What is the true clinical significance of plasma protein binding displacement interactions?
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Binding, Competitive↗
Biomedical subjects
L N Sansom
Publications and source records attributed to L N Sansom.
Identification of two human brain aryl sulfotransferase cDNAs.
A 1,179 bp and a 1,424 bp full-length aryl sulfotransferase cDNAs were isolated from a human brain cDNA library. Their coding domains are 93% identical, each encoding a cytosolic protein of 295 amino acids. Their deduced amino acid sequences of these cDNAs are also 93% identical. The 1179 bp brain cDNA has an identical coding domain to a previously reported human liver aryl sulfotransferase cDNA but it has a different 5' noncoding sequence. Northern blot analysis using a probe specific for the 1,424 bp cDNA identified a 1500 bp band in mRNA of human liver, colon, kidney and lung. In a human hepatocellular carcinoma the same band plus an extra larger band was also recognised. An intron of the gene encoding the 1424 bp cDNA was also identified.
Amino Acid Sequence↗
cDNA expression studies of rat liver aryl sulphotransferase.
A cDNA encoding an isoenzyme of rat liver aryl sulphotransferase was isolated from a rat liver bacteriophage Lambda gt 11 library by the polymerase chain reaction technique. The resulting cDNA was functionally expressed in COS-7 cells and characterised by determining the sulphating capacity of the cells with a range of substrates. The COS-expressed enzyme catalysed the sulphation of both phenol and dopamine with Kms of the same order as those obtained for the high affinity isozyme in rat liver cytosol, while low activity was observed with tyrosine methyl ester. The common food additive vanillin was also a good substrate for sulphate conjugation. The sulphation of vanillin catalysed by the COS-expressed enzyme was consistent with a single enzyme system, in contrast, the kinetics of the reaction catalysed by cytosolic sulphotransferase indicated that vanillin was sulphated by more than one isozyme.
Animals↗
Co-regulation of phenytoin and tolbutamide metabolism in humans.
1. The disposition of phenytoin and tolbutamide was compared in eighteen healthy young adults separately administered single therapeutic doses (sodium phenytoin 300 mg, tolbutamide 500 mg) of the two drugs. 2. Within the group, ratios of ranges of total and unbound areas under the plasma concentration-time curves were similar for both drugs. 3. There were significant (P < 0.001) correlations between total (r = 0.88) and unbound (r = 0.86) areas under the plasma phenytoin and tolbutamide concentration-time curves. 4. The results are consistent with the involvement of the same cytochrome P-450 isoenzyme(s) in the metabolism of tolbutamide and phenytoin.
Adult↗
The inhibition of drug oxidation by anhydroerythromycin, an acid degradation product of erythromycin.
The inhibition of steroid 6 beta-hydroxylase activity by anhydroerythromycin, an acid breakdown product of erythromycin, has been studied and compared to the effects of erythromycin using liver microsomes from control and dexamethasone pretreated rats and human liver microsomes. Both anhydroerythromycin and erythromycin were found to be demethylated, thus both fulfil the prerequisites for possible metabolite-cytochrome P450 complex information. The formation of a metabolite-cytochrome P450 complex was demonstrated for anhydroerythromycin by preincubating NADPH fortified microsomes with anhydroerythromycin. This complex formation could be reversed by incubating the microsomes in 50 microM potassium ferricyanide. Anhydroerythromycin was a more potent inhibitor of androst-4-ene-3,17-dione (androstenedione) 6 beta-hydroxylation than erythromycin. Kinetic analysis shows that there are probably two cytochromes P450 involved in androstenedione 6 beta-hydroxylation in control rat microsomes both of which are inhibited by anhydroerythromycin. There are at least two forms of cytochrome P450 responsible for androstenedione 6 beta-hydroxylation in microsomes from dexamethasone pretreated rats but only the high affinity form is inhibited by anhydroerythromycin. "Atypical" kinetics were observed in human microsomes but inhibition of androstenedione 6 beta-hydroxylation was observed with 5 microM anhydroerythromycin at all androstenedione concentrations used. Inconsistencies have been observed in the literature with respect to clinical interactions observed with erythromycin. Since anhydroerythromycin appears to be a more potent inhibitor of androstenedione 6 beta-hydroxylation than erythromycin, we speculate that the variable blood levels of anhydroerythromycin found after dosing with erythromycin may explain these discrepancies.
