Thebaine O-demethylation to oripavine: genetic differences between two rat strains

Update of the Scientific Opinion on opium alkaloids in poppy seeds

Poppy seeds are obtained from the opium poppy (Papaver somniferum L.). They are used as food and to produce edible oil. The opium poppy plant contains narcotic alkaloids such as morphine and codeine. Poppy seeds do not contain the opium alkaloids, but can become contaminated with alkaloids as a result of pest damage and during harvesting. The European Commission asked EFSA to provide an update of the Scientific Opinion on opium alkaloids in poppy seeds. The assessment is based on data on morphine, codeine, thebaine, oripavine, noscapine and papaverine in poppy seed samples. The CONTAM Panel confirms the acute reference dose (ARfD) of 10 μg morphine/kg body weight (bw) and concluded that the concentration of codeine in the poppy seed samples should be taken into account by converting codeine to morphine equivalents, using a factor of 0.2. The ARfD is therefore a group ARfD for morphine and codeine, expressed in morphine equivalents. Mean and high levels of dietary exposure to morphine equivalents from poppy seeds considered to have high levels of opium alkaloids (i.e. poppy seeds from varieties primarily grown for pharmaceutical use) exceed the ARfD in most age groups. For poppy seeds considered to have relatively low concentrations of opium alkaloids (i.e. primarily varieties for food use), some exceedance of the ARfD is also seen at high levels of dietary exposure in most surveys. For noscapine and papaverine, the available data do not allow making a hazard characterisation. However, comparison of the dietary exposure to the recommended therapeutical doses does not suggest a health concern for these alkaloids. For thebaine and oripavine, no risk characterisation was done due to insufficient data. However, for thebaine, limited evidence indicates a higher acute lethality than for morphine and the estimated exposure could present a health risk.

Rat CYP2D2, not 2D1, is functionally conserved with human CYP2D6 in endogenous morphine formation

The assumption that CYP2D1 is the corresponding rat cytochrome to human CYP2D6 has been revisited using recombinant proteins in direct enzyme assays. CYP2D1 and 2D2 were incubated with known CYP2D6 substrates, the three morphine precursors thebaine, codeine and (R)-reticuline. Mass spectrometric analysis showed that rat CYP2D2, not 2D1, catalyzed the 3-O-demethylation reaction of thebaine and codeine. In addition, CYP2D2 incubated with (R)-reticuline generated four products corytuberine, pallidine, salutaridine and isoboldine while rat CYP2D1 was completely inactive. This intramolecular phenol-coupling reaction follows the same mechanism as observed for CYP2D6. Michaelis-Menten kinetic parameters revealed high catalytic efficiencies for rat CYP2D2. These findings suggest a critical evaluation of other commonly accepted, however untested, CYP2D1 substrates.

Mammalian cytochrome P450 enzymes catalyze the phenol-coupling step in endogenous morphine biosynthesis

A cytochrome P450 (P450) enzyme in porcine liver that catalyzed the phenol-coupling reaction of the substrate (R)-reticuline to salutaridine was previously purified to homogeneity (Amann, T., Roos, P. H., Huh, H., and Zenk, M. H. (1995) Heterocycles 40, 425-440). This reaction was found to be catalyzed by human P450s 2D6 and 3A4 in the presence of (R)-reticuline and NADPH to yield not a single product, but rather (-)-isoboldine, (-)-corytuberine, (+)-pallidine, and salutaridine, the para-ortho coupled established precursor of morphine in the poppy plant and most likely also in mammals. (S)-Reticuline, a substrate of both P450 enzymes, yielded the phenol-coupled alkaloids (+)-isoboldine, (+)-corytuberine, (-)-pallidine, and sinoacutine; none of these serve as a morphine precursor. Catalytic efficiencies were similar for P450 2D6 and P450 3A4 in the presence of cytochrome b(5) with (R)-reticuline as substrate. The mechanism of phenol coupling is not yet established; however, we favor a single cycle of iron oxidation to yield salutaridine and the three other alkaloids from (R)-reticuline. The total yield of salutaridine formed can supply the 10 nm concentration of morphine found in human neuroblastoma cell cultures and in brain tissues of mice.

