Determination of the enantiomers of omeprazole in blood plasma by normal-phase liquid chromatography and detection by atmospheric pressure ionization tandem mass spectrometry.

An enantioselective assay of omeprazole in blood plasma using normal-phase liquid chromatography on a Chiralpak AD column and detection by mass spectrometry is described. Omeprazole is extracted by a mixture of dichloromethane and hexane and, after evaporation, redissolution and injection, separated into its enantiomers on the chiral stationary phase. Detection is made by a triple quadrupole mass spectrometer, using deuterated analogues as internal standards. The method enables determination in plasma down to 10 nmol/l (LOQ) and shows excellent consistency suited for pharmacokinetic studies in man.

Determination of candesartan cilexetil, candesartan and a metabolite in human plasma and urine by liquid chromatography and fluorometric detection.

Liquid chromatographic methods are described for the determination of a new effective anti-hypertensive drug candesartan (CV-11974), its prodrug candesartan cilexetil (TCV-116) and a metabolite, CV-15959 in human plasma and urine. The assays comprise liquid-liquid extraction and separation on a phenyl column with fluorometric detection. The methods give absolute recoveries of 70, 83 and 78% for candesartan cilexetil, candesartan and CV-15959, respectively, and the limit of quantification is 5, 1 and 3 nM of plasma (RSD < 20%), respectively. The methods were applied to plasma and urine samples from biopharmaceutical and clinical studies in man.

Lack of drug-drug interaction between three different non-steroidal anti-inflammatory drugs and omeprazole.

OBJECTIVE: To study, in three separate investigations, the potential interaction between omeprazole and three different non-steroidal anti-inflammatory drugs (NSAIDs; diclofenac, naproxen and piroxicam) in healthy male and female subjects. METHODS: Each investigation was an open, randomized, three-way cross-over study, in which the subjects were given omeprazole 20 mg once daily for 1 week, the NSAID in therapeutic daily doses (diclofenac 50 mg bid, naproxen 250 mg bid, or piroxicam 10 mg om), or a combination of omeprazole and each NSAID. The plasma concentrations of the NSAID as well as of omeprazole were determined on the last day of each investigation period. RESULTS: None of the NSAIDs studied had any effect on the plasma concentration versus time curve (AUC) of omeprazole. It was also demonstrated that omeprazole 20 mg daily had no significant influence on the pharmacokinetics of the NSAIDs. The AUC ratio, (NSAID +omeprazole):NSAID alone, was 1.11, 0.99, and 0.99 for diclofenac, naproxen, and piroxicam, respectively. CONCLUSION: Diclofenac, naproxen, and piroxicam can be administered together with omeprazole 20 mg daily without need for dosage alteration. There was no significant change in the bioavailability of theses NSAIDs during omeprazole therapy in this study.

Evaluation of an on-line solid-phase extraction method for determination of almokalant, an antiarrhythmic drug, by liquid chromatography.

A fully automated liquid chromatographic method based on a Prospekt solid-phase extraction unit is described for determination of the antiarrhythmic drug almokalant in plasma. The assay comprises solid-phase extraction on a C2 phase and separation on a C18 column with fluorometric detection. In the original procedure 40 samples a day could be run unattended but by modifying the sequence in the solid-phase extraction process it was possible to increase this number to 70. The method gives an absolute recovery of 92% and a repeatability (C.V.) of 2.9% at 75 nmol/l of plasma. The limit of quantitation is 2 nmol/l of plasma (C.V. < 20%). As regards accuracy and precision the performance of the method is as good as the manual method based on liquid-liquid extraction. The Prospekt method is, above all, faster and requires far less manual effort.

Ethnic and genetic determinants of omeprazole disposition and effect.

