[Clinical relevance of drug metabolism polymorphisms]

Individual variation in drug response is a substantial clinical problem. Research in pharmacogenetics is currently evolving in two directions, firstly to identify genes and gene products associated with certain diseases, which may serve as targets for new drugs, and secondly to identify genes and allelic variants of genes that affect the expected and adverse response to current and future drugs. In that respect drug metabolising enzymes play a key role. A readily and widely available pheno- and/or genotyping service for drug metabolism polymorphisms is currently not established and the clinical relevance has only been shown for a limited number of drugs. The overall pharmacologic effects are typically not monogenic traits, they are a result of an interaction of several genes encoding proteins involved in drug metabolism, disposition, and effects. Each individual represents a unique combination of polymorphic genes that are known to be involved in the metabolism and disposition of medications, in the target structures of drug therapy, and in the pathogenesis of diseases. This will lead to the development of DNA chips for genotyping which will guide the selection and dosing of drug therapy in the future.

Gabapentin enhances the analgesic effect of morphine in healthy volunteers

The most effective group of drugs for the treatment of severe pain is opioid analgesics. Their use, however, is limited by decreased effects in neuropathic and chronic pain as a result of increased pain and development of tolerance. Gabapentin (GBP) is effective in both experimental models of chronic pain and clinical studies of neuropathic pain. Therefore, we investigated, in a randomized, placebo-controlled, double-blinded study, the pharmacodynamic and pharmacokinetic interaction of GBP and morphine in 12 healthy male volunteers. Morphine (60 mg, controlled release) or placebo was administered at 8:00 AM, and GBP (600 mg) or placebo was administered at 10:00 AM, thus comparing the analgesic effect of placebo + GBP (600 mg) with placebo + placebo and morphine (60 mg) + GBP in comparison to morphine plus placebo by using the cold pressor test. The duration and intensity of the side effects were assessed by using visual analog scales. The analgesic effect was evaluated by the change in the area under the curve (h x %; 0% baseline before Medication 1) of pain tolerance. Placebo + GBP (18.9% x h, 95% confidence interval [CI]: -2.5 to 40.3) did not present any significant analgesic effect compared with placebo + placebo (4.7% x h, 95% CI: -16.7 to 26.1). A significant increase in pain tolerance was observed comparing the combination of morphine and GBP (75.5% x h, 95% CI: 54.0-96.9) with morphine + placebo (40.6% x h, 95% CI: 19. 2-62.0). The observed adverse events after placebo + GBP were not significantly different compared with placebo + placebo. Morphine + placebo led to the expected opioid-mediated side effects. They were significantly more pronounced compared with placebo + placebo but did not differ significantly compared with the combination of morphine + GBP. Concerning the pharmacokinetic variables of morphine and its glucuronides, no significant difference between morphine + placebo and morphine + GBP was observed, whereas the area under the curve of GBP (43.9 +/- 5.3 vs 63.4 +/- 16.2 microg. h(-1). mL(-1), P < 0.05) significantly increased, and apparent oral clearance (230.8 +/- 29.4 mL/min vs 178 +/- 97.9 mL/min, P = 0.06) and apparent renal clearance (86.9 +/- 20.6 vs 73.0 +/- 24.2 mL/min, P = 0.067) of GBP decreased when morphine was administered concomitantly. These results suggest two different sites for the pharmacokinetic interaction-one at the level of absorption and the other at the level of elimination. Our study reveals both a pharmacodynamic and pharmacokinetic interaction between morphine and GBP, leading to an increased analgesic effect of morphine + GBP. These results and the good tolerability of GBP should favor clinical trials investigating the clinical relevance of the combination of morphine and GBP for treating severe pain.

IMPLICATIONS

In a randomized, placebo-controlled, double-blinded trial with 12 healthy volunteers, we studied the interaction of morphine and gabapentin using the cold pressor test. The anticonvulsant gabapentin enhanced the acute analgesic effect of morphine. Furthermore, the plasma concentration of gabapentin was increased when morphine was administered concomitantly. Therefore, the well tolerated combination of gabapentin and morphine may improve pain therapy, especially in pain states, like chronic and neuropathic pain, which respond poorly to opioids.

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

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

Pharmacokinetics and pharmacodynamics of R- and S-gallopamil during multiple dosing

AIMS

Using a stable isotope technique we investigated the pharmacokinetics and pharmacodynamics of gallopamil after administration of 50 mg pseudoracemic gallopamil every 12 h for 7 doses (72 h).

METHODS

Six male healthy volunteers were studied. After the seventh dose the pharmacokinetics and pharmacodynamics were assessed. Serum levels of gallopamil were measured by gas chromatography/mass spectrometry. Effects of gallopamil were measured by ECG recording.