Animals↗
Relationship between phenytoin and tolbutamide hydroxylations in human liver microsomes.
1. The metabolic interaction of phenytoin and tolbutamide in human liver microsomes was investigated. 2. Phenytoin 4-hydroxylation (mean Km 29.6 microM, n = 3) was competitively inhibited by tolbutamide (mean Ki 106.2 microM, n = 3) and tolbutamide methylhydroxylation (mean Km 85.6 microM, n = 3) was competitively inhibited by phenytoin (mean Ki 22.6 microM, n = 3). 3. A significant correlation was obtained between phenytoin and tolbutamide hydroxylations in microsomes from 18 human livers (rs = 0.82, P less than 0.001). 4. Sulphaphenazole was a potent inhibitor of both phenytoin and tolbutamide hydroxylations with IC50 values of 0.4 microM and 0.6 microM, respectively. 5. Mephenytoin was a poor inhibitor of both phenytoin and tolbutamide hydroxylations with IC50 values greater than 400 microM for both reactions. 6. Anti-rabbit P450IIC3 IgG inhibited both phenytoin and tolbutamide hydroxylations in human liver microsomes by 62 and 68%, respectively. 7. These in vitro studies are consistent with phenytoin 4-hydroxylation and tolbutamide methylhydroxylation being catalysed by the same cytochrome P450 isozyme(s) in human liver microsomes.
Binding, Competitive↗
Effect of racemic ibuprofen dose on the magnitude and duration of platelet cyclo-oxygenase inhibition: relationship between inhibition of thromboxane production and the plasma unbound concentration of S(+)-ibuprofen.
1. Four healthy male subjects received racemic ibuprofen (200, 400, 800 and 1200 mg), orally, on four occasions, 2 weeks apart, according to a four-way Latin-square design, in order to investigate the influence of increasing dose of ibuprofen on the magnitude and duration of its antiplatelet effect as well as on the relationship between such effect and drug concentration. 2. The antiplatelet effect of ibuprofen was assessed by measuring the inhibition of platelet thromboxane B2 (TXB2) generation during the controlled clotting of whole blood. The plasma unbound concentration of S(+)-ibuprofen, the enantiomer shown in an in vitro study to be responsible for the inhibitory effect of platelet TXB2 generation, was measured using an enantioselective method. 3. The maximum percentage inhibition of TXB2 generation increased significantly with dose from a mean +/- s.d. of 93.4 +/- 1.2% after the 200 mg dose to 98.8 +/- 0.3% after the 1200 mg dose, and there was an increase with dose in the duration of inhibition of TXB2 generation. The effect of ibuprofen on platelet TXB2 generation was transient and mirrored the time-course of unbound S(+)-ibuprofen in plasma; on all but one of the 16 occasions, serum TXB2 concentrations returned to at least within 10% of the pretreatment concentrations within 24 h of ibuprofen administration. 4. For each subject, the relationship between the percentage inhibition of TXB2 generation and the unbound concentration of S(+)-ibuprofen in plasma was modelled according to a sigmoidal Emax equation. The mean plasma unbound concentration of S(+)-ibuprofen required to inhibit platelet TXB2 generation by 50% (EC50) was 9.8 +/- 1.0 micrograms l-1.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
Pharmacokinetic-pharmacodynamic modeling of the electroencephalographic effects of benzodiazepines. Correlation with receptor binding and anticonvulsant activity.
