Dihydrocodeine: a new opioid substrate for the polymorphic CYP2D6 in humans

Development of a Sensitive and Rapid HPLC-MS Method for Dihydrocodeine and Dihydromorphine: Application to Bioequivalence Studies

A ultraperformance liquid chromatography-tandem mass spectrometry method has been developed to determine dihydrocodeine (DHC) and dihydromorphine (DHM) in human plasma using dihydrocodeine-d6 and desomorphine as internal standards (IS). Acetonitrile-water-ammonium format was used as the mobile phase, in gradient elution on a C18 column. The concentration of DHC and DHM was determined in the positive ionization mode of mass spectrometry. The total chromatogram run time was 3.2 min, and the linear ranges of DHC and DHM were 1.000-400.0 ng/mL and 0.050-20.00 ng/mL, respectively. The method was fully validated concerning precision, accuracy, selectivity, linearity, recovery, stability and matrix effect. The method had been successfully applied to the bioequivalence test. In addition, we found that a high-fat diet impacts the Tmax and t1/2 of DHC.

Detection of pharmaceuticals in "dirty sprite" using gas chromatography and mass spectrometry

Dirty Sprite, also known as "lean" or "purple drank", is a preparation associated with the presence of codeine and promethazine. These drinks, predominantly used by young people, are mixtures of, for example, soft drinks, prescription medicines, and prescription cough syrups. The use of these illicit preparations started in Texas in the 1960s and become popularized in the 1990s. However, the misuse of these cocktails has become more common in other countries to date, for example, in Thailand. Given the illicit nature of these preparations and the lack of information available on the composition of these products, there is a need to identify and quantify the drugs that may be present. Three samples of Dirty Sprite were analyzed using GC-MS after liquid/liquid-extraction under acidic and basic conditions. Since the acidic extraction did not show the detection of relevant substances, samples were alkalized to pH ≥ 9, followed by extraction with 1-chlorobutane. GC-MS screening revealed the identification of codeine, dihydrocodeine, promethazine and impurities of cocaine. A selected ion monitoring method was developed for the quantification of these compounds using lemonade as a calibration matrix. Quantitative analysis showed concentrations of 130-mg/L codeine, 75-mg/L promethazine, and 3.4-mg/L cocaine in sample 1; 74-mg/L promethazine and 91-mg/L dihydrocodeine in sample 2; and 130-mg/L codeine combined with 68-mg/L promethazine in sample 3. The results also illustrate that the consumption of drugs detected in Dirty Sprite samples could lead to health risks given that these prescription medicines are consumed outside the medical environment.

Adult and infant pharmacokinetic profiling of dihydrocodeine using physiologically based pharmacokinetic modeling

We previously analysed the serum concentrations of dihydrocodeine in a 1-month-old infant with respiratory depression after being prescribed dihydrocodeine phosphate 2.0 mg/day divided t.i.d. for 2 days. The purpose was to develop a full physiologically based pharmacokinetic (PBPK) model that could account for these and other drug monitoring results. Based on experiments in Caco-2 cell monolayers, the effective permeability of dihydrocodeine in human jejunum was established as 1.28 × 10^-4 cm/s. The in vitro V_max /K_m values for dihydrocodeine demethylation mediated by recombinant cytochrome P450 2D6 and 3A4 were 0.19 and 0.066 μl/min/pmol, respectively, and for dihydrocodeine 6-O-glucuronidation mediated by recombinant UGT2B4 and 2B7, the V_max /K_m values were 0.14 and 0.22 μl/min/mg protein, respectively. Renal clearance was calculated as 5.37 L/h on the total clearance value multiplied by the fraction recovered in urine. The reported plasma concentration-time profiles of dihydrocodeine after intravenous administration in healthy volunteers were used to adjust the tissue partitioning ratios. The developed model simulated the pharmacokinetic profiles of dihydrocodeine after single and multiple oral administrations reasonably well in the same population. Subsequently, the validated model was used to simulate pharmacokinetic profiles for five pediatric cases, including the 1-month-old Japanese boy and a 14-year-old Japanese girl who took an overdose of dihydrocodeine phosphate (37 mg). The simulated pharmacokinetic profiles for five virtual pediatric subjects matching the age, gender, and P450 2D6 phenotype of each case approximately reflected the observed values. These results suggested that our dihydrocodeine PBPK model reproduced the results of clinical cases reasonably well for subjects.

