Determination of dexamethasone in human plasma and urine by electron-impact mass spectrometry

Past, present and future of mass spectrometry in the analysis of residues of banned substances in meat-producing animals

A residue is a trace (microg kg(-1), ng kg(-1)) of a substance, present in a matrix. Banned substances, such as growth promoters, which are abused in animal fattening and where this article is focused on, may be divided into four major groups: thyreostats, anabolics or anabolic steroids, corticosteroids and beta-agonists or repartitioning agents. The combination of chromatographic techniques with mass spectrometry (GC-MS(n), LC-MS(n), etc.) plays a key role in the production of specific results in residue analysis. In this review, the past, present and future of mass spectrometry in this area are discussed in the light of the impact of these substances on human health and the reliable production of analytical results, ready for challenge in a court.

Derivatization procedures for gas chromatographic-mass spectrometric determination of xenobiotics in biological samples, with special attention to drugs of abuse and doping agents

The development of low cost MS detectors in recent years has promoted an important increase in the applicability of GC-MS system to analyze for the presence of foreign substances in the human body. Drugs and toxic agents are in vivo metabolized in such a way that more polar compounds are usually formed. Derivatization of these metabolites is often an unavoidable requirement for gas chromatographic analysis. Application of derivatization methods in recent years has been relevant, especially for silylation, acylation, alkylation and the formation of cyclic or diastereomeric derivatives. Given the relevance of drug of abuse testing in modern toxicology, main derivatization procedures for opiates, cocaine, cannabis, amphetamines, benzodiazepines and LSD have been reviewed. Papers describing the analyses of drugs of abuse in matrixes other than blood, such as hair or sweat, have received special attention. Advances in derivatization for sports drug testing have been particularly relevant for anabolic steroids, diuretics and corticosteroids. Among the several methodologies applied, the formation of trimethylsilyl, perfluoroacyl or methylated derivatives have proved to be both versatile and extensively used. Further advances in derivatization for GC-MS applications in clinical and forensic toxicology will depend on the one hand on the degree of further use of GC-MS for routine applications and, on the other hand, on the alternative progress made for developments in LC-MS or CE-MS. Last but not least, the appearance of comprehensive libraries in which reference spectra for different derivatives of many drugs and their metabolites are collected will have an important impact on the expansion of derivatization in GC-MS for toxicological applications.

Increased vasoconstrictor sensitivity to glucocorticoids in essential hypertension

Glucocorticoids raise blood pressure but were thought not to play a pathophysiological role in essential hypertension when it was demonstrated that cortisol secretion rates and circulating concentrations are normal in this disease. However, recent observations suggest that increased tissue sensitivity to cortisol, mediated by either abnormal glucocorticoid receptors or impaired inactivation of cortisol by 11 beta-dehydrogenase, may allow cortisol to raise blood pressure despite normal circulating concentrations. We studied 11 patients with essential hypertension and 11 matched normotensive control subjects. Dermal vasoconstriction after topical application of both cortisol (16 +/- 4 versus 32 +/- 5 U, control subjects versus hypertensive patients; P < .02) and beclomethasone dipropionate (75 +/- 10 versus 100 +/- 7 U; P < .05) was increased in the hypertensive patients. Hypothalamic-pituitary glucocorticoid receptor sensitivity was normal, as judged by basal cortisol secretion rates and suppression of plasma cortisol during sequential overnight dexamethasone suppression tests. 11 beta-Dehydrogenase activity was impaired in essential hypertension, as judged by prolonged half-lives of [11 alpha-3H]cortisol (44 +/- 4 versus 58 +/- 4 minutes, control subjects versus hypertensive patients; P < .02). However, this did not correlate with the dermal vasoconstrictor response. We conclude that vasoconstrictor sensitivity to glucocorticoids is increased in essential hypertension and that this may initiate and/or sustain the increased peripheral vascular resistance that characterizes this disease. The mechanism of increased sensitivity remains uncertain, but it will be important to establish whether it relates to genetic abnormalities of the glucocorticoid receptor that have been observed in animal models and young individuals who are predisposed to essential hypertension.

Immunoaffinity chromatography combined with gas chromatography-negative ion chemical ionisation mass spectrometry for the confirmation of flumethasone abuse in the equine

Immunoaffinity chromatography using a synthesised immunosorbent was used to extract tritiated dexamethasone (with dexamethasone carrier) from equine urine at a recovery of 81.7 +/- 8.4% (mean +/- S.D.). A method utilising this procedure coupled to cool on-column injection gas chromatography-negative ion chemical ionisation mass spectrometry is also described for the confirmation of low levels of flumethasone in equine urine samples.

