Differential foetal development of the O- and N-demethylation of codeine and dextromethorphan in man

Developmental Pharmacogenetics of CYP2D6 in Chinese Children: Loratadine as a Substrate Drug

Objective: The elucidation of CYP2D6 developmental pharmacogenetics in children has improved, however, these findings have been largely limited to studies of Caucasian children. Given the clear differences in CYP2D6 pharmacogenetic profiles in people of different ancestries, there remains an unmet need to better understand the developmental pharmacogenetics in populations of different ancestries. We sought to use loratadine as a substrate drug to evaluate the effects of ontogeny and pharmacogenetics on the developmental pattern of CYP2D6 in Chinese pediatric patients.

Methods: Chinese children receiving loratadine treatment were enrolled in the present study. The metabolite-to-parent ratio (M/P ratio), defined as the molar ratio of desloratadine to loratadine of trough concentrations samples at steady-state condition, was used as a surrogate of CYP2D6 activity. Loratadine and desloratadine were determined by LC/MS/MS method. Variants of CYP2D6 were genotyped by polymerase chain reaction for CYP2D6 *4, *10, *41 and long polymerase chain reaction for CYP2D6 *5.

Results: A total of 40 patients were available for final analysis. The mean age was 4.50 (range 0.50–9.00) years and the mean weight was 19.64 (range 7.00–42.00) kg. The M/P ratio was significantly lower in intermediate metabolizers (IMs) compared to normal metabolizers (NMs) (10.18 ± 7.97 vs. 18.80 ± 15.83, p = 0.03). Weight was also found to be significantly associated with M/P ratio (p = 0.03).

Conclusion: The developmental pharmacogenetics of CYP2D6 in Chinese children was evaluated using loratadine as a substrate drug. This study emphasizes the importance of evaluating the developmental pharmacogenetics in populations of different ancestries.

Human Fetal Liver Metabolism of Oxycodone Is Mediated by CYP3A7

Oxycodone is an opioid analgesic that is commonly prescribed to pregnant women to treat moderate-to-severe pain. It has been shown to cross the placenta and distribute to the fetus. Oxycodone is mainly metabolized by CYP3A4 in the adult liver. Since CYP3A7 is abundantly expressed in the fetal liver and has overlapping substrate specificity with CYP3A4, we hypothesized that the fetal liver may significantly limit fetal exposure to oxycodone. This study showed that oxycodone is metabolized by CYP3A7 to noroxycodone in fetal liver microsomes (FLMs). The measured CYP3A7 expression was 191-409 pmol/mg protein in 14 FLMs, and an intersystem extrapolation factor (ISEF) for CYP3A7 was 0.016-0.066 in the panel of fetal livers using 6β-OH-testosterone formation as the probe reaction. Noroxycodone formation in the fetal liver was predicted from formation rate by recombinant CYP3A7, CYP3A7 expression level and the established ISEF value with average fold error of 1.25. Based on the intrinsic clearance of oxycodone measured in FLM, the fetal hepatic clearance (CLh) at term was predicted to be 495 (range: 66.4-936) μL/min, a value that is > 99% lower than the predicted adult liver CLh. The predicted fetal hepatic extraction ratio was 0.0019 (range: 0.00003-0.0036). These results suggest that fetal liver metabolism does not quantitatively contribute to the total systemic clearance of oxycodone in pregnant women nor does it provide a barrier for limiting fetal exposure to oxycodone. Additionally, since CYP3A7 forms noroxycodone, an inactive metabolite, the metabolism in the fetal liver is unlikely to affect fetal opioid activity.

Clinical pharmacology of midazolam in neonates and children: effect of disease-a review

Midazolam is a benzodiazepine with rapid onset of action and short duration of effect. In healthy neonates the half-life (t_1/2) and the clearance (Cl) are 3.3-fold longer and 3.7-fold smaller, respectively, than in adults. The volume of distribution (Vd) is 1.1 L/kg both in neonates and adults. Midazolam is hydroxylated by CYP3A4 and CYP3A5; the activities of these enzymes surge in the liver in the first weeks of life and thus the metabolic rate of midazolam is lower in neonates than in adults. Midazolam acts as a sedative, as an antiepileptic, for those infants who are refractory to standard antiepileptic therapy, and as an anaesthetic. Information of midazolam as an anaesthetic in infants are very little. Midazolam is usually administered intravenously; when minimal sedation is required, intranasal administration of midazolam is employed. Disease affects the pharmacokinetics of midazolam in neonates; multiple organ failure reduces the Cl of midazolam and mechanical ventilation prolongs the t_1/2 of this drug. ECMO therapy increases t_1/2, Cl, and Vd of midazolam several times. The adverse effects of midazolam in neonates are scarce: pain, tenderness, and thrombophlebitis may occur. Respiratory depression and hypotension appear in a limited percentage of infants following intravenous infusion of midazolam. In conclusion, midazolam is a safe and effective drug which is employed as a sedative, as antiepileptic agent, for infants who are refractory to standard antiepileptic therapy, and as an anaesthetic.

Neonatal withdrawal syndrome due to maternal codeine use

Neonatal withdrawal from maternal drugs and medications is not uncommon. Codeine-containing analgesic preparations given to pregnant mothers for headache have been identified as a cause of neonatal withdrawal syndrome. The present case highlights the importance of obtaining a detailed maternal drug history including prescription and nonprescription drugs, and highlights the need for prenatal counselling for women who are taking narcotic-containing analgesics.

Pharmacogenetic insights into codeine analgesia: implications to pediatric codeine use

Codeine has been used medicinally since the 1800s as an analgesic and antitussive agent. Although very few studies have methodically examined the safety of codeine use in the pediatric age group, it is nonetheless commonly prescribed to children and breastfeeding mothers. Empirical evidence over the last century has suggested variability in the efficacy of codeine, and recent genomic advancements have shed important light on the mechanisms leading to such variability. Aside from evaluating the role of genetic variability in drug-metabolizing enzymes, receptors and transporters, the development of the blood–brain-barrier and the ontogeny of drug-metabolizing enzymes must also be considered in newborns and young children.

The effects of maternally administered methadone, buprenorphine and naltrexone on offspring: review of human and animal data

Most women using heroin are of reproductive age with major risks for their infants. We review clinical and experimental data on fetal, neonatal and postnatal complications associated with methadone, the current "gold standard", and compare these with more recent, but limited, data on developmental effects of buprenorphine, and naltrexone. Methadone is a micro-opioid receptor agonist and is commonly recommended for treatment of opioid dependence during pregnancy. However, it has undesired outcomes including neonatal abstinence syndrome (NAS). Animal studies also indicate detrimental effects on growth, behaviour, neuroanatomy and biochemistry, and increased perinatal mortality. Buprenorphine is a partial micro-opioid receptor agonist and a kappa-opioid receptor antagonist. Clinical observations suggest that buprenorphine during pregnancy is similar to methadone on developmental measures but is potentially superior in reducing the incidence and prognosis of NAS. However, small animal studies demonstrate that low doses of buprenorphine during pregnancy and lactation lead to changes in offspring behaviour, neuroanatomy and biochemistry. Naltrexone is a non-selective opioid receptor antagonist. Although data are limited, humans treated with oral or sustained-release implantable naltrexone suggest outcomes potentially superior to those with methadone or buprenorphine. However, animal studies using oral or injectable naltrexone have shown developmental changes following exposure during pregnancy and lactation, raising concerns about its use in humans. Animal studies using chronic exposure, equivalent to clinical depot formulations, are required to evaluate safety. While each treatment is likely to have maternal advantages and disadvantages, studies are urgently required to determine which is optimal for offspring in the short and long term.