RESULTS

The apparent oral clearances (R: 4.8 l min-1 (95% CI: 2.9-6.8); S: 5.5 l min-1 (95% CI: 2.5-8.5)) and half-lives (R: 6.2 h; S: 7.2 h) of R- and S-gallopamil were similar (P >0.05). The serum protein binding (fu R: 0.035 (95% CI: 0.026-0. 045); S: 0.051 (95% CI: 0.033-0.069)) and the renal elimination (% of dose R: 0.49%; S: 0.71%) were enantioselective. Gallopamil had a potent effect on the PR interval (% prolongation 35.7% (95% CI: 14. 0-57.3)). No changes in other electrocardiographic or cardiovascular parameters were observed.

CONCLUSIONS

The pharmacokinetics and bioavailability of the racemic drug gallopamil are not stereoselective at steady-state and are therefore not substantially altered compared with the single dose administration of gallopamil.

Analysis of the CYP2D6 gene mutations and their consequences for enzyme function in a West African population

The data on differences in the metabolic handling of the CYP2D6 probe drugs sparteine and debrisoquine, and the relationship between phenotype and genotype and gene frequencies for several mutant CYP2D6 alleles in African populations are limited and sometimes controversial. Therefore, in a West African population (Ghana), we investigated (i) the phenotype for sparteine debrisoquine by phenotyping 201 individuals with both drugs and (iii) the genotype for CYP2D6 (n = 326) and debrisoquine (n = 201) oxidation, (ii) the coregulatory control of sparteine and alleles *3 and *4 in 133 individuals and for the alleles *1, *2, *3, *4, *5, *6, *7, *8, *9, *10, *14, *16, *17, *2b, *2xN, *2bxN in 193 individuals. Of the 326 individuals phenotyped with sparteine, eight had a metabolic ratio (MR)sp > 20 corresponding to a poor metabolizer frequency of 2.5% [95% (confidence interval) CI = 1.06-4.77]. The prevalence of the poor metabolizer phenotype for debrisoquine oxidation was 3% (95% CI = 1.1-6.39) with six of the 201 individuals having a MR greater than 12.6. The distribution of the MR of sparteine was trimodal whereas MR of debrisoquine was unimodally distributed with a pronounced kurtosis. In individuals phenotyped with both drugs, there was a significant correlation between the MRs (r(s) = 0.63, P < 0.001). The CYP2D6 alleles *1, *2 and *17 were the most common functional alleles occurring with frequencies of 43.7, 10.6 and 27.7%, respectively. The three other observed functional alleles *2xN, *10 and *20 had much lower frequencies (1.6%, 3.1% and 0.3%, respectively). Of the eight non-functional alleles, only *4 (6.3%) and *5 (6.0%) could be found. The allele *5 occurred with the same frequency as in Caucasian populations (4.1%) but the *4 allele had a much lower frequency (Caucasians 19.5%). One individual with *1/*1 was a poor metabolizer for sparteine and debrisoquine indicating the existence of as yet unknown non-functional alleles in this West African population. Although the prevalence of poor metabolizers and the number of heterozygotes for non-functional alleles was much lower in Ghanaians, the median MRsp of 0.7 was significantly higher in this population compared with a median MRsp of 0.4 in Caucasians, indicating a lower metabolic clearance for CYP2D6 substrates in the West Africans. The lower metabolic activity in Ghanaians could not be explained solely by the high frequency of the *17 allele, which is associated with an impairment of CYP2D6 enzyme function. In addition, a higher median MRsp of 0.5 corresponding to metabolic clearance of 346 ml/min was observed among extensive metabolizers with the genotype *1/*1. Thus, compared with the median of MRsp = 0.28 (CLmet 573 ml/min) in Caucasians homozygous for *1, the metabolic clearance of sparteine was 40% lower on average in respective Ghanaians.

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

AIMS

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

CONCLUSIONS

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

Highly sensitive gas chromatographic-tandem mass spectrometric method for the determination of morphine and codeine in serum and urine in the femtomolar range

A sensitive and specific method was developed for the determination of codeine and morphine in human serum and for the determination of trace amounts of endogenous morphine in human urine. The analytes were recovered from serum by a simple liquid-liquid extraction method. Urine samples were hydrolyzed, and purified by two liquid-liquid extraction steps and a solid-phase extraction. Samples were derivatized to the pentafluoropropionic esters and measured by gas chromatography tandem mass spectrometry. Using the deuterated analogues as internal standards a limit of quantification of 20 fmol/ml (5.7 pg/ml) morphine and 500 fmol/ml (150 pg/ml) codeine in human serum and of 2.5 fmol/ml (0.71 pg/ml) morphine in urine was achieved. The method was suitable for the determination of morphine and codeine in pharmacokinetic studies and for the determination of the urinary excretion of endogenous morphine.