Opioid Drug-Drug Interactions: A Review

Opioid analgesics are among the most commonly prescribed medications. Frequently, they are combined with other therapeutic agents and pharmacodynamic or pharmacokinetic interactions may ensue. This review summarizes published case reports and studies of potential opioid drug interactions. A MED-LINE computer literature search (1966-1998) was undertaken to retrieve all pertinent case reports and studies of opioid drug interactions published in the English language. The results of the search indicate that numerous compounds from various therapeutic classes may participate in clinically significant pharmacodynamic and pharmacokinetic drug-drug interactions. Pharmacodynamic interactions usually involved additive central nervous system depression. Additionally, propoxyphene and tramadol can potentiate a hyperserotonergic state when coadministered with the SSRIs and MAOIs. Pharmacokinetic interactions typically involved inhibition or induction by specific hepatic cytochrome P-450 isoenzymes. Agents with enzyme inhibiting ability such as erythromycin, cimetidine, and selective serotonin reuptake inhibitors have been shown to potentiate the effects of certain opioid analgesics while codeine, which requires metabolic conversion via CYP 2D6 for pharmacological effectiveness, has reduced analgesic efficacy in the presence of inhibitors. The enzyme inducers rifampin and several anticonvulsants have been involved in the emergence of methadone withdrawal when added to existing methadone treatment. Additionally, enzyme inducers can increase the formation of the toxic metabolite of meperidine. Genetic polymorphism also potentially impacts the effectiveness of agents such as codeine since reduced active metabolite formation and analgesic efficacy has been demonstrated in individuals who lack CYP 2D6 activity.

Pharmacogenetics and forensic toxicology

Large inter-individual variability in drug response and toxicity, as well as in drug concentrations after application of the same dosage, can be of genetic, physiological, pathophysiological, or environmental origin. Absorption, distribution and metabolism of a drug and interactions with its target often are determined by genetic differences. Pharmacokinetic and pharmacodynamic variations can appear at the level of drug metabolizing enzymes (e.g., the cytochrome P450 system), drug transporters, drug targets or other biomarker genes. Pharmacogenetics or toxicogenetics can therefore be relevant in forensic toxicology. This review presents relevant aspects together with some examples from daily routines.

Personalized therapy in pain management: where do we stand?

Genomic variations influencing response to pharmacotherapy of pain are currently under investigation. Drug-metabolizing enzymes represent a major target of ongoing research in order to identify associations between an individual’s drug response and genetic profile. Polymorphisms of the cytochrome P450 enzymes (CYP2D6) influence metabolism of codeine, tramadol, hydrocodone, oxycodone and tricyclic antidepressants. Blood concentrations of some NSAIDs depend on CYP2C9 and/or CYP2C8 activity. Genomic variants of these genes associate well with NSAIDs’ side effect profile. Other candidate genes, such as those encoding (opioid) receptors, transporters and other molecules important for pharmacotherapy in pain management, are discussed; however, study results are often equivocal. Besides genetic variants, further variables, for example, age, disease, comorbidity, concomitant medication, organ function as well as patients’ compliance, may have an impact on pharmacotherapy and need to be addressed when pain therapists prescribe medication. Although pharmacogenetics as a diagnostic tool has the potential to improve patient therapy, well-designed studies are needed to demonstrate superiority to conventional dosing regimes.

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Role of active metabolites in the use of opioids

The opioid class of drugs, a large group, is mainly used for the treatment of acute and chronic persistent pain. All are eliminated from the body via metabolism involving principally CYP3A4 and the highly polymorphic CYP2D6, which markedly affects the drug's function, and by conjugation reactions mainly by UGT2B7. In many cases, the resultant metabolites have the same pharmacological activity as the parent opioid; however in many cases, plasma metabolite concentrations are too low to make a meaningful contribution to the overall clinical effects of the parent drug. These metabolites are invariably more water soluble and require renal clearance as an important overall elimination pathway. Such metabolites have the potential to accumulate in the elderly and in those with declining renal function with resultant accumulation to a much greater extent than the parent opioid. The best known example is the accumulation of morphine-6-glucuronide from morphine. Some opioids have active metabolites but at different target sites. These are norpethidine, a neurotoxic agent, and nordextropropoxyphene, a cardiotoxic agent. Clinicians need to be aware that many opioids have active metabolites that will become therapeutically important, for example in cases of altered pathology, drug interactions and genetic polymorphisms of drug-metabolizing enzymes. Thus, dose individualisation and the avoidance of adverse effects of opioids due to the accumulation of active metabolites or lack of formation of active metabolites are important considerations when opioids are used.

Pharmacokinetic–pharmacodynamic modeling of the miotic effects of dihydrocodeine in humans

AIM

The purpose of this study was to evaluate the pharmacokinetic-pharmacodynamic interrelations of pupillary effects of dihydrocodeine by two different analytic approaches.

