Opiate receptor: demonstration in nervous tissue

Tritiated naloxone, a powerful opiate antagonist, specifically binds to an opiate receptor of mammalian brain and guinea pig intestine. Competition for the opiate receptor by various opiates and their antagonists closely parallels their pharmacological potency. The opiate receptor is confined to nervous tissue.

Codeine intoxication associated with ultrarapid CYP2D6 metabolism

Life-threatening opioid intoxication developed in a patient after he was given small doses of codeine for the treatment of a cough associated with bilateral pneumonia. Codeine is bioactivated by CYP2D6 into morphine, which then undergoes further glucuronidation. CYP2D6 genotyping showed that the patient had three or more functional alleles, a finding consistent with ultrarapid metabolism of codeine. We attribute the toxicity to this genotype, in combination with inhibition of CYP3A4 activity by other medications and a transient reduction in renal function.

Blood-brain barrier: penetration of morphine, codeine, heroin, and methadone after carotid injection

Labeled morphine, codeine, heroin, or methadone was injected as a bolus into the common carotid artery of the rat, and the rat was decapitated 15 seconds later. The brain uptake of the drug was calculated by measurement of the brain content of the drug as a percentage of a labeled, highly diffusible reference substance simultaneously injected. The uptake of morphine was below measurability; the uptake of codeine was 24 percent; heroin, 68 percent; and methadone, 42 percent. Brain uptakes of morphine and codeine were also studied after intravenous injection and correlated well with uptakes after carotid injection; the uptake of codeine being nearly complete by 30 seconds. These studies indicate that brain uptake of certain of these drugs is very rapid and that uptake of heroin injected intravenously is probably limited by the regional flow of blood in the brain. The possible relation of this rapid penetration of the blood-brain barrier by heroin to its strongly addictive properties is discussed.

Pharmacogenetics and human molecular genetics of opiate and cocaine addictions and their treatments

Opiate and cocaine addictions are major social and medical problems that impose a significant burden on society. Despite the size and scope of these problems, there are few effective treatments for these addictions. Methadone maintenance is an effective and most widely used treatment for opiate addiction, allowing normalization of many physiological abnormalities caused by chronic use of short-acting opiates. There are no pharmacological treatments for cocaine addiction. Epidemiological, linkage, and association studies have demonstrated a significant contribution of genetic factors to the addictive diseases. This article reviews the molecular genetics and pharmacogenetics of opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems that have been associated with or have evidence for linkage to opiate or cocaine addiction. This evidence has been marshalled either through identification of variant alleles that lead to functional alterations of gene products, altered gene expression, or findings of linkage or association studies. Studies of polymorphisms in the mu opioid receptor gene, which encodes the receptor target of some endogenous opioids, heroin, morphine, and synthetic opioids, have contributed substantially to knowledge of genetic influences on opiate and cocaine addiction. Other genes of the endogenous opioid and monoaminergic systems, particularly genes encoding dopamine beta-hydroxylase, and the dopamine, serotonin, and norepinephrine transporters have also been implicated. Variants in genes encoding proteins involved in metabolism or biotransformation of drugs of abuse and also of treatment agents are reviewed.

Evaluation of 3'-azido-3'-deoxythymidine, morphine, and codeine as probe substrates for UDP-glucuronosyltransferase 2B7 (UGT2B7) in human liver microsomes: specificity and influence of the UGT2B7*2 polymorphism

