Polymorphic formation of morphine from codeine in poor and extensive metabolizers of dextromethorphan: relationship to the presence of immunoidentified cytochrome P-450IID1

In vitro forecasting of drugs that may interfere with codeine bioactivation

The O-demethylation of codeine (methylmorphine) into morphine is mediated by the polymorphic cytochrome P450 DB1 (P450 IID6). By means of in vitro screening in human liver microsomes we have studied the effect on codeine bioactivation of several drugs used as analgesics or as adjuvants for pain control. In microsomes from an extensive metabolizer subject, paracetamol (acetaminophen) and NSAIDs (acetylsalicylic acid, diclofenac, indomethacin, piroxicam, and pirprofen), benzodiazepines (chlordiazepoxide, clonazepam, diazepam, flunitrazepam, and midazolam), and anticonvulsants (carbamazepine and phenytoin) did not alter the reaction. There was marked inhibition of in vitro morphine production by neuroleptics (chlorpromazine, haloperidol, levomepromazine, and thioridazine), metoclopramide, and tricyclic antidepressants (amitriptyline, clomipramine, desipramine, imipramine, and nortriptyline). Enzyme kinetics showed competitive inhibition by neuroleptics (chlorpromazine Ki = 0.5 microM) and antidepressants (clomipramine Ki = 6.8 microM), which are substrates of the polymorphic monooxygenase. Due to the low affinity of codeine for P450 DB1 (Km = 100-200 microM), its bioactivation in extensive metabolizers, and thus its analgesic efficacy, is liable to vary greatly when it is combined with any drug that has a high affinity for the polymorphic isozyme.

Infants and young children metabolise codeine to morphine. A study after single and repeated rectal administration

1. Codeine was administered rectally to thirteen infants and young children undergoing elective surgery. Nine infants (6-10 months old) received a 4 mg suppository and four children (3-4 years old) an 8 mg suppository. Codeine and its metabolite morphine were measured in plasma by GC/MS. 2. The mean concentrations of codeine at 3, 4 and 5 h after administration were 240, 163 and 123 nmol l-1 in the younger and 309, 251 and 169 nmol l-1 in the older patients. The corresponding concentrations of morphine were 8.3, 7.4 and 4.5 nmol l-1 and 6.8, 5.5 and 2.8 nmol l-1 respectively. One patient in each age group had no detectable amounts of morphine. 3. In the four children, the rectal dose was repeated 6-hourly for four doses. The plasma concentrations of codeine and morphine following the fifth dose were similar to those after the first dose. The mean AUC(0,5 h) of morphine was 1.6% that of codeine. 4. In the infants the mean plasma half-lives of codeine and morphine were 2.6 and 2.5 h. The two infants with the lowest body weights had the longest half-lives. 5. The mean morphine/codeine concentration ratio was 4.3% in the infants and 1.6% in the children, suggesting impaired glucuronidation of morphine in the former group. The hourly concentration ratios were almost identical following the first and fifth dose in the children. 6. We conclude that at the age of 6 months infants are capable of O-demethylating codeine to morphine.

The human hepatic cytochromes P450 involved in drug metabolism

The cytochromes P450 are a superfamily of hemoproteins that catalyze the metabolism of a large number of xenobiotics and endobiotics. The type and amount (i.e., the animal's phenotype) of the P450s expressed by the animal, primarily in the liver, thus determine the metabolic response of the animal to a chemical challenge. A majority of the characterized P450s involved in hepatic drug metabolism have been identified in experimental animals. However, recently at least 12 human drug-metabolizing P450s have been characterized at the molecular and/or enzyme level. The characterization of these P450s has made it possible to "phenotype" microsomal samples with respect to their relative levels of the various P450s and their metabolic capabilities. The purpose of this review is to compare and contrast the human P450s involved in drug metabolism with their related forms in the rat and other experimental species.

Pharmacokinetic considerations in abuse liability evaluation

The behavioral effects of a drug are related to three factors: its intrinsic pharmacological activity, its physicochemical properties, and its pharmacokinetic parameters. In many cases differences in absorption, distribution, metabolism, and elimination may explain the different abuse liability profiles of drugs from within the same pharmacological class. Rapid absorption rate and high lipid solubility are the most important factors contributing to early drug concentrations in the brain. Differences in drug metabolism may be related to dose-dependent kinetics, first-pass metabolism, and variations in genetic traits (e.g. poor or extensive metabolizers). Metabolic pathways may produce active metabolites with similar or greater pharmacological activity than the parent substance. Drugs with a rapid elimination rate have been associated with greater self-administration and with early emergence of withdrawal symptoms. More pharmacokinetic studies are needed in human drug abuse liability evaluations. Knowledge of the plasma concentrations of drugs and their pharmacokinetic parameters can be essential to interpret differences among similar drugs in human abuse liability assessments.