METHODS

Dihydrocodeine plasma concentrations and miotic effects were available from a previous study with 24-h measurements after administration of 60 mg dihydrocodeine to nine healthy young men. Plasma concentration versus time course was described either by a one-compartment model or by linear splines using NONMEM. Dihydrocodeine concentrations at the effect site were obtained by convolution of a first-order transfer function with the function describing the plasma concentration versus time courses, and miotic effects were related to effect-site concentrations by a sigmoidal pharmacodynamic model.

RESULTS

Bayesian individual fits of miotic effects were only slightly better with the spline approach than with the compartmental approach (median individual absolute weighted residuals 0.046 versus 0.058, respectively, Wilcoxon test p = 0.008; residual errors of an additive error model 0.0979 versus 0.184, respectively). Both approaches provided similar pharmacokinetic-pharmacodynamic population parameter values. The transfer half-life between plasma and effect site was 21.1 min (95% CI 11.1-34.7 min) and 19.8 min (95% CI 11.9-34 min) with spline and compartmental approaches, respectively, and miosis occurred with EC50 of 207 or 230 ng/ml, respectively.

CONCLUSION

Two modeling approaches to the miotic effects of dihydrocodeine provided similar transfer half-lives between plasma and effect site, which also agreed with previous independently estimated values obtained from analgesic effects, suggesting that pupil size is a valid biomarker to estimate the value of ke0 for opioid central nervous system (CNS) effects.

Pharmacogenetics of Opioids

Opioids are used for acute and chronic pain and dependency. They have a narrow therapeutic index and large interpatient variability in response. Genetic factors regulating their pharmacokinetics (metabolizing enzymes, transporters) and pharmacodynamics (receptors and signal transduction elements) are contributors to such variability. The polymorphic CYP2D6 regulates the O-demethylation of codeine and other weak opioids to more potent metabolites with poor metabolizers having reduced antinociception in some cases. Some opioids are P-glycoprotein substrates, whereas, ABCB1 genotypes inconsistently influence opioid pharmacodynamics and dosage requirements. Single-nucleotide polymorphisms in the mu opioid receptor gene are associated with increasing morphine, but not methadone dosage requirements and altered efficacy of mu opioid agonists and antagonists. As knowledge regarding the interplay between genes affecting opioid pharmacokinetics including cerebral kinetics and pharmacodynamics increases, our understanding of the role of pharmacogenomics in mediating interpatient variability in efficacy and side effects to this important class of drugs will be better informed. Opioid drugs as a group have withstood the test of time in their ability to attenuate acute and chronic pain. Since the isolation of morphine in the early 1800s by Friedrich Sertürner, a large number of opioid drugs beginning with modification of the 4,5-epoxymorphinan ring structure were developed in order to improve their therapeutic margin, including reducing dependence and tolerance, ultimately without success.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Drug disposition of chiral and achiral drug substrates metabolized by cytochrome P450 2D6 isozyme: case studies, analytical perspectives and developmental implications

The concepts of drug development have evolved over the last few decades. Although number of novel chemical entitities belonging to varied classes have made it to the market, the process of drug development is challenging, intertwined as it is with complexities and uncertainities. The intention of this article is to provide a comprehensive review of novel chemical entities (NCEs) that are substrates to cytochrome P450 (CYP) 2D6 isozyme. Topics covered in this review aim: (1) to provide a framework of the importance of CYP2D6 isozyme in the biotransformation of NCEs as stand-alones and/or in conjunction with other CYP isozymes; (2) to provide several case studies of drug disposition of important drug substrates, (3) to cover key analytical perspectives and key assay considerations to assess the role and involvement of CYP2D6, and (4) to elaborate some important considerations from the development point of view. Additionally, wherever applicable, special emphasis is provided on chiral drug substrates in the various subsections of the review.

Opioid metabolites

The metabolism of opioids closely relates to their chemical structure. Opioids are subject to O-dealkylation, N-dealkylation, ketoreduction, or deacetylation leading to phase-I metabolites. By glucuronidation or sulfatation, phase-II metabolites are formed. Some metabolites of opioids have an activity themselves and contribute to the effects of the parent compound. This can go as far that the main clinical activity is exerted through active metabolites while the parent compounds are only weak agonist at mu-opioid receptors, as in the case of codeine and tilidine. The clinical effects of tramadol also involve an important contribution of its active metabolite. With morphine, the active metabolite morphine-6-glucuronide exerts important clinical opioid effects when it accumulates in the plasma of patients with renal failure. However, after short-term administration of morphine, its contribution to the central nervous effects of morphine is probably poor. Morphine-6-glucuronide has recently been identified to exert important peripheral opioid effects. By this, it may play an important role in the clinical effects of morphine. Several other opioids, such as meperidine and perhaps also morphine and hydromorphone, produce metabolites with neuroexcitatory effects. In sum, the evidence suggests that the metabolites of several opioids account for an important part of the clinical effects that must be considered in clinical practice.