UDP-glucuronosyltransferase 2B7 (UGT2B7) is involved in the glucuronidation of a wide array of clinically important drugs and endogenous compounds in humans. The aim of this study was to identify an isoform-selective probe substrate that could be used to investigate genetic and environmental influences on glucuronidation mediated by UGT2B7. Three potential probe substrates [3'-azido-3'-deoxythymidine (AZT), morphine, and codeine], were evaluated using recombinant UGTs and human liver microsomes (HLMs; n = 54). Of 11 different UGTs screened, UGT2B7 was the principal isoform mediating AZT glucuronidation, morphine-3-glucuronidation, and morphine-6-glucuronidation. Codeine was glucuronidated equally well by UGT2B4 and UGT2B7. Enzyme kinetic analysis of these activities typically showed higher apparent Km values for HLMs (pooled and individual) compared with UGT2B7. This difference was least (less than 2-fold higher Km) for AZT glucuronidation and greatest (3- to 6-fold higher Km) for codeine glucuronidation. Microsomal UGT2B7 protein content correlated well with AZT glucuronidation (rs = 0.77), to a lesser extent with morphine-3-glucuronidation (rs = 0.50) and morphine-6-glucuronidation (rs = 0.51), but very weakly with codeine glucuronidation (rs = 0.33). Livers were also genotyped for the UGT2B7*2 (H268Y) polymorphism. No effect of genotype on microsomal glucuronidation or UGT2B7 protein content was observed. In conclusion, although both AZT and morphine can serve as in vitro probe substrates for UGT2B7, AZT appears to be more selective than morphine. Codeine is not a useful UGT2B7 probe substrate because of significant glucuronidation by UGT2B4. The UGT2B7*2 polymorphism is not a determinant of glucuronidation of AZT, morphine, or codeine in HLMs.

Mu receptor binding of some commonly used opioids and their metabolites

The binding affinity to the mu receptor of some opioids chemically related to morphine and some of their metabolites was examined in rat brain homogenates with 3H-DAMGO. The chemical group at position 6 of the molecule had little effect on binding (e.g. morphine-6-glucuronide Ki = 0.6 nM; morphine = 1.2 nM). Decreasing the length of the alkyl group at position 3 decreased the Ki values (morphine less than codeine less than ethylmorphine less than pholcodine). Analgesics with high clinical potency containing a methoxyl group at position 3 (e.g. hydrocodone, Ki = 19.8 nM) had relatively weak receptor binding, whilst their O-demethylated metabolites (e.g. hydromorphone, Ki = 0.6 nM) had much stronger binding. Many opioids may exert their pharmacological actions predominantly through metabolites.

Bioactivation of the narcotic drug codeine in human liver is mediated by the polymorphic monooxygenase catalyzing debrisoquine 4-hydroxylation (cytochrome P-450 dbl/bufI)

Codeine O-demethylation to its active moiety morphine was investigated in human liver microsomes from 1 poor and 5 extensive metabolizer subjects (debrisoquine-type of oxidation polymorphism). Apparent Km of the reaction in one extensive metabolizer's microsomes was 149 microM and Vmax 17.6 nmol X mg P-1 X hour-1 versus greater than 1 mM and 1.6 nmol X mg P-1 X hour-1 respectively in one poor metabolizer. In vitro morphine production was competitively inhibited by quinidine (Ki 15 nM), the selective inhibitor of cytochrome P-450 dbl/bufI. There was also an excellent correlation between dextromethorphan O-demethylation, a prototype reaction for cytochrome P-450 dbl/bufI activity, and codeine O-demethylation. These data allow to conclude that codeine bioactivation to morphine is dependent on the polymorphic monooxygenase known as cytochrome db1/bufI.

Pharmacogenetics of codeine metabolism in an urban population of children and its implications for analgesic reliability

BACKGROUND

Codeine analgesia is wholly or mostly due to its metabolism to morphine by the cytochrome P450 enzyme CYP2D6, which shows significant genetic variation in activity. The aims of this study were to investigate genotype, phenotype and morphine production from codeine in children undergoing adenotonsillectomy, and to compare analgesia from codeine or morphine combined with diclofenac.

METHODS

Ninety-six children received either codeine 1.5 mg kg(-1) or morphine 0.15 mg kg(-1) in a randomized, double-blind design. Genetic analysis was performed and plasma morphine concentrations at 1 h were determined. Postoperative analgesia and side-effects were recorded.

RESULTS

Forty-seven per cent of children had genotypes associated with reduced enzyme activity. Mean (SD) morphine concentrations were significantly lower (P<0.001) after codeine [4.5 (0.3) ng ml(-1)] than after morphine [24.7 (1.5) ng ml(-1)], and morphine and its metabolites were not detected in 36% of children given codeine. There was a significant relationship between phenotype and plasma morphine (P=0.02). More children required rescue analgesia after codeine at both 2 (P<0.05) and 4 h after administration (P<0.01). Fifty-six per cent of children vomited after morphine and 29% after codeine (P<0.01). Neither phenotype nor morphine concentration was correlated with either pain score or the need for rescue analgesia (r=-0.21, 95% confidence interval -0.4, -0.01).