Atypical parkinsonism on Guadeloupe, comparison with the parkinsonism-dementia complex of Guam, and environmental toxic hypotheses

INTRODUCTION

: On Guadeloupe, atypical parkinsonism is abnormally frequent, and represents 75% of progressive parkinsonism while Parkinson's disease (PD) accounts for only 25%, which is an inversed percentage in comparison with Europe. Herbal tea made with Annonaceae leaves (containing benzyltetrahydroisoquinolines (Be-TIQ), tetrahydroprotoberberines (THPB) and acetogenins (potent mitochondrial complex I inhibitors) are commonly used on Guadeloupe.

CLINICAL STUDY

: Of 265 patients studied on Guadeloupe, 66 (25%) had PD, and 199 (75%) had atypical parkinsonism. This latter group includes 58 patients (29%) with progressive supranuclear palsy (PSP), and 100 patients (50%) with unclassifiable parkinsonism (UP). This focus resembles the parkinsonism-dementia complex (PDC) described on Guam, where a very high prevalence of atypical parkinsonism has been reported since the second World War, including one-third of PSP. A preliminary case-control study on Guadeloupe showed a significant higher consumption of fruits and herbal tea of Annonaceae in atypical parkinsonian cases (PSP and unclassifiable parkinsonism, UP), compared to hospital controls and to idiopathic PD.

DISCUSSION

: The overrepresentation of atypical parkinsonism on Guadeloupe and Guam could be related to the consumption of plants containing (simultaneously) isoquinoline derivates which are toxic for dopaminergic neurons and inhibitors of the mitochondrial respiratory chain such as acetogenins. This hypothesis is in keeping with epidemiologic data and experimental studies showing neuronal loss after exposure to isoquinolines or acetogenins.

Regional and cellular expression of CYP2D6 in human brain: higher levels in alcoholics

Cytochrome P450 (CYP) 2D6 is expressed in liver, brain and other extrahepatic tissues where it metabolizes a range of centrally acting drugs and toxins. As ethanol can induce CYP2D in rat brain, we hypothesized that CYP2D6 expression is higher in brains of human alcoholics. We examined regional and cellular expression of CYP2D6 mRNA and protein by RT-PCR, Southern blotting, slot blotting, immunoblotting and immunocytochemistry. A significant correlation was found between mean mRNA and CYP2D6 protein levels across 13 brain regions. Higher expression was detected in 13 brain regions of alcoholics (n = 8) compared to nonalcoholics (n = 5) (anovap < 0.0001). In hippocampus this was localized in CA1-3 pyramidal cells and dentate gyrus granular neurons. In cerebellum this was localized in Purkinje cells and their dendrites. Both of these brain regions, and these same cell-types, are known to be susceptible to alcohol damage. For one case, a poor metabolizer (CYP2D6*4/*4), there was no detectable CYP2D6 protein, confirming the specificity of the antibody used. These data suggest that in alcoholics elevated brain CYP2D6 expression may contribute to altered sensitivity to centrally acting drugs and to the mediation of neurotoxic and behavioral effects of alcohol.

Precipitated withdrawal following codeine administration is dependent on CYP genotype

The role of metabolic polymorphism in the development of physical dependence to codeine was assessed in cytochrome P450 2D2 (CYP2D2) deficient Dark Agouti and CYP2D2 intact Sprague-Dawley rats by assessment of the severity of naloxone precipitated withdrawal after codeine and morphine administration. Plasma morphine concentrations after codeine were significantly higher (P<0.01) in Sprague-Dawley than in Dark Agouti rats with metabolic ratios of 0.71 +/- 0.27 and 0.07 +/- 0.04, respectively. Withdrawal after codeine resulted in significantly greater hypothermia (3.5-4 degrees C, P<0.0001) in Sprague-Dawley animals compared to the other groups. Body weight loss was similar for all groups ranging from 6.2 +/- 0.4 to 8.2 +/- 0.6 g. When strain and treatment data were combined, a relationship between body temperature and plasma morphine concentration could be described by the inverse Hill equation (r(2)=0.76, EC(50)=556 +/- 121 ng/ml, n=2.9 +/- 1.5). These data indicate that dependence and withdrawal after codeine administration are dependent on its bioconversion to morphine.

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

BACKGROUND

The opioid dihydrocodeine (DHC) is frequently used as an analgesic and antitussive agent. However, until now there have been no detailed data on dihydrocodeine metabolism in humans. We therefore investigated pathways that contribute to elimination of dihydrocodeine, and we tested the hypothesis that dihydrocodeine O-demethylation to dihydromorphine (DHM) is catalyzed by the polymorphic CYP2D6.

