Simultaneous determination of bosentan and its three major metabolites in various biological matrices and species using narrow bore liquid chromatography with ion spray tandem mass spectrometric detection

Development and Validation of a UHPLC UV Method for the In-Process Control of Bosentan Monohydrate Synthesis

Bosentan monohydrate (4-tert-butyl-N-[6-(2-hydroxyethoxy)-5-(2-methoxyphenoxy)-2-(pyrimidin-2-yl) pyrimidin-4-yl]benzene-1-sulfonamide monohydrate) is a dual endothelin receptor antagonist (ERA) applied in the treatment of pulmonary arterial hypertension. To achieve effective process control of the bosentan monohydrate synthesis, it was necessary to develop a selective and not highly time-consuming method for ultra-high performance liquid chromatography (UHPLC). The method is characterized by adequate sensitivity, reproducibility and selectivity for the determination of bosentan monohydrate and related compounds from all synthetic stages. The UHPLC separation was carried out by reversed phase chromatography on the Acquity BEH C18 column (100 mm × 2.1 mm, 1.7 µm) with a mobile phase composed of solvent A (0.1 %, v/v, acetic acid in water) and solvent B (methanol), in the gradient mode at the flow rate of 0.4 mL min⁻¹. Limits of detection and quantification for the compounds were ≤0.1 µg mL⁻¹ and 0.3 µg mL⁻¹, respectively. The linearity for all related compounds was investigated as in the range for the active pharmaceutical ingredient (API) and as in the range for the in-process control. The developed method was validated according to the current guidelines, proving the suitability of the method for its intended purpose.

Spectroscopic analysis of bosentan in biological samples after a liquid-liquid microextraction

INTRODUCTION

Microextraction processes with UV-Vis measurement have been developed and validated for analysis of bosentan in biological samples.

METHODS

In this work, liquid-liquid microextraction procedures (DLLME & USAEME) were employed for cleanup, pre-concentration, and determination of bosentan in biological samples by UV-Vis spectroscopy at 270 nm. The method was validated and applied to the determination of bosentan in spiked serum, exhaled breath condensate and urine samples.

RESULTS

Various experimental factors including type of extraction and dispersive solvents and their volumes, pH, sonication time and centrifuging time were investigated. Under the optimum conditions, the method was linear in the range of 1.0-5.0 μg.mL(-1), with coefficient of determination (R(2)) of > 0.998. The limit of detection (LOD) was 0.07 mg.L(-1). Recovery of the target analyte in biological samples was 106.2%. The method could be easily applied for higher concentration of bosentan and needs more improvement for application in the pharmacokinetic investigations where more sensitive methods are required.

CONCLUSION

A simple, low cost, precise and accurate spectrophotometric analysis of bosentan in biological samples after liquid-liquid microextraction were developed and validated for routine analyses.

ESI-MSn and LC-ESI-MS studies to characterize forced degradation products of bosentan and a validated stability-indicating LC-UV method

The present study reports the characterization of forced degradation products of bosentan and a validated stability-indicating HPLC method for the stability testing of bosentan tablets. The forced degradation was carried out under the conditions of hydrolysis, oxidation, dry heat and photolysis. The drug was found unstable in acid, alkali and oxidative media whereas stable to the hydrolysis in water, to dry heat and to photolysis. In total, six degradation products were formed in all conditions which were resolved in a single run on a C-18 column with gradient elution using ammonium acetate buffer (pH 4.5, 5.0mM), methanol and acetonitrile. Structures of all the degradation products were characterized through +ESI-MS(n) and LC-ESI-MS spectral data of bosentan as well as LC-ESI-MS spectral data of the products. The products II-VI were characterized as 6-amino-[2,2']bipyrimidinyl-4,5-diol, 6-amino-5-(2-methoxyphenoxy)-[2,2']-bipyrimidinyl-4-ol, 2-[6-amino-5-(2-methoxyphenoxy)-[2,2']-bipyrimidinyl-4-yloxy]-ethanol, 4-tert-butyl-N-[6-(1-methoxyethoxy)-5-(2-methoxyphenoxy)-[2,2']-bipyrimidinyl-4-yl]-benzenesulfonamide and 4-tert-butyl-N-[6-hydroxy-5-(2-methoxyphenoxy)-[2,2']bipyrimidinyl-4-yl]-benzenesulfonamide, respectively. The peak of the product I was found to be due to two secondary degradation products which co-eluted and were characterized as β-hydroxyethyl p-tert-butylphenylsulfonate (Ia) and 2-[2-(2-hydroxyethoxy)-phenoxy]-ethanol (Ib). These products were formed due to hydrolysis of sulfonamide and alkylaryl ether and the diaryl ether linkages as well as dehydration of the primary alcohol group. The most probable degradation mechanisms were proposed. The HPLC method was found to be stability-indicating, linear (2-100 μg ml(-1)), accurate, precise, sensitive, specific, rugged and robust for quantitation of the drug. The method was applied to the stability testing of the commercially available bosentan tablets successfully.

