Direct determination of codeine, norcodeine, morphine and normorphine with their corresponding O-glucuronide conjugates by high-performance liquid chromatography with electrochemical detection

Biotransformation, Using Recombinant CYP450-Expressing Baker's Yeast Cells, Identifies a Novel CYP2D6.10A122V Variant Which Is a Superior Metabolizer of Codeine to Morphine Than the Wild-Type Enzyme

CYP2D6, a cytochrome P450 (CYP) enzyme, metabolizes codeine to morphine. Within the human body, 0-15% of codeine undergoes O-demethylation by CYP2D6 to form morphine, a far stronger analgesic than codeine. Genetic polymorphisms in wild-type CYP2D6 (CYP2D6-wt) are known to cause poor-to-extensive metabolism of codeine and other CYP2D6 substrates. We have established a platform technology that allows stable expression of human CYP genes from chromosomal loci of baker's yeast cells. Four CYP2D6 alleles, (i) chemically synthesized CYP2D6.1, (ii) chemically synthesized CYP2D6-wt, (iii) chemically synthesized CYP2D6.10, and (iv) a novel CYP2D6.10 variant CYP2D6-C (i.e., CYP2D6.10^A122V) isolated from a liver cDNA library, were cloned for chromosomal integration in yeast cells. When expressed in yeast, CYP2D6.10 enzyme shows weak activity compared with CYP2D6-wt and CYP2D6.1 which have moderate activity, as reported earlier. Surprisingly, however, the CYP2D6-C enzyme is far more active than CYP2D6.10. More surprisingly, although CYP2D6.10 is a known low metabolizer of codeine, yeast cells expressing CYP2D6-C transform >70% of codeine to morphine, which is more than twice that of cells expressing the extensive metabolizers, CYP2D6.1, and CYP2D6-wt. The latter two enzymes predominantly catalyze formation of codeine's N-demethylation product, norcodeine, with >55% yield. Molecular modeling studies explain the specificity of CYP2D6-C for O-demethylation, validating observed experimental results. The yeast-based CYP2D6 expression systems, described here, could find generic use in CYP2D6-mediated drug metabolism and also in high-yield chemical reactions that allow the formation of regio-specific dealkylation products.

Morphine pharmacokinetics and pharmacodynamics in preterm and term neonates: secondary results from the NEOPAIN trial

BACKGROUND

Relationships between plasma morphine concentrations and neonatal responses to endotracheal tube (ETT) suctioning are unknown in preterm neonates.

METHODS

Ventilated preterm neonates (n=898) from 16 centres were randomly assigned to placebo (n=449) or morphine (n=449). After an i.v. loading dose (100 microg kg(-1)), morphine infusions [23-26 weeks postmenstrual age (PMA) 10 microg kg(-1) h(-1); 27-29 weeks 20 microg kg(-1) h(-1); and 30-32 weeks 30 microg kg(-1) h(-1)] were established for a maximum of 14 days. Open-label morphine (20-100 microg kg(-1)) was given for pain or agitation. Morphine assay and neonatal response to ETT suctioning was measured at 20-28 and 70-76 h after starting the drug infusion and at 10-14 h after discontinuation of the study drug. The concentration-effect response was investigated using non-linear mixed effects models.

RESULTS

A total of 5119 data points (1598 measured morphine concentrations and 3521 effect measures) were available from 875 neonates for analysis. Clearance was 50% that of the mature value at 54.2 weeks PMA (CLmat(50)) and increased from 2.05 litre h(-1) 70 kg(-1) at 24 weeks PMA to 6.04 litre h(-1) 70 kg(-1) at 32 weeks PMA. The volume of distribution in preterm neonates was 190 litre 70 kg(-1) (CV 51%) and did not change with age. There was no relationship between morphine concentrations (range 0-440 microg litre(-1)) and heart rate changes associated with ETT suctioning or with the Premature Infant Pain Profile.

CONCLUSIONS

A sigmoid curve describing maturation of morphine clearance is moved to the right in preterm neonates and volume of distribution is increased compared with term neonates. Morphine does not alter the neonatal response to ETT suctioning.

Morphine and its metabolites: Analytical methodologies for its determination

The present article reviews the methods of determination published for morphine and its metabolites covering the period from 1980 until at the first part of 2006. The overview includes the most relevant analytical determinations classified in the following two types: (1) non-chromatographic methods and (2) chromatographic methods.

Morphine metabolite pharmacokinetics during venoarterial extra corporeal membrane oxygenation in neonates

OBJECTIVE

To examine morphine metabolite serum concentrations in neonates undergoing venoarterial extra corporeal membrane oxygenation (ECMO) and to quantify clearance differences between these neonates and those subjected to noncardiac major surgery.