METHODS

A single oral dose of dihydrocodeine was administered to six extensive (metabolic ratio [MR] < or = 1), two intermediate (1 < MR < 20) and six poor metabolizers (MR > or = 20) of sparteine/debrisoquin. Serum concentrations of dihydrocodeine and dihydromorphine were measured up to 25 hours, and urinary excretion of conjugated and unconjugated dihydrocodeine, dihydromorphine, and nordihydrocodeine were determined.

RESULTS

There were no differences in the pharmacokinetics of dihydrocodeine between extensive and poor metabolizers. However, the area under the serum concentration-time curve (AUC), partial metabolic clearance, and total urinary recovery of dihydromorphine were significantly lower in poor metabolizers (10.3 +/- 6.1 nmol.hr/L; 7.0 +/- 4.1 ml/min; 1.3% +/- 0.9% of dose) compared with extensive metabolizers (75.5 +/- 42.9 nmol.hr/L; 49.7 +/- 29.9 ml/min; 8.9% +/- 6.2%; p < 0.01). There was a strong correlation between the AUCDHC/AUCDHM ratio and the urinary metabolic ratio of sparteine (rS = 0.89, p = 0.001). No significant differences between extensive and poor metabolizers were detected in urine for conjugated dihydrocodeine (extensive metabolizers, 27.7% of dose; poor metabolizers, 31.5%), unconjugated dihydrocodeine (extensive metabolizers, 31.1%; poor metabolizers, 31.1%), conjugated nordihydrocodeine (extensive metabolizers, 6.3%; poor metabolizers, 5.4%), or unconjugated nordihydrocodeine (extensive metabolizers, 15.8%; poor metabolizers, 19.5%).

CONCLUSIONS

Dihydrocodeine O-demethylation to dihydromorphine is impaired in poor metabolizers of sparteine. The main urinary metabolites after administration of dihydrocodeine are the parent compound and its conjugates in extensive and poor metabolizers.

Ethylmorphine O-deethylation in isolated rat hepatocytes. Involvement of codeine O-demethylation enzyme systems

The O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M) co-segregates with debrisoquine/sparteine genetic polymorphism in man. CD O-demethylation is catalysed by cytochrome P450 2D1 (CYP2D1) in rats. In the present study, the O-deethylation of EM was examined and compared with that of CD in suspensions of freshly-isolated hepatocytes prepared by a collagenase method from Wistar rats with and without CYP2D1 inhibitors. Isolated hepatocytes were also prepared from Dark Agouti (DA) rats deficient in CYP2D1, and were incubated with EM or CD. EM, CD and their metabolites were quantified by HPLC with UV detection. EM had a similar pattern of metabolism to that of CD in suspensions of hepatocytes from Wistar rats. Both EM and CD were O-dealkylated to form M plus morphine-3-glucuronide (M3G) and N-demethylated to form norethylmorphine (NEM) or norcodeine (NCD), respectively, which were further metabolized to normorphine (NM) and finally glucuronidated to normorphine-3-glucuronide (NM3G). As compared to hepatocytes from Wistar rats, DA rats were characterized by a markedly decreased formation (70 approximately 75% reduction) of M plus M3G from both EM and CD. Quinine, quinidine, propafenone and sparteine all inhibited EM O-deethylation as well as CD O-demethylation. Quinine was the most potent inhibitor of both these O-dealkylations (Ki = 0.2 microM for both EM and CD, respectively). Quinine as well as the other inhibitors inhibited both EM and CD O-dealkylation competitively and with small differences in Ki versus EM and CD, respectively. The metabolism of EM to M plus M3G and that of CD to M plus M3G was highly correlated when results from the various separate cell suspensions were plotted. In conclusion all findings indicated that the enzyme responsible for O-demethylation of CD, CYP2D1 was also responsible for the O-deethylation of EM to M.

Dextromethorphan metabolism in rat: interstrain differences and the fate of individually administered oxidative metabolites