Determination of corticosteroids in plasma by high-performance liquid chromatography after pre-column derivatization with 2-(4-carboxyphenyl)-5,6-dimethylbenzimidazole

2-(4-Carboxyphenyl)-5,6-dimethylbenzimidazole (CDB) was used as a pre-column derivatization reagent for corticosteroids analysed by high-performance liquid chromatography with fluorimetric detection. Eight corticosteroids were derivatized with CDB to their esters in acetonitrile containing 4-piperidinopyridine and 1-isopropyl-3-(3-dimethylaminopropryl)carbodiimide perchlorate. The resulting CDB esters were extracted with a Sep-Pak C18 cartridge and the esters were separated on a reversed-phase column (Zorbax ODS) with water-methanol (25:75, v/v) containing 5 mmol/l tetramethylammonium hydrogensulphate as the mobile phase. The limits of detection for steroids were 0.06-0.3 pg per 100 microliters of plasma (signal-to-noise = 3). The within-day relative standard deviations (n = 6) were 7.8-11.1%, and day-to-day relative standard deviations (n = 6) were 7.0-10.4%.

Stable isotope dilution mass spectrometry for the simultaneous determination of cortisol, cortisone, prednisolone and prednisone in plasma

A capillary gas chromatographic-mass spectrometric method for the simultaneous determination of cortisol, cortisone, prednisolone and prednisone in human plasma is described. [1,1,19,19,19-2H5]Cortisol, [1,1,19,19,19-2H5]cortisone, [1,19,19,19-2H4]prednisolone and [1,19,19,19-2H4]prednisone were used as internal standards. Formation of the bismethylenedioxy-3-heptafluoro-n-butyryl (BMD-HFB) derivatives made possible the separation of the four corticosteroids with good gas chromatographic behaviour. The new double derivatization has been demonstrated to be of value for sensitive and selective quantification by this technique. Detection was performed by monitoring the molecular ion (M+) of the BMD-HFB derivatives for cortisone and prednisolone, the [M - 18]+ ion for cortisol, and the [M - 30]+ ion for prednisone. The method requires no complex corrections for contributions and provides good accuracy and precision.

Simultaneous determination of beclomethasone, beclomethasone monopropionate and beclomethasone dipropionate in biological fluids using a particle beam interface for combining liquid chromatography with negative-ion chemical ionization mass spectrometry

A new simple and sensitive assay has been developed for the simultaneous quantitative measurement of beclomethasone dipropionate and its hydrolysis products in human plasma and urine. Beclomethasone 17.21-dipropionate, beclomethasone 17-monopropionate, beclomethasone and the internal standard, dexamethasone 21-acetate, were measured by combined liquid chromatography and negative-ion chemical ionization mass spectrometry with methane as the reagent gas. A particle beam interface from Hewlett Packard was used. Under mild operating conditions, abundant and stable characteristic high-mass ions were generated in the ion source of the mass spectrometer by a resonance electron-capture mechanism. The fast extraction procedure requires 1 ml of plasma or urine, and the quantification limit of the method is 1 ng ml-1 for the three tested compounds.

A robust assay for dexamethasone in plasma using a heterologous 125I radioligand and a magnetizable solid-phase antiserum

An assay using an antiserum raised against a dexamethasone 21-hemisuccinate conjugate and the heterologous radioligand dexamethasone 21-(carboxymethyl) ether was developed, validated, and used to study the pharmacokinetics of this steroid for 12 h following administration to patients with congenital adrenal hyperplasia. Coupling the antiserum to magnetizable cellulose allowed rapid separation of bound/free steroid. A C-21 rather than a C-3 antiserum was used to minimize interference with a main metabolite, 6 beta-hydroxydexamethasone. Close correspondence of assay (0.35 nmol/L) and curve (0.25 nmol/L) sensitivities suggests that interference by matrix effects is minimal. This was confirmed by good agreement in data from the in-house assay and that of a reference procedure. Good precision was demonstrated by the precision profile and Shewhart chart quality control data. The latter also demonstrated the assay was robust and reliable in routine practice.