PATIENTS AND METHODS

This was an observational study in level III referral centre. Fourteen neonates (< 7 days old) undergoing ECMO were included. Morphine and concomitant medications were given by protocol, adapted to the clinical conditions of the neonates. Pharmacokinetic findings were compared with those from a previous study in infants after noncardiac major surgery. Nonlinear mixed-effect modelling was used. Parameter estimates were standardised to a 70 kg person using allometric modeling

RESULTS

Morphine-3-glucuronide (M3G) was the predominant metabolite. Formation clearance to M3G at the start of ECMO on day 1 was lower than those in postoperative children, but matured more rapidly. After 10 days formation clearances of M3G in neonates on ECMO equalled those of postoperative children. Higher ECMO flows were associated with reduced formation clearances. Elimination clearances of M3G, but not morphine-6-glucuronide (M6G), were lower in the ECMO neonates; this was attributable to reduced renal clearance. These elimination clearances were correlated positively with ECMO flow and negatively with dopamine dose. Haemofiltration cleared M3G and M6G, but not morphine.

CONCLUSION

Formation clearance to M3G, the predominant metabolite, is reduced during the first 10 days of ECMO. Elimination clearance of M3G and M6G is related to creatinine clearance. ECMO flow had a small effect on metabolite clearance. Higher flows were associated with decreased formation clearances, possibly reflecting illness severity. Dopamine dose reflected decreased renal clearance.

Morphine pharmacokinetics during venoarterial extracorporeal membrane oxygenation in neonates

OBJECTIVE

To study morphine pharmacokinetics in neonates undergoing venoarterial ECMO and to quantify differences between these neonates and neonates subjected to noncardiac major surgery.

DESIGN AND SETTING

Observational study in a level III referral center.

PATIENTS AND METHODS

Pharmacokinetic estimates from 14 neonates undergoing ECMO were compared with findings from a previous study in 0- to 3-year-olds after noncardiac major surgery using a nonlinear mixed effect model. A one-compartment linear disposition model with zero-order input (infusion) and first-order elimination was used to describe all data.

RESULTS

Clearance in neonates (age <7 days) at the start of ECMO (2.2 l per hour per 70 kg) was lower than that in postoperative neonates (10.5 l per hour per 70 kg) but increased rapidly (maturation half-life 30 and 70 days, respectively) and equaled that of the postoperative group after 14 days. Clearance was affected by size and age only. Exchange transfusion, when used, contributed only 1.1% (CV 46%) of total clearance. Distribution volume increased with age and was 2.5 times (CV 102%) greater in ECMO children than in postoperative children. The between-subject variability values for volume of distribution and clearance were 49.4% and 38.7%. Weight and age information explained 83% of the overall clearance variability and 60% of overall distribution volume variability.

CONCLUSIONS

Morphine clearance is reduced in infants requiring ECMO, possibly reflecting severity of illness. Clearance maturation on ECMO is rapid and normalizes within 2 weeks. Initial morphine dosing may be guided by age and weight, but clearance and distribution volume changes (and their variability) during prolonged ECMO suggests that morphine therapy should be subsequently guided by clinical monitoring.

Developmental pharmacokinetics of morphine and its metabolites in neonates, infants and young children

BACKGROUND

Descriptions of the pharmacokinetics and metabolism of morphine and its metabolites in young children are scant. Previous studies have not differentiated the effects of size from those related to age during infancy.

METHODS

Postoperative children 0-3 yr old were given an intravenous loading dose of morphine hydrochloride (100 micro g kg(-1) in 2 min) followed by either an intravenous morphine infusion of 10 micro g h(-1) kg(-1) (n=92) or 3-hourly intravenous morphine boluses of 30 micro g kg(-1) (n=92). Additional morphine (5 micro g kg(-1)) every 10 min was given if the visual analogue (VAS, 0-10) pain score was >/=4. Arterial blood (1.4 ml) was sampled within 5 min of the loading dose and at 6, 12 and 24 h for morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G). The disposition of morphine and formation clearances of morphine base to its glucuronide metabolites and their elimination clearances were estimated using non-linear mixed effects models.

RESULTS

The analysis used 1856 concentration observations from 184 subjects. Population parameter estimates and their variability (%) for a one-compartment, first-order elimination model were as follows: volume of distribution 136 (59.3) litres, formation clearance to M3G 64.3 (58.8) litres h(-1), formation clearance to M6G 3.63 (82.2) litres h(-1), morphine clearance by other routes 3.12 litres h(-1) per 70 kg, elimination clearance of M3G 17.4 (43.0) litres h(-1), elimination clearance of M6G 5.8 (73.8) litres h(-1). All parameters are standardized to a 70 kg person using allometric 3/4 power models and reflect fully mature adult values. The volume of distribution increased exponentially with a maturation half-life of 26 days from 83 litres per 70 kg at birth; formation clearance to M3G and M6G increased with a maturation half-life of 88.3 days from 10.8 and 0.61 litres h(-1) per 70 kg respectively at birth. Metabolite formation decreased with increased serum bilirubin concentration. Metabolite clearance increased with age (maturation half-life 129 days), and appeared to be similar to that described for glomerular filtration rate maturation in infants.