OBJECTIVES: To compare the pharmacokinetics and dynamics of omeprazole in white and Chinese subjects. METHODS: This double-blind two-stage study, performed in the clinical research center of a university hospital, evaluated 15 healthy nonsmoking men (eight white subjects and seven Chinese extensive metabolizers of mephenytoin). Blood samples were obtained over 24 hours after the eighth omeprazole dose (40 mg/day). Omeprazole, omeprazole sulfone, and hydroxyomeprazole pharmacokinetics were calculated from the respective plasma concentration-time curves. Twelve- and 24-hour integrated plasma gastriun (AUCgas12 and AUCgas24) were calculated from the respective plasma gastrin concentrations. A week before the initiation of omeprazole the activities of CYP2D6, CYP2C19, and CYP3A4 were determined by previously established methods. RESULTS: Omeprazole concentrations were significantly lower (mean area under the plasma concentration time curve extrapolated to infinity [AUCO-infinity] +/- SEM; 7.53 +/- 1.21 versus 12.80 +/- 2.13 mumol.hr.L-1, respectively; p < 0.05) and its oral clearance greater (319 +/- 60 versus 183 +/- 35 ml/min, respectively; p < 0.05) in the white subjects than in the Chinese subjects. Omeprazole and omeprazole sulfone AUCO-infinity values were well correlated with the S/R mephenytoin ratio (r = 0.82 and r = 0.84, respectively; p < 0.001) and with urinary 4'-hydroxymephenytoin (r = -0.58 [p < 0.03] and r = -0.52 [p < 0.02], respectively). Fasting gastrin, AUCgas12, and AUCgas24 were significantly greater in the Chinese subjects than in the white subjects (30.0 +/- 6.4 versus 14.4 +/- 1.2 pmol, respectively [p < 0.02]; 661 +/- 114 versus 334 +/- 38 pmol.hr.L-1, respectively [p < 0.002]; and 1414 +/- 228 versus 747 +/- 99 pmol.hr.L-1, respectively [p < 0.004]). In addition, the S/R mephenytoin ratio and omeprazole AUCO-infinity correlated with the extent of omeprazole induced hypergastrinemia. CONCLUSION: The metabolism of omeprazole and the rise in gastrin concentration after its administration is genetically determined and ethnically dependent.

Control of gastric acid with high dose H2-receptor antagonists after omeprazole failure: report of two cases.

Successful omeprazole therapy in patients with symptomatic gastroesophageal reflux (GER) refractory to treatment with H2-receptor antagonists has often been reported. In contrast, successful treatment of GER by H2-receptor antagonists in patients resistant to the acid-suppressing effects of omeprazole is rarely reported. We describe two patients who demonstrated therapeutic responses to high dose H2-receptor antagonists after high dose omeprazole failed to suppress gastric acidity and GER.

High-performance liquid chromatographic assay for human liver microsomal omeprazole metabolism.

Assays for the measurement of omeprazole metabolites in plasma and urine have been reported, but when applied to the determination of omeprazole metabolites formed by human liver microsomal incubations there were obvious limitations in sensitivity. The present high-performance liquid chromatographic (HPLC) assay, which comprises extraction, evaporation and reconstitution, is several-fold more sensitive with a limit of detection of approximately 2 pmol (2 nM in incubate) for omeprazole sulphone and 25 pmol (25 nM in incubate) for hydroxyomeprazole. Extraction efficiency is essentially quantitative and is highly reproducible (coefficient of variation = 2.1% for both metabolites). The assay is linear over a wide range of concentrations and the formation of the metabolites is linear with respect to both time (to 15 min) and protein concentration (to 1.5 mg/ml). Two minor metabolites, one of which was identified tentatively as 5-O-desmethylomeprazole, were also formed by human liver microsomes and could be determined by this method. Preliminary studies of the formation of omeprazole sulphone and hydroxyomeprazole showed that the formation kinetics in human liver microsomes were biphasic for both metabolites, suggesting that at least two different cytochrome P450 isoforms are involved in their formation.

Enantioselective assay of warfarin in blood plasma by liquid chromatography on Chiralcel OC.

