Effect of pentobarbital on the formation of diastereomeric oxazepam glucuronides in man: analysis by high performance liquid chromatography

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Influence of Stereochemistry on the Bioactivation and Glucuronidation of 4-Ipomeanol

A potential CYP4B1 suicide gene application in engineered T-cell treatment of blood cancers has revived interest in the use of 4-ipomeanol (IPO) in gene-directed enzyme prodrug therapy, in which disposition of the administered compound may be critical. IPO contains one chiral center at the carbon bearing a secondary alcohol group; it was of interest to determine the effect of stereochemistry on 1) CYP4B1-mediated bioactivation and 2) (UGT)-mediated glucuronidation. First, (R)-IPO and (S)-IPO were synthesized and used to assess cytotoxicity in HepG2 cells expressing rabbit CYP4B1 and re-engineered human CYP4B1, where the enantiomers were found to be equipotent. Next, a sensitive UPLC-MS/MS assay was developed to measure the IPO-glucuronide diastereomers and product stereoselectivity in human tissue microsomes. Human liver and kidney microsomes generated (R)- and (S)-IPO-glucuronide diastereomers in ratios of 57:43 and 79:21, respectively. In a panel of 13 recombinantly expressed UGTs, UGT1A9 and UGT2B7 were the major isoforms responsible for IPO glucuronidation. (R)-IPO-glucuronide diastereoselectivity was apparent with each recombinant UGT, except UGT2B15 and UGT2B17, which favored the formation of (S)-IPO-glucuronide. Incubations with IPO and the UGT1A9-specific chemical inhibitor niflumic acid significantly decreased glucuronidation in human kidney, but only marginally in human liver microsomes, consistent with known tissue expression patterns of UGTs. We conclude that IPO glucuronidation in human kidney is mediated by UGT1A9 and UGT2B7. In human liver, it is mediated primarily by UGT2B7 and, to a lesser extent, UGT1A9 and UGT2B15. Overall, the lack of pronounced stereoselectivity for IPO's bioactivation in CYP4B1-transfected HepG2 cells, or for hepatic glucuronidation, suggests the racemate is an appropriate choice for use in suicide gene therapies.

Stereoselectivity in drug metabolism and action: effects of enzyme inhibition and induction

Many synthetic drugs that contain a chiral center are marketed as racemates. Taking into account the composite nature of chiral rugs has improved our understanding of the dose/effect relationship of these compounds. It is widely accepted that drug metabolism is often responsible for stereoselective disposition and hence factors modulating the activity of drug-metabolizing enzymes can modify this effect. The present article highlights the influence of some of these factors (induction and inhibition of drug-metabolizing enzymes) on stereoselective drug disposition. The latter topic includes an enantiomer/enantiomer interaction that alters drug disposition and drug action.

Factors and conditions affecting the glucuronidation of oxazepam

The aim of the present work was to investigate the impact of disease states and environmental and host factors on the glucuronidation of oxazepam. Glucuronidation represents quantitatively one of the most important metabolic conjugation pathways (phase II) in man for the inactivation and detoxication of xenobiotics and endogenous compounds and the liver is the major site for it to take place. Far less attention has been paid to the conjugation reactions in previous clinical research in this field compared to the immense interest in the oxidative biotransformation pathways (phase I). This fact is mainly due to the latter giving rise to active or reactive metabolites with a toxicological potential. The metabolism of oxazepam expresses exclusively the capacity for glucuronide formation. It was a prerequisite to establish the bioavailability of oxazepam prior to succeeding studies on the oral disposition of the drug. A preparation for intravenous administration was created. Clearance was chosen as measurement of the capacity to glucuronidate oxazepam. Severe decompensated liver disease was associated with a significant decrease in oxazepam clearance, that became even more obvious when corrected for by a diminished binding to plasma proteins. This increase in free fraction of oxazepam was substantial and could mainly be accounted for by low plasma albumin values. The results are in part a settlement with earlier studies on glucuronidation in liver disease and they may undoubtedly be ascribed to the severe degree of liver disease. For the first time it was shown that hypothyroidism led to a decline in the clearance and metabolism of oxazepam and paracetamol that is mainly biotransformed by glucuronidation. It was concluded that the enzymes responsible for glucuronidation in hypothyroidism are under the influence of thyroid hormones as is the case with oxidative enzymes. Further studies focused on the effect of host and environmental factors on glucuronidation. A commercially available very low calorie product for the treatment of obesity resulted in a decrease in oxazepam clearance and a lack of co-factors as a consequence of the low calorie intake was explanatorily proposed. Beta-adrenoceptor antagonists are often prescribed together with other drugs and close knowledge on interactions is mandatory but insufficient in regard of drugs being glucuronidated. Despite the mutual metabolic pathway labetalol exerted no dispositional alterations concerning oxazepam. It was moreover suggested that very elderly subjects between the age of 80 to 94 years had a reduced clearance of oxazepam.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification and quantitation of intact diastereoisomeric benzodiazepine glucuronides in biological samples by high-performance liquid chromatography

