Development and validation of a gas chromatography–mass spectrometry method for the simultaneous determination of buprenorphine, flunitrazepam and their metabolites in rat plasma: application to the pharmacokinetic study

Metabolites replace the parent drug in the drug arena. The cases of fonazepam and nifoxipam

Fonazepam (desmethylflunitrazepam) and nifoxipam (3-hydroxy-desmethylflunitrazepam) are benzodiazepine derivatives and active metabolites of flunitrazepam. They recently invaded the drug arena as substances of abuse and alerted the forensic community after being seized in powder and tablet forms in Europe between 2014 and 2016. A review of all the existing knowledge of fonazepam and nifoxipam is reported, concerning their chemistry, synthesis, pharmacology and toxicology, prevalence/use, biotransformation and their analysis in biological samples. To our knowledge, fonazepam and nifoxipam-related intoxications, lethal or not, have not been reported in the scientific literature. All the available information was gathered through a detailed search of PubMed and the World Wide Web.

Issues pertaining to the analysis of buprenorphine and its metabolites by gas chromatography-mass spectrometry

"Substitution therapy" and the use of buprenorphine (B) as an agent for treating heroin addiction continue to gain acceptance and have recently been implemented in Taiwan. Mature and widely utilized gas chromatography-mass spectrometry (GC-MS) technology can complement the low cost and highly sensitive immunoassay (IA) approach to facilitate the implementation of analytical tasks supporting compliance monitoring and pharmacokinetic/pharmacogenetic studies. Issues critical to GC-MS analysis of B and norbuprenorphine (NB) (free and as glucuronides), including extraction, hydrolysis, derivatization, and quantitation approaches were studied, followed by comparing the resulting data against those derived from IA and two types of liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods. Commercial solid-phase extraction devices, highly effective for recovering all metabolites, may not be suitable for the analysis of free B and NB; acetyl-derivatization products exhibit the most favorable chromatographic, ion intensity, and cross-contribution characteristics for GC-MS analysis. Evaluation of IA, GC-MS, and LC-MS/MS data obtained in three laboratories has proven the 2-aliquot GC-MS protocol effective for the determination of free B and NB and their glucuronides.

Formation of the N-methylpyridinium derivative to improve the detection of buprenorphine by liquid chromatography-mass spectrometry

The legally defensible identification of the narcotic, analgesic buprenorphine, in biological specimen requires considerable sensitivity due to its low therapeutic dosages and corresponding target concentrations. Application of liquid chromatography-electrospray ionisation-mass spectrometry, which became the default method for buprenorphine detection, is impeded by the disadvantageous fragmentation of the stable precursor ion producing unspecific product ions of comparatively low abundance. A chemical modification to form the N-methylpyridinium ether derivative of buprenorphine is presented to improve the selectivity and sensitivity of its detection by liquid chromatography-mass spectrometry (LC-MS). The reaction of buprenorphine with 2-fluoro-1-methyl-pyridinium-p-toluene-sulfonate and triethylamine as catalyst was accomplished in acetonitrile at an ambient temperature yielding a chemically stable derivative. Fragmentation of the permanently charged precursor ion (m/z = 559) leads to the formation of diagnostic and abundant fragments (e.g. m/z = 443 and 450) representing all parts of the molecule. The application of the technique to the identification of buprenorphine in hair samples demonstrates a high specificity, availability of sufficient qualifier ions and a significant (approximately 8-fold) improvement of detection limits with respect to comparable experiments based on underivatised buprenorphine.

Pharmacokinetic and pharmacodynamic properties of buprenorphine after a single intravenous administration in healthy volunteers: a randomized, double-blind, placebo-controlled, crossover study

BACKGROUND

Buprenorphine is used as an analgesic for postoperative and chronic pain. The usual sublingual dose is 0.2 to 0.8 mg, and the usual parenteral dose is 0.3 mg for acute postoperative pain. The pharmacokinetic and related pharmacodynamic properties of buprenorphine at these doses have not been characterized.

OBJECTIVE

The aim of this study was to assess the pharmacokinetic properties of buprenorphine 0.002 mg/kg IV (0.15 mg/70 kg) and its antinociceptive and psychomotor effects.

METHODS

Healthy male volunteers received 0.002 mg/kg buprenorphine IV in a randomized, double-blind, placebo-controlled, crossover design. Blood samples were collected at 0.5, 1, 1.5, 1.75, 2, 2.5, 3, 4, 5, 6, and 8 hours for the determination of plasma concentrations. Pharmacokinetic parameters were estimated by a compartmental model using specialized software. Antinociceptive and psychomotor effects were determined for 8 hours. Quantitative sensory testing with thermal and electrical (nociceptive flexion RIII reflex) stimulations was performed. The cold pressor test was used to assess pain tolerance to a tonic, intense pain stimulation. Psychomotor performance was assessed by the digit symbol substitution test (DSST). Participants also rated sedation on an 11-point numeric scale (0 = none to 10 = severe). A selective liquid chromatography-tandem mass spectrometry assay was developed for the determination of buprenorphine; the limit of quantification was 0.05 ng/mL using a 0.25-mL plasma aliquot. Participants were instructed to report adverse effects, which were recorded for type, time of onset, seriousness, and duration.