A stereoselective method for the two enantiomers of warfarin has been developed. Warfarin is extracted into dichloromethane-hexane from blood plasma. After concentration by evaporation, the enantiomers are resolved by liquid chromatography on Chiralcel OC employing a non-polar mobile phase. By fluorometric detection, concentrations in plasma down to 80 nmol/l (25 ng/ml) can be determined with a coefficient of variation of less than 15%. At the 200 nmol/l level, a coefficient of variation of ca. 4% was found.

Direct chiral separation of almokalant on Chiralcel OD and Chiralpak AD for liquid chromatographic assay of biological samples.

The four isomers of almokalant, a new antiarrhythmic substance under investigation, were separated by liquid chromatography on a Chiralcel OD and a Chiralpak AD column containing cellulose and amylose tris(3,5-dimethylphenylcarbamate), respectively. Both chiral stationary phases separate almokalant into the four isomers, but the retention orders are different if the carbamate is derivatized on cellulose or amylose. The Chiralcel OD column was used for the separation and determination of the isomers in urine at levels down to 100 nmol/l for the first three eluted and 200 nmol/l for the last with a relative standard deviation of less than 15%. The fluorescence response was increased by post-column ionization after stereoselective separation on the Chiralpak AD column. The isomers of almokalant could be determined at levels down to 10 nmol/l in plasma with a relative standard deviation of less than 15%.

A study of the interaction of omeprazole and warfarin in anticoagulated patients.

1. Thirty-five patients on continuous therapy with warfarin were given omeprazole 20 mg once daily and placebo each for 3 weeks according to a two-centre randomised double-blind cross-over design. 2. Blood samples were obtained once weekly during the run-in and follow-up periods as well as during the first 2 weeks of each treatment period, and twice during the last week of each treatment period. Plasma concentrations of R- and S-warfarin were measured by h.p.l.c. and the anticoagulant effect was assessed using the Trombotest. 3. Twenty-eight patients were evaluated. The mean plasma concentration of R-warfarin was increased by 9.5% during omeprazole treatment compared with placebo, while that of S-warfarin, the more active isomer, was unaffected. The coagulation time was not significantly changed (106 s during omeprazole and 98 s during placebo). Corresponding TT-values (Trombotest) were 8.8 and 9.9 (NS).

Lidocaine for treatment of severe seizures in newborn infants. II. Blood concentrations of lidocaine and metabolites during intravenous infusion.

The blood concentrations of lidocaine and its main active metabolites, methylethylglycinexylidide (MEGX) and glycinexylidide (GX), were measured in 24 newborn infants during anticonvulsive treatment with an iv infusion of lidocaine. After a bolus dose of 1.5-2.2 mg/kg and continuous infusion of lidocaine (4.7-6.3 mg/kg/h) there was accumulation of the drug and MEGX within 24 h. After termination of the iv infusion, both lidocaine and the metabolites were eliminated within 24-48 h. The anticonvulsive effectiveness--estimated by clinical observation and continuous amplitude integrated EEG monitoring (cerebral function monitor)--was immediate in 15 infants (nine term and six preterm). There was no correlation between blood concentrations of lidocaine and metabolites, and anticonvulsive effect (i.e. good, intermediate or no response). No differences in blood concentrations were found between full-term and preterm babies, or between infants with or without birth asphyxia. In combination with a fast withdrawal of the drug, few adverse reactions were seen with the dosages used, even though blood concentrations were high. Routine measurements of lidocaine concentrations during anticonvulsive treatment in neonates seem to be of little clinical value. For evaluation of the anticonvulsive effect and for early detection of seizure activity during lidocaine withdrawal, continuous EEG monitoring is preferable.

Omeprazole treatment does not affect the metabolism of caffeine.