1. Dextromethorphan undergoes O- and N-demethylation, with the resultant metabolites being further N- and O-demethylated respectively to 3-hydroxymorphinan. The polymorphically expressed O-demethylation reaction is catalysed by P4502D1 in the Sprague-Dawley (SD) rat. The Dark-Agouti (DA) rat lacks this enzyme. 2. The aims were: (1) to determine if there were strain differences also in the Hooded Wistar (HW) and Albino Wistar (AW) rats with respect to the four demethylation reactions after dextromethorphan 20 mg/kg intraperitoneally; (2) to investigate the inhibition of the demethylation reactions by quinine and quinidine (each 40 mg/kg i.p.) in the above strains; and (3) to investigate the fate of separately administered metabolites (5 mg/kg i.p.) of dextromethorphan in the SD strain. 3. The total recovery of dextromethorphan and metabolites in the four strains ranged from 38 to 64% of the dose. The O-demethylation ratios (expressed as the ratio of urinary total dextrorphan divided by dextromethorphan) in the AW and DA strains were similar but less than in the SD/HW strains; the N-demethylation ratios (expressed as the ratio of urinary total 3-hydroxymorphinan plus 3-methoxymorphinan divided by dextromethorphan) in the DA and SD strains were similar but greater than in the AW and HW strains. Quinine and quinidine significantly reduced the O-demethylation ratio in the SD and DA rat strains, and the N-demethylation ratio in the SD strain. 4. In the SD rat the major metabolic route was via O-demethylation to dextrorphan. The source of 3-hydroxymorphinan is primarily from N-demethylation of dextromethorphan to 3-methoxymorphinan and its subsequent O-demethylation to 3-hydroxymorphinan. The O-demethylation metabolic ratio for dextromethorphan should be calculated as the quotient of urinary total dextrorphan divided by dextromethorphan.

An evaluation of cytochrome P450 isoform activities in the female dark agouti (DA) rat: relevance to its use as a model of the CYP2D6 poor metaboliser phenotype

The female dark agouti (DA) rat lacks CYP2D1, the equivalent enzyme in the rat to human CYP2D6 (debrisoquine hydroxylase), and shows impaired metabolism of a number of CYP2D6 substrates. However, from the data available in the literature it is not entirely clear whether the enzyme deficiency in the DA rat is restricted to CYP2D1, and whether factors such as age and substrate concentration are important determinants of interstrain differences in the activity of this enzyme. Given that the female DA rat is used as a model of the human CYP2D6 poor metaboliser phenotype, there is a need for a systematic evaluation of the P450 activities in the DA rat, and of its suitability as a model of the PM phenotype. In the present study metoprolol was used as a probe substrate to investigate CYP2D1 activity since both the alpha-hydroxylation and O-demethylation of this drug are catalysed by CYP2D6 in man. Formation of alpha-hydroxymetoprolol (AHM) and O-demethylmetoprolol (ODM) was 10- and 2.5-fold lower in liver microsomes from female DA rats compared with microsomes from age-matched female Wistar rats, the latter representing the extensive metaboliser strain. Kinetic analysis suggested that in both strains of rat both the alpha-hydroxylation and O-demethylation of metoprolol were catalysed by more than one enzyme. By using quinine as a specific inhibitor of the enzyme, CYP2D1 was identified as an intermediate affinity site in the Wistar strain and was shown to have impaired activity in the DA strain. The activities of lower and higher affinity sites were similar in the two strains. Thus, the only difference between the two strains with respect to both routes of metoprolol metabolism appeared to be in the activity of CYP2D1. Interstrain differences were found to be highly dependent on the choice of substrate concentration, being more marked at lower concentrations. We have also investigated the metabolism of a number of probe compounds for some of the other P450 isoforms commonly involved in drug metabolism to determine the selectivity of the deficiency in the DA strain. p-Nitrophenol hydroxylation and erythromycin N-demethylation were catalysed at higher rates by DA than by Wistar liver microsomes, indicating higher levels of activity of CYP2E1 and CYP3A in the former strain. Felodipine oxidation, tolbutamide hydroxylation and both the hydroxylation and N-demethylation of S-mephenytoin were catalysed at similar rates by microsomes from the two strains, indicating similar activities of enzymes in the CYP2C and CYP3A families.(ABSTRACT TRUNCATED AT 400 WORDS)

Primary and secondary oxidative metabolism of dextromethorphan. In vitro studies with female Sprague-Dawley and Dark Agouti rat liver microsomes

