A comparison of the pharmacokinetics of codeine and its metabolites in healthy Chinese and Caucasian extensive hydroxylators of debrisoquine

Physiologically based pharmacokinetic modeling to predict the pharmacokinetics of codeine in different CYP2D6 phenotypes

Objectives

Codeine, a prodrug used as an opioid agonist, is metabolized to the active product morphine by CYP2D6. This study aimed to establish physiologically based pharmacokinetic (PBPK) models of codeine and morphine and explore the influence of CYP2D6 genetic polymorphisms on the pharmacokinetics of codeine and morphine.

Methods

An initial PBPK modeling of codeine in healthy adults was established using PK-Sim® software and subsequently extrapolated to CYP2D6 phenotype-related PBPK modeling based on the turnover frequency (Kcat) of CYP2D6 for different phenotype populations (UM, EM, IM, and PM). The mean fold error (MFE) and geometric mean fold error (GMFE) methods were used to compare the differences between the predicted and observed values of the pharmacokinetic parameters to evaluate the accuracy of PBPK modeling. The validated models were then used to support dose safety for different CYP2D6 phenotypes.

Results

The developed and validated CYP2D6 phenotype-related PBPK model successfully predicted codeine and morphine dispositions in different CYP2D6 phenotypes. Compared with EMs, the predicted AUC0-∞ value of morphine was 98.6% lower in PMs, 60.84% lower in IMs, and 73.43% higher in UMs. Morphine plasma exposure in IMs administered 80 mg of codeine was roughly comparable to that in EMs administered 30 mg of codeine. CYP2D6 UMs may start dose titration to achieve an optimal individual regimen and avoid a single dose of over 20 mg. Codeine should not be used in PMs for pain relief, considering its insufficient efficacy.

Conclusion

PBPK modeling can be applied to explore the dosing safety of codeine and can be helpful in predicting the effect of CYP2D6 genetic polymorphisms on drug-drug interactions (DDIs) with codeine in the future.

Hydrocodone, Oxycodone, and Morphine Metabolism and Drug-Drug Interactions

Awareness of drug interactions involving opioids is critical for patient treatment as they are common therapeutics used in numerous care settings, including both chronic and disease-related pain. Not only do opioids have narrow therapeutic indexes and are extensively used, but they have the potential to cause severe toxicity. Opioids are the classical pain treatment for patients who suffer from moderate to severe pain. More importantly, opioids are often prescribed in combination with multiple other drugs, especially in patient populations who typically are prescribed a large drug regimen. This review focuses on the current knowledge of common opioid drug-drug interactions (DDIs), focusing specifically on hydrocodone, oxycodone, and morphine DDIs. The DDIs covered in this review include pharmacokinetic DDI arising from enzyme inhibition or induction, primarily due to inhibition of cytochrome p450 enzymes (CYPs). However, opioids such as morphine are metabolized by uridine-5'-diphosphoglucuronosyltransferases (UGTs), principally UGT2B7, and glucuronidation is another important pathway for opioid-drug interactions. This review also covers several pharmacodynamic DDI studies as well as the basics of CYP and UGT metabolism, including detailed opioid metabolism and the potential involvement of metabolizing enzyme gene variation in DDI. Based upon the current literature, further studies are needed to fully investigate and describe the DDI potential with opioids in pain and related disease settings to improve clinical outcomes for patients. SIGNIFICANCE STATEMENT: A review of the literature focusing on drug-drug interactions involving opioids is important because they can be toxic and potentially lethal, occurring through pharmacodynamic interactions as well as pharmacokinetic interactions occurring through inhibition or induction of drug metabolism.

Model-based meta-analysis of ethnic differences and their variabilities in clearance of oral drugs classified by clearance mechanism

In this study, the ethnic ratios (ERs) of oral clearance between Japanese and Western populations were subjected to model-based meta-analysis (MBMA) for 81 drugs evaluated in 673 clinical studies. The drugs were classified into eight groups according to the clearance mechanism, and the ER for each group was inferred together with interindividual variability (IIV), interstudy variability (ISV), and inter-drug variability within a group (IDV) using the Markov chain Monte Carlo (MCMC) method. The ER, IIV, ISV, and IDV were dependent on the clearance mechanism, and, except for particular groups such as drugs metabolized by polymorphic enzymes or their clearance mechanism is not confirmative, the ethnic difference was found to be generally small. The IIV was well-matched across ethnicities, and the ISV was approximately half of the IIV as the coefficient of variation. To adequately assess ethnic differences in oral clearance without false detections, phase I studies should be designed with full consideration of the mechanism of clearance. This study suggests that the methodology of classifying drugs based on the mechanism that causes ethnic differences and performing MBMA with statistical techniques such as MCMC analysis is helpful for a rational understanding of ethnic differences and for strategic drug development.

Pharmacokinetics of CYP2C9, CYP2C19, and CYP2D6 substrates in healthy Chinese and European subjects

PURPOSE

The aim of this analysis is to compare the pharmacokinetics of drug substrates in healthy Chinese and European subjects of aligned CYP2C9, CYP2C19, or CYP2D6 enzyme activity, providing further insight into drivers of interethnic differences in pharmacokinetics.

METHODS

Following identification of appropriate drug substrates, a comprehensive and structured literature search was conducted to identify single-dose pharmacokinetic data in healthy Chinese or European subjects with reported CYP2C9, CYP2C19, or CYP2D6 activity (genotype or phenotype). The ratio of drug AUC in the Chinese and European subjects classified with aligned enzyme activity was calculated (ethnicity ratio (ER)).