CONCLUSIONS

Reduced ability for codeine metabolism may be more common than previously reported. Plasma morphine concentration 1 h after codeine is very low, and related to phenotype. Codeine analgesia is less reliable than morphine, but was not well correlated with either phenotype or plasma morphine in this study.

Impact of environmental and genetic factors on codeine analgesia

The polymorphic cytochrome P-450 DB1 (P-450 IID6) is responsible for the O-demethylation of codeine to morphine by human liver microsomes. The influence of P-450 DB1 variable activity on the bioactivation of codeine in vivo to morphine and on its analgesic effect was investigated in phenotyped healthy volunteers--7 extensive [EM] and 1 poor [PM] metabolizer of debrisoquine. After pretreatment with oral placebo or quinidine sulphate 50 mg, codeine phosphate 100 mg or placebo were administered orally according to a double-blind randomized crossover design. In EM subjects the plasma morphine Cmax was 17.9 nmol/l, whereas virtually no morphine was detectable after quinidine pretreatment (1.5 nmol/l), and in the PM subject (0.60 nmol/l). In EM codeine significantly increased subjective (VAS) and objective (R-III reflex) pain thresholds in response to selective transcutaneous nerve stimulation, whereas no significant analgesia was detected after placebo, or after codeine with quinidine pretreatment, or in the PM. In PM of genetic origin, or due to environmental alteration of the phenotypic expression (i.e. drug interaction), codeine is not activated into morphine and is an inefficient analgesic.

Pharmacogenetics affects dosing, efficacy, and toxicity of cytochrome P450-metabolized drugs

Drug-metabolizing enzyme activity is one of many factors affecting patient response to medications. The objective of this review is to highlight the potential for genetic variability in cytochrome P450 enzyme activity that can lead to interperson differences in response to drugs. Awareness and application of this knowledge will improve drug use in clinical practice and provide the physician with further appreciation that standard drug dosing may not be appropriate in all patients.

Affinity profiles of morphine, codeine, dihydrocodeine and their glucuronides at opioid receptor subtypes

The affinity of morphine, codeine, dihydrocodeine and their glucuronides for mu-, delta-, and kappa-opioid receptors was investigated. Binding was studied on guinea-pig brain homogenates with [3H]DAMGO, [3H]DPDPE, and [3H]U69593. The substitution of the free phenolic group of morphine caused a decrease in binding at opioid receptors without affecting the mu/delta-ratio nor that of mu/kappa. Glucuronidation of the 6-hydroxyl group of morphine, codeine or dihydrocodeine did not affect the affinity to mu-receptors, slightly increased the affinity for delta-receptors and reduced the affinity for kappa-receptors. The 6-glucuronides possess a decreased selectivity for mu-receptors over delta-receptors whereas that for mu- over kappa-receptors was increased. It is concluded that chemical variations at 3- and 6-position of morphine independently affect the affinity to opioid receptor subtypes.

The pharmacogenetics of codeine hypoalgesia

Codeine is an old drug that is still widely used to treat mild and moderate pain. It is mainly metabolised by glucuronidation, but minor pathways are N-demethylation to norcodeine and O-demethylation to morphine. The latter pathway depends on the genetically polymorphic CYP2D6 which is absent in 7% of the white population (PM) and present in the remainder (EM). Lack of influence of codeine on experimental pain in PM as well as in EM treated with the CYP2D6 blocker quinidine, who are both practically unable to convert codeine to morphine, has supported an old hypothesis that codeine acts through metabolically formed morphine. Possibly, local codeine O-demethylation in the CNS is of major importance for its hypoalgesic effect. Such a local morphine formation from codeine, which supposedly is also catalysed by CYP2D6, could explain why the hypoalgesic effect of codeine stems from morphine despite relatively low plasma levels of morphine after standard hypoalgesic doses of codeine. Dependence of codeine hypoalgesia on morphine formation via CYP2D6 makes this effect liable to interaction with drugs that are inhibitors of CYP2D6. Examples of potent inhibitors of CYP2D6 are quinidine, some selective serotonin reuptake inhibitors and some neuroleptics. Less potent inhibitors, such as tricyclic antidepressants, will probably also reduce the pain relieving effect of codeine, since codeine has a low affinity for CYP2D6. Biosynthesis of morphine in humans may also include steps catalyse by CYP2D6. Experimental studies in large groups of EM and PM indicate that this may lead to interphenotype differences in pain tolerance.