The ontogeny of drug metabolism enzymes and implications for adverse drug events

Profound changes in drug metabolizing enzyme (DME) expression occurs during development that impacts the risk of adverse drug events in the fetus and child. A review of our current knowledge suggests individual hepatic DME ontogeny can be categorized into one of three groups. Some enzymes, e.g., CYP3A7, are expressed at their highest level during the first trimester and either remain at high concentrations or decrease during gestation, but are silenced or expressed at low levels within one to two years after birth. SULT1A1 is an example of the second group of DME. These enzymes are expressed at relatively constant levels throughout gestation and minimal changes are observed postnatally. ADH1C is typical of the third DME group that are not expressed or are expressed at low levels in the fetus, usually during the second or third trimester. Substantial increases in enzyme levels are observed within the first one to two years after birth. Combined with our knowledge of other physiological factors during early life stages, knowledge regarding DME ontogeny has permitted the development of robust physiological based pharmacokinetic models and an improved capability to predict drug disposition in pediatric patients. This review will provide an overview of DME developmental expression patterns and discuss some implications of the data with regards to drug therapy. Common themes emerging from our current knowledge also will be discussed. Finally, the review will highlight gaps in knowledge that will be important to advance this field.

Expression of CYP3A isoforms and P-glycoprotein in human stomach, jejunum and ileum

1. CYP3A isoforms metabolise a diverse array of clinically important drugs and P-glycoprotein (P-gp), a transmembrane efflux pump, can extrude a wide variety of drugs from the cell. It has been suggested that the function of CYP3A4 is complementary to that of P-gp along the gastrointestinal (GI) tract, together forming a coordinated intestinal barrier against xenobiotics. Therefore, the expression of CYP3A4, CYP3A5, CYP3A7 and ABCB1 (P-gp) genes were quantified in five normal samples from the human stomach, seven from the jejunum and eight from the ileum by real-time reverse transcription-polymerase chain reaction and western blot analysis. 2. In the tissues examined, measurable mRNA expression of CYP3A was found in almost all samples from the stomach, jejunum and ileum. The rank order for CYP3A mRNA expression was CYP3A4 > CYP3A5 > CYP3A7 in the GI tract studied, whereas median mRNA CYP3A4 expression was highest in the small intestine and lowest in the stomach. Expression of ABCB1 mRNA was found in almost all samples and the median mRNA expression level was comparable in the jejunum and ileum, but lower in the stomach. Our data also show a significant correlation between all mRNA transcripts studied and a wide interindividual variation. 3. At the protein level, CYP3A4 was detected in all stomach and small intestine samples, the levels being substantially higher in the small intestine than in the stomach. P-Glycoprotein was detected in all GI samples, but no statistically significant difference was found along the GI tract considered. 4. Collectively, these results demonstrate that CYP3A4 is the main CYP3A expressed in the GI tract investigated, an extensive interindividual variability in the expression of the different CYP3A isoforms in all tissues examined and P-gp apoprotein levels similar in the stomach, jejunum and ileum.

Insights into drug metabolism by cytochromes P450 from modelling studies of CYP2D6-drug interactions

The cytochromes P450 (CYPs) comprise a vast superfamily of enzymes found in virtually all life forms. In mammals, xenobiotic metabolizing CYPs provide crucial protection from the effects of exposure to a wide variety of chemicals, including environmental toxins and therapeutic drugs. Ideally, the information on the possible metabolism by CYPs required during drug development would be obtained from crystal structures of all the CYPs of interest. For some years only crystal structures of distantly related bacterial CYPs were available and homology modelling techniques were used to bridge the gap and produce structural models of human CYPs, and thereby obtain useful functional information. A significant step forward in the reliability of these models came seven years ago with the first crystal structure of a mammalian CYP, rabbit CYP2C5, followed by the structures of six human enzymes, CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2D6 and CYP3A4, and a second rabbit enzyme, CYP2B4. In this review we describe as a case study the evolution of a CYP2D6 model, leading to the validation of the model as an in silico tool for predicting binding and metabolism. This work has led directly to the successful design of CYP2D6 mutants with novel activity-including creating a testosterone hydroxylase, converting quinidine from inhibitor to substrate, creating a diclofenac hydroxylase and creating a dextromethorphan O-demethylase. Our modelling-derived hypothesis-driven integrated interdisciplinary studies have given key insight into the molecular determinants of CYP2D6 and other important drug metabolizing enzymes.

Expression of Cytochromes P450 3A and P-Glycoprotein in Human Large Intestinse in Paired Tumour and Normal Samples

Our objective was to investigate the expression of different cytochromes P450 3A (CYP3A4, CYP3A5, and CYP3A7) and P-glycoprotein (ABCB1) genes along the human large intestine in paired tumour and normal samples. Real-time reverse transcriptase-polymerase chain reaction was used to measure CYP3A4-, CYP3A5-, CYP3A7- and ABCB1-specific mRNA expression, and Western blot analysis was used to measure membrane protein levels of CYP3A4/7, CYP3A5 and P-glycoprotein. Levels of mRNA and membrane protein fractions in the large intestine were compared with those of normal human liver. The mRNA expressions of CYP3A4, CYP3A5, CYP3A7 and ABCB1 in the large intestine were found to be highly variable, but overall the levels were significantly lower than those measured in liver (P < 0.0001, P < 0.001, P < 0.0001 and P < 0.01, respectively). At the membrane protein level, CYP3A4/7 was detected in all large intestine samples examined and the levels were substantially higher than those of the liver (P < 0.01). Although expression of CYP3A5 was detected in all large intestine samples, in most the levels were too low to allow quantification. P-glycoprotein was readily detected at levels slightly higher than those of liver (P < 0.05). Comparison between paired samples of normal and tumour in large intestine showed no significant differences in either the mRNA or membrane protein levels of these genes. In conclusion, this work suggests a potential role of the large intestine in the absorption and metabolism of xenobiotics and nutrients and no difference in the CYP3A and P-glycoprotein membrane protein fractions and mRNA expression between normal and tumour tissues.

Differences in absorption, distribution, metabolism and excretion of xenobiotics between the paediatric and adult populations

In children, the therapeutic benefits and potential risks associated with drug treatment may be different from those in adults and will depend on the exposure, receptor sensitivity and relationship between effect and exposure. In this paper, key factors undergoing maturational changes accounting for differences in drug metabolism and disposition in the paediatric population compared with adults are reviewed. Gastric and duodenal pH, gastric emptying time, intestinal transit time, secretion and activity of bile and pancreatic fluid, bacterial colonisation and transporters, such as P-glycoprotein (P-gp), are important factors for drug absorption, whereas key factors explaining differences in drug distribution between the paediatric population and adults are organ size, membrane permeability, plasma protein concentration and characteristics, endogenous substances in plasma, total body and extracellular water, fat content, regional blood flow and transporters such as P-gp, which is present not only in the gut, but also in liver, kidney, brain and other tissues. As far as drug metabolism is concerned, important differences have been found in the paediatric population compared with adults both for phase I enzymes (oxidative [e.g., cytochrome P450 (CYP)1A2, and CYP3A7 versus -3A4], reductive and hydrolytic enzymes) and phase II enzymes (e.g., N-methyltransferases and glucuronosyltransferases). Generally, the major enzyme differences observed in comparison with the adult age are in newborn infants, although for some enzymes (e.g., glucuronosyltransferases and other phase II enzymes) important differences still exist between infants and toddlers and adults. Finally, key factors undergoing maturational changes accounting for differences in renal excretion in the paediatric population compared with adults are glomerular filtration and tubular secretion. The ranking of the key factors varies according to the chemical structure and physicochemical properties of the drug examined, as well as to the characteristics of its formulation. It would be important to generate additional information on the developmental aspects of renal P-gp and of other renal transporters, as has been done and is still being done with the different -isozymes involved in drug metabolism.