RESULTS

For 22/25 drugs identified, the ERs calculated indicated no or only limited interethnic differences in exposure (<twofold) in Chinese and European subjects with aligned polymorphic enzyme activity. The interethnic differences observed can reflect differences across populations in additional determinants of pharmacokinetics, although the notable between study variation and change over time in methods used to assign enzyme activity may also be contributing factors. There was no association between drug substrate fraction metabolized (fm) for CYP2C9, CYP2C19, or CYP2D6 and the ERs calculated.

CONCLUSION

The spectrum of pharmacokinetic determinants for each drug substrate and their differences across ethnic groups must be considered on a case-by-case basis in addition to metabolism by CYP2C9, CYP2C19, or CYP2D6. This analysis has also highlighted the challenges which arise when comparing published datasets if consistent methods to assign polymorphic enzyme activity have not been used.

The Pharmacokinetics of Morphine and Codeine in Human Plasma and Urine after Oral Administration of Qiangli Pipa Syrup

Papaveris pericarpium, a natural source of morphine and codeine, is the principal active component in many antitussive traditional Chinese medicines. We herein report the first PK study of papaveris pericarpium in human plasma and urine following oral administration of single (15, 30, 60 mL) and multiple dose (15 mL) of Qiangli Pipa Syrup (MOR 0.1 mg/mL, COD 0.028 mg/mL) by monitoring morphine and codeine using a HPLC-MS/MS method. Their T_max and t_1/2 values are independent of dosages, while the AUC_0-t linearly increased with higher dosages, indicating linear PK characteristics. AUC_0-t increased obviously after multiple doses, indicating possible risk of accumulative toxicity. Urine studies suggested risks of positive opiate drug tests with a cutoff of 300 ng/mL, which lasted 6-14 h at different doses. These results provide important information for clinical safety, efficacy and rational drug use of Qiangli Pipa Syrup and also guide the related judicial expertise of its administration.

Toxicological Investigation of Drug-Related Cases in Greece: Interpretation of Analytical Findings

The process of toxicological analysis of postmortem specimens can reveal some special difficulty compared to the clinically derived specimens. Many drugs are not stable and the chemical changes that occur in the specimens, due to the hydrolysis processing, the time passed, the drug metabolism, and matrix effect, even when the postmortem interval is short, may affect the interpretation of the toxicological results. This interpretation may be critical, not only to the thorough investigation of different kind of forensic cases, but also to clinical or other cases as it provides very significant challenges to the scientists. This article reviews (a) particular toxicological issues associated with some toxic substances responsible for common lethal or nonlethal poisonings, such as opiates, cannabis, and cocaine and the vast number of factors that affect drug concentration; and (b) focuses on toxicological issues associated with the analytical findings of certain postmortem specimens. The toxic substances cited in the present paper are the most commonly found in forensic cases in Greece. The investigation of these drug-related deaths has revealed that heroin, alone or in combination with other psychoactive substances, such as cannabis and cocaine, is the main drug involved in these deaths.

Urinary bromophenol glucuronide and sulfate conjugates: Potential human exposure molecular markers for polybrominated diphenyl ethers

One possible source of urinary bromophenol (BP) glucuronide and sulfate conjugates in mammalian animal models and humans is polybromodiphenyl ethers (PBDEs), a group of additive flame-retardants found ubiquitously in the environment. In order to study the correlation between levels of PBDEs in human blood plasma and those of the corresponding BP-conjugates in human urine, concentrations of 17 BDE congeners, 22 OH-BDE and 13 MeO-BDE metabolites, and 3 BPs in plasma collected from 100 voluntary donors in Hong Kong were measured by gas chromatograph tandem mass spectrometry (GC-MS). Geometric mean concentration of ΣPBDEs, ΣOH-BDEs, ΣMeO-BDEs and ΣBPs in human plasma were 4.45 ng g(-1) lw, 1.88 ng g(-1) lw, 0.42 ng g(-1) lw and 1.59 ng g(-1) lw respectively. Concentrations of glucuronide and sulfate conjugates of 2,4-dibromophenol (2,4-DBP) and 2,4,6-tribromophenol (2,4,6-TBP) in paired samples of urine were determined by liquid chromatography tandem triple quadrupole mass spectrometry (LC-MS/MS). BP-conjugates were found in all of the parallel urine samples, in the range of 0.08-106.49 μg g(-1)-creatinine. Correlations among plasma concentrations of ΣPBDEs/ΣOH-BDEs/ΣMeO-BDEs/ΣBPs and BP-conjugates in urine were evaluated by multivariate regression and Pearson product correlation analyses. These urinary BP-conjugates were positively correlated with ΣPBDEs in blood plasma, but were either not or negatively correlated with other organobromine compounds in blood plasma. Stronger correlations (Pearson's r as great as 0.881) were observed between concentrations of BDE congeners having the same number and pattern of bromine substitution on their phenyl rings in blood plasma and their corresponding BP-conjugates in urine.