This study was performed to investigate the possible influence of repeated omeprazole dosing on the metabolism of caffeine, which has been shown to reflect the activity of one specific enzyme within the hepatic cytochrome P450 family, P450IA2. Ten healthy, nonsmoking young men participated in this placebo-controlled double-blind trial. Each subject was given omeprazole, 20 mg, every morning for 1 week and placebo every morning for 1 week in random order and separated by a 2-3 week washout period. On the sixth and seventh days of each period urine was collected twice daily, and urinary metabolites of caffeine were determined by high-performance liquid chromatography. The urinary metabolite ratio of three paraxanthine 7-demethylation products relative to a paraxanthine-hydroxylation product corresponds to caffeine clearance and, therefore, to P450IA2 activity. This calculated ratio was 4.8 (95% confidence interval, 3.9-5.6) in the placebo and 4.6 (95% confidence interval, 3.6-5.5) in the omeprazole period. These results show that the metabolism of caffeine was unaltered following omeprazole treatment, indicating that omeprazole treatment has no influence on cytochrome P450IA2 activity in the clinical situation.

Liquid chromatographic separation of the enantiomers of metoprolol and its alpha-hydroxy metabolite on Chiralcel OD for determination in plasma and urine.

The two enantiomers of metoprolol and the four enantiomeric forms of alpha-hydroxymetoprolol were separated by liquid chromatography on a Chiralcel OD column containing a cellulose tris(3,5-dimethyl-phenylcarbamate) chiral stationary phase. The column efficiency was strongly dependent on the flow-rate and the enantioselectivity was influenced by temperature. Of utmost importance for the chiral separation was the water content of the mobile organic phase. The separation system was used for the separation and determination of the enantiomers in plasma and urine samples. The metoprolol enantiomers could be determined by fluorescence down to 10 nmol/l of each in plasma with a relative standard deviation of less than 15%.

A study of the interaction between omeprazole and phenytoin in epileptic patients.

This study was performed to determine the effect of omeprazole, given in therapeutically recommended doses, on the steady-state plasma levels of phenytoin in epileptic patients. Five men and three women of median age 34 years participated in the study. Steady-state plasma levels of phenytoin were measured once a week for 2 weeks before and after, respectively, and during 3 weeks of concomitant omeprazole treatment with 20 mg daily. Urinary excretion of phenytoin and its metabolite [5-(p-hydroxyphenyl)-5-phenyl-hydantoin] were determined before and at the end of the omeprazole treatment period. The steady-state plasma phenytoin levels as well as urinary excretion of phenytoin and its main metabolite were unchanged during omeprazole treatment. The results from this study suggest that concomitant omeprazole treatment in therapeutically recommended doses (20 mg daily) will not significantly affect the steady-state plasma levels of phenytoin in epileptic patients.

Pharmacokinetics and bioavailability of omeprazole after single and repeated oral administration in healthy subjects.

1. Ten healthy subjects were given 20 mg omeprazole EC (enteric coated) granules once daily for 8 days. An i.v. tracer dose of [14C]-omeprazole was given simultaneously with the first and last oral doses and blood sampling was performed thereafter. In order to study the extent of absorption at minimal acid exposure, a single dose of 20 mg omeprazole was also given as a buffered solution, before and after the treatment with EC granules. 2. Kinetic parameters of omeprazole after the i.v. tracer dose were unchanged on repeated dosing while AUC increased by approximately 40% for the solution and 60% for the EC granules. 3. The increased AUC is caused by an increased systemic availability, which may be explained by a decreased first-pass elimination during repeated treatment and/or by a reduced degradation of omeprazole in the stomach secondary to the profound decrease in intragastric acidity caused by the drug. 4. The implication of these findings is that the antisecretory effect of therapeutic doses of omeprazole must be studied during repeated administration and not judged from studies using single doses only.

The pharmacokinetics of omeprazole in humans--a study of single intravenous and oral doses.

