated with the unbound fraction in plasma (r = 0.95 and 0.80, respectively). Thus DZ and DMDZ concentrations in CSF, presumed to reflect concentrations at the site of action, are determined by unbound plasma concentrations. The intensity of pharmacologic action is more likely to correlate with unbound than with total plasma concentrations.