CYP2D6 polymorphisms and codeine analgesia in postpartum pain management: a pilot study

BACKGROUND

Codeine, a common opiate prescribed for pain postcesarean section (c-section), is biotransformed by the highly polymorphic Cytochrome P450 enzyme 2D6 (CYP2D6). Ultrarapid metabolizers (UMs), individuals with multiple active copies of CYP2D6, can biotranform up to 50% more codeine into morphine than normal individuals can. In contrast, poor metabolizers (PMs), individuals who have no active CYP2D6 genes, convert almost no codeine into morphine and as a result may take multiple doses of codeine without attaining analgesia.

OBJECTIVE

The aim was to study the relationship between CYP2D6 genotype and codeine analgesia among women recovering from c-section.

METHODS

Forty-five mothers prescribed codeine provided a blood sample for CYP2D6 genotyping and recorded their pain level 4 times a day for 3 days immediately after a c-section. Codeine was used on an as-needed basis; doses and times were recorded. The relationship between CYP2D6 genotype, pain scores, need for codeine, and adverse events was studied. Theoretical morphine dose, based on CYP2D6 genotype, was estimated.

RESULTS

Women at the genotypic extremes reported codeine effects consistent with their genotype: the 2 PMs of codeine reported no analgesia as a result of taking codeine, whereas 2 of the 3 UMs reported immediate pain relief from codeine but stopped taking it due to dizziness and constipation. Much larger numbers are needed to study similar correlations among extensive and intermediate metabolizers.

CONCLUSIONS

In this pilot study, the extreme CYP2D6 genotypes (PMs and UMs) seemed to predict pain response and adverse events. Larger sample sizes are needed to correlate the range of genotypes with pain response.

Biomarkers of opiate use

The interpretation of toxicological findings is critical for the thorough investigation of the use and abuse of psychoactive substances. A positive analytical result for a sample taken could usually result in criminal proceedings and a punitive outcome for the defendant whose sample was analysed. The detection of markers of illicit opiate misuse is important both in the management of substance misuse and in the postmortem identification of illicit opiate use. The aim of this study was to emphasise the role of opiate biomarkers available at the laboratory and in the clinical environment. Urine remains the biological tool of choice for qualitative detection of illicit drug use in a clinical setting, while quantitative accuracy remains strictly the domain of blood. Accurate interpretation of the screening tests within a clinical setting alongside other relevant information remains the key to the usefulness of any test. Moreover, the finding of a morphine/codeine concentration ratio in blood exceeding unity is a strong evidence that the person had used heroin, as opposed to having taken a prescription analgesic drug containing codeine.

Opioid rotation: the science and the limitations of the equianalgesic dose table

Opioid rotation refers to a switch from one opioid to another in an effort to improve the response to analgesic therapy or reduce adverse effects. It is a common method to address the problem of poor opioid responsiveness despite optimal dose titration. Guidelines for opioid rotation are empirical and begin with the selection of a safe and reasonably effective starting dose for the new opioid, followed by dose adjustment to optimize the balance between analgesia and side effects. The selection of a starting dose must be based on an estimate of the relative potency between the existing opioid and the new one. Potency, which is defined as the dose required to produce a given effect, differs widely among opioids, and among individuals under varying conditions. To effectively rotate from one opioid to another, the new opioid must be started at a dose that will cause neither toxicity nor abstinence, and will be sufficiently efficacious in that pain is no worse than before the change. The estimate of relative potency used in calculating this starting dose has been codified on "equianalgesic dose tables," which historically have been based on the best science available and have been used with little modification for more than 40 years. These tables, and the clinical protocols used to apply them to opioid rotation, may need revision, however, as the science underlying relative potency evolves. Review of these issues informs the use of opioid rotation in the clinical setting and defines key areas for future research.

Drug dosing error with drops: severe clinical course of codeine intoxication in twins

In spite of the lack of evidence for its efficacy, and of sporadic reports of severe adverse events, codeine is still widely used as an antitussive agent in children. A 3-year-old boy (twin 1) was found lying in vomit and apnoeic at night; he was resuscitated and immediately transferred to our paediatric intensive care unit (PICU). Two and a half hours later, his twin brother (twin 2) was found dead in his bed at home. Twin 1 required mechanical ventilation for 3 days, but he eventually made a full recovery; autopsy in twin 2 showed massive aspiration of gastric content. History revealed that the monozygotic twins had an upper respiratory tract infection for several days and had both been given codeine at a dose of "10 drops per day" by their mother. The blood of both twins was found to contain high levels of codeine and its metabolites. The weight of "10 drops" was determined experimentally and was found to range from 494 to 940 mg. Thus, the highest possible dose given by mother was 23.5 mg of codeine instead of the recommended 10 mg. The twins had identical CYP2D6 gene polymorphisms corresponding to the "extensive metaboliser" type.

CONCLUSIONS

Because of the variability of drop size drug dosage, dosage "by drops" is unprecise and may result in accidental overdose. The combination of repeated overdosing and extensive metabolism to morphine is likely to have caused apnoea in these twins. These cases illustrate the danger of codeine as an antitussive in young children.

The Interface of Multiculturalism and Psychopharmacology

The impact of culture and ethnicity on psychopharmacological drug response continues to be a topic of interest and research. Diagnostic issues among patients of different races and cultures and also the influence of race and culture of the treating clinician are factors to consider before pharmacotherapy is even prescribed, although it also appears to affect the type of pharmacotherapy prescribed as well. Culture and ethnicity may also influence the response rates to treatment with pharmacotherapy along with affecting the reporting of adverse effects, compliance with the treatment regimen, and perception of need for such treatments compared to alternative health beliefs. African Americans may be diagnosed with a more severe disorder compared to Caucasians, and African Americans may also receive comparatively different, and higher, doses for the same level of symptoms compared to white patients. Asian patients may require different doses of psychotropics compared to Caucasian patients. Some of these dosing differences may be explained by pharmacogenetic differences, whereas some may be explained by cultural perceptions of illness among the different patient populations. This interface between biology, ethnicity, and cultural issues poses a challenge for the practitioner to pay attention to the multiple factors that may influence an individual's response to pharmacotherapy.

