Bioavailability of two licensed paediatric rifampicin suspensions: implications for quality control programmes.

SETTING: To assess the revised World Health Organization-recommended dose of 10-20 mg/kg rifampicin (RMP), we studied the steady state pharmacokinetics of RMP in South African children who received standard treatment for drug-susceptible tuberculosis (TB). OBJECTIVE: To determine the formulation effect on the pharmacokinetics of RMP. DESIGN: RMP plasma concentrations were characterised in 146 children (median age 1.4 years, range 0.2-10.2). The morning dose on the day of the pharmacokinetic evaluation was administered as one of two RMP single-drug oral suspensions. RESULTS: While one formulation achieved 2 h concentrations in the range of those observed in adults (median 6.54 mg/l, interquartile range [IQR] 4.47-8.84), the other attained a median bioavailability of only 25% of this, with a median 2 h concentration of 1.59 mg/l (IQR 0.89-2.38). CONCLUSION: RMP is a key drug for the treatment of TB. It is critical that the quality of RMP suspensions used to treat childhood TB is ensured.

Single and divided doses of penbutolol.

Penbutolol, a nonselective beta-adrenoreceptor antagonist, induced reduction of exercise-induced heartbeats for at least 24 hr after a single 40-mg oral dose, and was equipotent with respect to a 2 X 20-mg regimen over the same period. Ingestion for 7 days did not influence the pharmacodynamics or pharmacokinetics of penbutolol, and there was no cumulation of drug in serum. A relationship was found between the logarithms of measurable serum concentrations of penbutolol and the percentage reduction of total heartbeats. Absorption of oral penbutolol appeared to be reduced when administered in the evening. Since beta-adrenoceptor activity was relatively unchanged between 13 and 24 hr after a single 40-mg dose of penbutolol, there is a possibility that an active metabolite or metabolites may contribute to prolonged duration of action.

Antithrombotic activity of Bay u3405, a thromboxane A2-antagonist, in patients with Dacron aortic grafts: a random controlled clinical trial.

Twelve patients with Dacron aortic grafts participated in a placebo controlled, crossover trial to investigate the effect of Bay u3405, a thromboxane A2 receptor antagonist, on graft thrombogenicity. During each treatment period (seven days, Bay u3405 or placebo), 111In-platelet survival and platelet deposition on the grafts were measured daily by gamma-camera imaging and blood radioactivity analysis. Bay u3405 substantially reduced the deposition of platelets and the thrombogenic index, while platelet survival remained unchanged. The ex vivo platelet aggregation response to ADP and epinephrine was significantly inhibited. The bleeding time increased slightly but not to any clinically relevant extent, and no adverse side effects were recorded. Bay u3405 seems to be a safe and effective drug for the inhibition of platelet deposition on aortic Dacron grafts. The use of quantitative imaging techniques is also more sensitive than the measurement of platelet survival for the assessment of antiplatelet drug efficacy in patients with aortic grafts.

No pharmacokinetic or pharmacodynamic interaction between rivastatin and warfarin.

Twenty-one healthy, male volunteers completed this double-blind, randomized, two-period, crossover study to determine the possible pharmacodynamic and pharmacokinetic interaction of the concomitant administration of rivastatin and warfarin sodium in healthy volunteers. The study comprised 2 treatment periods of 8 days each, with a medication-free period of 14 days between the 2 treatment periods. According to the randomization, the volunteers received either 300 micrograms of rivastatin or matching placebo once daily during the treatment periods. On day 4 of each treatment period, the volunteers also received a single oral dose of 25 mg of warfarin sodium together with rivastatin or matching placebo. The effect of rivastatin on both the pharmacokinetics and pharmacodynamics (prothrombin time and clotting factor VII activity) of warfarin sodium, and the effect of warfarin sodium on the pharmacokinetics of rivastatin were investigated. Blood sample assays included the analysis of both R- and S-warfarin, because it is known that the enantiomers differ in anticoagulant potency. The study results indicate that the concomitant administration of rivastatin and warfarin does not affect the pharmacokinetics of R- and S-warfarin, or the pharmacodynamics of warfarin. Furthermore, the administration of warfarin sodium does not affect the pharmacokinetics of rivastatin.