Determination of dexamethasone in bovine tissues by coupled-column normal-phase high-performance liquid chromatography and capillary gas chromatography-mass spectrometry

A new method for the determination of dexamethasone in bovine liver and muscle tissues has been developed. Crude tissue extracts were obtained by means of a three-phase liquid-liquid extraction scheme. The resulting residue was subjected to coupled-column normal-phase high-performance liquid chromatography which served to isolate the drug for the purpose of screening and quantification. Sample was injected onto the first column of the system, a phenyl column, from which a heart-cut was diverted to a short silica column which retained dexamethasone. The contents of this column were backflushed onto a cyanopropyl column which isolated dexamethasone. Mobile phases consisted of hexane modified with 2-propanol, acetic acid, and water. Analysis of each sample was completed in 15 min. Quantitation was performed by external standard calibration of ultraviolet response at 239 nm. Limits of detection were estimated to be 4 and 6 ppb in muscle and liver, respectively. In addition to screening and quantitation, the coupled-column system purified tissue extracts for gas chromatographic-mass spectrometric analysis which, in the selected-ion monitoring mode, confirmed the identity of the trimethylsilyl-enol-trimethylsilyl derivative of dexamethasone.

A rapid and highly sensitive method for the quantitative determination of dexamethasone in plasma, synovial fluid and tissues by combined gas chromatography/negative ion chemical ionization mass spectrometry

A new, simple and highly sensitive assay is developed for the quantitative measurement of very low levels of dexamethasone in human plasma, synovial fluid and tissues following a topical administration of the drug. Dexamethasone and the internal standard, flumethasone, are measured by gas chromatography/negative ion chemical ionization mass spectrometry with methane as the reagent gas. After a three-step extraction procedure, the two compounds of interest are converted to their trimethylsilyl ether derivatives using trimethylsilylimidazole and formamide as the base catalyst. Under soft derivatization conditions only one chromatographic peak corresponding to the trisubstituted derivative is observed. The mass spectrometer is focused to monitor abundant and stable characteristic high-mass ions (m/z 446 and 464) which are generated in the ion source by an electron capture process. This assay requires only 1 ml of plasma or 0.5 ml of synovial fluid and the detection limit of the method is equal to 0.1 ng ml-1 with a relative standard deviation lower than 6%.

Direct radioimmunoassay procedure for plasma dexamethasone with a sensitivity at the picogram level

A simple radioimmunoassay (RIA) for the direct quantitation of plasma dexamethasone (DEX) at the picogram level has been developed. An antiserum with high specificity and avidity was obtained by the immunization of a carefully synthesized dexamethasone-21-succinyl-thyroglobulin with a high incorporation ratio. As little as 1 pg of DEX in 50 microL of plasma sample can be detected directly by this RIA without extraction and other purification procedures. Intra- and interassay coefficients of variation were 2.1 and 3.3% for plasma levels at 2.93 ng/mL or 2.3 and 7.2% for plasma levels at 0.88 ng/mL. Blank values for plasma of normal or pre-DEX patients were always under the detection limit (20 pg/mL). Excellent linearity (r = 0.9991-0.9999) was demonstrated between the serial dilutions of six plasma samples and their corresponding DEX concentrations. In single-dose DEX (0.25-1 mg) pharmacokinetic studies, plasma DEX was consistently detectable up to 24 h post dose. Compared with existing methods, this direct RIA demonstrates superior performance with regard to simplicity, sensitivity, specificity, and reproducibility. It also enables high sample throughput and has proven robust in our hands. This assay should be readily transferable to other laboratories for clinical or research purposes.

Determination of cortisol in human plasma by stable isotope dilution mass spectrometry

Cortisol selectively labelled with 2H at the 19-methyl and the C-1 position (cortisol-d5) was synthesized and used as an internal standard in stable isotope dilution mass spectrometry in order to determine cortisol in human plasma. A capillary gas chromatographic/mass spectrometric method provided a sensitive and reliable technique with good accuracy, precision and reproducibility without complex corrections for contributions by using cortisol-d5 as an internal standard. For calculation of plasma cortisol, peak areas were measured by selected ion monitoring on the characteristic fragment ions of the dimethoxime tri(trimethylsilyl) derivatives of cortisol and cortisol-d5 (m/z 605 and 610, respectively). The sensitivity of the gas chromatographic/mass spectrometric assay was 1.02 ng per injection, with a signal-to-noise ratio of about 6. The inter- and intra-assay coefficients of variation for plasma sample were 3.07% and 1.77%, respectively.