Determination of the enantiomeric composition of a new insulin sensitizer in plasma samples from non-clinical and clinical investigations using chiral HPLC with electrospray tandem mass spectrometric detection

Two drug assays were developed and applied to assess the enantiomeric composition of an insulin sensitizer drug in plasma after administration of its racemate to man, and in human and animal plasma and serum samples generated after in vitro experiments. The sample preparation for the assays consisted either of protein precipitation and column-switching, or liquid-liquid extraction and direct injection. Subsequently, both assays employed chiral HPLC coupled to atmospheric pressure ionization mass spectrometry. An interconversion of the racemate to a mixture enriched with the (+)-enantiomer could be confirmed for all species and biological matrices. The individual enantiomers could be quantified in the concentration range 0.5-500 ng/ml, starting with a 100-microl plasma aliquot. Inter- and intra-assay precision and accuracy were in the range 0.1-7.9 and 88.8-106.0%, respectively. Run times of 5 min for a single sample allows the analysis of more than 200 samples overnight.

Effective on-line purification for cationic compounds in rat bile using a column-switching LC technique

An on-line purification method for cationic compounds and their metabolites in rat bile was investigated using a column-switching technique. 8-Hydroxyquinoline and its glucuronide were used as test compounds. Bile samples were injected directly into the system and successful on-line extraction with high purification efficiency for analytes was achieved using two-dimensional extraction LC; that is, reversed-phase chromatography followed by cation-exchange chromatography. After removal of the endogenous component by extraction LC, chromatographic separation of the target analyte was performed on an analytical ODS column, followed by identification using UV detection. The quantitative ability of the method was evaluated on the basis of injection repeatability, linearity and accuracy. This novel method was also applied to LC/MS analysis in order to characterise the pharmacokinetics of propranolol in rats, and the metabolites were successfully identified.

Clinical pharmacology of bosentan, a dual endothelin receptor antagonist

Bosentan, a dual endothelin receptor antagonist, is indicated for the treatment of patients with pulmonary arterial hypertension (PAH). Following oral administration, bosentan attains peak plasma concentrations after approximately 3 hours. The absolute bioavailability is about 50%. Food does not exert a clinically relevant effect on absorption at the recommended dose of 125 mg. Bosentan is approximately 98% bound to albumin and, during multiple-dose administration, has a volume of distribution of 30 L and a clearance of 17 L/h. The terminal half-life after oral administration is 5.4 hours and is unchanged at steady state. Steady-state concentrations are achieved within 3-5 days after multiple-dose administration, when plasma concentrations are decreased by about 50% because of a 2-fold increase in clearance, probably due to induction of metabolising enzymes. Bosentan is mainly eliminated from the body by hepatic metabolism and subsequent biliary excretion of the metabolites. Three metabolites have been identified, formed by cytochrome P450 (CYP) 2C9 and 3A4. The metabolite Ro 48-5033 may contribute 20% to the total response following administration of bosentan. The pharmacokinetics of bosentan are dose-proportional up to 600 mg (single dose) and 500 mg/day (multiple doses). The pharmacokinetics of bosentan in paediatric PAH patients are comparable to those in healthy subjects, whereas adult PAH patients show a 2-fold increased exposure. Severe renal impairment (creatinine clearance 15-30 mL/min) and mild hepatic impairment (Child-Pugh class A) do not have a clinically relevant influence on the pharmacokinetics of bosentan. No dosage adjustment in adults is required based on sex, age, ethnic origin and bodyweight. Bosentan should generally be avoided in patients with moderate or severe hepatic impairment and/or elevated liver aminotransferases. Ketoconazole approximately doubles the exposure to bosentan because of inhibition of CYP3A4. Bosentan decreases exposure to ciclosporin, glibenclamide, simvastatin (and beta-hydroxyacid simvastatin) and (R)- and (S)-warfarin by up to 50% because of induction of CYP3A4 and/or CYP2C9. Coadministration of ciclosporin and bosentan markedly increases initial bosentan trough concentrations. Concomitant treatment with glibenclamide and bosentan leads to an increase in the incidence of aminotransferase elevations. Therefore, combined use with ciclosporin and glibenclamide is contraindicated and not recommended, respectively. The possibility of reduced efficacy of CYP2C9 and 3A4 substrates should be considered when coadministered with bosentan. No clinically relevant interaction was detected with the P-glycoprotein substrate digoxin. In healthy subjects, bosentan doses >300 mg increase plasma levels of endothelin-1. The drug moderately reduces blood pressure, and its main adverse effects are headache, flushing, increased liver aminotransferases, leg oedema and anaemia. In a pharmacokinetic-pharmacodynamic study in PAH patients, the haemodynamic effects lagged the plasma concentrations of bosentan.