CONCLUSION

M3G is the predominant metabolite of morphine in young children and total body morphine clearance is 80% that of adult values by 6 months. A mean steady-state serum concentration of 10 ng ml(-1) can be achieved in children after non-cardiac surgery in an intensive care unit with a morphine hydrochloride infusion of 5 micro g h(-1) kg(-1) at birth (term neonates), 8.5 micro g h(-1) kg(-1) at 1 month, 13.5 micro g h(-1) kg(-1) at 3 months and 18 micro g h(-1) kg(-1) at 1 year and 16 micro g h(-1) kg(-1) for 1- to 3-yr-old children.

Age- and therapy-related effects on morphine requirements and plasma concentrations of morphine and its metabolites in postoperative infants

BACKGROUND

To investigate clinical variables such as gestational age, sex, weight, the therapeutic regimens used and mechanical ventilation that might affect morphine requirements and plasma concentrations of morphine and its metabolites.

METHODS

In a double-blind study, neonates and infants stratified for age [group I 0-4 weeks (neonates), group II > or =4-26 weeks, group III > or =26-52 weeks, group IV > or =1-3 yr] admitted to the paediatric intensive care unit after abdominal or thoracic surgery received morphine 100 micro g kg(-1) after surgery, and were randomly assigned to either continuous morphine 10 micro g kg(-1) h(-1) or intermittent morphine boluses 30 micro g kg(-1) every 3 h. Pain was measured using the COMFORT behavioural scale and a visual analogue scale. Additional morphine was administered on guidance of the pain scores. Morphine, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) plasma concentrations were measured before, directly after, and at 6, 12 and 24 h after surgery.

RESULTS

Multiple regression analysis of different variables revealed that age was the most important factor affecting morphine requirements and plasma morphine concentrations. Significantly fewer neonates required additional morphine doses compared with all other age groups (P<0.001). Method of morphine administration (intermittent vs continuous) had no significant influence on morphine requirements. Neonates had significantly higher plasma concentrations of morphine, M3G and M6G (all P<0.001), and significantly lower M6G/morphine ratio (P<0.03) than the older children. The M6G/M3G ratio was similar in all age groups.

CONCLUSIONS

Neonates have a narrower therapeutic window for postoperative morphine analgesia than older age groups, with no difference in the safety or effectiveness of intermittent doses compared with continuous infusions in any of these age groups. In infants >1 month of age, analgesia is achieved after morphine infusions ranging from 10.9 to 12.3 micro g kg(-1) h(-1) at plasma concentrations of <15 ng ml(-1).

LC determination of morphine and morphine glucuronides in human plasma by coulometric and UV detection

A reversed-phase high-performance liquid chromatographic method with coulometric and UV detection has been developed for the simultaneous determination of morphine, morphine-3-glucuronide and morphine-6-glucuronide. The separation was carried out by using a Supelcosil LC-8 DB reversed-phase column and 0.1 M potassium dihydrogen phosphate (pH 2.5)--acetonitrile--methanol (94:5:1 v/v) containing 4 mM pentanesulfonic acid as the mobile phase. The compounds were determined simultaneously by coulometry for morphine and with UV detection for morphine-3-glucuronide and morphine-6-glucuronide. Morphine, morphine glucuronides and the internal standard were extracted from human plasma using Bond-Elut C18 (1 ml) solid-phase extraction cartridges. In the case of coulometric detection, the detection limit was 0.5 ng/ml for morphine; in the case of UV detection the detection limit was 10 ng/ml for morphine-3-glucuronide and for morphine-6-glucuronide, too.

Simultaneous determination of codeine and ethyl morphine HCL in tablet formulations using LC

A reverse phase high performance liquid chromatography (HPLC) method was developed for the simultaneous determination of codeine (methyl morphine) and dionin (ethyl morphine hydrochloride) in antitussive analgesic tablet formulations. A C(18) column and methanol-water (1:2) mixture mobile phase (pH 3.0) were used. Spectrophotometric detection was carried out at 210 nm. The total elution time was shorter than 7 min. This method was found to be quite precise and reproducible. This proposed method was successfully applied to the determination of codeine and ethyl morphine hydrochloride in tablets produced by the Turkish Army Drug Factory.