The pharmacokinetics of omeprazole, hydroxyomeprazole, omeprazolesulfone, and "remaining metabolites" have been studied in eight young healthy subjects following an acute i.v. and oral dose of 10 and 20 mg of 14C-labeled drug, respectively. The oral dose was given as a buffered solution. Two subjects exhibited essentially higher and more sustained plasma levels of omeprazole than the others. This was due to a higher bioavailability, lower clearance, and longer t1/2 of omeprazole in these two subjects. Maximum concentration (0.7-4.6 mumol/L) was reached between 10 and 25 min after oral dosing. The median bioavailability was 39% (25-117%) and the median systemic plasma clearance was 624 ml/min (range of 59-828 ml/min). The corresponding t1/2 for the i.v. dose was 35 min (16-150 min) and 39 min (14-186 min) after oral administration. The drug was rapidly distributed to extravascular sites (mean t1/2 lambda 1 = 3.0 +/- 0.8 min). Mean Vss was 0.23 +/- 0.04 L/kg. Low systemic clearance of omeprazole was associated with a decreased formation rate of hydroxyomepraxole and "remaining metabolites" while omeprazolesulfone formation seemed to be less affected. However, there was a clear-cut correlation between the t1/2 of omeprazole and of its omeprazolesulfone metabolite, indicating that the elimination of these two compounds is mediated by the same isoenzyme. The mean urinary recovery of the radioactive dose during 96 h was 78.3 +/- 2.3 and 75.7 +/- 2.6% for the i.v. and oral dose, respectively. Insignificant amounts were due to unchanged drug and omeprazolesulfone. The excretion of hydroxyomeprazole during the first 12 h varied between 4.6 to 15.5% of a given dose. The mean recovery of radioactivity in the feces was 19.3 +/- 3.1% of a given i.v. dose and 18.2 +/- 2.3% when given orally. It is concluded that omeprazole is mainly eliminated metabolically and that there is a substantial interindividual variation in the rate of formation of primary and secondary metabolites. This variation in omeprazole disposition is probably of limited clinical importance. The half-life, with a maximum of approximately 3 h, is too short to cause accumulation when the drug is administered in a once-daily regimen.

Enantioselective determination of metoprolol in plasma by liquid chromatography on a silica-bonded alpha 1-acid glycoprotein column.

The enantiomers of metoprolol were determined in plasma samples after direct resolution on a silica bonded alpha 1-acid glycoprotein column. Metoprolol was extracted from plasma into a diethyl ether-dichloromethane mixture and after back extraction to dilute phosphoric acid and adjustment of pH the sample was injected on a Chiral-AGP column for separation of R- and S-metoprolol. It was possible to measure down to 2 nmol per litre plasma with a relative standard deviation of less than 15% by use of gradient elution and fluorescence detection. The analytical method was employed to study the pharmacokinetics of the metoprolol enantiomers after administration of the racemate to humans.

Binding of the beta-blockers timolol and H 216/44 to ocular melanin.

The eyes from pigmented rabbits were instilled with the beta-adrenoceptor antagonists H 216/44 or timolol. After a single instillation (1.9 mumol), the iris and ciliary body contained H 216/44, which decreased with a half-life of approx. 43 days. Daily instillation caused a gradual increase in the content of H 216/44 and timolol in the iris and ciliary body, the steady-state concentration of timolol being 10 times higher than that of H 216/44. The concentrations of H 216/44 were seven times higher in the iris and ciliary body of pigmented rabbits than in albino animals. H 216/44 was reversibly bound to the melanosomes from the iris and ciliary body and not metabolized in this tissue. In vitro binding of timolol and H 216/44 to bovine melanosomes showed comparable multi-site binding curves. The binding of chlorpromazine was substantially higher. The beta-blockers could be more readily released from the melanosomes with aqueous solutions of salt and ethanol than with distilled water. It is concluded that both H 216/44 and timolol bind reversibly to ocular melanin. The differences in binding characteristics in vitro may only partly explain the differences in the in vivo binding to ocular melanin in the rabbit eye.

Determination of metoprolol and two major metabolites in plasma and urine by column liquid chromatography and fluorometric detection.

Metoprolol and its alpha-hydroxy metabolite were determined in plasma down to 2 nmol/l (S.D. 10-15%) after solvent extraction and bonded-phase liquid chromatography with fluorometric detection. The major metabolite with a carboxylic function was also measured in plasma when liquid-solid extraction on a column activated with dodecyl sulphate was applied. In urine the three components were assayed by direct injection of a diluted sample.