Automated high-performance liquid chromatographic method for the determination of acyclovir in plasma.

Owing to the low plasma concentrations of acyclovir obtained during pharmacokinetic studies after low-dosage oral administration, a sensitive, automated HPLC method had to be developed for determining acyclovir in a large number of plasma samples. Extraction and injection of the samples were done automatically by a Gilson ASPEC system using tC18, 100-mg Sep-Pak Vac extraction columns. The extracts were chromatographed on a Nova-Pak C18 column with sodium octanesulphonate and methanol in the mobile phase. The analyte was detected at 250 nm. The calibration graphs were linear up to at least 1200 ng/ml and the limit of quantification was 10 ng/ml.

Determination of fluspirilene in human plasma by liquid chromatography-tandem mass spectrometry with electrospray ionisation.

An ultra-sensitive method for the determination of fluspirilene in plasma was established, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted with hexane/isoamyl alcohol, separated on a Phenomenex Luna C18 5 mu 150 x 2.1 mm column with a mobile phase consisting of methanol-water-acetic acid (600:400:1) at a flow-rate of 0.3 ml/min. Detection was achieved by a Finnigan Matt mass spectrometer (LCQ) at unit resolution in full scan mode scanning the product ion spectrum from m/z 130-500 and monitoring the transition of the protonated molecular ion at m/z 476.2, to the sum of the largest product ions m/z 371, 342 and 274 (MS-MS). Electrospray ionisation was used for ion production. The mean recovery for fluspirilene was 90% with a lower limit of quantification of 21.50 pg/ml using 1 ml plasma for extraction. This is the first chromatographic method described for the determination of fluspirilene in plasma that is accurate and sensitive enough to be used in pharmacokinetic studies.

Rapid and sensitive liquid chromatography-tandem mass spectrometry method for the quantitation of domperidone in human plasma.

A rapid and sensitive method for the determination of domperidone in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometry detection. The samples were rendered basic with 1 M Na2CO3 and the domperidone extracted using tert.-butyl methyl ether, followed by back-extraction into formic acid (2% in water). Chromatography was performed on a Phenomenex Luna C8 (2), 5 microm, 150x2 mm column with a mobile phase consisting of acetonitrile-0.02% formic acid (300:700, v/v), delivered at 0.2 ml/min. Detection was performed using an Applied Biosystems Sciex API 2000 mass spectrometer set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recovery of domperidone was +/- 100%, with a lower limit of quantification set at 0.189 ng/ml. This assay method makes use of the increased sensitivity and selectivity of tandem mass spectrometric detection resulting in a rapid (extraction and chromatography) and sensitive method for the determination of domperidone in human plasma, which is more sensitive than previously described methods.

Highly specific and sensitive liquid chromatography-tandem mass spectrometry method for the determination of 3-desmethylthiocolchicine in human plasma as analyte for the assessment of bioequivalence after oral administration of thiocolchicoside.

A sensitive method for the determination of 3-desmethylthiocolchicine in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The plasma samples were extracted with ethyl acetate and separated on a Phenomenex Luna C18(2) 5 microm, 150x2 mm column with a mobile phase consisting of acetonitrile-0.005% formic acid (350:650, v/v) at a flow rate of 0.35 ml/min. Detection was achieved by an Applied Biosystems API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recovery for 3-desmethylthiocolchicine was 70%, with a lower limit of quantification set at 0.39 ng/ml. The increased selectivity of mass spectrometric (MS-MS) detection allowed us to distinguish between thiocolchicoside and its primary metabolite 3-desmethylthiocolchicine in human plasma, thereby giving more insight about the pharmacokinetics of the drug in humans.

Sensitive liquid chromatography-tandem mass spectrometry method for the determination of loratadine and its major active metabolite descarboethoxyloratadine in human plasma.