On the assessment of drug metabolism by assays of codeine and its main metabolites

Codeine and its main metabolites appear to have advantages for assessing drug metabolic phenotypes. The authors have further developed a high-performance liquid chromatography (HPLC) method for the quantification of codeine and six of its metabolites in urine. Quantification was performed by electrochemical detection for morphine, normorphine, morphine-6-glucuronide, and the internal standard 4-O-methyldopamine; and by ultraviolet detection for codeine, norcodeine, and morphine-3-glucuronide. The method had a detection limit of 2 nmol/L(-1) for morphine and normorphine, 4 nmol/L(-1) for morphine-6-glucuronide, 3 nmol/L for the internal standard, 20 nmol/L(-1) for morphine-3-glucuronide, and 60 nmol/L(-1) for codeine and norcodeine. The coefficients of variations were <9% for intraday and <10% for interday analyses. The recovery of codeine and its metabolites ranged from 55% (for morphine-3-glucuronide) to 90% (for codeine, norcodeine, morphine, and morphine-6-glucuronide). Eleven healthy volunteers were phenotyped for CYP2D6 using codeine as well as debrisoquine and dextromethorphan. Ten subjects were extensive metabolizers (EM) and one a poor metabolizer (PM) of codeine, debrisoquine, and dextromethorphan. Significant correlations between the metabolic ratios (MRs) of the different probe drugs were obtained (r2 > 0.95, p < 0.001). This HPLC method is simple, sensitive, accurate, and reproducible for assessing the CYP2D6 phenotype.

Immunoaffinity extraction of morphine, morphine-3-glucuronide and morphine-6-glucuronide from blood of heroin victims for simultaneous high-performance liquid chromatographic determination

The development of an immunoaffinity-based extraction method for the determination of morphine and its glucuronides in human blood is described. For the preparation of an immunoadsorber, specific antisera (polyclonal, host: rabbit) against morphine, morphine-3-glucuronide and morphine-6-glucuronide were coupled to 1,1'-carbonyldiimidazole-activated tris-acrylgel and used for immunoaffinity extraction of morphine and its glucuronides from coronary blood. The resulting extracts were analysed by HPLC with native fluorescence detection. The mean recoveries from spiked blood samples were 71%, 76% and 88% for morphine, morphine-3-glucuronide and morphine-6-glucuronide, respectively. The limit of detection was 3 ng/g blood and the limit of quantitation was 10 ng/g blood for all three analytes. The results of the analysis of coronary blood samples from 23 fatalities due to heroin are presented.

Fully automated analytical method for codeine quantification in human plasma using on-line solid-phase extraction and high-performance liquid chromatography with ultraviolet detection

A simple, sensitive and fully automated analytical method for the analysis of codeine in human plasma is presented. Samples are added with oxycodone, used as internal standard (I.S.), and directly loaded in the autosampler tray. An on-line sample clean-up system based on solid-phase extraction (SPE) cartridges (Bond-Elut C2, 20 mg) and value switching (Prospekt) is used. Isocratic elution improved reproducibility and allowed the recirculation of the mobile phase. A Hypersil BDS C18, 3 microns, 10 x 0.46 cm column was used and detection was done by UV monitoring at 212 nm. Retention times of norcodeine (codeine metabolite), codeine and oxycodone (I.S.) were 5.5, 6.4 and 9.1 min, respectively. Morphine was left to elute in the chromatographic front. Detection limit for codeine was 0.5 microgram l-1 and inter-assay precision (expressed as relative standard deviation) and accuracy (expressed as relative error) measured at 2 micrograms l-1 were 5.03% and 1.82%. Calibration range was 2-140 micrograms l-1.

Determination of drugs of abuse in blood

The detection and quantitation of drugs of abuse in blood is of growing interest in forensic and clinical toxicology. With the development of highly sensitive chromatographic methods, such as high-performance liquid chromatography (HPLC) with sensitive detectors and gas chromatography-mass spectrometry (GC-MS), more and more substances can be determined in blood. This review includes methods for the determination of the most commonly occurring illicit drugs and their metabolites, which are important for the assessment of drug abuse: Methamphetamine, amphetamine, 3,4-methylenedioxymethamphetamine (MDMA), N-ethyl-3,4-methylenedioxyamphetamine (MDEA), 3,4-methylenedioxy-amphetamine (MDA), cannabinoids (delta-9-tetrahydrocannabinol, 11-hydroxy-delta-9-tetrahydrocannabinol, 11-nor-9-carboxy-delta-9-tetrahydrocannabinol), cocaine, benzoylecgonine, ecgonine methyl ester, cocaethylene and the opiates (heroin, 6-monoacetylmorphine, morphine, codeine and dihydrocodeine). A number of drugs/drug metabolites that are structurally close to these substances are included in the tables. Basic information about the biosample assayed, work-up, GC column or LC column and mobile phase, detection mode, reference data and validation data of each procedure is summarized in the tables. Examples of typical applications are presented.