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.

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

Affinities of dihydrocodeine and its metabolites to opioid receptors

Dihydrocodeine is metabolized to dihydromorphine, dihydrocodeine-6-O-, dihydromorphine-3-O- and dihydromorphine-6-O-glucuronide, and nordihydrocodeine. The current study was conducted to evaluate the affinities of dihydrocodeine and its metabolites to mu-, delta- and kappa-opioid receptors. Codeine, morphine, d,1-methadone and levomethadone were used as controls. Displacement binding experiments were carried out at the respective opioid receptor types using preparations of guinea pig cerebral cortex and the specific opioid agonists [5H]DAMGO (mu-opioid receptor), [3H]DPDPE (delta-opioid receptor) and [3H]U69,593 (K-opioid receptor) as radioactive ligands at concentrations of 0.5, 1.0 and 1.0 nmol/l, respectively. All substances had their greatest affinity to the mu-opioid receptor. The affinities of dihydromorphine and dihydromorphine-6-O-glucuronide were at least 70 times greater compared with dihydrocodeine (Ki 0.3 micromol/l), whereas the other metabolites yielded lower affinities. For the delta-opioid receptor, the order of affinities was similar with the exception that dihydrocodeine-6-O-glucuronide revealed a doubled affinity in relation to dihydrocodeine (Ki 5.9 micromol/l). In contrast, for the K-opioid receptor, dihydrocodeine-6-O- and dihydromorphine-6-O-glucuronide had clearly lower affinities compared to the respective parent compounds. The affinity of nordihydrocodeine was almost identical to that of dihydrocodeine (Ki 14 micromol/l), whereas dihydromorphine had a 60 times higher affinity. These results suggest that dihydromorphine and its 6-O-glucuronide may provide a relevant contribution to the pharmacological effects of dihydrocodeine. The O-demethylation of dihydrocodeine to dihydromorphine is mediated by the polymorphic cytochrome P-450 enzyme CYP2D6, resulting in different metabolic profiles in extensive and poor metabolizers. About 7% of the caucasian population which are CYP2D6 poor metabolizers thus may experience therapeutic failure after standard doses.

Contribution of dihydrocodeine and dihydromorphine to analgesia following dihydrocodeine administration in man: a PK-PD modelling analysis

AIMS

It is not clear whether the analgesic effect following dihydrocodeine (DHC) administration is due to either DHC itself or its metabolite, dihydromorphine (DHM). We examined the relative contribution of DHC and DHM to analgesia following DHC administration in a group of healthy volunteers using a PK-PD link modelling approach.

METHODS

A single oral dose of DHC (90 mg) was administered to 10 healthy volunteers in a randomised, double-blind, placebo-controlled study. A computerized cold pressor test (CPT) was used to measure analgesia. On each study day, the volunteers performed the CPT before study medication and at 1.25, 2.75, 4.25 and 5.75 h postdose. Blood samples were taken at 0.25 h (predose) and then at half hourly intervals for 5.75 h postdose. PK-PD link modelling was used to describe the relationships between DHC, DHM and analgesic effect.

RESULTS

Mean pain AUCs following DHC administration were significantly different to those following placebo administration (P = 0.001). Mean pain AUC changes were 91 score x s(-1) for DHC and -17 score x s(-1) for placebo (95% CI = +/- 36.5 for both treatments). The assumption of a simple linear relationship between DHC concentration and effect provided a significantly better fit than the model containing DHM as the active moiety (AIC = 4.431 vs 4.668, respectively). The more complex models did not improve the likelihood of model fits significantly.

CONCLUSIONS

The findings suggest that the analgesic effect following DHC ingestion is mainly attributed to the parent drug rather than its DHM metabolite. It can thus be inferred that polymorphic differences in DHC metabolism to DHM have little or no effect on the analgesic affect.

Strategies to manage the adverse effects of oral morphine: an evidence-based report

Successful pain management with opioids requires that adequate analgesia be achieved without excessive adverse effects. By these criteria, a substantial minority of patients treated with oral morphine (10% to 30%) do not have a successful outcome because of (1) excessive adverse effects, (2) inadequate analgesia, or (3) a combination of both excessive adverse effects along with inadequate analgesia. The management of excessive adverse effects remains a major clinical challenge. Multiple approaches have been described to address this problem. The clinical challenge of selecting the best option is enhanced by the lack of definitive, evidence-based comparative data. Indeed, this aspect of opioid therapeutics has become a focus of substantial controversy. This study presents evidence-based recommendations for clinical-practice formulated by an Expert Working Group of the European Association of Palliative Care (EAPC) Research NETWORK: These recommendations highlight the need for careful evaluation to distinguish between morphine adverse effects from comorbidity, dehydration, or drug interactions, and initial consideration of dose reduction (possibly by the addition of a co analgesic). If side effects persist, the clinician should consider options of symptomatic management of the adverse effect, opioid rotation, or switching route of systemic administration. The approaches are described and guidelines are provided to aid in selecting between therapeutic options.