A sensitive method for the simultaneous determination of loratadine and its major active metabolite descarboethoxyloratadine (DCL) in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted from plasma with toluene followed by back-extraction into formic acid (2%) for DCL after which the toluene containing the loratadine was evaporated, the analyte reconstituted and combined with the DCL back-extract. Chromatography was performed on a Phenomenex Luna C18 (2) 5-microm, 150x2.1-mm column with a mobile phase consisting of acetonitrile-0.1% formic acid using gradient elution (10 to 90% acetonitrile in 2 min) at a flow-rate of 0.3 ml/min. Detection was achieved by a Perkin-Elmer API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recovery for loratadine and descarboethoxyloratadine was 61 and 100%, respectively, with a lower limit of quantification at 0.10 ng/ml for both the analyte and its metabolite. This is the first assay method described for the simultaneous determination of loratadine and descarboethoxyloratadine in plasma using one chromatographic run. The method is sensitive and reproducible enough to be used in pharmacokinetic studies.

Sensitive liquid chromatographic-tandem mass spectrometric method for the determination of fluoxetine and its primary active metabolite norfluoxetine in human plasma.

A sensitive method for the simultaneous determination of fluoxetine and its major active metabolite norfluoxetine in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted from alkalised plasma with hexane-isoamyl alcohol (98:2, v/v) followed by back-extraction into formic acid (2%). Chromatography was performed on a Phenomenex Luna C18 (2) 5 microm, 150x2 mm column with a mobile phase consisting of acetonitrile-0.02% formic acid (340:660, v/v) at a flow-rate of 0.35 ml/min. Detection was achieved by a Perkin-Elmer Sciex API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode. TurbolonSpray ionisation was used for ion production. The mean recoveries for fluoxetine and norfluoxetine were 98 and 97%, respectively, with a lower limit of quantification set at 0.15 ng/ml for the analyte and its metabolite. This assay method makes use of the increased sensitivity and selectivity of mass spectrometric (MS-MS) detection to allow for a more rapid (extraction and chromatography) and sensitive method for the simultaneous determination of fluoxetine and norfluoxetine in human plasma than has previously been described.

Extractionless and sensitive method for high-throughput quantitation of cetirizine in human plasma samples by liquid chromatography-tandem mass spectrometry.

Following a single 10-mg oral dose of cetirizine dihydrochloride to 24 healthy volunteers, the analyte was quantified in human plasma. Protein precipitation using acetonitrile (ACN) was followed by reversed-phase liquid chromatography and tandem mass spectrometry. The MS/MS method was optimised using a PE Sciex API 2000 triple quadrupole mass spectrometer in selected reaction monitoring (SRM) mode, using electrospray with positive ionisation. Oxybutynin was used as the internal standard. The assay method represents a robust, high-throughput, highly specific and sensitive quantitative assay procedure, with 0.5 ng/ml being the lowest plasma concentration that could be reliably quantified. The procedure involves minimal sample preparation, and is well suited to clinical studies of the drug involving large numbers of generated samples. Pre-dose as well as post-dose samples up to and including 48 h were quantified, and the data generated were used to determine the pharmacokinetic profile of the drug.

Determination of carbamazepine and carbamazepine 10,11-epoxide in human plasma by tandem liquid chromatography-mass spectrometry with electrospray ionisation.

A sensitive method for the determination of carbamazepine and carbamazepine 10,11-epoxide in plasma is described, using high-performance liquid chromatographic separation with tandem mass spectrometry. Samples were purified using liquid-liquid extraction and separated on a Phenomenex Luna C18 5 microm. 150 x 2 mm column with a mobile phase consisting of acetonitrile, methanol and formic acid (0.1%) (10:70:20, v/v). Detection was performed by a Micromass Quattro Ultima mass spectrometer in the MRM mode (LC-MS-MS) using electro spray ionisation (ESI+), monitoring the transition of the protonated molecular ion for carbamazepine at m/z 237.05 and carbamazepine 10,11-epoxide at m/z 253.09 to the predominant ions of m/z 194.09 and 180.04, respectively. The mean recovery was 95% for carbamazepine and 101% for carbamazepine 10,11-epoxide, with a lower limit of quantification of 0.722 ng/ml for carbamazepine and 5.15 ng/ml for carbamazepine 10,11-epoxide, when using 0.5 ml plasma. This high-throughput method was used to quantify 230 samples per day, and is sufficiently sensitive to be employed in pharmacokinetic studies.