Simultaneous determination of codeine and its seven metabolites in plasma and urine by high-performance liquid chromatography with ultraviolet and electrochemical detection

A sensitive and selective high-performance liquid chromatography method has been developed for the measurement of codeine and its seven metabolites, norcodeine, morphine, normorphine, codeine-6-glucuronide, morphine-6-glucuronide, morphine-3-glucuronide and norcodeine glucuronide, in plasma and urine. The compounds were recovered from plasma and urine using solid-phase extraction with C18 cartridges and separated on a reversed-phase C8 column with a mobile phase consisting of 77% buffer (5 mM sodium phosphate monobasic and 0.70 mM sodium dodecyl sulfate, pH 2.35) and 23% acetonitrile. Codeine, norcodeine, codeine-6-glucuronide, norcodeine glucuronide and morphine-3-glucuronide were detected by ultraviolet detection at 214 nm, with a detection limit of 0.02 nmol/ml for each compound in plasma. Morphine-6-glucuronide, normorphine and morphine were monitored by electrochemical detection at 350 mV, with a detection limit of 0.003 nmol/ml for each compound in plasma. The assay showed good reproducibility and accuracy using external standardization. The recovery and inter-day variation for all compounds in plasma samples were 63.40-77.90% and 3.49-16.77% (R.S.D.) and while in urine were 64.98-90.13% and 2.93-9.96% (R.S.D.), respectively.

Determination of 6-monoacetylmorphine and morphine in plasma, whole blood and urine using high-performance liquid chromatography with electrochemical detection

6-Monoacetylmorphine and morphine were determined simultaneously in plasma, whole blood and urine, after solid-phase extraction, by high-performance liquid chromatography using amperometric detection at 600 mV oxidation potential. The recoveries ranged from 92 to 99%. The reproducibility study indicated that the coefficients of variation were less than 11% for morphine and 12.4% for 6-monoacetylmorphine. The determination limits were 1 ng/ml for morphine and 4 ng/ml for 6-monoacetylmorphine. The method had a good selectivity towards opiate and nonopiate analgesics and other drugs. The stability of the analytes in methanol (standard solutions), in samples (plasma, whole blood and urine) at -20 degrees C and at 20 degrees C, and in samples after enzymatic hydrolysis at 37 degrees C, was also studied. For sample containing 6-monoacetylmorphine, inadequate storage or hydrolysis could lead to overestimation of morphine or its conjugates. The technique described can be applied for the study of the pharmacokinetics of heroin; it is also available for forensic toxicology to distinguish heroin use from medical prescription of morphine and other opiate drugs.

Simultaneous determination of dihydrocodeine and its metabolites in dog plasma by high-performance liquid chromatography with electrochemical and ultraviolet detection

An HPLC method with electrochemical and UV detection was established for the simultaneous determination of dihydrocodeine and its metabolites, dihydronorcodeine, dihydromorphine, and dihydrocodeine glucuronide, in dog plasma using N-ethylnormorphine as the internal standard. The method involved sample pretreatment with a C18-bonded disposable column, and the injected fraction was separated and detected on the C18-bonded column with serially coupled UV and coulometric detectors. Dihydromorphine was detected with the coulometric detector at 0.4 V, and dihydrocodeine and dihydronorcodeine at 0.8 V. Dihydrocodeine glucuronide was detected with UV at 210 nm. Recoveries of the studied compounds were quantitative at the individual assay ranges, and validation of the assay gave results that were satisfactory in terms of within-run or between-run precision and accuracy. Lower limits of quantitation were 2 ng/ml for dihydrocodeine and dihydronorcodeine, 0.5 ng/ml for dihydromorphine, and 200 ng/ml for dihydrocodeine glucuronide.

Determination of codeine and its metabolites in microsomal incubates by high-performance liquid chromatography

A rapid and sensitive HPLC method has been developed for the determination of codeine, norcodeine and morphine in small volumes of a biological matrix, using a cyanopropyl column and a combination of coulometric and UV detection. The compounds were isolated using C18 solid-phase extraction cartridges prior to quantitative analysis. The limit of detection was 250 pg/ml for morphine and 5 ng/ml for both norcodeine and codeine. Recovery of each compound was greater than 90% and intra- and inter-assay precision was better than 10%. The method has been used to study the metabolism of codeine in microsomal incubations.