[Pain management in subarachnoid haemorrhage: a survey of French analgesic practices]

OBJECTIVE

Pain management in patients having a subarachnoid haemorrhage was assessed in French intensive care unit of neurosurgical centres.

STUDY DESIGN

Nationwide survey.

METHODS

A standardized postal questionnaire was sent to senior doctor of every neurosurgical centres in France inquiring pain scores assessment, analgesics used and their routes of administration, centre's opinion about efficacy of pain management.

RESULTS

Of the 34 centres, 24 returned completed questionnaires. Fifty four per cent of the centres evaluated pain intensity with a non valid pain score. In the case of patients in the comatose, pain was not evaluated in fifty four per cent of the centres. Paracetamol and morphine were the most currently used analgesics drugs. Morphine was administered subcutaneously by 75% of the centres. Six centres used also PCA. Thirty-seven percent of the centres were reluctant to use opioids and 75% to use NSAIDS.

CONCLUSION

The majority of the centres considered pain management in patient suffering from subarachnoid haemorrhage (SAH) was not optimal and stressed the need to establish a well validated pain rating scale dedicated to SAH patients.

Individualizing analgesic prescription Part I: pharmacogenetics of opioid analgesics

The current use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic processes, and each of these components, in addition to pain perception and processing, seem to be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After usual dose, drug toxicity, as well as inefficacy, can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. Thus, cytochrome P450 (CYP)2D6 polymorphism influences codeine and tramadol analgesic effects, CYP2C9 has an impact on the disposition of some nonsteroidal anti-inflammatory drugs, and opioid receptor polymorphism (118A>G) may reduce morphine potency. Moreover, drug interaction mimics genetic deficiency and contributes to the variability in response to analgesics. This two-part review summarizes the available data on the pharmacokinetic-pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes (CYP and uridine diphosphate glucuronosyltransferase), drug transporters (multidrug resistance proteins, multidrug resistance-associated proteins, organic anion-transporting polypeptides, and serotonin transporters), relevant drug targets (such as µ-opioid receptor, serotonin receptor and cyclooxygenases) and other nonopioid biological systems, on currently prescribed central and peripheral analgesics.

Classification of orally administered drugs on the World Health Organization Model list of Essential Medicines according to the biopharmaceutics classification system

Since its inception in 1995, the biopharmaceutical classification system (BCS) has become an increasingly important tool for regulation of drug products world-wide. Until now, application of the BCS has been partially hindered by the lack of a freely available and accurate database summarising solubility and permeability characteristics of drug substances. In this report, orally administered drugs on the Model list of Essential Medicines of the World Health Organization (WHO) are assigned BCS classifications on the basis of data available in the public domain. Of the 130 orally administered drugs on the WHO list, 61 could be classified with certainty. Twenty-one (84%) of these belong to class I (highly soluble, highly permeable), 10 (17%) to class II (poorly soluble, highly permeable), 24 (39%) to class III (highly soluble, poorly permeable) and 6 (10%) to class IV (poorly soluble, poorly permeable). A further 28 drugs could be provisionally assigned, while for 41 drugs insufficient or conflicting data precluded assignment to a specific BCS class. A total of 32 class I drugs (either certain or provisional classification) were identified. These drugs can be further considered for biowaiver status (drug product approval based on dissolution tests rather than bioequivalence studies in humans).

Differences in drug pharmacokinetics between East Asians and Caucasians and the role of genetic polymorphisms

Interethnic variability in pharmacokinetics can cause unexpected outcomes such as therapeutic failure, adverse effects, and toxicity in subjects of different ethnic origin undergoing medical treatment. It is important to realize that both genetic and environmental factors can lead to these differences among ethnic groups. The International Conference on Harmonization (ICH) published a guidance to facilitate the registration of drugs among ICH regions (European Union, Japan, the United States) by recommending a framework for evaluating the impact of ethnic factors on a drug's effect, as well as its efficacy and safety at a particular dosage and dosage regimen. This review focuses on the pharmacokinetic differences between East Asians and Caucasians. Differences in metabolism between East Asians and Caucasians are common, especially in the activity of several phase I enzymes such as CYP2D6 and the CYP2C subfamily. Before drug therapy, identification of either the genotype and/or the phenotype for these enzymes may be of therapeutic value, particularly for drugs with a narrow therapeutic index. Furthermore, these differences are relevant for international drug approval when regulatory agencies must decide if they accept results from clinical trials performed in other parts of the world.