Evaluation of urinary dihydrocodeine excretion in human by gas chromatography-mass spectrometry

Urinary metabolic pattern after the therapeutic peroral dose of dihydrocodeine tartrate to six human volunteers has been explored. Using the GC-MS analytical method, we have found that the major part of the dose administered is eliminated via urine within the first 24 h. However, the analytical monitoring of dihydrocodeine and its metabolites in urine was still possible 72 h after the dose was administered. The dihydrocodeine equivalent amounts excreted in urine in 72 h ranged between 32 and 108% of the dose, on average 62% in all individuals. The major metabolite excreted into urine was a 6-conjugate of dihydrocodeine, then in a lesser amount a 6-conjugate of nordihydrocodeine (both conjugated to approximately 65%). The O-demethylated metabolite dihydromorphine was of a minor amount and was 3,6-conjugated in 85%. Traces of nordihydromorphine and hydrocodone were confirmed as other metabolites of dihydrocodeine in our study. This information can be useful in interpretation of toxicological findings in forensic practice.

Pharmacogenetic diagnostics of cytochrome P450 polymorphisms in clinical drug development and in drug treatment

The current use and future perspectives of molecular genetic characterisation of cytochrome P450 enzymes (CYP) for drug development and drug treatment are summarised. CYP genes are highly polymorphic and the enzymes play a key role in the elimination of the majority of drugs from the human body. Frequent variants of some enzymes, CYP2A6, 2C9, 2C19 and 2D6, should be analysed in participants of clinical trials whenever these enzymes may play a role. It is suggested that a CYP genotype certificate is handed out to the volunteers or patients to avoid replicate analyses, and to allow that this information is available for future research and also for treatment with eventually needed drugs. Guidelines on what CYP alleles have to be analysed in drug development, as well as on analytical validation and CYP genotype data handling will be required. Treatment with several drugs may be improved by prior genotyping. The concepts and problems of CYP genotype-based clinical dose recommendations are presented and illustrated for selected drugs. The requirement for prospective trials on the medical and economic benefits of routine CYP genotyping is emphasised. Specific operationally defined recommendations dependent on genotype are a prerequisite for such studies and this review presents tentative CYP genotype-based dose recommendations systematically calculated from published data. Because of the multiplicity of factors involved, these doses will not be the optimal doses for each given individual, but should be more adequate than doses generally recommended for an average total population. Those CYP alleles and polymorphically metabolised drugs which are currently most interesting in drug development and drug treatment are reviewed, and more complete information is available from websites cited in this article.

Morphine-related metabolites differentially activate adenylyl cyclase isozymes after acute and chronic administration

Morphine-3- and morphine-6-glucuronide are morphine's major metabolites. As morphine-6-glucuronide produces stronger analgesia than morphine, we investigated the effects of acute and chronic morphine glucuronides on adenylyl cyclase (AC) activity. Using COS-7 cells cotransfected with representatives of the nine cloned AC isozymes, we show that AC-I and V are inhibited by acute morphine and morphine-6-glucuronide, and undergo superactivation upon chronic exposure, while AC-II is stimulated by acute and inhibited by chronic treatment. Morphine-3-glucuronide had no effect. The weak opiate agonists codeine and dihydrocodeine are also addictive. These opiates, in contrast to their 3-O-demethylated metabolites morphine and dihydromorphine (formed by cytochrome P450 2D6), demonstrated neither acute inhibition nor chronic-induced superactivation. These results suggest that metabolites of morphine (morphine-6-glucuronide) and codeine/dihydrocodeine (morphine/dihydromorphine) may contribute to the development of opiate addiction.