Development of a method for simultaneous determination of eflucimibe and its three major metabolites in rat plasma by liquid chromatography/electrospray tandem mass spectrometry: a preliminary study

A high-performance liquid chromatography/electrospray ionisation tandem mass spectrometry (HPLC/ESI-MS/MS) method has been developed for the simultaneous determination of eflucimibe, a powerful acyl-coenzyme A cholesterol O-acyltransferase (ACAT) inhibitor, and its main metabolites, in plasma. The ESI and MS/MS parameters were investigated and optimised for each of the four compounds in the positive ion mode. Plasma samples were deproteinised by precipitation with acetonitrile and directly analysed by HPLC/ESI-MS/MS in less than 4 min. Quantitation was performed in the multiple reaction monitoring (MRM) mode for highest sensitivity, selecting the protonated molecules [M+H](+) as precursor ions. The method was demonstrated to be specific and sensitive, and a linear response was observed within a 1-25 ng/mL concentration range. Correlation coefficients (r(2)) greater than 0.9960 were obtained by least-squares regression, and limits of detection down to 0.2 ng/mL were calculated. Therefore, this HPLC/ESI-MS/MS method appears to be an efficient tool, able to provide valuable information for a pharmacological purpose.

Inhibition of Organic Anion Transporting Polypeptide-Mediated Hepatic Uptake Is the Major Determinant in the Pharmacokinetic Interaction between Bosentan and Cyclosporin A in the Rat

In clinical trials, a significant interaction between the endothelin receptor antagonist bosentan and the immunosuppressant cyclosporin A was observed, which could not be rationalized in terms of inhibition of drug-metabolizing enzymes. We present here a study performed in rats investigating the mechanisms underlying this interaction, including the inhibition of active drug transport processes as part of the gastrointestinal absorption and disposition into the liver. In vitro, the majority of bosentan uptake into liver cells was shown to depend on active transport and to be efficiently inhibited by cyclosporin A. All known members of the organic anion transporting polypeptide (oatp) transport protein family expressed in rat liver, i.e., oatp1, oatp2, and oatp4, were shown to be involved in the uptake of bosentan. Results from both series of experiments point to inhibition of active bosentan uptake into the liver by cyclosporin A as the major underlying mechanism for this pharmacokinetic interaction that is in line with reports on other oatp-transported drugs. Significant contributions of other mechanisms such as inhibition of mdr1-mediated drug efflux during gastrointestinal absorption, inhibition of bosentan metabolism, or inhibition of hepatobiliary excretion seemed to be unlikely. The interaction between bosentan and cyclosporin A is a rare example of a pharmacokinetic interaction, which can mostly be attributed to the inhibition of transport processes in the liver. It also demonstrates that inhibition of uptake into the liver might become rate-limiting in the overall elimination process even for compounds whose clearance is dependent on metabolism. The relevance of these findings in the rat for clinical use remains to be explored. It is, however, clear that inhibition of CYP3A4-mediated metabolism by cyclosporin A alone is insufficient to explain the increased bosentan concentrations and that inhibition of hepatocellular uptake offers an attractive mechanistic alternative also in human.

Determination of tezosentan, a parenteral endothelin receptor antagonist, in human plasma by liquid chromatography-tandem mass spectrometry

An analytical method was developed for the quantification of tezosentan in human plasma obtained in clinical studies. The method was linear in the range 1 to 512 ng/ml. After liquid-liquid extraction, the samples were analyzed by reversed-phase HPLC with tandem mass spectrometry. The limit of quantification was 1 ng/ml and the extraction recovery was at least 88.2%. Intra- and inter-assay coefficients of variation were below 10%. Stability tests revealed that tezosentan is stable under the different conditions tested.

Investigation of the mutual pharmacokinetic interactions between bosentan, a dual endothelin receptor antagonist, and simvastatin

BACKGROUND

In vitro, bosentan has been shown to be a mild inducer of cytochrome P450 (CYP) 2C9 and 3A4.

PURPOSE

To investigate in vivo the mutual pharmacokinetic interactions between bosentan and simvastatin, a CYP3A4 substrate.

METHODS

Nine healthy male subjects were treated in a three-period randomised crossover study with: (A) bosentan 125 mg twice daily for 5.5 days; (B) simvastatin 40 mg once daily for 6 days; and (C) bosentan 125 mg twice daily and simvastatin 40 mg once daily for 5.5 and 6 days, respectively. Plasma concentration-time profiles of bosentan and its metabolites (treatments A and C) and simvastatin and beta-hydroxyacid simvastatin (treatments B and C) were determined on day 6.