RESULTS

The study enrolled 12 participants, all of whom were white. Mean (SD) age was 26 (3.5) years, and mean weight was 67 (9) kg. None of the participants had a history of opiate abuse. Buprenorphine significantly increased the objective (nociceptive flexion RIII reflex) and subjective pain thresholds for >4 hours and pain tolerance (cold pressor test) for 2 hours. The mean (SD) RIII reflex threshold and subjective threshold at baseline were 31.6 (9.5) mA and 45.5 (22.3) mA, respectively. The maximum increases (mean [SD]) were +14.1 (17.5) mA for the RIII reflex (P = 0.02) and +24.2 (21.7) mA for the subjective threshold (P = 0.02), corresponding to mean (SEM) percentages of 53.7% (20.2%) and 74.7% (20.4%) of the baseline values, respectively. The maximum increases were observed at 120 minutes for both measures. The effect of buprenorphine on pain tolerance peaked at 30 minutes. Mean (SEM) latency before withdrawal of the hand was 69 (10) seconds, corresponding to a mean increase of 63.8% (14.4%) from baseline (P = 0.003). Buprenorphine had a significant effect on the DSST. The mean maximum decrease in the total number of symbols drawn was -6 (14.5%; P = 0.005) at 1 hour. The participants reported high levels of sedation: at peak effect (120 minutes), mean scores increased from 2.9 to 6.4 (SEM 0.7) (P = 0.005). Levels returned to baseline values by the end of the session, unlike for the nociceptive tests. The onset of effects occurred during the distribution phase for all the measures, and their duration was observed across a wide range of concentrations during the elimination phase. The most likely explanation for this finding is the high affinity of buprenorphine at mu-opioid receptors, and possibly distribution to the brain. Buprenorphine t(l/2) was 2.75 hours. A secondary peak in concentration was observed at 90 minutes, suggesting enterohepatic circulation of buprenorphine. A 2-compartment model adequately described buprenorphine pharmacokinetics.

CONCLUSIONS

A clinically relevant analgesic dose of 0.002 mg/kg (0.15 mg/70 kg) of buprenorphine had a significant effect on nociception and psychomotor performance in these healthy male volunteers. A 2-compartment model satisfactorily characterized buprenorphine pharmacokinetics, and we found evidence of enterohepatic circulation.

Simultaneous determination of buprenorphine, norbuprenorphine and the enantiomers of methadone and its metabolite (EDDP) in human plasma by liquid chromatography/mass spectrometry

A previously reported enantioselective LC-MS assay for the determination of (R)- and (S)-methadone [Met] and (R)- and (S)-2-ethylidene-1,5-dimethyl-3,3-diphenyl-pyrrolidine [EDDP] (the primary metabolite of Met) has been adapted for use in the simultaneous determination of the plasma concentrations of Met, EDDP, buprenorphine (Bu) and norbuprenorphine (norBu). All of the target compounds were separated within 15 min using an alpha1-acid glycoprotein chiral stationary phase, a mobile phase composed of acetonitrile: ammonium acetate buffer [10 mM, pH 7.0] in a ratio of 18:82 (v/v), a flow rate of 0.9 ml/min at 25 degrees C. Deuterium labeled compounds were used as internal standards [d4-Bu, d3-norBu, (R,S)-d3-Met and (R,S)-d3-EDDP] and linear relationships between peak height ratios and drug concentrations were obtained for Bu and norBu in the range 0.2-12 ng/ml with correlation coefficients greater than 0.999. The relative standard deviations (%R.S.D.) for the intra- and inter-day precision of the method were <4.5% and for accuracy was <4.0%. The method was validated and used to analyze plasma samples obtained from opioid dependent methadone-maintained adults enrolled in a research study.

An automated and fully validated LC-MS/MS procedure for the simultaneous determination of 11 opioids used in palliative care, with 5 of their metabolites

A fully validated liquid chromatographic procedure coupled with electrospray ionization-tandem mass spectrometry (LC-ESI-MS/MS) is presented for quantitative determination of the opioids buprenorphine, codeine, fentanyl, hydromorphone, methadone, morphine, oxycodone, oxymorphone, piritramide, tilidine, and tramadol together with their metabolites bisnortilidine, morphine-glucuronides, norfentanyl, and nortilidine in blood plasma after an automatically performed solid-phase extraction (SPE). Separation was achieved in 35 min on a Phenomenex C12 MAX-RP column (4 microm, 150 x 2 mm) using a gradient of ammonium formiate buffer (pH 3.5) and acetonitrile. The validation data were within the required limits. The assay was successfully applied to authentic plasma samples, allowing confirmation of the diagnosis of overdose situations as well as monitoring of patients' compliance, especially in patients under palliative care.