Risk-benefit assessment of opioids in chronic noncancer pain

Opioids have been accepted as appropriate treatment for acute and cancer pain, but their role in the management of chronic nonmalignant pain is the subject of much debate, mainly due to concerns about waning efficacy, the potential for neuropsychological impairment and the development of drug addiction. Controlled clinical trials demonstrated that opioids may be effective in both nociceptive and neuropathic noncancer pain, although the former responded more consistently than the latter. Gastrointestinal and CNS adverse effects were frequent in most studies. Observational studies have generated contradictory findings regarding efficacy and safety as well as the risk of drug addiction in patients with chronic noncancer pain receiving long term opioid therapy. However, they suggest that opioids may be effective in individual cases, whichever the pathophysiological mechanism of pain. Taken together, the available data indicate that the outcomes associated with opioid therapy vary markedly across patients experiencing chronic nonmalignant pain. The main consensus is that a subset of these patients may gain substantial benefit from opioid analgesics without requiring rapidly escalating doses or developing intolerable adverse effects or drug addiction. Prescribing guidelines have been developed to assist practitioners in selecting the appropriate patients and ensuring an acceptable risk : benefit ratio of opioid therapy. Finally, it must be emphasised that chronic pain is a complex entity wherein analgesics, including opioids, are only part of the treatment.

Pharmacokinetics of dihydrocodeine and its active metabolite after single and multiple oral dosing

AIMS

The pharmacokinetics of dihydrocodeine (DHC) and its active metabolite dihydromorphine (DHM) were assessed after a single oral dose of DHC and after increasing doses of DHC at steady-state. Methods Twelve healthy male volunteers (18-45 years, CYP2D6 extensive metabolizers (EMs), MR<1 took a single oral dose (s.d.) of DHC 60 mg after breakfast. After 60 h DHC 60 mg was administered twice daily for 3 days, the dose was increased to 90 mg twice daily for 3 days, the final dose of 120 mg was administered twice daily for 3 days (multiple dose: m.d.). Blood sampling and urine collection: during 60 h after s.d. and during 12 h after m.d. Results No significant differences in the area under the curve (AUC) of both, DHC and DHM could be detected after a single oral dose of 60 mg DHC (AUC (0,infinity)) and during steady-state doses of 60 mg DHC (AUC(0,12 h)). During increasing steady-state doses of DHC, the data showed a dose linearity of AUC, maximal serum concentration (Cmax ) and minimal steady-state serum levels (Cssmin) of both, DHC and DHM (P<0.0001), point estimates of DHC dose corrected AUCs were well within the bioequivalence range (60 mg: 0.989; 90%CI 0.951-1. 028, 90 mg: 0.997; 90%CI 0.959-1.036, 120 mg: 0.977; 90%CI 0.940-1. 016). O-demethylation from DHC to DHM remained constant within the increasing steady-state doses of DHC in the 12 extensive metabolizers of CYP2D6.

CONCLUSIONS

In the studied dose range (60-120 mg) the pharmacokinetics of DHC and its active metabolite DHM are linear in EMs of CYP2D6.

Pharmacokinetics of opioids in liver disease

The liver is the major site of biotransformation for most opioids. Thus, the disposition of these drugs may be affected in patients with liver insufficiency. The major metabolic pathway for most opioids is oxidation. The exceptions are morphine and buprenorphine, which primarily undergo glucuronidation, and remifentanil, which is cleared by ester hydrolysis. Oxidation of opioids is reduced in patients with hepatic cirrhosis, resulting in decreased drug clearance [for pethidine (meperidine), dextropropoxyphene, pentazocine, tramadol and alfentanil] and/or increased oral bioavailability caused by a reduced first-pass metabolism (for pethidine, dextropropoxyphene, pentazocine and dihydrocodeine). Although glucuronidation is thought to be less affected in liver cirrhosis, and clearance of morphine was found to be decreased and oral bioavailability increased. The consequence of reduced drug metabolism is the risk of accumulation in the body, especially with repeated administration. Lower doses or longer administration intervals should be used to remedy this risk. Special risks are known for pethidine, with the potential for the accumulation of norpethidine, a metabolite that can cause seizures, and for dextropropoxyphene, for which several cases of hepatotoxicity have been reported. On the other hand, the analgesic activity of codeine and tilidine depends on transformation into the active metabolites, morphine and nortilidine, respectively. If metabolism is decreased in patients with chronic liver disease, the analgesic action of these drugs may be compromised. Finally, the disposition of a few opioids, such as fentanyl, sufentanil and remifentanil, appears to be unaffected in liver disease.