Determination of cortisol in human plasma by capillary gas chromatography-mass spectrometry using [2H5] cortisol as an internal standard

A capillary gas chromatographic-mass spectrometric method for the determination of cortisol in human plasma using cortisol M + 5 as an internal standard is described. For calculation of plasma cortisol, peak areas were measured by selected-ion monitoring of the characteristic fragment ions of the dimethoxime-tri (trimethylsilyl) derivatives of cortisol and cortisol M + 5 (m/z 605 and 610, respectively). The inter- and intra-assay coefficients of variation for plasma sample were 3.07 and 1.77%, respectively. The method needed no complex corrections for contributions and provides a sensitive and reliable technique with good accuracy, precision and reproducibility.

Clinical pharmacokinetic considerations in the treatment of increased intracranial pressure

Life-threatening increased intracranial pressure can be reversed by a variety of drugs. These compounds all have some disadvantages, producing rebound effects, severe coma or cardiovascular depression and electrolyte imbalance. However, reduction of intracranial pressure is a prerequisite for recovery and the benefits of treatment outweigh the risks. Dexamethasone is rapidly eliminated, the short half-life (about 3 hours) indicating that dosage intervals should be kept small. As yet, however, its therapeutic efficacy has not been clearly demonstrated. Therefore, an association between pharmacokinetics and pharmacodynamics cannot be established. Osmotic diuretics are the most widely used agents for reduction of intracranial pressure. Pharmacokinetics show a very close relationship to changes in serum osmolality, but there are large variations in the clearance. For the use of osmotics, the blood-brain barrier must be intact. Osmotic diuretics may lead to intracerebral oedema or to acute renal failure as serum osmolality increases. Considering the pharmacokinetics of each drug, and the dynamics of intracerebral pressure and osmolality, an intermittent, individually titrated dosage should be administered, with simultaneous monitoring of intracranial pressure. Frusemide (furosemide) can be used as an adjunct, to enhance the effect of osmotic diuretics. Its pharmacokinetics are limited by renal function, depending on age as well as on the extent of renal impairment. Altered renal elimination of concomitantly administered drugs, and electrolyte imbalances should be anticipated when diuretics are used. Barbiturates are certain to decrease intracranial pressure in humans by an as yet unknown mechanism. Their administration is recommended for patients that do not respond to conventional therapy. As barbiturates can result in deep coma, knowledge of their pharmacokinetics is of great importance for recovery. Following single doses, pentobarbitone has a relatively long elimination half-life (about 22 hours). However, after repeated administration for several days, its elimination may be enhanced due to autoinduction. Thiopentone kinetics are characterised by distribution and redistribution into deep peripheral compartments. Administration of high and frequent doses leads to considerably delayed recovery. This is not true for methohexitone, which shows comparable pharmacokinetics after single and multiple dose administration. Elimination depends on liver blood flow. Thus, recovery from methohexitone-coma is rapid. Rapid elimination is also an important characteristic of etomidate and alphaxalone/alphadolone, two non-barbiturate hypnotics.(ABSTRACT TRUNCATED AT 400 WORDS)

Dexamethasone bioavailability: implications for DST research

The bioavailability of dexamethasone (DEX) has recently been demonstrated to be a critical factor in determining Dexamethasone Suppression Test (DST) status in psychiatric patients. This brief review focuses on several aspects of DEX bioavailability as they relate to the use of the DST in neuroendocrine research. Several methodologies, including radioimmunoassay, high-performance liquid chromatography, and gas chromatography-mass spectrometry are available for quantification of DEX in biological fluids, although few detailed comparisons between methods have been reported. Surprisingly, little systematic research on the metabolism of DEX has been reported, but it appears that hepatic rather than renal mechanisms are the major source of DEX elimination. The marked variability in serum DEX levels following oral administration in psychiatric patients is also observed in normal controls and patients with Cushing's syndrome. A variety of drugs can modify serum DEX levels and thereby after the effectiveness of DEX in suppressing serum cortisol levels. Simultaneous measurement of serum DEX and cortisol levels appears to be necessary for the appropriate evaluation of DST results. This procedure may help explain many of the inconsistencies in recent DST research.

Identification and quantification of 6 beta-hydroxydexamethasone as a major urinary metabolite of dexamethasone in man

Identification of 6 beta-hydroxydexamethasone as a major urinary metabolite of dexamethasone in man has been accomplished by nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. Mass fragmentographic measurements revealed that more than 30% of the intravenously or orally administered dexamethasone dose was excreted in the 24-h urine as 6 beta-hydroxydexamethasone, while only a small fraction of the dose was excreted as unchanged dexamethasone and its glucuronic acid conjugate.