The relevance of EEG effect parameters as a measure of pharmacological effect intensity of benzodiazepines was evaluated. The concentration-EEG effect relationships of four benzodiazepine agonists, flunitrazepam, midazolam, oxazepam and clobazam, were quantified in individual rats and correlated with receptor affinity and anticonvulsant effect intensity of these compounds. Male Wistar-derived rats received a single i.v. dose of flunitrazepam (2.5 mg/kg), midazolam (5 mg/kg), oxazepam (10 mg/kg) and clobazam (20 mg/kg). Arterial blood samples were drawn frequently and EEG was monitored continuously until it had returned to preadministration levels. The concentrations of the benzodiazepines were determined by chromatographic means. Plasma protein binding was determined at 37 degrees C by ultrafiltration. The amplitudes in the 11.5 to 30 Hz frequency range, determined by aperiodic analysis, was used as EEG effect measure. Concentration-EEG effect relationships were derived by a pharmacokinetic-pharmacodynamic modeling procedure and characterized by the sigmoidal Emax model. The EC50 based on free drug concentrations (EC50,U, mean +/- S.E.) calculated for flunitrazepam (4.2 +/- 0.7 ng/ml) and midazolam (3.7 +/- 0.5 ng/ml) were similar and significantly less than the values for oxazepam (49 +/- 4 ng/ml) and clobazam (277 +/- 34 ng/ml) and illustrates the importance of using parameters referenced to unbound drug for comparative purposes. The maximal responses (Emax) for midazolam, oxazepam and clobazam were significantly less than for flunitrazepam suggesting that these three drugs may be regarded as partial agonists when compared to flunitrazepam. Receptor affinity was determined based on displacement of [3H] flumazenil in a washed brain homogenate at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)
Animals↗
Phenytoin 4-hydroxylation by rabbit liver P450IIC3 and identification of orthologs in human liver microsomes.
The ability of rabbit liver microsomes to 4-hydroxylate phenytoin to 5-(4-hydroxyphenyl)-5-phenylhydantoin was studied. No significant difference was observed between the capacity of control and rifampicin, phenobarbital, acetone, 2,3,7,8-tetrachlorodibenzo-p-dioxin and phenytoin induced rabbit liver microsomes to 4-hydroxylate phenytoin. In reconstitution experiments using six purified rabbit cytochromes P450 isozymes, only P450IIC3 was capable of 4-hydroxylating phenytoin whereas P450IA1, P450IA2, P450IIB4, P450IIIA6, and P450IVB1 were inactive. Further, anti-P450IIC3 IgG completely inhibited phenytoin 4-hydroxylase activity in rabbit liver microsomes. The above data suggest a major role for the constitutive isozyme P450IIC3 in phenytoin 4-hydroxylase activity in rabbit liver. In human liver microsomes P450IIC3 IgG inhibited phenytoin 4-hydroxylase activity by 66%, suggesting that an ortholog to rabbit P450IIC3 is in part responsible for this activity in man.
Animals↗
Bioavailability of a new sustained-release theophylline capsule in fasted and non-fasted healthy subjects: single and multiple dosing studies.
Eighteen healthy, non-smoking, adult volunteers participated in single and multiple dose three-way crossover studies to evaluate a sustained-release, pellet-filled capsule of theophylline, Austyn. The effect of food on the bioavailability of the sustained-release capsule was investigated by administering 300 mg single doses of Austyn, with a high-fat meal and without food and a divided 300 mg dose of the reference product Elixophyllin elixir, given after fasting. Plasma theophylline concentrations were measured by fluorescence polarization immunoassay (FPIA) which had been validated against HPLC. The single dose study data showed that there were no significant differences (n = 18, ANOVA, p greater than 0.05) between the three regimens with respect to AUC0-infinity values (mg h l-1), (mean +/- SD); Elixophyllin fasting = 97.1 +/- 33.7, Austyn with food = 90.9 +/- 31.3, Austyn fasting = 91.2 +/- 33.8. Similarly, multiple dosing with rapid-release Nuelin tablets, Austyn capsules, and sustained-release Theo-Dur tablets demonstrated that there were no significant differences between regimens with respect to AUC0-24h, AUC0-12h, and AUC12-24h values calculated from the steady-state concentrations (5th day, 24 h sampling). However, the percentage fluctuation at steady-state over the total blood sampling period was significantly less for treatment with the sustained-release capsule. Austyn, compared with the sustained-release tablet, Theo-Dur (Austyn = 36.7 +/- 13.7 per cent, Theo-Dur = 53.1 +/- 14.1 per cent). The results of the single and multiple dose studies indicate that Austyn capsules demonstrate complete bioavailability, and good controlled release characteristics not influenced by concomitant intake of a high-fat meal and with no evidence of dose dumping.