Ethnic Differences in Genetic Polymorphisms of CYP2D6, CYP2C19, CYP3As and MDR1/ABCB1

Metabolic capacities for debrisoquin, sparteine, mephenytoin, nifedipine, and midazolam, which are substrates of polymorphic CYP2D6, CYP2C19, and CYP3A, have been reported to exhibit, in many cases, remarkable interindividual and ethnic differences. These ethnic differences are partly associated with genetic differences. In the case of the drug transporter ABCB1/MDR1, interindividual differences in its transporter activities toward various clinical drugs are also attributed to several ABCB1/MDR1 genetic polymorphisms. In this review, the existence and frequency of various low-activity alleles of drug metabolizing enzymes as well as populational drug metabolic capacities are compared among several different races or ethnicities. Distribution of nonsynonymous ABCB1/MDR1 SNPs and haplotype frequency in various races are summarized, with the association of nonsynonymous SNPs with large functional alterations as a rare event.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Extemporaneous compounding of pain and symptom control medications

This paper introduces a new series in the Journal on extemporaneously compounded dosage forms for symptom control. Some advantages and limitations of compounded medications are described and issues that clinicians should consider are mentioned. Topics that will be discussed in future papers in this series are described. Changes of compounding-related chapters of the United States Pharmacopeia from advisory statements to enforceable standards are discussed. As an example of important formulation considerations, some physical-chemical characteristics and route of administration characteristics of opioid analgesics are discussed.

A review and assessment of potential sources of ethnic differences in drug responsiveness

The International Conference on Harmonization (ICH) E5 guidelines were developed to provide a general framework for evaluating the potential impact of ethnic factors on the acceptability of foreign clinical data, with the underlying objective to facilitate global drug development and registration. It is well recognized that all drugs exhibit significant inter-subject variability in pharmacokinetics and pharmacologic response and that such differences vary considerably among individual drugs and depend on a variety of factors. One such potential factor involves ethnicity. The objective of the present work was to perform an extensive review of the world literature on ethnic differences in drug disposition and responsiveness to determine their general significance in relation to drug development and registration. A few examples of suspected ethnic differences in pharmacokinetics or pharmacodynamics were identified. The available literature, however, was found to be heterologous, including a variety of study designs and research methodologies, and most of the publications were on drugs that were approved a long time ago.

Plasma and Oral Fluid Pharmacokinetics and Pharmacodynamics after Oral Codeine Administration

Background: The ease, noninvasiveness, and safety of oral fluid collection have increased the use of this alternative matrix for drugs-of-abuse testing; however, few controlled drug administration data are available to aid in the interpretation of oral fluid results.

Methods: Single oral codeine doses (60 and 120 mg/70 kg) were administered to 19 volunteers. Oral fluid and plasma were analyzed for free codeine, norcodeine, morphine, and normorphine by solid-phase extraction combined with gas chromatography–mass spectrometry (SPE/GC-MS). Physiologic and subjective effects were examined.

Results: Mean (SE) peak codeine concentrations were 214.2 ± 27.6 and 474.3 ± 77.0 μg/L in plasma and 638.4 ± 64.4 and 1599.3 ± 241.0 μg/L in oral fluid. The oral fluid-to-plasma ratio for codeine was relatively constant (∼4) from 1 to 12 h. The mean half-life (t1/2) of codeine was 2.2 ± 0.10 h in plasma and 2.2 ± 0.16 h in oral fluid. Significant dose-related miosis and increases in sedation, psychotomimetic effect, and “high” occurred after the high dose. Mean codeine oral fluid detection time was 21 h with a 2.5 μg/L cutoff, longer than that of plasma (12–16 h). Detection times with the proposed Substance Abuse and Mental Health Services Administration cutoff (40 μg/L) were only 7 h. Norcodeine, but not morphine or normorphine, was quantified in both plasma and oral fluid.

Conclusions: The disposition of codeine over time was similar in plasma and oral fluid, but because of high variability, oral fluid codeine concentrations did not reliably predict concurrent plasma concentrations. Oral fluid testing is a useful alternative matrix for monitoring codeine exposure with a detection window of 7–21 h for single doses, depending on cutoff concentrations. These controlled drug administration data should aid in the interpretation of oral fluid codeine results.

Concentration Ratios of Morphine to Codeine in Blood of Impaired Drivers as Evidence of Heroin Use and not Medication with Codeine

Background: Both the illicit drug heroin and the prescription drug codeine are metabolized to morphine, which tends to complicate interpretation of opiate-positive samples. We report here the concentrations of morphine and codeine, the morphine/codeine ratios, and 6-acetylmorphine (6-AM) in blood specimens from individuals arrested for driving under the influence of drugs (DUID) in Sweden. The results were compared with positive findings of 6-AM in urine as evidence of heroin intake.

Methods: In 339 DUID suspects, both blood and urine specimens were available for toxicologic analysis. In another 882 cases, only blood was available. All specimens were initially analyzed by immunoassay, and the positive results were verified by isotope-dilution gas chromatography–mass spectrometry. In routine casework, the limits of quantification (LOQs) for unconjugated opiates were 5 ng/g for blood and 20 μg/L for urine.

Results: The median concentration of morphine in blood was 30 ng/g with 2.5 and 97.5 percentiles of 5 and 230 ng/g, respectively (n = 979). This compares with a median codeine concentration of 20 ng/g and 2.5 and 97.5 percentiles of 5 and 592 ng/g, respectively (n = 784). The specific metabolite of heroin, 6-AM, was identified in only 16 of 675 blood specimens (2.3%). This compares with positive findings of 6-AM in 212 of 339 urine samples (62%) from the same population of DUID suspects. When 6-AM was identified in urine, the morphine/codeine ratio in blood was always greater than unity (median, 6.0; range, 1–66). In 18 instances, 6-AM was present in urine, although morphine and codeine were below the LOQ in blood. The morphine/codeine ratio in blood was greater than unity in 85% of DUID cases when urine was not available (n = 506), and the median morphine and codeine concentrations were 70 ng/g and 10 ng/g, respectively. When morphine/codeine ratios in blood were less than unity (n = 76), the median morphine and codeine concentrations were 10 ng/g and 180 ng/g, respectively.