Comparative metabolic capabilities and inhibitory profiles of CYP2D6.1, CYP2D6.10, and CYP2D6.17

Polymorphisms in the cytochrome P450 2D6 (CYP2D6) gene are a major cause of pharmacokinetic variability in human. Although the poor metabolizer phenotype is known to be caused by two null alleles leading to absence of functional CYP2D6 protein, the large variability among individuals with functional alleles remains mostly unexplained. Thus, the goal of this study was to examine the intrinsic enzymatic differences that exist among the several active CYP2D6 allelic variants. The relative catalytic activities (enzyme kinetics) of three functionally active human CYP2D6 allelic variants, CYP2D6.1, CYP2D6.10, and CYP2D6.17, were systematically investigated for their ability to metabolize a structurally diverse set of clinically important CYP2D6-metabolized drugs [atomoxetine, bufuralol, codeine, debrisoquine, dextromethorphan, (S)-fluoxetine, nortriptyline, and tramadol] and the effects of various CYP2D6-inhibitors [cocaine, (S)-fluoxetine, (S)-norfluoxetine, imipramine, quinidine, and thioridazine] on these three variants. The most significant difference observed was a consistent but substrate-dependent decease in the catalytic efficiencies of cDNA-expressed CYP2D6.10 and CYP2D6.17 compared with CYP2D6.1, yielding 1.32 to 27.9 and 7.33 to 80.4% of the efficiency of CYP2D6.1, respectively. The most important finding from this study is that there are mixed effects on the functionally reduced allelic variants in enzyme-substrate affinity or enzyme-inhibitor affinity, which is lower, higher, or comparable to that for CYP2D6.1. Considering the rather high frequencies of CYP2D6*10 and CYP2D6*17 alleles for Asians and African Americans, respectively, these data provide further insight into ethnic differences in CYP2D6-mediated drug metabolism. However, as with all in vitro to in vivo extrapolations, caution should be applied to the clinical consequences.

The nonelectrolyte permeability of planar lipid bilayer membranes

The permeability of lecithin bilayer membranes to nonelectrolytes is in reasonable agreement with Overton's rule. The is, Pd alpha DKhc, where/Pd is the permeability coefficient of a solute through the bilayer, Khc is its hydrocarbon:water partition coefficient, and D is its diffusion coefficient in bulk hydrocarbon. The partition coefficients are by far the major determinants of the relative magnitudes of the permeability coefficients; the diffusion coefficients make only a minor contribution. We note that the recent emphasis on theoretically calculated intramembranous diffusion coefficients (Dm'S) has diverted attention from the experimentally measurable and physiologically relevant permeability coefficients (Pd'S) and has obscured the simplicity and usefulness of Overton's rule.

Quick onset of severe abdominal pain after codeine in an ultrarapid metabolizer of debrisoquine

The authors describe a 33-year-old woman who experienced severe pain in the epigastrium after codeine intake. This side-effect is consistent with that of morphine. Later, the patient was phenotyped and genotyped as an ultrarapid metabolizer with high capacity to metabolize codeine to morphine.

Codeine O-demethylation co-segregates with polymorphic debrisoquine hydroxylation