Adult↗
Plasma and cerebrospinal fluid concentration of temazepam following oral drug administration.
Thirteen male patients were administered 20 mg of temazepam orally 1 to 2 h prior to undergoing spinal anaesthesia for a urological procedure. Samples of blood and CSF were drawn just before insertion of the spinal and the concentration of drug estimated in these two media. The results obtained indicated that a highly significant correlation existed between the unbound concentration of temazepam in plasma and the concentration of drug present in CSF. Temazepam appeared to be an effective light pre-medicant in all of the subjects studied.
Administration, Oral↗
Stereoselective plasma protein binding of ibuprofen enantiomers.
We have developed a novel and reproducible method for determining the plasma protein binding of the two ibuprofen enantiomers in the presence of each other. The method involves the use of radiolabelled racemic ibuprofen, equilibrium dialysis, derivatization of the enantiomers to diastereomeric amides, high-performance liquid chromatography, and radiochemical analysis. We have determined the plasma protein binding of R(-)- and S(+)-ibuprofen in 6 healthy male volunteers after the oral administration of 800 mg racemic ibuprofen. The mean time-averaged percentage unbound of the R(-)-enantiomer, 0.419 was significantly less than that of the S(+)-enantiomer, 0.643, consistent with stereoselective plasma protein binding. The percentage unbound of each ibuprofen enantiomer was concentration-dependent over the therapeutic concentration range and was influenced by the presence of its optical antipode.
Adult↗
Lack of effect of cimetidine on the pharmacokinetics of R(-)- and S(+)-ibuprofen.
1. To investigate the effect of cimetidine on the pharmacokinetics of R(-)- and S(+)-ibuprofen, six healthy male volunteers received orally 800 mg racemic ibuprofen both in the drug-free state (control phase, C) and on the second day of a 3 day course of oral cimetidine, 1 g daily (treatment phase, T). The two phases (14 days apart) were randomised in a balanced cross-over manner. 2. The plasma concentrations of R(-)- and S(+)-ibuprofen were measured by high-performance liquid chromatography (h.p.l.c.). The protein binding of the enantiomers was assessed in a selection of plasma samples from each volunteer. Following alkaline hydrolysis of glucuronide conjugates, the urinary recoveries of ibuprofen and its major metabolites were measured by h.p.l.c. 3. There was no difference (P greater than 0.05, two-tailed Student's t-test; data expressed as mean +/- s.d.) between C and T phases in the total area under the plasma concentration-time curve of R(-)-ibuprofen (C 4514 +/- 1063 mg 1(-1) min vs T 4665 +/- 1435 mg 1(-1) min) and S(+)-ibuprofen (C 6460 +/- 1063 mg 1(-1) min vs T 6886 +/- 1207 mg 1(-1) min). Similarly, for each enantiomer, there was no difference between the two phases in the terminal half-life, the maximum plasma concentration or the time of its occurrence. 4. Cimetidine treatment had no effect (P greater than 0.05) on the time-averaged percent unbound in plasma of R(-)-ibuprofen (C 0.419 +/- 0.051% vs T 0.435 +/- 0.060%) and S(+)-ibuprofen (C 0.643 +/- 0.093% vs T 0.633 +/- 0.053%). (ABSTRACT TRUNCATED AT 250 WORDS)
Absorption↗
Pharmacokinetics of a new sublingual formulation of temazepam.
The pharmacokinetics of a new 10 mg sublingual tablet formulation of temazepam and those of a currently marketed 10 mg oral capsule formulation were evaluated in a group of ten healthy volunteers. No significant differences were observed between the two formulations with respect to any of the pharmacokinetic parameters assessed. Lethargy and somnolence were reported on both capsule and tablet by several subjects at a time which corresponded with the maximum concentration of drug in plasma. The data indicate that the sublingual tablet and orally administered capsule have a similar pharmacokinetic and pharmacodynamic profile.