Conclusions: Only 2.3% of opiate-positive DUID suspects were verified as heroin users on the basis of positive findings of 6-AM in blood. A much higher proportion (62%) were verified heroin users from 6-AM identified in urine. When urine was not available for analysis, finding a morphine/codeine concentration ratio in blood above unity suggests heroin use and not medication with codeine. This biomarker indicated that 85% of opiate-positive DUID blood samples were from heroin users.

Molecular basis of ethnic differences in drug disposition and response

Ethnicity is an important demographic variable contributing to interindividual variability in drug metabolism and response. In this rapidly expanding research area many genetic factors that account for the effects of ethnicity on pharmacokinetics, pharmacodynamics, and drug safety have been identified. This review focuses on recent developments that have improved understanding of the molecular mechanisms responsible for such interethnic differences. Genetic variations that may provide a molecular basis for ethnic differences in drug metabolizing enzymes (CYP 2C9, 2C19, 2D6, and 3A4), drug transporter (P-glycoprotein), drug receptors (adrenoceptors), and other functionally important proteins (eNOS and G proteins) are discussed. A better understanding of the molecular basis underlying ethnic differences in drug metabolism, transport, and response will contribute to improved individualization of drug therapy.

Influence of stiripentol on cytochrome P450-mediated metabolic pathways in humans: in vitro and in vivo comparison and calculation of in vivo inhibition constants

OBJECTIVE

The spectrum of cytochrome P450 inhibition of stiripentol, a new anticonvulsant, was characterized in vitro and in vivo.

METHODS

Stiripentol was incubated in vitro with (R)-warfarin, coumarin, (S)-warfarin, (S)-mephenytoin, bufuralol, p-nitrophenol, and carbamazepine as probes for CYPs 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4, respectively. Caffeine demethylation and the 6 beta-hydroxycortisol/cortisol ratio were monitored in vivo before and after 14 days of treatment with stiripentol as measures of CYP1A2 and CYP3A4 activity, and dextromethorphan O- and N-demethylation were used to measure CYP2D6 and CYP3A4 activity, respectively. In vivo inhibition constants for CYP3A4 were calculated with use of data that previously documented the interaction between stripentol and carbamazepine.

RESULTS

In vitro, stiripentol inhibited CYPs 1A2, 2C9, 2C19, 2D6, and 3A4, with inhibition constant values at or slightly higher than therapeutic (total) concentrations of stiripentol, but it did not inhibit CYPs 2A6 and 2E1 even at tenfold therapeutic concentrations. In vivo inhibition of caffeine demethylation and dextromethorphan N-demethylation were consistent with inhibition of CYP1A2 and CYP3A4, respectively. The 6 beta-hydroxycortisol/cortisol ratio did not provide a reliable index of CYP3A4 inhibition. Inhibition of CYP2D6-mediated O-demethylation was not observed in vivo. With use of carbamazepine, in vivo inhibition constants for CYP3A4 ranged between 12 and 35 mumol/L, whereas the corresponding in vitro value was 80 mumol/L.

CONCLUSIONS

Stiripentol appears to inhibit several CYP450 enzymes in vitro and in vivo. In vivo inhibition constants show that stiripentol inhibition of CYP3A4 is linearly related to plasma concentration in patients with epilepsy.

Quantification of the O- and N-demethylated and the glucuronidated metabolites of codeine relative to the debrisoquine metabolic ratio in urine in ultrarapid, rapid, and poor debrisoquine hydroxylators

The O-demethylation of codeine is polymorphic and catalyzed by CYP2D6. The metabolites of codeine formed through O- and N-demethylation as well as glucuronidation were quantified in the ultrarapid metabolizers of debrisoquine and compared with the normal extensive (EM) and poor metabolizers (PM). The urinary codeine and its seven metabolites were detected after 25 mg codeine in 24 healthy Caucasian subjects with low debrisoquine metabolic ratios (MR, < or = 0.11) and a group of 132 subjects tested earlier with codeine and debrisoquine including 114 EMs (MR < 12.6) and 18 PMs (MR > 12.6). Whereas the O-demethylated metabolites accounted for < 0.4% of the total recovery on average in the PMs and 1.7% to 8.7% in the EMs, they accounted for 15.3% in the 24 subjects with ultrarapid metabolism of debrisoquine. This study suggests that the ultrarapid debrisoquine hydroxylators may develop increased O-demethylated metabolite-dependent effects or side-effects of codeine.