O-demethylation of tramadol in the first months of life

OBJECTIVE

Assess in vivo O-demethylation activity in the first months of life.

METHODS

Time-concentration profiles of tramadol (M) and O-demethyl tramadol (M1) in plasma and urine were simultaneously collected in the first 24 h of continuous intravenous tramadol administration in neonates and young infants. M and M1 were determined by high performance liquid chromatography. Correlations between perinatal characteristics [postnatal age (PNA), postmenstrual age (PMA)] and the contribution of metabolites (M, M1) to overall tramadol elimination and to the plasma and urine log M/M1 were calculated.

RESULTS

Plasma samples were available in 20/29 and complete 24-h urine collections were available in 25/29 neonates (25-53 weeks PMA). Mean plasma log M/M1 value (>4 h, n=86) was 0.8 (SD 0.4). A significant correlation between plasma log M/M1 and PMA (r=-0.73, P<0.0001) and PNA (r=-0.58, P<0.005) was observed. In a multiple regression model, only PMA remained an independent variable. Mean urine log M/M1 was 0.94 (SD 0.7). Significant correlations of the urine log M/M1 ratio with PMA (r=-0.73, P<0.0001) and PNA (r=-0.56, P=0.0035) were observed. In a multiple regression model with the urine log M/M1 ratio as dependent variable, only PMA remained an independent variable. The maturational half-life of the log M/M1 ratio in early neonatal life in the age range evaluated is about 12-16 weeks without plateau.

CONCLUSIONS

O-demethylation activity was already observed in early neonatal life. A significant correlation with PMA was documented, but PMA can only partially explain the observed variability in O-demethylation activity. Polymorphism therefore likely already contributes to the interindividual variability observed in neonates.

Dextromethorphan differentially affects opioid antinociception in rats

Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N-methyl-D-aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. The antinociceptive effects of the mu-opioid receptor agonists morphine (5 mg kg(-1), s.c.), meperidine (25 mg kg(-1), s.c.) and codeine (25 mg kg(-1), s.c.), and the kappa-opioid agonists nalbuphine (8 mg kg(-1), s.c.) and U-50,488H (20 mg kg(-1), s.c.) were studied using the tail-flick test in male Sprague-Dawley rats. Coadministration of DM (20 mg kg(-1), i.p.) with these opioids was also performed and investigated. The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC.It was found that DM potentiated the antinociceptive effects of some mu-opioid agonists but not codeine or kappa-opioid agonists in rats. DM potentiated morphine's antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.

Tramadol disposition in the very young: an attempt to assess in vivo cytochrome P-450 2D6 activity

BACKGROUND

Tramadol is potentially a very useful pain relief medication in neonates and infants. It is primarily metabolized into O-demethyl tramadol (M1) by CYP2D6. Data concerning tramadol disposition and CYP2D6 activity in young infants are not available.

METHODS

A population pharmacokinetic analysis of tramadol and M1 time-concentration profiles was undertaken using non-linear mixed-effects models (NONMEM), based on newly collected data on tramadol and M1 time-concentration profiles in neonates and young infants (n=20) and published studies on intravenous tramadol in children and adults. M1 formation served as a surrogate for CYP2D6 activity.

RESULTS

Tramadol clearance was described using a two-compartment linear model with zero-order input and first-order elimination. Clearance increased from 25 weeks post-conception age (PCA) (5.52 litre h(-1) [70 kg](-1)) to reach 84% of the mature value by 44 weeks PCA (standardized to a 70 kg adult using allometric '1/4 power' models). The central volume of distribution decreased from 25 weeks PCA (256 litre [70 kg](-1)) to reach 120% of its mature value by 87 weeks PCA. Formation clearance to M1 contributed 43% of tramadol clearance, but had no relationship with PCA. There was a weak non-linear relationship between PCA and M1 metabolite clearance.

CONCLUSIONS

Maturational clearance of tramadol is almost complete by 44 weeks PCA. A target concentration of 300 microg litre(-1) is achieved after a bolus of tramadol hydrochloride 1 mg kg(-1) and can be maintained by infusion of tramadol hydrochloride 0.09 mg kg(-1) h(-1) at 25 weeks PCA, 0.14 mg kg(-1) h(-1) at 30 weeks PCA, 0.17 mg kg(-1) h(-1) at 35 weeks PCA, 0.18 mg kg(-1) h(-1) at 40 weeks, 0.19 mg kg(-1) h(-1) at 50 weeks PCA to 1 yr, 0.18 mg kg(-1) h(-1) at 3 yr and 0.12 mg kg(-1) h(-1) in adulthood. CYP2D6 activity was observed as early as 25 weeks PCA, but the impact of CYP2D6 polymorphism on the variability in pharmacokinetics, metabolism and pharmacodynamics of tramadol remains to be established.

Progress in cytochrome P450 active site modeling

Models capable of predicting the possible involvement of cytochromes P450 in the metabolism of drugs or drug candidates are important tools in drug discovery and development. Ideally, functional information would be obtained from crystal structures of all the cytochromes P450 of interest. Initially, only crystal structures of distantly related bacterial cytochromes P450 were available-comparative modeling techniques were used to bridge the gap and produce structural models of human cytochromes P450, and thereby obtain some useful functional information. A significant step forward in the reliability of these models came four years ago with the first crystal structure of a mammalian cytochrome P450, rabbit CYP2C5, followed by the structures of two human enzymes, CYP2C8 and CYP2C9, and a second rabbit enzyme, CYP2B4. The evolution of a CYP2D6 model, leading to the validation of the model as an in silico tool for predicting binding and metabolism, is presented as a case study.

Atomoxetine pharmacokinetics in children and adolescents with attention deficit hyperactivity disorder

OBJECTIVE

Atomoxetine is indicated for the treatment of attention deficit hyperactivity disorder in children, adolescents, and adults. This study was conducted, in part, to evaluate the single-dose and steady-state pharmacokinetics of atomoxetine in pediatric patients.

METHODS

This was an open-label, dose-titration study in pediatric patients with attention deficit hyperactivity disorder. Eligible patients could elect to participate in a single-dose or steady-state discontinuation pharmacokinetic evaluation including serial plasma sample collection over 24 hours. Plasma concentrations of atomoxetine, 4-hydroxyatomoxetine, and N-desmethylatomoxetine were determined using an atmospheric pressure chemical ionization liquid chromatography/mass spectrometry/mass spectrometry assay. Pharmacokinetic parameters were calculated using noncompartmental analysis.