RESULTS

Steady-state conditions for bosentan and its metabolites were attained on day 4 of treatment. The pharmacokinetic parameters of bosentan and its metabolites were not influenced by concomitant treatment with simvastatin: areas under the plasma concentration-time curve over one administration interval of 12 hours (AUC(tau)) [geometric mean and 95% CI] were 4586 (3719-5656) and 4928 (3945-6156) micro g * h/L. In contrast, bosentan significantly reduced exposure to simvastatin and beta-hydroxyacid simvastatin by 34 and 46%, respectively. AUC(tau) values for simvastatin were 30.5 (23.1-40.2) and 20.0 (15.9-25.1) micro g * h/L and for beta-hydroxyacid simvastatin 43.0 (32.1-57.8) and 23.4 (16.7-32.6) micro g * h/L in treatments B and C, respectively.

CONCLUSIONS

Concomitant treatment with bosentan reduces the exposure to simvastatin and beta-hydroxyacid simvastatin by approximately 40%, indicating that in vivo bosentan is also a mild inducer of CYP3A4.

Influence of mild liver impairment on the pharmacokinetics and metabolism of bosentan, a dual endothelin receptor antagonist

The purpose of the study was to investigate the effect of mild liver impairment on the pharmacokinetics and metabolism of bosentan. Eight patients with mild liver impairment and 8 matching healthy subjects were treated with single and multiple oral 125-mg doses of bosentan. The pharmacokinetic parameters of bosentan and its metabolites were similar in both groups: geometric means for Cmax and AUC for bosentan were 2534 and 1980 ng/ml and 11,957 and 10,781 ng.h/ml after single doses and were 1831 and 1715 ng/ml and 7216 and 7838 ng.h/ml after multiple doses, respectively, in healthy subjects and patients. In both groups, the exposure to the metabolites was low when compared to that to bosentan. The decrease in exposure to bosentan after multiple dosing, indicative of autoinduction, tended to be less pronounced in patients as compared to healthy subjects. Bosentan was well tolerated in this study. In conclusion, the pharmacokinetics, metabolism, and tolerability of bosentan are similar in healthy subjects and patients with mild liver impairment.

Single- and multiple-dose pharmacokinetics of bosentan and its interaction with ketoconazole

AIMS

The present study was conducted to characterize the single- and multiple-dose pharmacokinetics of bosentan, a dual endothelin receptor antagonist, and to investigate a possible pharmacokinetic interaction with ketoconazole.

METHODS

In a randomized, two-way crossover study, 10 healthy male subjects received treatments A and B. Treatment A consisted of a single dose of 62.5 mg bosentan on day 1 followed by 62.5 mg twice daily for 5.5 days. Treatment B consisted of bosentan (62.5 mg twice daily) for 5.5 days plus concomitant ketoconazole (200 mg once daily) for 6 days. Plasma concentrations of bosentan and its three metabolites were measured on days 1 and 7 of treatment A and on day 6 of treatment B.

RESULTS

Bosentan was absorbed and eliminated with a tmax of 4.5 h (range 3.5-6.0 h) and a t(1/2) of 5.4 h (95% CI; 4.5, 6.6). Upon multiple dosing, the exposure to bosentan was reduced by 33% without change in tmax and t(1/2). Concomitant administration of ketoconazole increased the Cmax and AUC of bosentan 2.1- (95% CI; 1.5, 2.7) and 2.3-fold (95% CI; 1.8, 2.9), respectively. Exposure to the metabolites was low and represented less than 25% of that to bosentan both after single and multiple doses. In the presence of ketoconazole, formation of the metabolites was inhibited.

DISCUSSION

The multiple-dose pharmacokinetics of bosentan are consistent with the phenomenon of auto-induction. In the presence of CYP3A4 inhibitors, bosentan concentrations may be increased 2-fold.

Development of a combined protein and pharmacophore model for cytochrome P450 2C9

A combined protein and pharmacophore model for cytochrome P450 2C9 (CYP2C9) has been derived using various computational chemistry techniques. A combination of pharmacophore modeling (using 31 metabolic pathways for 27 substrates), protein modeling (using the rabbit CYP2C5/3 crystal structure), and molecular orbital calculations was used to derive a model that incorporated steric, electronic, and chemical stability properties. The initial pharmacophore model (based on a subset of 17 metabolic pathways for 16 substrates) and the protein model used to construct the combined model were derived independently and showed a large degree of complementarity. The combined model is in agreement with experimental results concerning the substrates used to derive the model and with site-directed mutagenesis data available for CYP2C9. The model has been successfully used to predict the metabolism of substrates not used to construct the model, of which four examples are discussed in detail. The model has also been successful in explaining the differences in substrate specificity between CYP2C9 and CYP2C19.