Liquid chromatographic-electrospray ionization mass spectrometric quantitative analysis of buprenorphine, norbuprenorphine, nordiazepam and oxazepam in rat plasma

A liquid chromatographic-mass spectrometric method with electrospray ionization is presented for the simultaneous determination of buprenorphine, nordiazepam and their pharmacologically active metabolites, norbuprenorphine and oxazepam, in rat plasma. The drugs were extracted from plasma by liquid-liquid extraction and chromatographically separated using a gradient elution of aqueous ammonium formate and acetonitrile. Following electrospray ionization, the analytes were quantified in the single ion storage mode. The assay was validated according to current acceptance criteria for bioanalytical method validation. It was proved to be linear from 0.7 to 200 ng/ml plasma for buprenorphine, 1.0 to 200 ng/ml for norbuprenorphine, 2.0 to 200 ng/ml for nordiazepam, and from 5.0 to 200 ng/ml for oxazepam. The average recoveries of buprenorphine, norbuprenorphine, nordiazepam and oxazepam were 89, 39, 88 and 82%, respectively, with average coefficients of variation ranging from 1.8 to 14.3%. The limits of quantitation for these drugs were 0.7, 1.0, 2.0 and 5.0 ng/ml, respectively, with associated precisions within 17% and accuracies within +/-18% of the nominal values. Both the intra- and inter-assay precision values did not exceed 11.3% for the four analytes. Intra- and inter-assay accuracies lay within +/-15% of the nominal values. The validated method was applied to the determination of buprenorphine, norbuprenorphine, nordiazepam and oxazepam in plasma samples collected from rats at various times after intravenous administration of buprenorphine and nordiazepam.

Liquid chromatography–mass spectrometric analysis of buprenorphine and its N-dealkylated metabolite norbuprenorphine in rat brain tissue and plasma

INTRODUCTION

A specific, accurate, and reproducible liquid chromatography-mass spectrometric (LC/MS) method was developed and validated that allows simultaneous measurement of the centrally acting analgesic buprenorphine and its major metabolite, norbuprenorphine, in rat brain and plasma samples.

METHODS

A 96-well plate solid phase extraction (SPE) procedure was developed for buprenorphine and norbuprenorphine using mixed-mode cation-exchange reversed-phase sorbent. An LC method using a C8 column with isocratic mobile phase (80:20 water/acetonitrile with 20 mM ammonium acetate and 0.1% acetic acid) was developed for reproducible and selective separation. A quadrupole mass spectrometer with atmospheric electrospray ionization source under positive ion mode was used for detection. d4-Buprenorphine and d3-norbuprenorphine were used as internal standards.

RESULTS

The calibration curves for buprenorphine and norbuprenorphine in plasma and brain tissue were linear within the range of 7 to 8333 ng/ml (plasma) and 5 to 5000 ng/g (brain). The lower limit of quantification for both buprenorphine and norbuprenorphine from brain tissue was 5 ng/g, and from plasma was 7 ng/ml. Assay accuracy and precision of back-calculated standards were within +/-15%.

DISCUSSION

This method will be useful for investigation of buprenorphine's mechanism of action and clinical profile.

Flunitrazepam does not alter cerebral distribution of buprenorphine in the rat

Deaths have been reported among heroin addicts related to combined buprenorphine and flunitrazepam use. The aim of this study was to determine the existence of a drug-drug interaction during the distribution phase of buprenorphine. Arterial blood gases were measured after intravenous administration of buprenorphine alone (30 mg/kg), flunitrazepam alone (40 mg/kg) or both drugs in rats. Buprenorphine kinetics was studied in plasma and in striatum using cerebral microdialysis, both alone and after rat pretreatment with flunitrazepam. In contrast to buprenorphine or flunitrazepam alone, buprenorphine in combination with flunitrazepam induced a significant, rapid and sustained respiratory depression. Arterial PCO2 was increased at 1.5 min (6.7+/-0.2 versus 5.4+/-0.3 and 5.5+/-0.3 kPa, respectively, P=0.04) (mean+/-S.E.M.), and arterial pH decreased (7.37+/-0.02 versus 7.45+/-0.02 and 7.45+/-0.01, respectively, P=0.03). Plasma buprenorphine kinetics was well described by a three-compartment linear model, with a distribution half-life of 7.4+/-2.7 min and an elimination half-life of 463.9+/-152.3 min. However, neither plasma nor striatal buprenorphine kinetics were significantly altered by pre-administration of flunitrazepam. The adverse interaction between flunitrazepam and buprenorphine cannot be explained by a pharmacokinetic drug-drug interaction during the distribution phase of buprenorphine.

Current literature in mass spectrometry

In order to keep subscribers up‐to‐date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (5 Weeks journals ‐ Search completed at 8th. Sept. 2004)