Drug metabolism and interactions in abuse liability assessment

The interpretation of acute abuse liability studies and drug interaction studies would be importantly strengthened by the routine inclusion of drug concentration measurements at appropriate sampling times. Reliance on mean kinetic data misrepresents the variation in drug kinetics and fails to take experimental advantage of the natural differences in the population which may represent the extremes of abuse risk. Pharmacokinetic-pharmacodynamic studies to better understand the relationship of plasma drug concentration, drug concentration in the receptor biophase and specific drug reinforced behaviour will ensure proper study design and yield useful theoretic information. Multi- and poly-drug abuse (including heavy smoking and heavy ethanol use) are very common. Such patterns of use can have quite large effects on drug kinetics. Because of the potentially large qualitative and quantitative differences in drug metabolism and kinetics between pre-clinical species and the human, data should be gathered at the earliest possible time with respect to human metabolic rates, patterns, and identification of inhibitors. The availability of human liver microsomes facilitates such studies.

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

1. Codeine and dextromethorphan were N-demethylated in human foetal liver microsomes at high rates which were close to the activities in adult livers. In contrast, foetal liver microsomes did not catalyze the O-demethylation of these drugs at mid-gestation. 2. The metabolic data were in accordance with the absence of P450IID6 and the presence of P450 IIIA as determined by Western blotting with anti-human P450 IID6 (MAb 114/2) and anti-rat P450 IIIA (PCN 2-13-1/C2) monoclonal antibodies, respectively. 3. The inhibitory effects of midazolam and dehydroepiandrosterone support the contention that the N-demethylase is a human foetal form of the cytochrome P450 IIIA family. Consistent with this we found that blotting with the MAb PCN 2-13-1/C2, which recognizes an epitope specific for the P450 III family, correlated well with the N-demethylase activities.

Impact of environmental and genetic factors on codeine analgesia

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

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

1. The kinetics of codeine and metabolites were studied in eight unrelated healthy Chinese subjects following a single oral dose of 50 mg codeine phosphate. The data were compared with those from eight Caucasian subjects who were matched with the Chinese group according to their metabolic ratio (MR) of debrisoquine. 2. Mean values of Cmax (445 nmol l-1) and AUC (1660 nmol l-1 h) of codeine in the Chinese were significantly higher than those in the Caucasians (292 nmol l-1 and 1010 nmol l-1 h). Thus plasma clearance was significantly lower (P less than 0.02) and the plasma half-life was longer (P less than 0.05) in the Chinese. 3. Partial clearance by glucuronidation was significantly lower (0.79 +/- 0.14 s.d. vs 1.42 +/- 0.48 s.d. 1 h-1 kg-1) in Chinese than in Caucasians. 4. The total urinary recovery of drug-related material in 48 h urine was similar in Chinese (82.2%) and Caucasians (84.4%). The recovery of unchanged codeine was significantly higher in Chinese (5.7%) than in Caucasians (3.3%). 5. The AUC ratios of codeine relative to its 6-glucuronide, morphine and norcodeine were 1:9, 35:1 and 4:1, respectively in Chinese. The corresponding ratios in Caucasians were 1:15, 50:1 and 6:1. 6. There was no significant difference between Chinese and Caucasians in the renal clearances of codeine and its primary metabolites. 7. Large interethnic differences in the kinetics of codeine have been shown. The Chinese are less able to metabolise codeine mainly because of a lower efficiency in glucuronidation.

Interethnic variation of drug metabolism

There are many examples of differences between ethnically defined populations with regard to drug-metabolizing enzymes i.e. enzymes that serve as defences against exogenous chemicals. The prevalence of variants of enzymes such as monooxygenases, dehydrogenases, esterases and some transferases is not uniform, and this leads to differences in metabolism of drugs such as aspirin and diazepam, among others. Werner Kalow explains why the mere counting of a genetic variant may not be sufficient to uncover more major problems such as may be posed by multiple allelism or heterozygosity, and why it may be predicted that a large number of drugs will display interethnic differences in function.