RESULTS

Twenty-one cytochrome P450 2D6 extensive metabolizer patients participated in these single-dose and steady-state pharmacokinetic evaluations. Atomoxetine was rapidly absorbed, with peak plasma concentrations occurring 1 to 2 hours after dosing. Half-life averaged 3.12 and 3.28 hours after a single dose and at steady state, respectively. Minimal accumulation occurred in plasma after multiple twice-daily dosing in extensive metabolizer pediatric patients, as expected based on single-dose pharmacokinetics. As the dose (in mg/kg) increased, proportional increases in area under the curve were observed.

CONCLUSIONS

The pharmacokinetics of atomoxetine in extensive metabolizer patients were well characterized over a wide range of doses in this study. Atomoxetine pharmacokinetics in pediatric patients and adult subjects were similar after adjustment for body weight.

Drug metabolism and disposition in children

Key factors undergoing maturational changes accounting for differences in drug metabolism and disposition in the pediatric population compared with adults are reviewed. Gastric and duodenal pH, gastric emptying time, intestinal transit time, bacterial colonization and probably P-glycoprotein are important factors for drug absorption, whereas key factors explaining differences in drug distribution between the pediatric population and adults are membrane permeability, plasma protein concentration and plasma protein characteristics, endogenous substances in plasma, total body and extracellular water, fat content, regional blood flow and probably P-glycoprotein, mainly that present in the gut, liver and brain. As far as drug metabolism is concerned, important differences have been found in the pediatric population compared with adults both for phase I enzymes [oxidative (e.g. cytochrome CYP3A7 vs. CYP3A4 and CYP1A2), reductive and hydrolytic enzymes] and phase II enzymes (e.g. N-methyltransferases and glucuronosyltransferases). Finally, key factors undergoing maturational changes accounting for differences in renal excretion in the pediatric population compared with adults are glomerular filtration and tubular secretion. It would be important to generate information on the developmental aspects of renal P-glycoprotein and of other renal transporters as done and still being done with the different isozymes involved in drug metabolism.

Ontogeny of hepatic and renal systemic clearance pathways in infants: part I

Dramatic developmental changes in the physiological and biochemical processes that govern drug pharmacokinetics and pharmacodynamics occur during the first year of life. These changes may have significant consequences for the way infants respond to and deal with drugs. The ontogenesis of systemic clearance mechanisms is probably the most critical determinant of a pharmacological response in the developing infant. In recent years, advances in molecular techniques and an increased availability of fetal and infant tissues have afforded enhanced insight into the ontogeny of clearance mechanisms. Information from these studies is reviewed to highlight the dynamic and complex nature of developmental changes in clearance mechanisms in infants during the first year of life. Hepatic and renal elimination mechanisms constitute the two principal clearance pathways of the developing infant. Drug metabolising enzyme activity is primarily responsible for the hepatic clearance of many drugs. In general, when compared with adult activity levels normalised to amount of hepatic microsomal protein, hepatic cytochrome P450-mediated metabolism and the phase II reactions of glucuronidation, glutathione conjugation and acetylation are deficient in the neonate, but sulfate conjugation is an efficient pathway at birth. Parturition triggers the dramatic development of drug metabolising enzymes, and each enzyme demonstrates an independent rate and pattern of maturation. Marked interindividual variability is associated with their developmental expression, making the ontogenesis of hepatic metabolism a highly variable process. By the first year of life, most enzymes have matured to adult activity levels. When compared with adult values, renal clearance mechanisms are compromised at birth. Dramatic increases in renal function occur in the ensuing postpartum period, and by 6 months of age glomerular filtration rate normalised to bodyweight has approached adult values. Maturation of renal tubular functions exhibits a more protracted time course of development, resulting in a glomerulotubular imbalance. This imbalance exists until adult renal tubule function values are approached by 1 year of age. The ontogeny of hepatic biliary and renal tubular transport processes and their impact on the elimination of drugs remain largely unknown. The summary of the current understanding of the ontogeny of individual pathways of hepatic and renal elimination presented in this review should serve as a basis for the continued accruement of age-specific information concerning the ontogeny of clearance mechanisms in infants. Such information can only help to improve the pharmacotherapeutic management of paediatric patients.

Impact of incorporating the 2C5 crystal structure into comparative models of cytochrome P450 2D6

Cytochrome P450 2D6 (CYP2D6) metabolizes approximately one third of the drugs in current clinical use. To gain insight into its structure and function, we have produced four different sets of comparative models of 2D6: one based on the structures of P450s from four different microorganisms (P450 terp, P450 eryF, P450 cam, and P450 BM3), another on the only mammalian P450 (2C5) structure available, and the other two based on alternative amino acid sequence alignments of 2D6 with all five of these structures. Principal component analysis suggests that inclusion of the 2C5 crystal structure has a profound effect on the modeling process, altering the general topology of the active site, and that the models produced differ significantly from all of the templates. The four models of 2D6 were also used in conjunction with molecular docking to produce complexes with the substrates codeine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP); this identified Glu 216 [in the F-helix; substrate recognition site (SRS) 2] as a key determinant in the binding of the basic moiety of the substrate. Our studies suggest that both Asp 301 and Glu 216 are required for metabolism of basic substrates. Furthermore, they suggest that Asp 301 (I-helix, SRS-4), a residue thought from mutagenesis studies to bind directly to the basic moiety of substrates, may play a key role in positioning the B'-C loop (SRS-1) and that the loss of activity on mutating Asp 301 may therefore be the result of an indirect effect (movement of the B'-C loop) on replacing this residue.

Growth hormone replacement therapy induces codeine clearance

BACKGROUND

The increasing clinical use of growth hormone (GH) has raised questions about other than growth-related metabolic effects of this treatment. GH regulates the expression of several hepatic drug metabolising enzymes in the rat, but it is not known whether GH treatment alters the expression of such liver enzymes in man. We have investigated the effects of GH on codeine clearance and two enzymes of the cytochrome P450 (CYP) family, CYP3A and CYP2D6, and UDP-glucuronosyl transferase (UDPGT). These enzymes have a superior importance in hepatic biotransformation of numerous drugs. In addition, CYP3A and UDPGT are catalysts of many reactions with endobiotics such as steroid hormones.

METHODS

We used codeine as a probe drug for assessment of the enzyme activities. Codeine was administered as a single-dose prior to, and after 3 months of GH substitution in GH-deficient patients. Total clearance, and clearance along each of the three primary metabolic pathways of codeine, was assessed.

RESULTS

Three months of GH substitution increased the total clearance of codeine (21%, P < 0.01) and clearance catalysed by UDPGT significantly (31%, P < 0.05). The treatment tended to increase the clearance via the CYP3A pathway (83%, P = 0.05).

CONCLUSIONS

The effects of GH replacement therapy on drug metabolism may have clinical implications when combined with drugs that are substrates of UDPGT and CYP3A. Effects on steroid hormone metabolism with endocrine consequences can not be ruled out.

The ontogeny of human drug-metabolizing enzymes: phase I oxidative enzymes

Although some patterns are beginning to emerge, our knowledge of human phase I drug-metabolizing enzyme developmental expression remains far from complete. Expression has been observed as early as organogenesis, but this appears restricted to a few enzymes. At least two of the enzyme families that are expressed in the fetal liver exhibit a temporal switch in the immediate perinatal period (e.g., CYP3A7 to CYP3A4/3A5 and FMO1 to FMO3), whereas others show a progressive change in isoform expression through gestation (e.g., the class I alcohol dehydrogenases). Many of the phase I drug-metabolizing enzyme exhibit dynamic perinatal expression changes that are regulated primarily by mechanisms linked to birth and secondarily to maturity. A few of these enzymes are not detectable until well after birth, suggesting that birth is necessary but not sufficient for the onset of expression (e.g., CYP1A2). Tissue-specific expression adds to the complexity during ontogeny. For example, CYP3A7 expression is restricted to the fetal liver. However, with few exceptions, complete temporal relationship information during development is not known. Furthermore, most studies have concentrated on hepatic expression and much less is known about extrahepatic developmental events.