Pharmacokinetics of morphine and its glucuronides following intravenous administration of morphine in patients undergoing continuous ambulatory peritoneal dialysis

BACKGROUND

Conjugation with glucuronic acid represents the major route of biotransformation of morphine. The glucuronides morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) are eliminated via the kidneys. Therefore, chronic renal failure should affect the disposition of M3G and M6G. Numerous patients undergoing long-term continuous ambulatory peritoneal dialysis (CAPD) require pain treatment with morphine. There are only limited data available about the disposition of morphine and its active metabolites M6G and M3G in patients on CAPD. We therefore investigated the pharmacokinetics of morphine and its metabolites in CAPD patients.

METHODS

This was a single intravenous dose pharmacokinetic study in 10 CAPD patients (1 female, 9 male, age 31-69 years). Morphine-hydrochloride (Mo) (10 mg) was administered intravenously. Serum, urine, and dialysate samples were collected during 24 h. GC-MS-MS and HPLC-MS methods were used to quantify respectively morphine and morphine glucuronides.

RESULTS

While systemic clearance of morphine (1246+/-240 ml/min) was in the range observed in patients with normal kidney function, both M3G and M6G showed substantial accumulation. The area under the concentration-time curve (AUC) ratio of M3G:Mo (33.4+/-7.1) and of M6G:Mo (12.2+/-3.2) was 5.5 and 13.5 times higher than in patients with normal kidney function. Renal clearances of morphine, M3G, and M6G (morphine 3.0+/-2.5 ml/min; M3G 3.9+/-2.2 ml/min; M6G 3.6+/-2.2 ml/min) and dialysate clearances (morphine 4.1+/-1.3 ml/min; M3G 3.2+/-0.7 ml/min; M6G 3.0+/-0.8 ml/min) were extremely low. Therefore the accumulation of M6G and M3G is readily explained by kidney failure which is not compensated by CAPD.

CONCLUSION

Accumulation of M3G and M6G is due to the substantially lowered clearance by residual renal function and peritoneal dialysis. In view of the accumulation of potential active metabolites, subsequent investigations have to assess the frequency of side-effects in patients on CAPD.

Chronopharmacology of intravenous and oral modified release verapamil

AIMS

Using a stable isotope technique which allows simultaneous and differential measuring of orally and intravenously administered drugs we compared the pharmacokinetics and pharmacodynamics of unlabelled modified release verapamil p.o. (steady state) and deuterated verapamil i.v. (single dose) following morning and evening administration.

METHODS

Twelve female and 12 male healthy volunteers were studied in a randomized, crossover design. During the last day of each treatment period (day 6 and day 10) pharmacokinetics and pharmacodynamics (PR interval) of verapamil were assessed; 1 h before ingestion of a new R/S-verapamil 240 mg modified release formulation (08.00 h vs 20.00 h) a single dose of 10 mg d7-R/S-verapamil was administered intravenously. Serum levels of unlabelled and labelled R/S-verapamil were measured by gas chromatography/mass spectrometry. In selected samples of serum which were chosen at tmin,po and tmax,po the enantiomers were separated by chiral high-performance liquid chromatography in order to calculate R- to S-verapamil serum concentration ratios.

RESULTS

We observed no significant differences in pharmacokinetics (AUCpo, Cmax, tmax, CLo, F and R/S enantiomer ratio) between morning and evening treatment with modified release verapamil and there was no influence of time of dosing on mean prolongation of PR interval. AUCiv, CL, Vss and d7-R/d7-S enantiomer ratio following verapamil i.v. did not show circadian variation. t1/2 was slightly but statistically significantly increased after the morning infusion. PR-prolongation was significantly greater after verapamil i.v. in the morning than in the evening. The 90% confidence intervals of the differences between morning and evening administration in AUCpo, Cmax and AUCiv were within the equivalence range of 0.8-1.25.

CONCLUSIONS

Time of dosing has no significant influence on pharmacokinetics and pharmacodynamics of this new modified release formulation of verapamil. Circadian variation in presystemic metabolism of verapamil was not observed.

Rapid, highly sensitive method for the determination of morphine and its metabolites in body fluids by liquid chromatography-mass spectrometry