Administration, Oral↗
Lack of effect of erythromycin on the pharmacokinetics of single oral doses of phenytoin.
The effect of erythromycin on the pharmacokinetics of phenytoin was studied in eight healthy volunteers using a balanced randomised cross-over design. Volunteers received a single oral dose of 400 mg of phenytoin sodium during each phase. During the treatment phase, the phenytoin sodium dose was administered 4.5 days after the commencement of a 7 day course of erythromycin base (250 mg every 6 h). There was no significant difference between the control and treatment phases (P greater than 0.05) with respect to the area under the plasma phenytoin concentration-time curve, the fraction of phenytoin unbound in plasma, the area under the unbound phenytoin concentration-time curve, the elimination half-life of phenytoin or the fraction of the dose excreted in urine as free and conjugated hydroxyphenylphenylhydantoin. This single dose study indicated that the intrinsic clearance of unbound phenytoin was unaffected by the concurrent administration of erythromycin.
Administration, Oral↗
Stereoselective drug disposition: potential for misinterpretation of drug disposition data.
1. Although it is well recognised that the enantiomers of a chiral drug may possess different pharmacokinetic and pharmacodynamic properties, many studies dealing with chiral drugs which are administered as their racemates rely on non-stereoselective analytical techniques. 2. We present a theoretical analysis to illustrate the potential which exists for misinterpretation of drug disposition and plasma drug concentration-effect data generated for a racemic drug using a non-stereoselective assay. 3. It was shown that the use of such an analytical method can lead to the collection of data which may be both quantitatively and qualitatively inaccurate with respect to the individual enantiomers. For example, the clearance of the unresolved drug may indicate concentration- and time-dependence even though this pharmacokinetic process is concentration- and time-independent for each of the enantiomers. 4. The problems discussed emphasise the need to consider stereoselectivity in clinical pharmacological studies involving racemic drugs.
Humans↗
The pharmacokinetics of ketoconazole after chronic administration in adults.
We have studied the pharmacokinetics of the anti-mycotic ketoconazole in seven patients who took it for 1-6 months at a dose of 200 mg daily. The mean elimination half-life of the drug was 3.3 h, and although the ketoconazole was given only once daily, a satisfactory clinical response was obtained in all seven individuals. Only a small fraction of the absorbed drug (mean 0.22%) was excreted unchanged in the urine, suggesting almost complete metabolism. Our results support the concept that anti-mycotic activity in the tissues continues after the plasma drug concentration has fallen below a critical level. Our results also support the concept of a change in pharmacokinetics with chronic dosing.
Adult↗
Moxalactam kinetics during continuous ambulatory peritoneal dialysis after intraperitoneal administration.
Moxalactam kinetics during continuous ambulatory peritoneal dialysis (CAPD) was followed in eight patients after a single intraperitoneal dose of 1 g. Approximately 60% of the dose was absorbed after a dwell time of 4 h. Dialysis solutions were exchanged at 4-h intervals with an overnight dwell of 8 h. The mean (+/- standard deviation) elimination half-life was 13.2 +/- 2.9 h, and the mean apparent volume of distribution was 0.22 +/- 0.08 liters/kg. Mean total clearance was 11.5 +/- 2.4 ml/min, with a mean dialysis clearance of 2.3 +/- 0.5 ml/min. The maximum concentration in plasma ranged from 24.5 to 54.1 micrograms/ml. Moxalactam concentrations in the peritoneal dialysis fluid were above 80 micrograms/ml during the first exchange and above 2 micrograms/ml for a further three exchanges. A suggested intraperitoneal dose regimen for patients undergoing CAPD is 1 g initially, followed by 15 to 25% of the recommended dose for normal patients given at the same time intervals, or 30 to 50% of the recommended dose at twice the usual intervals. Moxalactam is suggested for initial treatment of peritonitis in CAPD patients who do not have ready access to the antibiotic of choice.
Adult↗