A rapid and simple high-performance liquid chromatographic method for the simultaneous detection of intact glucuronides of different benzodiazepines is described. Separation of the diastereomers of the following benzodiazepine glucuronides can be achieved on a reversed-phase column (octadecyl or select B): oxazepam, nordiazepam, temazepam, lorazepam and 3-hydroxyprazepam. If the sample contains both S-lorazepam and R-temazepam glucuronides, or both S-temazepam and nordiazepam glucuronides, further separation on a beta-cyclodextrin column is required. The detection limit ranges between 5 and 10 ng of glucuronides per ml of plasma or urine, respectively.

Direct high pressure liquid chromatographic analysis and preliminary pharmacokinetics of enantiomers of oxazepam and temazepam with their corresponding glucuronide conjugates

Three high pressure liquid chromatographic systems for the separation of oxazepam, temazepam and their glucuronides (system A), the separation of their R,S glucuronide diastereomers (system B) and the chiral separation of the parent drugs (system C) are described. Preliminary pharmacokinetics of R,S-oxazepam and R,S-temazepam in a human volunteer reveal that the protein binding of the glucuronides is lower than that of the parent drugs, but that there is no difference in protein binding between the R-oxazepam/temazepam and S-oxazepam/temazepam and their corresponding glucuronides. The S-glucuronide is the main metabolite formed and excreted by man. The plasma ratio R/S-glucuronide is 1:1 for both oxazepam and temazepam. The renal clearance of R-temazepam, and S-temazepam are similar, and those of R-oxazepam and S-oxazepam tend to be different.

Drug glucuronidation in humans

Glucuronidation is a major metabolic pathway for a large number of drugs in humans. Conjugation of drugs and other chemicals with glucuronic acid is catalyzed by the multigene UDP-glucuronosyltransferase family. It is believed that a number (unspecified at present) of glucuronosyltransferase isozymes, which probably differ in terms of substrate specificity and regulation, contribute to drug glucuronidation. Factors known to influence the pharmacokinetics of glucuronidated drugs in man, presumably via an effect on specific glucuronosyltransferases, include age (especially the neonatal period), cigarette smoking, diet, certain disease states, coadministered drugs, ethnicity, genetics and hormonal effects.

HPLC analysis and pharmacokinetics of the enantiomers of R,S-oxazepam and R,S-temazepam with their corresponding glucuronide conjugates in urine and plasma of man

R,S-Oxazepam and R,S-temazepam can be separated in their enantiomers by means of a chiral AGP column. The corresponding R- and S-glucuronide conjugates can be separated on a normal reversed-phase C18 column. Man conjugates the S-enantiomer of oxazepam and temazepam both better than the R-enantiomer. The urinary recovery of the glucuronides following either R,S,-oxazepam or R,S-temazepam almost amounts to 100% of the dose administered.