Determination of peptide 520 in human plasma using post-column photolysis with electrochemical detection in liquid chromatography

A simple LC method for the determination of peptide 520 in human plasma was developed. Based on micellar chromatography, sodium octyl sulphate (SOS) was added into the mobile phase in order to separate the peptide from human plasma components. The procedure was fast and sensitive for the determination of the peptide in untreated human plasma. The electrochemical (EC) detection limit for peptide 520 in human plasma was 0.5 microgram ml-1. Linearity of the calibration plot for peptide 520 in human plasma was 0.999. This approach represents a direct injection technique for the potential detection and analysis of numerous peptides in biofluids, besides just plasma, with absolute quantification.

Pharmacokinetics of naproxen, its metabolite O-desmethylnaproxen, and their acyl glucuronides in humans

The aim of this investigation was to assess the pharmacokinetics of naproxen in 10 human subjects after an oral dose of 500 mg using a direct HPLC analysis of the acyl glucuronide conjugates of naproxen and its metabolite O-desmethylnaproxen. The mean t1/2 of naproxen in 9 subjects was 24.7 +/- 6.4 h (range 16 to 36 h). The t1/2 of 7.4 as found in subject number 10 must, therefore, be regarded as an extraordinary case (p < 0.0153). Naproxen acyl glucuronide accounts for 50.8 +/- 7.32 per cent of the dose, its isomerized conjugate isoglucuronide for 6.5 +/- 2.0 per cent, O-desmethylnaproxen acyl glucuronide for 14.3 +/- 3.4 per cent, and its isoglucuronide for 5.5 +/- 1.3 per cent (n = 10; 100 h collection period). Naproxen and O-desmethylnaproxen are excreted in negligible amounts (< 1 per cent). Even though urine pH of the subjects was kept acid (range pH 5.0-5.5) in order to stabilize the acyl glucuronides, isomerization takes place in blood when the acyl glucuronide is released from the liver for excretion by the kidney. Binding to plasma proteins was measured as 98 per cent and 100 per cent, respectively for the unconjugated compounds naproxen and O-desmethylnaproxen. Binding of the acyl glucuronides was less, being 92 per cent; for naproxen acyl glucuronide, 66 per cent for naproxen isoglucuronide, 72 per cent for O-desmethylnaproxen acyl glucuronide and 42 per cent for O-desmethylnaproxen isoglucuronide.

Determination of indomethacin, its metabolites and their glucuronides in human plasma and urine by means of direct gradient high-performance liquid chromatographic analysis. Preliminary pharmacokinetics and effect of probenecid

Indomethacin is metabolized in humans by O-demethylation, and by acyl glucuronidation to the 1-O-glucuronide. Indomethacin, its metabolite O-desmethylindomethacin (DMI) and their conjugates can be measured directly by gradient high-performance liquid chromatographic analysis without enzymic deglucuronidation. The glucuronide conjugates were isolated by preparative HPLC from human urine samples. In plasma only indomethacin was present. No isoglucuronides were present in acidic urine of the volunteer. The possible metabolite deschlorobenzoylindomethacin (DBI) was not detectable in urine. Calibration curves were constructed by enzymic deconjugation of samples containing different concentrations of isolated indomethacin acyl glucuronide, DMI acyl glucuronide and DMI ether glucuronide. The limit of quantitation of indomethacin in plasma is 0.060 microgram/ml. The limits of quantitation in urine are: indomethacin 0.053 microgram/ml, DMI 0.065 microgram/ml, DMI acyl glucuronide 0.065 microgram/ml and DMI ether glucuronide 0.254 microgram/ml. A pharmacokinetic profile of indomethacin is shown, and some preliminary pharmacokinetic parameters of indomethacin obtained from one human volunteer are given. Probenecid inhibits the formation of both the ether and the acyl glucuronide of DMI.

Automated kinetic-spectrofluorimetric method for the determination of morphine in urine

The continuous addition of reagent technique was used with spectrofluorimetric detection for the kinetic determination of morphine in urine samples. The method thus developed is based on the oxidative dimerization of morphine to fluorescent pseudomorphine by potassium hexacyanoferrate(III) in a basic medium. The optimum pH, oxidant concentration and instrumental variables were determined. The method permits the sensitive determination of morphine over a wide concentration range (15-925 ng ml-1) with high precision (relative standard deviation approximately 2%), selectivity and sample throughput (48 samples h-1). A novel sorption--desorption procedure was used to isolate morphine from whole urine, which provided recoveries of between 85 and 100%. The results showed the usefulness of the proposed method for controlling a wide variety of medicinal problems (e.g., therapeutic, overdose and doping analysis).

High-performance liquid chromatographic determination of morphine and its metabolites in plasma using diode-array detection

An isocratic high-performance liquid chromatographic method has been developed for the determination of morphine, codeine, normorphine, morphine 3-glucuronide and morphine 6-glucuronide in plasma using a diol column and diode-array detection. Samples were extracted using solid-phase extraction with recoveries in excess of 90%. The limit of determination was 1 ng/ml for morphine, codeine and morphine 3-glucuronide, and 10 ng/ml for normorphine and morphine 6-glucuronide. Inter- and intra-day precision were better than 10%.