Formation and clearance of active and inactive metabolites of opiates in humans

The results of recent investigations of the analgesic and the nonanalgesic effects of opioid glucuronides are relevant to the research on drug abuse in forensic toxicology. As has been shown for heroin, knowledge of the state of distribution and elimination of active and inactive metabolites and glucuronides offers new possibilities in forensic interpretation of analytic results. Because of similar metabolic degradation, calculation of the time-dependent ratio of the concentration of morphine and its glucuronide metabolites in blood or serum allows a rough estimation of increased dosage and of time elapsed since the last application. Drug effects can be examined with respect to individual case histories, including overdose and survival time if the patient died. However, different methods of administration and the strong influence of different volumes or compartments of distribution of parent compounds and metabolites on concentrations in human body tissues require careful use of glucuronide concentration data. In Germany, dihydrocodeine (DHC) is prescribed as a heroin substitute, and relative overdoses are needed to be effective. DHC metabolism was studied in three patients who died from overdoses. All metabolites (dihydrocodeine-6-glucuronide [DHC6], nor-DHC [NDHC], dihydromorphine [DHM], nor-DHM [NDHM], and DHM-3- and 6-glucuronide [DHM3G, DHM6G]) were determined using HPLC and fluorescence detection. Concentrations of DHM (0.16 mg/L to 0.22 mg/L serum) were found. The DHM glucuronide ratios were similar to those of morphine. Receptor binding studies showed that the binding affinity of DHM to porcine mu-receptor was higher than that of morphine, and DHM6G's binding affinity was as high as that of morphine-6-glucuronide (M6G). Metabolites may play an important role in the effectiveness of DHC in substitution and toxicity. Because of enzyme polymorphism, the formation of DHC poses a risk for proper dosage in patients who are either poor or extensive metabolizers. The distribution of opioid glucuronides in cerebral spinal fluid in relation to transcellular transport in central nervous tissue is discussed with respect to the receptor binding of opiates and drug effect.

Postmortem distribution of dihydrocodeine and metabolites in a fatal case of dihydrocodeine intoxication

A report of a fatal dihydrocodeine ingestion under substitution therapy is given. Quantitation of dihydrocodeine, dihydromorphine, N-nordihydrocodeine, dihydrocodeine-6-, dihydromorphine-6- and dihydromorphine-3-glucuronide was performed simultaneously after solid-phase extraction prior to HPLC analysis, and the analytes were detected using their native fluorescence. Postmortem concentrations of blood samples from different sampling sites as well as from liver, kidney and cerebrum are reported. A hair sample was investigated to prove long-term use of the substitute drug. Site-to-site differences of the analytes from blood samples were very small. The partition behavior of the opioid glucuronides depended on the hematocrit value of the particular blood sample. Most important findings seemed that dihydromorphine and dihydromorphine-6-glucuronide concentrations decisively contributed to the toxicity of dihydrocodeine. This case report outlines that in dihydrocodeine related deaths the concentrations of the pharmacologically active metabolites should additionally be determined for reliable interpretation.

Pharmacokinetics and the pharmacodynamic action of midazolam in young and elderly patients undergoing tooth extraction

OBJECTIVE

To determine whether age-dependent pharmacokinetic and pharmacodynamic alterations account for a more pronounced response to benzodiazepines among elderly patients.

METHODS

Twelve young patients and 10 elderly patients received an intravenous dose of 0.05 or 0.03 mg/kg midazolan, respectively, before third molar extraction. Postoperative pain was treated with 30 mg dihydrocodeine. Serum concentrations of midazolam and sedative effects were monitored with visual analog scales and choice reaction time measurements for 6 hours. Test values above baseline were integrated, and pharmacokinetic-pharmacodynamic analysis was performed. Heart rate, blood pressure, arterial oxygen saturation, and amnesia also were assessed.

RESULTS

There were no significant age-dependent differences in disposition of midazolam between young and elderly patients (apparent volume of distribution, 1.3 +/- 0.2 versus 1.1 +/- 0.4 L/kg; halflife, 3.3 +/- 1.5 hours versus 3.7 +/- 2.2 hours; total body clearance, 451 +/- 186 ml/min versus 343 +/- 137 ml/min). However, higher values of area under the effect curve (AUEC) and AUEC divided by area under the serum concentration-time curve (AUC) (sensitivity index) were observed among the elderly as follows: AUEC for reaction time (AUECRT) (573 versus 261; p = 0.042), AUEC for visual analog scale (AUECVAS) (37.7 versus 14.4; p = 0.011), AUECRT/AUC (6.3 versus 1.8; p = 0.007), and AUECVAS/AUC (0.40 versus 0.11; p = 0.009) compared with the young group. Likewise, mean concentration at half-maximal effect for sedation was lower (p = 0.025) among older patients (20.5 +/- 2.2 ng/ml) than among younger (29.7 +/- 6.6 ng/ml) patients. Amnesia was observed among 86% of patients and oxygen saturation was always 95% or more of basal value. There were no age-related differences in concentration of dihydrocodeine and its active metabolite dihydromorphine, but dihydromorphone levels were much lower in there intermediate metabolizers (455 to 879 fmol/l) and especially in five poor metabolizers (65 to 498 fmol/L) than among extensive metabolizer of cytochrome p450 2D6 (1604 to 6490 fmol/L).