Pharmacokinetics of fosinoprilat in Chinese and whites after intravenous administration

The pharmacokinetics of fosinoprilat was studied in 12 healthy Chinese men after a 7.5 mg intravenous dose of fosinoprilat. The data were compared with those from an earlier study using the same protocol in nine healthy white men. Blood and urine samples were obtained before and at various time intervals after fosinoprilat administration up to 24 hours and 48 hours, respectively. Pharmacokinetic parameters were calculated by fitting the plasma or serum concentrations to a three-compartment model. The total clearance (Clt), renal clearance (ClT), and nonrenal clearance (ClNR) were significantly lower in Chinese (16.29 +/- 6.92, 6.85 +/- 2.97, and 9.44 +/- 5.08 mL.hr-1.kg-1) than those obtained in whites (29.88 +/- 6.36, 13.55 +/- 3.45, and 16.33 +/- 5.07 mL.hr-1.kg-1). The Chinese subjects had a significantly lower volume of distribution (Vc [volume of distribution of central compartment] and Vdss [volume of distribution at steady state]) (29.38 +/- 21.12 and 73.67 +/- 40.20 mL/kg) than white men (58.14 +/- 15.01 and 152.49 +/- 24.89 mL/kg). The Chinese men also had a shorter elimination half-life than whites, although not statistically significant. The respective half-lives in Chinese and whites were 5.51 +/- 1.53 and 8.24 +/- 1.99 hours. The significant differences in ClNR and ClR may be related to lower liver elimination function and lower kidney excretory function, respectively. Plasma protein binding may contribute to part of the difference in the volume of distribution. Chinese men have smaller volume of distribution and clearances of fosinoprilat after intravenous administration compared with white men. The cumulative urine excretion of fosinoprilat was not different between Chinese and whites. Chinese may require a lower fosinoprilat dosage to obtain plasma concentrations similar to whites after intravenous administration. However, since a relatively high variation was found in fosinopril oral absorption, the oral dosage of fosinopril for Chinese and whites may not be different. Further study is obviously needed to elucidate whether the pharmacodynamic effect may be different between Chinese and whites.

Pharmacokinetic changes in the elderly. Do they contribute to drug abuse and dependence?

The elderly frequently use psychoactive drugs including alcohol (ethanol), benzodiazepines and opioid analgesics, which have a propensity to cause abuse and dependence. Theoretically, the changes in pharmacokinetics of these agents in the elderly may modify their abuse and dependence potential. In the elderly, blood alcohol concentrations following an oral dose are higher, alcohol withdrawal syndrome follows a more severe and protracted clinical course and requires treatment with higher doses of chlordiazepoxide than needed for younger adults. However, there is no direct evidence that supports an increased direct abuse and dependence potential of alcohol because of its altered kinetics in the elderly. In the case of oxidatively metabolised benzodiazepine, both age-related pharmacokinetics and pharmacodynamic changes may increase their clinical effects in the elderly. The hypothesis that benzodiazepines have an increased abuse and dependence potential in the elderly has not been tested. Many of the benzodiazepines (e.g. alprazolam, triazolam and midazolam) are metabolised by the cytochrome P450 (CYP)3A subfamily. The pharmacokinetics of these agents may be modified by inhibition of CYP3A due to concurrently administered medications such as selective serotonin reuptake inhibitors. Unfortunately, data on the direct measures of abuse and dependence potential of benzodiazepines are not available in the elderly. Thus, a conclusive statement on the contribution of age-related pharmacokinetic changes to benzodiazepine abuse and dependence cannot be made at the present time. The clinical effects of codeine do not appear to change with age. Codeine is O-demethylated to its active metabolite morphine by the genetically polymorphic CYP2D6 isozyme. The activity of this isozyme is unaltered by age, gender or smoking habits; however, it is subject to potent inhibition by some of the frequently used medications in the elderly, such as the antidepressants paroxetine and fluoxetine. This may result in an impairment in O-demethylation of codeine to morphine and may lead to a decrease in the abuse and dependence potential of codeine. Conversely, those with a very rapid CYP2D6 catalytic activity may have an increased potential for codeine abuse and dependence. The clinical significance of age-related pharmacokinetic changes should be evaluated within the context of clinical practice. Most physicians are inclined to prescribe lower doses to the elderly, which may offset the potential impact of altered pharmacokinetics on the abuse and dependence potential of psychoactive agents. In summary, the available data are not sufficient for a definitive conclusion on whether the pharmacokinetic changes in the elderly translate to an increase in the abuse and dependence potential of alcohol, benzodiazepines or opioids. In particular, the data on age-associated changes in direct measures of abuse potential of these agents are missing. Future comparative systemic pharmacokinetic-pharmacodynamic studies assessing pertinent outcome measures on abuse and dependence potential of commonly used psychoactive drugs are required to resolve the ongoing controversy on risk factors for drug abuse and dependence in the elderly.

Formation of morphine from codeine in Chinese subjects of different CYP2D6 genotypes

Codeine and morphine pharmacokinetics among different CYP2D6 genotypes was compared in this study. Polymerase chain reaction tests were used to determine CYP2D6 genotypes in leukocyte deoxyribonucleic acid in 32 unrelated volunteers. Based on the genotypes, subjects were categorized into three groups: homozygous C/C188 (n = 8), heterozygous C/T188 (n = 12), and homozygous T/T188 (n = 12). Each subject was given a single oral dose of 30 mg codeine phosphate tablet after overnight fasting. Plasma concentration of codeine and 24-hour urinary morphine recovery were measured with HPLC. All three genotypes of subjects showed almost identical time profiles of plasma codeine. Urinary morphine glucuronide was hydrolyzed with beta-glucuronidase. The total recovered amount of morphine and glucuronides was 4349 +/- 646, 2564 +/- 242, and 1127 +/- 164 nmol (mean +/- SEM), respectively, for C/C188, C/T188, and T/T188 subjects (p < 0.05). The significant lower amount of urinary morphine but identical codeine plasma concentration suggested a lower partial clearance of the formation of morphine from codeine in T/T188 subjects. The results suggest a future study to assess the analgesic effect of codeine in different genotypes of CYP2D6 extensive metabolizers.