An improved extraction method for the HPLC determination of morphine and its metabolites in plasma

A new, simple and rapid extraction procedure coupled with a combined coulometric-fluorescence HPLC assay is described for the simultaneous determination of morphine (M) and morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G), and normorphine (NM) in plasma. The effect of concentration and pH of selected ion-pairing agents on the extraction of these compounds from plasma by solid-phase extraction was investigated. The extraction procedure was optimized in terms of recovery, reproducibility and lack of interference from endogenous materials. The optimized method uses tetrabutylammonium hydrogen sulphate (TBAHS) at pH 10 followed by separation on a single C18 solid-phase extraction cartridge. For routine analysis the procedure provides high and reproducible recoveries over a concentration range of 1.0-1000 ng ml-1 for morphine, M6G and normorphine and 20-1000 ng ml-1 for M3G. The method was used successfully to analyse plasma samples from a pharmacokinetic study in which sheep had received an intravenous dose of 0.015 mg kg-1 of M6G.

Influence of hydrolysis procedures on the urinary concentrations of codeine and morphine in relation to doping analysis

A method is described for the GC-NPD determination of urinary codeine and morphine after derivatization with trifluoroacetic anhydride. The lower limit for accurate quantitative determination was 0.05 microgram ml-1. After the oral administration of Bisolvon Griblettes corresponding to 30 mg codeine phosphate to seven subjects maximum codeine concentrations were obtained after 1-2 h and codeine remained detectable generally 24 h post dosing. The mean maximum level was 5.1 +/- 2.8 micrograms ml-1 found after enzymatic hydrolysis with Suc Helix pomatia juice (SHP). Based on these and previous results (mean 6.3 +/- 3.4 micrograms ml-1) a threshold level for codeine of 16 micrograms ml-1 is proposed. Significant differences were noticed between urinary codeine concentrations found after enzymatic hydrolysis with SHP, beta-glucuronidase from Patella vulgata and acid hydrolysis, respectively. Generally, highest values were obtained after SHP, while beta-glucuronidase and especially acid hydrolysis resulted in much lower levels. No morphine could be detected after acid hydrolysis. Concerning doping analysis, in particular the uniformity of methods and interpretation of the results, it is recommended that the hydrolysis method should be specified in the rules of those sporting federations allowing codeine and/or morphine.

Contribution of the human kidney to the metabolic clearance of drugs

OBJECTIVE

To demonstrate that the human kidney is capable not only of filtering and secreting drugs and their metabolites, but also of carrying out conjugation reactions such as acyl glucuronidation, N-glucuronidation, and glycination.

DATA SOURCES

Plasma concentrations and renal excretion rates of drugs are measured and renal clearance is calculated in a series of selected pharmacokinetic studies in healthy human volunteers (some studies were conducted in the authors' laboratory and others were reported in the literature). BACKGROUND THEORY: It is generally agreed that the liver plays the dominant role in drug metabolism, and that the function of the kidneys is limited to excretion of parent drug and metabolites. This can be easily understood when a metabolite is present in both plasma and urine. When the metabolite is present in urine but is not measurable in plasma, then the possibility exists that the metabolite is formed by the kidneys.

RESULTS

"Simple" excretion by the kidneys is demonstrated for sulfatroxazole/sulfamethoxazole. Ether glucuronides of codeine are formed in the liver, and the resulting glucuronide is excreted by the kidneys. Possible formation of N1- and N2-glucuronides by the kidneys is demonstrated for sulfadimethoxine, sulfametomidine, and sulfaphenazole. Acyl glucuronidation of probenecid and nalidixic acid is carried out by the kidneys. The acyl glucuronidation of probenecid shows a capacity-limited formation/excretion rate of 46 mg/h, which is subject dependent. During this process, the acyl glucuronidation of co-administered nalidixic acid is reduced from 53 to 16 percent compared with that of nalidixic acid alone. Probenecid and its acyl glucuronidation do not inhibit the ether glucuronidation of codeine in the liver, but only interfere with the active tubular secretion process. The acyl glucuronidation of the nonsteroidal antiinflammatory drug naproxen and its metabolite, O-desmethylnaproxen, may be carried out by the liver and kidneys. Glycination of benzoic acid and salicylic acid is carried out in both the liver and kidneys.

CONCLUSIONS

It is difficult to recognize renal drug metabolism in the intact human body (in vivo); the glucuronides or conjugates must be measured via direct HPLC analysis. In cases where the metabolite is present in high concentrations in urine but not in blood, there may be an indication that the kidneys are responsible for the formation of the metabolite. Impaired kidney function not only affects renal excretion but may also affect renal metabolism.