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.

Variable cytochrome P450 2D6 expression and metabolism of codeine and other opioid prodrugs: implications for the Australian anaesthetist

Codeine is a popular opioid prodrug dependent on the activity of the specific cytochrome P450 enzyme 2D6 (CYP2D6). This enzyme catalyses the production of the potent analgesic metabolite morphine, but genetic studies have demonstrated that individuals from different ethnic groups exhibit considerable variability in the functional capacities of their expressed CYP2D6 enzymes, and pharmacological studies have shown many commonly prescribed drugs can reduce the action of CYP2D6 enzymes. These findings have significant clinical implications for the rational prescription of effective analgesia, especially in a multicultural country like Australia.

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

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

Drug interactions in palliative care

PURPOSE

This review of drug interactions in palliative care examines the relevant literature in this area and summarizes the information on interactions of drugs, nutrients, and natural products that are used in the palliative care setting. Particular emphasis is placed on describing the newer information on the cytochrome P450 (CYP) system and the interactions of opioids, antidepressants, and the antitussive, dextromethorphan.

METHODS

We performed a search of the MEDLINE database of the time period from 1966 until April 1998, using medical subject headings such as the names of selective serotonin reuptake inhibitors and other relevant medications in palliative care. Literature reviewed included both human and animal articles as well as non-English literature. Bibliographies of these articles and the personal libraries of several palliative care specialists were reviewed. Software developed by The Medical Letter-The Drug Interaction Program was also used.

RESULTS

Drug interactions can be categorized in several ways. Drug-drug interactions are the most well known and can be kinetic, dynamic, or pharmaceutical. Pharmacokinetic interactions can involve CYP 2D6, which acts on drugs such as codeine and is responsible for its conversion to morphine. Poor metabolizers, either genotypic or due to phenocopying, are at risk for undertreatment if not recognized. Pharmacodynamic interactions with dextromethorphan may produce serotonin syndrome.

CONCLUSION

Drug interactions are important in palliative care as in other aspects of medicine. These interactions are similar to those seen in other areas of medical care but have significant consequences in pain management. Failure to recognize these interactions can lead to either overdosing or undertreatment.

Fetal hepatic drug elimination

The majority of studies of fetal hepatic elimination have concentrated on the expression and activity of the metabolizing enzymes, but the unique physiologic milieu of the fetal liver should also be considered. The basic structure of the liver is formed by the end of the first trimester. The fetal hepatic circulation differs substantially from that of the adult in that there is an extra input vessel, the umbilical vein, and there is shunting of 30-70% of hepatic blood flow via the ductus venosus. The left and right lobes of the fetal liver seem to function independently with respect to a variety of biochemical parameters, due at least in part to the lower oxygen supply to the right lobe. The zonation of drug-metabolizing enzymes along the hepatic acinus, which is prominent in the adult liver, is absent in the fetal liver. Unlike rodent species, the human fetal liver has a significant capacity for drug metabolism. Of the oxidative enzymes, CYP3A7 accounts for up to 50% of total fetal hepatic cytochrome P450 content. Expression of this enzyme decreases dramatically after birth. CYP1A1 and CYP2D6 have also been detected in human fetal liver, but whether CYP2E1 is expressed remains controversial. Several other cytochrome P450s have been identified and await characterization. Fetal hepatic drug conjugation may prolong fetal exposure to the metabolites produced, which, being more water soluble, do not readily cross the placenta back to the mother and, if excreted in fetal urine, can be recycled in the fetus via amniotic fluid and fetal swallowing. Limited activity of glucuronidation enzymes has been demonstrated in human fetal liver in contrast to the activity of sulfation enzymes, which is significant. Limited in vivo studies in fetal sheep have demonstrated significant fetal hepatic drug elimination, and this has been confirmed in studies of the isolated perfused fetal sheep liver. Our understanding of fetal hepatic elimination processes has advanced steadily over the years. Future developments, however, should consider more fully the influence of the unique physiological milieu of the fetal liver, in addition to the expression and activity of drug metabolizing enzymes.

Insulin-like growth factor 1 (IGF-I) effects on sex-specific cytochrome P450 enzymes in normal and hypophysectomised male rats

The role of growth hormone (GH) in the regulation of the sex-differentiated rat cytochrome P450 (CYP) enzymes has been extensively studied. However, little is known about the involvement of insulin-like growth factor I (IGF-I) as a mediator in this regulation. We wanted to study if IGF-I had effects on sex-differentiated CYP enzymes and to compare the effects of IGF-I to the effects of GH. IGF-I, GH or saline was administered continuously via osmotic minipumps to normal and hypophysectomised rats for seven days. After treatment, the expression of several sex-differentiated liver enzymes (CYP2C11, CYP2C12), the female-dominant steroid 5alpha-reductase, and the male-dominant CYP3A2 enzyme was studied at mRNA, protein and/or functional levels. Our results demonstrate that IGF-I has marked effects on the sex-specific expression of CYP2C11 and CYP2C12. The effects of IGF-I were similar to those of GH. In contrast, in hypophysectomised rats IGF-I gave effects opposite to those observed after GH treatment to normal rats on the CYP3A-associated cortisol 6beta-hydroxylation. No effects of IGF-I on the steroid 5alpha-reductase activity were observed.

Selective effects of somatostatin analogs on human drug-metabolizing enzymes

Pharmacologic or surgical manipulation with growth hormone secretion or with the physiologic release of somatostatin and growth hormone-releasing hormone affects some rat liver enzymes, especially the sex-differentiated ones. We investigated the effects of two somatostatin analogs on several enzyme functions in six patients with carcinoid syndrome, using codeine as a probe drug. Codeine was given intravenously and its N- and O-demethylation, as well as 6-glucuronidation catalyzed by CYP3A, CYP2D6, and uridine diphosphate-glucuronosyltransferase, respectively, were studied before and during treatment with somatostatins. After 3 days of treatment with somatostatins the partial metabolic clearance of codeine by N-demethylation decreased by 21% to 64% in all patients (mean change, 44%; p < 0.05), and the clearance by O-demethylation was decreased by 20% to 69% in five of the patients (mean change in all patients, 35%; p < 0.05). In contrast, the partial clearance by 6-glucuronidation and the total systemic clearance of codeine were unchanged. Our results may be caused by the inhibition of growth hormone secretion induced by the somatostatins, inasmuch as direct metabolic interactions with these peptide drugs are improbable. The decline in CYP3A4 and CYP2D6 activity might have clinical implications when substrates of these enzymes with low therapeutic indices are combined with somatostatin analogs. Because the formation of morphine from codeine was altered, the analgesic effect of this drug may be reduced during concomitant treatment with somatostatins.