Sensitive and rapid liquid chromatography-tandem mass spectrometry method for the determination of stavudine in human plasma.

A sensitive method for the determination of stavudine in plasma was developed, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. The samples were extracted from plasma with Waters, Sep-Pak Vac, 100 mg, tC(18) solid-phase extraction (SPE) columns. Chromatography was performed on a Supelco Discovery C(18), 5 microm, 150 x 2 mm column with a mobile phase consisting of ammonium acetate (0.01 M)-acetonitrile-methanol (800:100:100, v/v/v) at a flow-rate of 0.3 ml/min. Detection was achieved by an Applied Biosystems API 2000 mass spectrometer (LC-MS-MS) set at unit resolution in the multiple reaction monitoring mode (MRM). Atmospheric pressure chemical ionization (APCI) was used for ion production. The mean recovery for stavudine was 94% with a lower limit of quantification set at 4 ng/ml. This assay method makes use of the increased sensitivity and selectivity of mass spectrometric (MS-MS) detection to allow for a more rapid (extraction and chromatography) and selective method for the determination of stavudine in human plasma than has previously been described.

Sensitive liquid chromatography-tandem mass spectrometry method for the determination of clarithromycin in human plasma.

A sensitive method for the determination of clarithromycin in plasma is described, using high-performance liquid chromatographic separation with tandem mass spectrometric detection. Samples were prepared using liquid-liquid extraction and separated on a Supelco Discovery C18 column with a mobile phase consisting of acetonitrile, methanol and acetic acid. Detection was performed by a PE SCIEX API 2000 mass spectrometer in the multiple reaction monitoring (MRM) mode (LC-MS-MS) using TurbolonSpray ionization and monitoring the transition of the protonated molecular ion for clarithromycin at m/z 748.5 (M+1) to the predominant product ion of m/z 158.2. The mean recovery of clarithromycin was 87.3%, with a lower limit of quantification of 2.95 ng/ml when using 0.3-ml plasma. This high-throughput method was used to quantify 230 samples per day, and is sufficiently sensitive to be employed in pharmacokinetic studies.

Sensitive and rapid liquid chromatography-tandem mass spectrometry method for the determination of meloxicam in human plasma.

Meloxicam was quantified in human plasma after a single 15 mg oral dose of the drug was given to 26 healthy volunteers. An Applied Biosystems Sciex API 2000 triple quadrupole mass spectrometer in multiple reaction monitoring (MRM) mode, using TurboIonSpray (TIS) in the positive ion mode, was used. Protein precipitation with acetonitrile was followed by C(18) reverse phase liquid chromatography and tandem mass spectrometry. The mean recovery for meloxicam was 92% with a lower limit of quantification of 8.96 ng/ml. Piroxicam was used as the internal standard. This assay method makes use of the increased sensitivity and selectivity of tandem mass spectrometry (MS-MS) detection to allow for a more rapid (extraction and chromatography) and selective method for the determination of meloxicam in human plasma than has previously been described.

Selective, sensitive and rapid liquid chromatography-tandem mass spectrometry method for the determination of alfuzosin in human plasma.

A selective, sensitive and rapid liquid chromatography-tandem mass spectrometry method for the determination of alfuzosin in plasma was developed. A PE Sciex API 2,000 triple quadrupole mass spectrometer in multiple reaction monitoring (MRM) mode, using TurboIonSpray with positive ionisation was used. Using prazosin as an internal standard, liquid-liquid extraction was followed by C(18) reversed-phase liquid chromatography and tandem mass spectrometry. The mean recovery for alfuzosin was 82.9% with a lower limit of quantification set at 0.298 ng/ml, the calibration range being between 0.298 and 38.1 ng/ml. This assay method makes use of the increased sensitivity and selectivity of tandem mass spectrometric (MS-MS) detection to allow for a more rapid (extraction and chromatography) and selective method for the determination of alfuzosin in human plasma than has previously been described. The assay method was used to quantify alfuzosin in human plasma samples generated in a multiple-dose (5 mg bd.) study at steady state.