Capacity-limited renal glucuronidation of probenecid by humans. A pilot Vmax-finding study

Probenecid shows dose-dependent pharmacokinetics. When in one volunteer the dose is increased from 250 to 1,500 mg orally, the t1/2 increased from 3 to 6 h. The Cmax was 14 micrograms/ml with a dosage of 250 mg, 31 micrograms/ml with 500 mg, 70 micrograms/ml with 1,000 mg and 120 micrograms/ml with 1,500 mg. The tmax remained 1 h for all four dosages. The AUC/dose ratio increased with the dose, indicating nonlinear elimination. The total body clearance declined from 64.5 ml/min for 250 mg to 26.0 ml/min for 1,500 mg. The renal clearance of probenecid remained constant, 0.6-0.8 ml/min. Protein binding of probenecid is high (91%) and independent of the dose. The phase I metabolites show lower protein binding values (34-59%). The protein binding of probenecid glucuronide in vitro (spiked plasma) is 75%. Probenecid is metabolized by cytochrome P-450 to three phase I metabolites. Each of the metabolites accounts for less than 10% of the dose administered; the percentage recovered in the urine is independent of the dose. The main metabolite probenecid glucuronide is only present in urine and not in plasma. The renal excretion rate--time profile of probenecid glucuronide shows a plateau value of approximately 700 micrograms/min (46 mg/h) with acidic urine pH. The duration of this plateau value depends on the dose: 2 h at 500 mg, 10 h at 1,000 mg and 20 h at 1,500 mg. It is demonstrated that probenecid glucuronide must be formed in the kidney during its passage of the tubule. The plateau value in the renal excretion rate of probenecid value reflects its Vmax of formation.

Pharmacokinetics and metabolism of codeine in humans

Codeine (30 mg phosphate) was metabolized by eight human volunteers to the following six metabolites: codeine-6-glucuronide 81.0 +/- 9.3 per cent, norcodeine 2.16 +/- 1.44 per cent, morphine 0.56 +/- 0.39 per cent, morphine-3-glucuronide 2.10 +/- 1.24 per cent, morphine-6-glucuronide 0.80 +/- 0.63 per cent, and normorphine 2.44 +/- 2.42 per cent. Two out of eight volunteers were unable to O-dealkylate codeine into morphine and lack therefore the cytochrome P450 IID6 isoenzyme. The half-life of codeine was 1.47 +/- 0.32 h, that of codeine-6-glucuronide 2.75 +/- 0.79 h, and that of morphine-3-glucuronide 1.71 +/- 0.51 h. The systemic clearance of codeine was 2280 +/- 840 ml min-1, the renal clearance of codeine was 93.8 +/- 29.8 ml min-1, and that of codeine-6-glucuronide was 122 +/- 39.2 ml min-1. The plasma AUC of codeine-6-glucuronide is approximately 10 times higher than that of codeine. Protein binding of codeine and codeine-6-glucuronide in vivo was 56.1 +/- 2.5 per cent and 34.0 +/- 3.6 per cent, respectively. The in vitro protein binding of norcodeine was 23.5 +/- 2.9 per cent; of morphine, 46.5 +/- 2.4 per cent; of normorphine, 23.5 +/- 3.5 per cent; of morphine-3-glucuronide, 27.0 +/- 0.8 per cent; and of morphine-6-glucuronide, 36.7 +/- 3.8 per cent.

Direct measurement of probenecid and its glucuronide conjugate by means of high pressure liquid chromatography in plasma and urine of humans

Probenecid with its phase-I metabolites, and phase-II glucuronide conjugate can be analysed by a gradient high pressure liquid chromatographic method. Probenecid glucuronide in plasma with pH 7.4 is not stable and declines to 10% of the original value within 6 h (t1/2 approximately 1 h). Probenecid glucuronide is stable in urine with pH 5.0, moderately unstable at pH 6.0 (t1/2 approximately 10 h), and unstable at pH 8.0 (t1/2 approximately 0.5 h). Probenecid glucuronide is stable in water and 0.01 mol/l phosphoric acid in the autosampler of the high pressure liquid chromatograph. The decrease in concentration in water is 5.5% during 9 h and 0% in diluted acid. Probenecid glucuronide and the phase-I metabolites were not detectable in plasma. The main compound in fresh urine is the phase-II conjugate probenecid glucuronide (62% of a 500 mg dose); the phase-I metabolites are present and only a trace of probenecid is present. The percentage of the dose of the phase-I metabolites varies between 5 and 10, while hardly any probenecid is excreted unchanged (0.33%).