Developmental expression of cytochrome P450 enzymes in human liver

Drug-metabolizing cytochrome P450 enzymes, the major phase I enzymes, are active in human liver already at very early stages of intrauterine development, although presumably at fairly low concentrations and in low numbers. During maturation, these enzymes go through various developmental programmes towards adulthood. The major increase both in abundance as well as in number of different enzymes takes place after birth, probably during the first year of life. Detailed information concerning these developmental changes is still limited. The major drug-metabolizing P450 enzymes appear to be primarily members of the CYP3A subfamily in all stages of development. The balance between different members of this subfamily, however, undergoes significant switches from the foetal predominant CYP3A7 to the major adult form CYP3A4. The ontogeny of the other cytochrome P450 enzymes is less well characterized, but the major switch-on appears to occur mainly after birth. Developmental expression of P450 enzymes is one of the key factors determining the pharmacokinetic status of developing individuals both pre- and postnatally.

Xenobiotic-metabolizing cytochrome P450 enzymes in the human feto-placental unit: role in intrauterine toxicity

Practically all lipid-soluble xenobiotics enter the conceptus through placental transfer. Many xenobiotics, including a number of clinically used drugs, are known to cause unwanted effects in the embryo or fetus, including in utero death, initiation of birth defects, and production of functional abnormalities. It is well established that numerous xenobiotics are not necessarily toxic as such, but are enzymatically transformed in the body to reactive and toxic intermediates. The cytochrome P450 (CYP) enzymes are known to catalyze oxidative metabolism of a vast number of compounds, including many proteratogens, procarcinogens, and promutagens. About 20 xenobiotic-metabolizing CYP forms are known to exist in humans. Most of these forms are most abundant in the liver, but examples of exclusively extrahepatic CYP forms also exist. Unlike rodents, the liver of the human fetus and even embryo possesses relatively well-developed metabolism of xenobiotics. There is experimental evidence for the presence of CYP1A1, CYP1B1, CYP2C8, CYP2D6, CYP2E1, CYP3A4, CYP3A5, and CYP3A7 in the fetal liver after the embryonic phase (after 8 to 9 weeks of gestation). Significant xenobiotic metabolism occurs also during organogenesis (before 8 weeks of gestation). Also, some fetal extrahepatic tissues, most notably the adrenal, contain substantial levels of CYP enzymes. The full-term human placenta is devoid of many CYP activities present in liver. Placental CYP1A1 is highly inducible by maternal cigarette smoking. Other forms present in full-term placenta include CYP4B1 and CYP19 (steroid aromatase), which also contribute to the oxidation of some xenobiotics. At earlier stages of pregnancy, the placenta may express a wider array of CYP genes, including CYP2C, CYP2D6, and CYP3A7. Due to the small size of the fetus and low abundance of CYPs in placenta, the contribution of feto-placental metabolism to overall gestational pharmacokinetics of drugs is probably minor. In contrast, several toxic outcomes have been ascribed to altered metabolic patterns in the feto-placental unit, including a putative association between reduced placental oxidative capacity and birth defects. Examples of human teratogens that are substrates for CYP enzymes include thalidomide, phenytoin, ethanol, and several hormonal agents. Recent studies have improved our understanding of the expression and regulation of individual CYP genes in the fetus and placenta, and the stage is set for applying this knowledge with more precision to the role of xenobiotic metabolism in abnormal intrauterine development in humans.

A combination of mutations in the CYP2D6*17 (CYP2D6Z) allele causes alterations in enzyme function

In many black African populations, the capacity for CYP2D6-dependent drug metabolism is generally reduced. A specific variant of the CYP2D6 gene (CYP2D6*17) that carries three functional mutations (T107I, R296C, and S486T) has been found to be present in Zimbabwean subjects with impaired CYP2D6-dependent hydroxylase activity. To evaluate whether the CYP2D6*17 allele was the major cause behind the decreased rate of drug metabolism and to examine the role of the different mutations, CYP2D6 cDNAs containing all eight combinations of the mutations were created. Expression of the cDNAs in COS-1 cells revealed that the CYP2D6 17 enzyme displayed only 20% of the wild-type (CYP2D6 1) activity, whereas the T107I substitution on its own had no significant effect on enzyme function. Expression in yeast showed that the three possible single amino-acid mutant CYP2D6 variants all had properties similar to CYP2D6 1 when the kinetics of bufuralol hydroxylation was examined. However, enzymes containing both the T107I and R296C mutations exhibited a more than 5-fold higher K(m) for bufuralol than the wild-type enzyme, whereas the S486T mutation was of little importance. In contrast, when codeine was used as a substrate, the T107I substitution alone was sufficient to cause a significant increase in the apparent K(m), indicating a differential effect for this substitution depending on the CYP2D6 substrate. In conclusion, the CYP2D6*17 allele represents the first human cytochrome P450 polymorphic variant in which a combination of substitutions is required to alter the enzyme's catalytic properties and is the first case in which a decreased CYP2D6 activity, as monitored in vivo, has been documented to be caused by an enzyme with altered affinity for CYP2D6 substrates.

Urinary excretion of codeine, ethylmorphine, and their metabolites: relation to the CYP2D6 activity

The formation of morphine from codeine and ethylmorphine is mainly mediated by the polymorphic enzyme CYP2D6. The objective of this study was to investigate whether CYP2D6 poor metabolizers (PM) and CYP2D6 extensive metabolizers (EM) would respond differently during testing for opiate drugs of abuse in urine after intake of these drugs. Five PM and five EM of dextromethorphan were administered single oral doses of codeine (25 mg) and ethylmorphine (25 mg), and the urinary excretion of parent compounds and selected metabolites was observed for 72 hours. Analysis was performed with GC-MS after hydrolysis of the glucuronide conjugates. Selected urine samples were screened for the presence of opiates by the Abbott ADx immunoassay method. The results from one PM and one EM were excluded because of technical analytical problems. EM excreted significantly more morphine than PM after intake of both codeine (6.5% vs. 1.1% of the dose; p < 0.05) and ethylmorphine (11.0% vs. 3.0% of the dose; p < 0.05). Screening results were positive significantly longer for EM than for PM after codeine intake (mean, 33 hours vs. 17 hours; p < 0.05), and the same trend, albeit nonsignificantly, was noted for ethylmorphine (mean, 33 hours vs. 24 hours). Regardless of CYP2D6 phenotype, significantly more morphine was formed after intake of ethylmorphine than after intake of codeine (7.0% vs. 3.8% of the dose; p < 0.05). There were high correlations between dextromethorphan metabolic ratios and the ratios of codeine to morphine, ethylmorphine to morphine, norcodeine to normorphine, and norethylmorphine to normorphine (r = 0.80 to 0.92; p = 0.030 to 0.001). Although this study should be interpreted with caution because of the few subjects included and the single-dose design, it demonstrates that the CYP2D6 phenotype clearly affects the results when testing for opiates in urine after intake of codeine and ethylmorphine.

Induction of CYP2D6 in pregnancy

Expression of the drug-metabolizing enzyme cytochrome P4502D6 (CYP2D6) is predominantly under genetic control, and enzyme-inducing drugs have little influence on its activity. We studied the activity of CYP2D6 during pregnancy. One hundred forty pregnant women were genotyped for CYP2D6. Seventeen of them (four poor metabolizers, seven heterozygous extensive metabolizers, and six extensive metabolizers) were phenotyped with dextromethorphan in late pregnancy and 7 to 11 weeks after parturition. During pregnancy the dextromethorphan/dextrorphan metabolic ratio was significantly reduced (p = 0.0015) among homozygous and heterozygous extensive metabolizers, indicating increased CYP2D6 activity. In contrast, poor metabolizers showed an increased metabolic ratio during pregnancy. These results are consistent with previous findings of a marked increase in metabolism of the CYP2D6 substrate metoprolol during pregnancy. Both studies indicate an increase in CYP2D6 activity during pregnancy, which may be caused by an induction of the CYP2D6 enzyme.