Ethnic differences in analgesic consumption for postoperative pain

OBJECTIVE

To examine the influence of patient ethnicity on receipt of postoperative analgesia.

METHOD

Chart review of 250 consecutive patients hospitalized for open reduction and internal fixation of a limb fracture. Analgesics consumption in the postoperative period was measured in terms of morphine equivalents.

RESULTS

There were significant differences (p < .005) in analgesics administered to black, Hispanic, and white patients. Whites received 22 mg/day, blacks 16 mg/day, and Hispanics 13 mg/day. These ethnic differences persisted after controlling for possible confounding variables.

CONCLUSION

Ethnicity exerts a powerful impact on medical care. We cannot determine from our data whether these differences stem from differences in pain behavior of the patients or differences in medical staff's perception and treatment of such patients.

Determination of codeine and metabolites in plasma and urine using ion-pair high-performance liquid chromatography

A reversed-phase ion-pair high-performance liquid chromatographic method for the simultaneous determination of codeine and seven metabolites is described. The samples are purified by reversed-phase solid-phase extraction. Codeine, norcodeine, codeine-6-glucuronide, norcodeine-6-glucuronide and morphine-3-glucuronide are measured with UV detection. Detection limits are 3 nmol/l (morphine-3-glucuronide) to 20 nmol/l (codeine). Morphine, normorphine and morphine-6-glucuronide are measured with electrochemical detection. Detection limits are 0.4 nmol/1 (morphine-6-glucuronide) to 1.0 nmol/l (normorphine). Correlation coefficients better than 0.998 are normally obtained for all compounds. The method was applied to the determination of the kinetics of codeine and its metabolites in plasma and urine samples from healthy volunteers.

Molecular basis of polymorphic drug metabolism

Genetic polymorphisms with functional effects occur in many of the genes encoding drug metabolizing enzymes and are an important cause of adverse drug reaction. Recent advances in the understanding of the molecular genetics of drug-metabolizing enzymes, particularly the cytochromes P450, has enabled the molecular basis of several polymorphisms to be elucidated and genotyping assays using the polymerase chain reaction to be developed. Polymorphisms in this category include those in the cytochrome P450 genes CYP2D6, CYP2C19, CYP2A6, CYP2C9 and CYP2E1, the glutathione S-transferase genes GSTM1 and GSTT1 and the N-acetyltransferase gene NAT2. The molecular basis and importance to drug metabolism of the various polymorphisms as well as evidence for the existence of polymorphisms in other genes encoding drug-metabolizing enzymes such as the UDP-glucuronosyltransferases, the sulphotransferases and the methyltransferases are discussed.

Elimination studies of antipyrine and its metabolites in healthy Saudi Arabians

1. We measured the antipyrine clearance in 18 healthy Saudi subjects and determined the urinary excretion of three of its metabolites: 4-hydroxyantipyrine (4-OH AP), norantipyrine (NOR AP) and 3 hydroxymethylantipyrine (3-OHM AP) in 21 subjects. 2. The mean +/- SD of the antipyrine clearance was 2.4 +/- 1.1 h-1 (range 1.0-5.5 l h-1) and the corresponding value per kg body weight was 0.6 +/- 0.2 ml min-1 kg-1. Urinary excretion of antipyrine (AP), 4-OH AP, NOR AP and 3-OHM AP expressed as a percentage of the oral dose of antipyrine given was 2.8 +/- 2.2, 14.5 +/- 6.9, 12.3 +/- 5.6 and 7.6 +/- 3.2 respectively. 3. Compared to Africans, Saudis preferentially metabolize AP to NOR AP and compared to Caucasians to 3-OHM AP, rather than to 4-OH AP. These discrepancies may reflect age differences between the study populations rather than genetic or ethnic variations.

Metabolic polymorphisms

Polymorphisms have been detected in a variety of xenobiotic-metabolizing enzymes at both the phenotypic and genotypic level. In the case of four enzymes, the cytochrome P450 CYP2D6, glutathione S-transferase mu, N-acetyltransferase 2 and serum cholinesterase, the majority of mutations which give rise to a defective phenotype have now been identified. Another group of enzymes show definite polymorphism at the phenotypic level but the exact genetic mechanisms responsible are not yet clear. These enzymes include the cytochromes P450 CYP1A1, CYP1A2 and a CYP2C form which metabolizes mephenytoin, a flavin-linked monooxygenase (fish-odour syndrome), paraoxonase, UDP-glucuronosyltransferase (Gilbert's syndrome) and thiopurine S-methyltransferase. In the case of a further group of enzymes, there is some evidence for polymorphism at either the phenotypic or genotypic level but this has not been unambiguously demonstrated. Examples of this class include the cytochrome P450 enzymes CYP2A6, CYP2E1, CYP2C9 and CYP3A4, xanthine oxidase, an S-oxidase which metabolizes carbocysteine, epoxide hydrolase, two forms of sulphotransferase and several methyltransferases. The nature of all these polymorphisms and possible polymorphisms is discussed in detail, with particular reference to the effects of this variation on drug metabolism and susceptibility to chemically-induced diseases.

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.