GLC determination of phendimetrazine in serum.

A sensitive, specific, and quantitative GLC method for the determination of phendimetrazine in serum is described. The procedure involves the addition of an internal standard to serum samples, followed by extraction at pH 13 into toluene. The extracted bases are back-extracted into 1 ml of 4 M hydrochloric acid and again into 100 mu1 of chloroform after making the 4 M hydrochloric acid extract basic with 1.5 ml of 4 M sodium hydroxide. The sensitivity of the method is such that 25 ng of material can be detected in 5 ml of serum.

Pharmacokinetic characteristics for extent of absorption and clearance in drug/drug interaction studies.

Many aspects of drug/drug interaction studies, including aspects of the design, choice of pharmacokinetic characteristics, and statistical analysis can be adapted from bioequivalence studies [Steinijans et al. 1991]. However, an important difference between drug/drug interaction studies and bioequivalence studies is that two formulations in bioequivalence studies generally do not differ with respect to the clearance of the drug under investigation, but in drug/drug interaction studies an effect of one drug on the clearance of another drug is not only possible, but the likely mechanism of interaction for many classes of drugs. Thus, while in bioequivalence studies two formulations are conventionally compared with respect to the rate and extent of absorption of the drug, in drug/drug interaction studies equivalence has to be shown with respect to not only the rate and extent of absorption, but also, and in particular, with respect to the clearance of the drug. Consequently, in drug/drug interaction studies the area under the curve is not a pure characteristic of the extent of absorption, but a composite characteristic of extent of absorption and clearance. This should be taken into account when interpreting the results of drug/drug interaction studies. Apart from standard characteristics such as Cmax and AUC used in bioequivalence studies, for drug/drug interaction studies we suggest the elimination half-life as a characteristic for the clearance, and the ratio of AUC and the elimination half-life as a characteristic for the extent of absorption of a drug.

Absolute bioavailability of a new high dose methylprednisolone tablet formulation.

This was a single-blind, single-dose, randomized crossover study to determine the absolute bioavailability of Medrol, a new high dose (100 mg) methylprednisolone tablet product, by comparing it with 100 mg methylprednisolone from an intravenous formulation, Solu-Medrol. Fourteen healthy, non-smoking, Caucasian male volunteers took part. On treatment days volunteers remained recumbent for 4 hours after drug administration, with food and fluid intake standardized over this period. Serial blood samples were drawn over a 14-hour period after drug administration. Plasma methylprednisolone concentrations were determined by high performance liquid chromatography. The geometric means of AUCi.v. and AUCtablet were 4,049 and 3,334 ng.h/ml, respectively. The absolute bioavailability of the tablet product was 82%, which is in agreement with published data for other oral dosage forms of methylprednisolone. Volunteers displayed the expected rise in peripheral blood neutrophil count, but no other clinically relevant changes in hematology or clinical chemistry were observed. No adverse drug reactions were recorded. It is concluded that the tablet product can be used as a substitute for parenteral methylprednisolone in situations requiring high-dose therapy.

No influence of pantoprazole on the pharmacokinetics of phenytoin.

Twenty-three healthy, male volunteers completed this doubleblind, randomized, placebo controlled, 2-period crossover study to assess the influence of multiple doses of pantoprazole on single-dose phenytoin pharmacokinetics. During each treatment period, the volunteers received either one 40 mg pantoprazole tablet or placebo for 7 days. In addition, a single-dose of 300 mg (3 x 100 mg capsules) phenytoin sodium was administered on day 4 of each treatment period. A 14-day wash-out period was allowed between phenytoin administrations. The results indicate that pantoprazole neither affects the rate nor the extent of absorption, nor the elimination of phenytoin.