Expression of CYP3A in the human liver--evidence that the shift between CYP3A7 and CYP3A4 occurs immediately after birth

CYP3A isoforms are responsible for the biotransformation of a wide variety of exogenous chemicals and endogenous steroids in human tissues. Two members of the CYP3A subfamily display developmentally regulated expression in the liver; CYP3A7 is expressed in the fetal liver, whereas CYP3A4 is the major cyrochrome P-450 isoform present in the adult liver. To gain insight into the descriptive ontogenesis of CYP3A isoforms during the neonatal period, we have developed several approaches to explore a neonatal liver bank. Although CYP3A4 and CYP3A7 are structurally closely related, they differ in their capacity to carry out monooxygenase reactions. We have cloned CYP3A4 and CYP3A7 and established stable transfectants in Ad293 cells to investigate their substrate specificities. The 16alpha hydroxylation of dehydroepiandrosterone is catalyzed by both proteins, but CYP3A7 has a higher affinity and maximal velocity than CYP3A4. Conversely, the conversion of testosterone into its 6beta derivative is essentially supported by CYP3A4. We used these two probes to determine the ontogenic evolution at the protein level; CYP3A7 was very active in the fetal liver and its activity was maximal during the first week following birth before to progressively decline and reached a very low level in adult livers. Conversely, the activity of CYP3A4 was extremely weak in the fetus and began to raise after birth to reach 30-40% of the adult activity after one month. CYP3A4 RNA accumulation displays a similar pattern of evolution; when probed with an oligonucleotide, its concentration increased rapidly after birth to reach a plateau as soon as the first week of age. These data supports the assumption that CYP3A4 expression is transcriptionally activated during the first week after birth and is accompanied by a simultaneous decrease of CYP3A7 expression, in such a way that the overall CYP3A protein content and the level of pentoxyresorufin dealkylase catalyzed by the two proteins remain nearly constant.

Pharmacogenetics in pediatrics. Implications for practice

Cumulative experience with pharmacotherapy in children indicates that it is difficult to prescribe medications rationally solely on the basis of patient age. Furthermore, the apparent drug biotransformation phenotype may be influenced by disease (e.g., infection), environmental factors (e.g., diet and environmental contaminants), and concurrent medications. Therefore, characterization of drug biotransformation pathways during development and, at a given developmental stage, the effects of known modulators of drug biotransformation are essential for optimum treatment. This is particularly true when one considers that altered drug biotransformation may contribute significantly to therapeutic failure (e.g., graft rejection with inadequate serum and tissue concentrations of cyclosporin and myelotoxicity consequent to a relative inability to metabolize normal doses of certain antineoplastic agents). Accordingly, the goals of coordinated clinical and basic investigations should be to characterize important drug biotransformation pathways for compounds under development and intended for use in pediatrics and to identify the population extremes or "outliers" to aid in selection of an appropriate dosage range for efficacy studies. Acquired knowledge should then be incorporated into the drug-design process to further maximize the efficacy-toxicity ratio. The development of acceptable, preferably noninvasive, phenotyping procedures for all age ranges including neonates, infants, and older children is a major challenge for investigators but, if met, will be rewarded with improved pediatric pharmacotherapy.

Dextromethorphan metabolism in Jordanians: dissociation of dextromethorphan O-demethylation from debrisoquine 4-hydroxylation

The concentrations of dextromethorphan (DM) and its metabolites dextrorphan (DRP), 3-methoxymorphinan (MM) and 3-hydroxymorphinan (HM) were measured in 8 h urine samples from 266 unrelated healthy Jordanian subjects following oral administration of 30 mg dextromethorphan hydrobromide and using a rapid, sensitive and precise HPLC method with fluorometric detection. The frequency of the 'poor' metabolizer status of DM-O-demethylation as judged by log DM/DRP was found to be 6.8% with a 95% confidence interval of 3.8-9.8%. There was a strong correlation between log DM/DRP and log total non-O-demethylated compounds (NODM)/total O-demethylated metabolites (ODM) metabolic ratios (r = 0.96, P < 0.01). However, one subject with log DM/DRP of 0.05 that classifies him as a poor metabolizer was found to have a log NODM/ODM of -0.73 which is in the range of extensive metabolizer status suggesting the presence of another cytochrome P450 isoenzyme involved in dextromethorphan O-demethylation. Dextromethorphan N-demethylation to 3-methoxymorphinan was detected in 55.3% of individuals. Furthermore, a dissociation between dextromethorphan O-demethylation and debrisoquine (D) 4-hydroxylation has been observed. Among the 116 subjects phenotyped with both dextromethorphan and debrisoquine, 7 were poor metabolizers of both, three were poor metabolizers of debrisoquine and extensive metabolizers of dextromethorphan whilst 4 were poor metabolizers of dextromethorphan and extensive metabolizers of debrisoquine, one of whom was reclassified as an extensive metabolizer of dextromethorphan using log NODM/ODM to characterize dextromethorphan metabolizer status.

A model for human cytochrome P450 2D6 based on homology modeling and NMR studies of substrate binding

The cytochrome P450 responsible for the debrisoquine/sparteine polymorphism (P450 2D6) has been produced in large quantities by expression of a modified cDNA in baculovirus. A polyhistidine extension was incorporated at the C-terminus of the expressed protein, which, after purification of the protein on a nickel-agarose column, could be removed proteolytically by treatment with thrombin. Purified yields of P450 2D6 were 2.4 mg from 700 mL of cell culture. The protein had a greater than 90% heme content and was fully active, having no residual absorbance at 420 nm in the reduced CO complex. The quantities produced allowed direct study of the interaction of the substrate codeine with the enzyme by paramagnetic relaxation effects on the NMR spectrum of the substrate. Distances between the heme iron atom and substrate protons were calculated from these experiments, and the orientation of the substrate in the binding pocket was determined. This showed that codeine was bound with the methoxy group of the molecule closest to the heme iron (iron-methyl proton distance of 3.1 +/- 0.1 A), consistent with the observed O-demethylation to morphine. A model of the complex Of P450 2D6 with codeine was built from a multiple sequence and structure alignment of the known crystal structures for P450s, incorporating the experimental constraints derived from the NMR studies. This showed that the overall fold Of P450 2D6 is more similar to that of P450 BM3 than to either P450 cam or P450 terp. Codeine binds to P450 2D6 so that the methoxy group is directly above the A ring of the heme, while the basic nitrogen interacts with the carboxylate of aspartate 301.

Dextromethorphan as an in vivo probe for the simultaneous determination of CYP2D6 and CYP3A activity

Dextromethorphan (DM) is O-demethylated into dextrorphan (DEX) in humans by the cytochrome P450 designated as CYP2D6 and N-demethylated into 3-methoxymorphinan (3MM) via CYP3As. Clinically, DM has been successfully used as an index of CYP2D6 and this paper describes analytical and clinical data that will help evaluate the use of DM hydrobromide as a probe of CYP3A activity. DM and its three demethylated metabolites were measured in a 4-h spot urine sample using a HPLC method employing solid-phase extraction (C(18)), analysis on a phenyl column [mobile phase, methanol-acetonitrile-phosphate buffer (10 mM, pH 3.5, 20:25:55, v/v)] and fluorescence detection (excitation at lambda=228 nm, no emission cut-off filter). The urinary molar ratio DM-DEX was used to assess CYP2D6 activity while DM-3MM was used for CYP3As. The DM-3MM ratios were sensitive to the co-administration of selective CYP3A inhibitors grapefruit juice and erythromycin. In addition, in healthy volunteers and cancer patients, the N-demethylation of DM correlated with the CYP3A-mediated metabolism of verapamil and tamoxifen. DM appears to be a promising way to simultaneously phenotype patients for CYP2D6 and CYP3As.