Assessment of enzyme induction and enzyme inhibition in humans: toxicological implications

1. The principal methods used for the assessment of enzyme induction and enzyme inhibition are measurement of the pharmacokinetics of a model compound (probe drug), analysis of drug metabolism in vitro, and determination of changes in the disposition of, and endogenous substrate for, the enzyme of interest. 2. Probe drugs that have been used for this purpose include antipyrine, aminopyrine, tolbutamide, caffeine, theophylline, warfarin, oxazepam and paracetamol. Measurement of the excretion of metabolites of cortisol and oestradiol, which are endogenous substrates for cytochrome P450 IIIA enzymes, provides a non-invasive means of assessing enzyme induction or inhibition. 3. Combined pharmacokinetic/pharmacodynamic studies are required to assess the pharmacological relevance of either induction or inhibition of the enzymes involved in drug metabolism. 4. At present it is difficult to assess the toxicological implications of enzyme induction and inhibition in man. Safe probe drugs are required for the enzymes primarily responsible for drug detoxication, such as epoxide hydrolase and glutathione transferase, in order to identify individuals particularly at risk.

Stereochemical characterization of the diastereomers of the phenobarbital N-beta-D-glucose conjugate excreted in human urine

The absolute configuration of the N-beta-D-glucoside metabolites of phenobarbital was determined by methylation of the diastereomers to make mephobarbital N-beta-D-glucosides, followed by oxidative removal of glucose to give the optical isomers of mephobarbital. Following a single oral dose of phenobarbital to two male subjects, both phenobarbital N-beta-D-glucosides were excreted in the urine. The absolute configuration (C-5 position) of the major phenobarbital N-beta-D-glucoside excreted in the urine was the S form. A pronounced stereoselective formation and/or urinary excretion occurs for the N-glucoside conjugates of phenobarbital in humans.

Glucuronidation of the enantiomers of E-10-hydroxynortriptyline in human and rat liver microsomes

Conjugation of racemic E-10-hydroxynortriptyline (E-10-OH-NT) with glucuronic acid was studied in the liver microsomal fraction of rats and humans. The diastereomeric glucuronides of E-10-OH-NT were resolved and quantitated by HPLC. Only the (+)-enantiomer was glucuronidated in liver microsomes from humans. Rat liver microsomes catalyzed the formation of both glucuronides. Phenobarbital pretreatment of rats increased the glucuronidation of both enantiomers about five-fold. The formation rate of (+)-E-10-OH-NT glucuronide varied from 5.5 to 33.2 pmol/mg x min, in microsomes from 13 humans. High activity was found in individuals previously treated with pentobarbital. Inhibition experiments with human liver microsomes showed that amitriptyline is a potent competitive inhibitor of (+)-E-10-OH-NT glucuronidation. p-Nitrophenol, paracetamol and 2-hydroxydesipramine also inhibited this reaction.

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)

Paracetamol as a test drug to determine glucuronide formation in man. Effects of inducers and of smoking

A simple, noninvasive procedure was developed to monitor glucuronidation and sulphation in patients using paracetamol as the test drug. Urinary paracetamol and its metabolites were determined by UV absorption and electrochemical detection after separation by HPLC. The metabolite to paracetamol ratio (M/P) was used as an approximation of the partial clearance due to metabolite formation. In 14 healthy volunteers, all nonsmokers without medication, M/P was 18 +/- 5 for glucuronides and 12 +/- 4 for sulphate esters. The test was validated in patients treated with enzyme inducers. In 10 patients with epilepsy given phenytoin 0.3 g/day, and in 10 patients with tuberculosis treated with rifampicin 0.6 g/day, the M/P value for glucuronidation was significantly increased to 41 +/- 11 and 35 +/- 7, respectively. In contrast, M/P values for sulphation were not significantly different from untreated controls. In 9 heavy smokers (about 40 cigarettes/day) M/P values for glucuronidation were also significantly increased to 33 +/- 11. However, in 4 moderate smokers (about 10 cigarettes/day) no significant increase was found. The results suggest that in man glucuronidation of paracetamol is inducible both by phenobarbital- and 3-methylcholanthrene-type inducers. Monitoring the ratios of various urinary paracetamol conjugates/paracetamol may be useful as a new tool for the evaluation of factors determining glucuronide and sulphate ester formation in man.