The O-demethylation of dextromethorphan (DM) to dextrorphan (DR) is catalysed by the polymorphic CYP2D6 (cytochrome P4502D6) isozyme in man. DM is commonly used as a probe for phenotyping subjects as either poor or extensive metabolizers for the debrisoquine/sparteine oxidative polymorphism via CYP2D6. The enzyme kinetics of DM O- and N-demethylation, and the N- and O-demethylations of the primary metabolites DR and 3-methoxymorphinan (3MM), respectively, were studied in liver microsomes from female Dark Agouti (DA) rats, the poor metabolizer counterpart, and female Sprague-Dawley (SD) rats, the extensive metabolizer counterpart. The formation of metabolites was quantified by HPLC with fluorescence detection and kinetic parameters were calculated. The intrinsic clearance (Vmax/Km) of the O-demethylation of 3MM to 3-hydroxymorphinan (3OHM) was 180-fold lower in DA rats (0.11 vs 20.77 mL/hr/mg) due to a 60-fold higher Km (108.7 vs 1.76 microM) and 3-fold lower Vmax (11.5 vs 35.95 nmol/mg/hr). The kinetics for DR N-demethylation to 3OHM did not differ between rat strains. The Michaelis-Menten constant (Km) for DM N-demethylation to 3MM was similar between SD and DA rats (85.04 vs 68.99 microM); however, SD rats displayed a 2-fold higher Vmax (83.37 vs 35.49 nmol/mg/hr) and intrinsic clearance (0.96 vs 0.51 mL/hr/mg). The O-demethylation of DM to DR in SD rats showed a high and low affinity enzyme component, with the high affinity intrinsic clearance contributing 98% of the total intrinsic clearance in these rats. DM O-demethylation in DA rats was characterized by a single enzyme system. The high affinity O-demethylating enzyme in SD rats showed a 20-fold lower Km (2.5 vs 55.6 microM) and a three-fold higher Vmax (51.04 vs 16.84 nmol/mg/hr) resulting in a 66-fold higher intrinsic clearance (20.04 vs 0.31 mL/hr/mg) compared to DA rats. Quinine, dextropropoxyphene, (+/-)methadone and (+/-)propafenone were shown to be potent inhibitors of 3MM and DM O-demethylation but did not inhibit DR or DM N-demethylation at similar concentrations. SD and DA rats showed a clear strain difference in 3MM O-demethylation and DM O-demethylation. In contrast, DR N-demethylation and DM N-demethylation do not appear to be under genetic control in the female SD-DA rat model. Kinetic parameters and inhibition studies suggest that 3MM and DM O-demethylation pathways in the rat may be mediated by the same cytochrome P450 isozyme.

Speculations on the substrate structure-activity relationship (SSAR) of cytochrome P450 enzymes

This brief review attempts to define the SSAR of two families of cytochrome P450. With P4502D catalytic competence is achieved by tight ionic binding which gives the enzyme high regioselectivity. In contrast P4503A achieves catalytic competence by a flexible binding site relying on hydrophobic forces that allow chemically vulnerable sites to be the principal sites of metabolism. In general, the different binding mechanism should be reflected in the enzyme, such that substrates of P4502D should have lower Km values than substrates of P4503A. Thus, routes of metabolism catalysed by P4502D may be saturated at substrate concentrations lower than routes catalysed by P4503A. The apparent differences between P4502D and P4503A in terms of substrate specificity bring into question what relationships govern other families of cytochrome P450. Our analysis of data suggests that the other principal form involved, generally, in the metabolism of pharmaceuticals in humans is P4502C9 (possibly 2C8 and 2C10). The enzyme is responsible for the metabolism of phenytoin, tolbutamide, tienilic acid [4], naproxen, ibuprofen, diclofenac [38], the 7-hydroxylation of S-warfarin [39] and the 7-hydroxylation of delta 1-tetrahydrocannabinol [40]. These compounds all have areas of strong hydrogen bond [4] forming potential (Fig. 8), all distanced 5-10A from the site of metabolism. Moreover the carboxylic acid function of naproxen, ibuprofen and diclofenac (pKa 4.5) and the sulfonylurea of tolbutamide (pKa 5.4) render the compounds ionized at physiological pH. The ionised group is positioned 7-11A from the site of metabolism. It is likely, therefore, that hydrogen bonding and possibly ion-pair interactions play a major role in determining the SSAR of the P4502C isoenzymes. These interactions would suggest that the P4502C enzymes are analogous to P4502D rather than P4503A. In this regard it is noteworthy that P4502C9 is selectively and potently inhibited by sulfaphenazole (IC50 of 0.6 microM), a compound that is structurally related (Fig. 8) to the substrates in terms of potential hydrogen bonding regions [4, 41]. Simplistically we suggest that the SSAR of the various P450 enzymes ranges from the highly selective enzymes dealing with endogenous substrates, through the enzymes metabolising exogenous substrates with narrow substrate structure requirements such as P4502D to P4503A with its broad substrate structure range. It would seem logical that animals and humans would evolve such combinations of isoenzymes to deal with the vast array of exogenous xenobiotics.