The assessment of flavin-containing monooxygenase activity in intact animals

A large number of drug metabolising enzymes with different substrate specificities and induction and inhibition characteristics have been described, suggesting that specific test drugs, i.e. probes, should be used for assessing the activity of distinct metabolising enzymes. The flavin-containing monooxygenase (FMO) and cytochrome P-450 (P-450) are the two main microsomal enzyme systems involved in the oxidation of xenobiotics. FMO is present in liver and other tissues of most vertebrates. It catalyses the oxidation of a wide range of xenobiotics, especially soft nucleophiles bearing nitrogen and sulphur centres. There is substantial information on both in vitro and in vivo probes for cytochrome P-450. For example antipyrine has been widely used for assessing the activity of P-450 in vivo by utilising pharmacokinetic parameters as indices of enzyme activity. In more recent years, isozyme specific probes have also been developed for some of the P-450s. Whereas a number of substrates are available for measuring FMO activity in vitro (e.g. N,N-dimethylaniline), probes for assessing FMO activity in vivo are limited. In this review a background to the use of in vitro and in vivo probes for assessing the activity of FMO is presented, and approaches and criteria for development of potential pharmacokinetic probes for FMO are described. Preliminary data on the development of ethyl methyl sulphide (EMS) and trimethylamine (TMA) as potential pharmacokinetic probes for assessing FMO activity in rats are discussed in detail. Clinical implications of modulation of FMO activity are discussed, and arguments presented as to why the development of FMO probes for use in man will be useful additions to the range of other compounds available for assessment of liver metabolic function.

The emerging role of cytochrome P450 3A in psychopharmacology

Recent advances in molecular pharmacology have allowed the characterization of the specific isoforms that mediate the metabolism of various medications. This information can be integrated with older clinical observations to begin to develop specific mechanistic and predictive models of psychotropic drug interactions. The polymorphic cytochrome P450 2D6 has gained much attention, because competition for this isoform is responsible for serotonin reuptake inhibitor-induced increases in tricyclic antidepressant concentrations in plasma. However, the cytochrome P450 3A subfamily and the 3A3 and 3A4 isoforms (CYP3A3/4) in particular are becoming increasingly important in psychopharmacology as a result of their central involvement in the metabolism of a wide range of steroids and medications, including antidepressants, benzodiazepines, calcium channel blockers, and carbamazepine. The inhibition of CYP3A3/4 by medications such as certain newer antidepressants, calcium channel blockers, and antibiotics can increase the concentrations of CYP3A3/4 substrates, yielding toxicity. The induction of CYP3A3/4 by medications such as carbamazepine can decrease the concentrations of CYP3A3/4 substrates, yielding inefficiency. Thus, knowledge of the substrates, inhibitors, and inducers of CYP3A3/ and other cytochrome P450 isoforms may help clinicians to anticipate and avoid pharmacokinetic drug interactions and improve rational prescribing practices.

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

Involvement of CYP2D6, CYP3A4, and other cytochrome P-450 isozymes in N-dealkylation reactions

Metabolic N-dealkylation is a commonly observed biotransformation with tertiary and secondary amine drugs and related N-alkylated amides, but surprisingly little is known about the cytochrome P-450 isozymes involved in these dealkylation reactions. In this review, evidence is provided that supports the involvement of various P-450 isozymes, but especially CYP3A4 and other isozymes of the CYP3A subfamily. Although CYP2D6 is generally not considered to be capable of catalyzing the N-dealkylation of basic drugs, some examples of the involvement of this important isozyme in N-dealkylation reactions are identified. Procedures used to identify individual P-450 isozymes involved in N-dealkylation reactions are discussed.

Characterization of dextromethorphan N-demethylation by human liver microsomes. Contribution of the cytochrome P450 3A (CYP3A) subfamily

In an effort to identify the human cytochromes P450 involved in the N-demethylation of dextromethorphan, the kinetics of 3-methoxymorphinan formation were studied in microsomal enzyme systems. Under initial rate conditions, 3-methoxymorphinan formation demonstrated single enzyme Michaelis-Menten kinetics using microsomes obtained from three human livers (Km: 0.52-0.71 mM; Vmax: 375-812 pmol/mg protein/min). B-lymphoblastoid cells expressing CYP3A4 incubated with 0.4 mM dextromethorphan catalyzed the formation of 3-methoxymorphinan at a rate of 22 pmol product/mg protein/min. Midazolam, a prototypic substrate for CYP3A4 and CYP3A5, competitively inhibited dextromethorphan N-demethylation by two human liver microsomal samples with Ki values of 46 +/- 10 and 63 +/- 8 microM. At a dextromethorphan concentration of 0.4 mM, gestodene (100 microM) inhibited 3-methoxymorphinan formation by approximately 50%. Immunoinhibition of dextromethorphan N-demethylation using rabbit anti-CYP3A4 antibodies resulted in a 60% decrease in 3-methoxymorphinan formation at a dextromethorphan concentration of 0.4 mM. Additional inhibition studies using furafylline, coumarin, sulfaphenazole, mephenytoin, quinidine, and diethyldithiocarbamic acid, which are selective inhibitors of CYP1A2, CYP2A6, CYP2C8/9, CYP2Cmp, CYP2D6, and CYP2E1, respectively, demonstrated no substantial inhibition of dextromethorphan N-demethylation. Correlation analysis was performed using the rate of 3-methoxymorphinan formation at a concentration of 1 mM dextromethorphan and immunoquantified levels of CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 and their associated characteristic catalytic activities. A significant correlation was observed between dextromethorphan N-demethylase activity and midazolam 1'- and 4-hydroxylase activity (r2 = 0.77 and 0.69 respectively, N = 19, P < 0.01); the exclusion of those samples containing both CYP3A4 and CYP3A5 increased the correlation significantly (r2 = 0.87 and 0.91 respectively, N = 12, P < 0.01). In the absence of CYP3A5, a significant correlation was observed between 3-methoxymorphinan formation and the sample's erythromycin N-demethylase activity (r2 = 0.94, N = 12, P < 0.01), testosterone 6 beta-hydroxylase activity (r2 = 0.96, N = 7, P < 0.01) and relative immunoquantified levels of CYP3A4 (r2 = 0.96, N = 12, P < 0.01). Inclusion of those samples expressing CYP3A5 in addition to CYP3A4 reduced the magnitude of the observed correlation. No significant correlation between 3-methoxymorphinan formation and the sample's relative immunoquantified levels of or form-selective activity associated with CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19 (or CYP2Cmp), CYP2D6, and CYP2E1 was observed. In conclusion, dextromethorphan N-demethylation appears to be catalyzed primarily by CYP3A4 and to a lesser extent by CYP3A5 in vitro in humans.(ABSTRACT TRUNCATED AT 400 WORDS)