1. A single oral dose of codeine (25 mg) was given to 132 healthy Swedish Caucasians who had previously been phenotyped with respect to debrisoquine hydroxylation. The 'metabolic ratios' (MR) in urine of codeine O-demethylation (codeine/(morphine (M) + morphine-3- and 6-glucuronides (M3G and M6G) + normorphine], N-demethylation (codeine/(norcodeine (NC) + norcodeine glucuronide + normorphine (NM]) and glucuronidation (codeine/codeine-6-glucuronide (C6G] were calculated following h.p.l.c. analysis of urine samples collected over 8 h. 2. There was a significant correlation between the log MR for debrisoquine hydroxylation and the log MR for codeine O-demethylation (rs = 0.77, P less than 0.001). The poor debrisoquine hydroxylators had MRs of codeine O-demethylation between 8.3 and 55.1, while the values for extensive hydroxylators were between 0.4 and 5.5. 3. The poor debrisoquine hydroxylators excreted significantly less M, M3G, M6G and NM, while the urinary recovery of C6G and NC was significantly higher in these subjects compared to the extensive hydroxylators. 4. The MRs for glucuronidation and N-demethylation did not exhibit a bimodal distribution, and were not related to the MR of debrisoquine hydroxylation. 5. No associations were found between sex, body-weight, smoking habits, age, urine volume or urine pH and the O-demethylation of codeine. 6. The O-demethylation of codeine to form M appears to be under the same polymorphic genetic control as the 4-hydroxylation of debrisoquine.

Disposition and metabolism of codeine after single and chronic doses in one poor and seven extensive metabolisers

1. The pharmacokinetics, metabolism and partial clearances of codeine to morphine, norcodeine and codeine-6-glucuronide after single (30 mg) and chronic (30 mg 8 h for seven doses) administration of codeine were studied in eight subjects (seven extensive and one poor metaboliser of dextromethorphan). Codeine, codeine-6-glucuronide, morphine and norcodeine were measured by high performance liquid chromatographic assays. 2. After the single dose, the time to achieve maximum plasma codeine concentrations was 0.97 +/- 0.31 h (mean +/- s.d.) and for codeine-6-glucuronide it was 1.28 +/- 0.49 h. The plasma AUC of codeine-6-glucuronide was 15.8 +/- 4.5 times higher than that of codeine. The AUC of codeine in saliva was 3.4 +/- 1.1 times higher than that in plasma. The elimination half-life of codeine was 3.2 +/- 0.3 h and that of codeine-6-glucuronide was 3.2 +/- 0.9 h. 3. The renal clearance of codeine was 183 +/- 59 ml min-1 and was inversely correlated with urine pH (r = 0.81). These data suggest that codeine undergoes filtration at the glomerulus, tubular secretion and passive reabsorption. The renal clearance of codeine-6-glucuronide was 55 +/- 21 ml min-1, and was not correlated with urine pH. Its binding to human plasma was less than 10%. These data suggest that codeine-6-glucuronide undergoes filtration at the glomerulus and tubular reabsorption. This latter process is unlikely to be passive. 4. After chronic dosing, the pharmacokinetics of codeine and codeine-6-glucuronide were not significantly different from the single dose pharmacokinetics. 5. After the single dose, 86.1 +/- 11.4% of the dose was recovered in urine, of which 59.8 +/- 10.3% was codeine-6-glucuronide, 7.1 +/- 1.1% was total morphine, 6.9 +/- 2.1% was total norcodeine and 11.8 +/- 3.9% was unchanged codeine. These recoveries were not significantly different (P greater than 0.05) after chronic administration. 6. After the single dose, the partial clearance to morphine was 137 +/- 31 ml min-1 in the seven extensive metabolisers and 8 ml min-1 in the poor metaboliser; to norcodeine the values were 103 +/- 33 ml min-1 and 90 ml min-1; to codeine-6-glucuronide the values were 914 +/- 129 ml min-1 and 971 ml min-1; and intrinsic clearance was 1568 +/- 103 ml min-1 and 1450 ml min-1. These values were not significantly (P greater than 0.05) altered by chronic administration.(ABSTRACT TRUNCATED AT 400 WORDS)

The quantitative analysis of heroin, methadone and their metabolites and the simultaneous detection of cocaine, acetylcodeine and their metabolites in human plasma by high-performance liquid chromatography coupled with tandem mass spectrometry