CONCLUSION

Elderly patients are more sensitive to the sedative action of midazolam than young patients, and the sensitivity is caused by age-dependent pharmacodynamic alterations. The age-adjusted doses used are both effective (for sedative amnesia) and safe (in terms of arterial oxygen saturation, heart rate, and blood pressure.

Glucuronidation of dihydrocodeine by human liver microsomes and the effect of inhibitors

1. Glucuronidation is the major route of metabolism of dihydrocodeine (DHC) and accounts for 25-30% of an oral dose in urine. The kinetics of DHC-6-glucuronide formation in liver microsomes from five human donors and the effect of a number of potential inhibitor drugs were examined using a newly developed and validated HPLC assay. 2. The formation of DHC-6-glucuronide exhibited atypical kinetics that conformed to the Hill equation. The mean intrinsic dissociation constant (Ks) and maximum velocity (Vmax) values were 1566 micromol/L and 0.043 micromol/min per g, respectively. The Ks and Vmax values varied 1.5- and 3.5-fold, respectively. 3. Seven drugs were tested for inhibitory effects on DHC glucuronidation at low (50 micromol/L) and high (500 micromol/L) concentrations. At 50 micromol/L, only diclofenac produced greater than 50% inhibition, while at concentrations of 500 micromol/L inhibition was greater than 35% for diclofenac, amitriptyline, oxazepam, naproxen, chloramphenicol and probenecid, but not paracetamol. 4. The present study found little interindividual variation in the activity of human liver microsomes for glucuronidation of DHC. Comparison of the results from the inhibition studies with those reported previously for codeine and morphine suggest that the UDP-glucuronosyltransferase isoform UGT2B7 is involved in the glucuronidation of DHC.

Determination of the dihydrocodeine metabolites, dihydromorphine and nordihydrocodeine, in hepatic microsomal incubations by high-performance liquid chromatography

A high-performance liquid chromatographic assay for the oxidative metabolites of dihydrocodeine, nordihydrocodeine and dihydromorphine, formed in human liver microsomal incubations, is described. A simple solvent extraction followed by reversed-phase high-performance liquid chromatography with UV detection allows quantification of both metabolites in a single assay. Standard curve concentration ranges for dihydromorphine and nordihydrocodeine were 0.05-5 and 0.2-20 microM, respectively. Assay performance was assessed by intra- and inter-day accuracy and precision of quality control (QC) samples. The difference between the calculated and the actual concentration and the relative standard deviation were less than 15% at low QC concentrations and less than 10% at medium and high QC concentrations for both analytes. The method provides good precision, accuracy and sensitivity for use in kinetic studies of the oxidative metabolism of dihydrocodeine in human liver microsomes.

Effect of codeine on gastrointestinal motility in relation to CYP2D6 phenotype

BACKGROUND

Codeine is widely used as an analgesic and antitussive drug. The analgesic effect of codeine is mediated by its metabolite morphine, which is formed by the polymorphically expressed enzyme CYP2D6; therefore poor metabolizers have no analgesia after administration of codeine. Like other opiates, codeine causes a delay of gastric emptying and spastic constipation. It is not yet known whether the effect on gastrointestinal motility is mediated by codeine or its metabolite morphine.

METHODS

To test the hypothesis that the metabolite morphine is responsible for the effects of codeine on gastrointestinal motility, a randomized, double-blind, two-way crossover study was performed. The orocecal transit time was studied in five extensive and five poor metabolizers of sparteine with the sulfasalazine-sulfapyridine method, assuming that no effects are observed in poor metabolizers because negligible amounts of morphine are formed.

RESULTS

No differences of orocecal transit times were observed between extensive metabolizers and poor metabolizers after oral placebo administration. However, after oral codeine administration orocecal transit time was significantly prolonged in extensive metabolizer but not poor metabolizer subjects. All pharmacokinetic parameters of codeine showed no differences between extensive metabolizers and poor metabolizers. The pharmacokinetic parameters (mean +/- SD) of the metabolite morphine were significantly different between extensive metabolizer and poor metabolizer subjects (peak serum concentration, 13.9 +/- 10.5 versus 0.68 +/- 0.15 pmol/ml; area under the serum concentration-time curve, 27.8 +/- 16.0 versus 1.9 +/- 0.7 hr.pmol/ml; total amount of morphine excreted in urine, 0.160 +/- 0.036 versus 0.015 +/- 0.007 mumol).

CONCLUSIONS

Because the orocecal transit time prolongation after codeine administration was observed only in extensive metabolizers, the effect of codeine on gastrointestinal motility, like the analgesia, is mediated by its metabolite morphine.