For a pharmacokinetic-pharmacodynamic study in opioid tolerant patients, who were treated with heroin in combination with methadone, a liquid chromatographic assay with tandem mass spectrometry detection (LC-MS/MS) was developed for the simultaneous determination of heroin, methadone, heroin metabolites 6-monoacetylmorphine, morphine, and morphine-6 and 3-glucuronide and methadone metabolite EMDP. To detect any abuse of substances besides the prescribed opioids the assay was extended with the detection of cocaine, its metabolites benzoylecgonine and norcocaine and illicit heroin adulterants acetylcodeine and codeine. Heroin-d6, morphine-d3, morphine-3-glucuronide-d3 and methadone-d9 were used as internal standards. The sample pre-treatment consisted of solid phase extraction using mixed mode sorbent columns (MCX Oasis). Chromatographic separation was performed at 25 degrees C on a reversed phase Zorbax column with a gradient mobile phase consisting of ammonium formate (pH 4.0) and acetonitrile. The run time was 15 min. MS with relatively mild electrospray ionisation under atmospheric pressure was applied. The triple quadrupole MS was operating in the positive ion mode and multiple reaction monitoring (MRM) was used for drug quantification. The method was validated over a concentration range of 5-500 ng/mL for all analytes. The total recovery of heroin varied between 86 and 96% and of the heroin metabolites between 76 and 101%. Intra-assay and inter-assay accuracy and precision of all analytes were always within the designated limits (< or =20% at lower limit of quantification (LLQ) and < or =15% for other samples). This specific and sensitive assay was successfully applied in pharmacokinetic studies with medically prescribed heroin and toxicological cases.

Interindividual and interethnic differences in the demethylation and glucuronidation of codeine

1. The 8 h urinary excretion of codeine and seven of its metabolites was compared in 149 healthy Swedish Caucasians and 133 healthy Chinese following a single oral dose of 25 mg codeine phosphate. 2. The total 8 h urinary recovery of drug-related material was 74 +/- 24% in the Caucasians and 60 +/- 14% in the Chinese (P less than 0.001). The excretion of unchanged codeine was significantly higher in the Chinese (7.2%) compared with the Caucasians (4.3%, P less than 0.001). 3. The Caucasians excreted significantly greater proportions of codeine-6-glucuronide (C6G) (62%) than the Chinese (44%) (P less than 0.001). The frequency distribution of the log metabolic ratio (MR) for glucuronidation (codeine/C6G) was shifted towards higher values in the Chinese population. Males in both groups and Chinese smokers had significantly lower glucuronidation MRs than females and non-smokers in the respective populations (P less than 0.001). 4. The frequency distribution of the MR for O-demethylation (codeine/morphine (M) + M-3 and M-6-glucuronide (M3G and M6G) + normorphine (NM) was highly skewed in the Caucasians, suggestive of a bimodal distribution. There was a 160-fold interindividual variation in this MR. A unimodal distribution of the log O-demethylation MR was observed in Chinese. The Caucasians excreted less M and more M6G than did the Chinese (P less than 0.001). 5. Significantly more norcodeine (NC) and less NC-glucuronide (NCG) were excreted in the Chinese compared with the Caucasians (P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of codeine and its metabolites in Caucasian healthy volunteers: comparisons between extensive and poor hydroxylators of debrisoquine

1. The kinetics of codeine and seven of its metabolites codeine-6-glucuronide (C6G), norcodeine (NC), NC-glucuronide (NCG), morphine (M), M-3 (M3G) and 6-glucuronides (M6G), and normorphine (NM) were investigated after a single oral dose of 50 mg codeine phosphate in 14 healthy Caucasian subjects including eight extensive (EM) and six poor (PM) hydroxylators of debrisoquine. The plasma and urine concentrations of codeine and the metabolites were measured by h.p.l.c. 2. The mean area under the curve (AUC), half-life and total plasma clearance of codeine were 1020 +/- 340 nmol l-1 h, 2.58 +/- 0.57 h and 2.02 +/- 0.73 l h-1 kg-1, respectively. There were no significant differences between EM and PM in these aspects. 3. PM had significantly lower AUC of M3G, the active metabolites M6G, NM and M (P less than 0.0001), and lower partial metabolic clearance by O-demethylation (P less than 0.0001). In contrast, the PM had higher AUC of NC (P less than 0.05) than the EM. There was no difference between PM and EM in the AUC of C6G and NCG, nor in the partial clearances by N-demethylation and glucuronidation. 4. Among EM, the AUC of C6G was 15 times higher than that of codeine, which in turn was 50 times higher than that of M. The AUCs of M6G and NM were about 6 and 10 times higher than that of M, respectively. The partial clearance by glucuronidation was about 8 and 12 times higher than those by N- and O-demethylations, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

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.