A rapid, highly sensitive method for the determination of morphine and its metabolites morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G) and normorphine has been developed using high-performance liquid chromatography-electrospray mass spectrometry, with the deuterated analogues as internal standards. The analytes were extracted automatically using end-capped C2 solid-phase extraction cartridges. Baseline separation of morphine, M3G and M6G was achieved on a LiChrospher 100 RP-18 end-capped analytical column (125x3 mm I.D., 5 microm particle size) with water-acetonitrile-tetrahydrofuran-formic acid (100:1:1:0.1, v/v) as the mobile phase. Morphine and normorphine coeluate and were separated mass spectrometrically. The mass spectrometer was operated in the selected-ion monitoring mode using m/z 272 for normorphine, m/z 286 for morphine, m/z 462 for morphine-6-glucuronide. Due to an interfering peak, M3G was measured by tandem mass spectrometry in the daughter-ion mode. The limits of quantitation achieved with this method were 1.3 pmol/ml for morphine, 1.5 pmol/ml for normorphine, 1.0 pmol/ml for M6G and 5.4 pmol/ml for M3G in serum or cerebrospinal fluid. The limits of quantitation achieved in urine were 10 pmol/ml for morphine, 20 pmol/ml for normorphine and M6G and 50 pmol/ml for M3G using a sample size of 100 microl. The method described was successfully applied to the determination of morphine and its metabolites in human serum, cerebrospinal fluid and urine in pharmacokinetic and drug interaction studies.

Same incidence of adverse drug events after codeine administration irrespective of the genetically determined differences in morphine formation

The analgesic effect and adverse events of the weak opioid codeine is assumed to be mediated by its metabolite morphine. The cytochrome P-450 enzyme CYP2D6 catalysing the formation of morphine exhibits a genetic polymorphism. Two distinct phenotypes, the extensive (EMs) and poor metabolisers (PMs), are present in the population. The prevalence of PMs in the Caucasian population is 7% to 10%. Since PMs do not express functional CYP2D6, they have a severely impaired capacity to metabolise drugs which are substrates of this enzyme. Provided the analgesic effect and the adverse events of codeine are mediated by its metabolite morphine, large phenotype-related differences are to be expected and PMs, as they form only trace amounts of morphine, can serve as a model to test the hypothesis whether the analgesia and adverse events of codeine are mediated by the parent drug or its metabolite morphine. Therefore we have studied in a randomised placebo-controlled double-blind trial the analgesic effect of 170 mg codeine (p.o.) compared to 20 mg morphine (p.o.) and placebo in 9 EMs and 9 PMs using the cold pressor test. The duration and intensity of the side effects were assessed using visual analogue scales (VAS). Codeine and morphine concentrations were measured in serum and urine. Compared to placebo, 20 mg morphine caused a significant increase in pain tolerance in both phenotypes, EMs and PMs (16.2+/-27.4 vs. -0.66+/-27.4 s x h, n=18). However, following administration of codeine, analgesia was only observed in EMs but not in PMs (EMs: 54.9+/-42.2 vs. 1.7+/-4.2 s x h, P < 0.01; PMs: 9.6+/-10.9 vs. 3.3+/-23.7 s x h, not significant). Adverse events were significantly more pronounced after morphine and codeine compared to placebo in both EMs and PMs. In contrast to the phenotype-related differences in the analgesic effect of codeine, however, no difference in adverse events between the phenotypes could be observed. In the pharmacokinetic studies, significant differences between the two phenotypes in the formation of morphine after codeine administration could be observed. Whereas morphine plasma concentrations were similar in PMs (Cmax: 44+/-13 nmol/l: AUC: 199+/-45 nmol x h/l) and EMs (Cmax: 48+/-17 nmol/l); AUC: 210+/-65 nmol x h/l) after morphine administration, following 170 mg codeine, morphine plasma concentrations comparable to those after morphine application were only observed in EMs (Cmax: 38+/-16 nmol/l; AUC: 173+/-90 nmol x h/l). In PMs only traces of morphine could be detected in plasma (Cmax: 2+/-1 nmol/l; AUC: 10+/-7 nmol x h/l). The percentage of the codeine dose converted to morphine and its metabolites was 3.9% in EMs and 0.17% in PMs. The interindividual variability in analgesia of codeine which is related to genetically determined differences in the formation of morphine clearly indicate that this metabolite is responsible for the analgesic effect of codeine. In contrast to the analgesic effect, frequency and intensity of the adverse events did not present significant differences between the two phenotypes. These findings have implications for the clinical use of codeine. Since side effects occurred in both EM and PM subjects, the use of codeine as an analgesic will expose 7% to 10% of patients who are PMs to the side effects of the drug without providing any beneficial analgesic effects.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Assessment of the predictive power of genotypes for the in-vivo catalytic function of CYP2D6 in a German population

The polymorphic cytochrome P450 CYP2D6 catalyses the biotransformation of at least 40 drugs. The CYP2D6 genetic polymorphism is responsible for pronounced interindividual differences in plasma concentrations and, hence, in drug action and side-effects after administration of the same dose. Provided there is a close relationship between CYP2D6 genotypes and catalytic function, genotyping could be used in the clinical setting for individualization of drug dose. In the present study, we evaluated the relationship between the in-vivo enzyme activity and 35 different genotypes in order to determine whether genotyping can be used to predict a person's metabolic capacity for CYP2D6-catalysed drug oxidation using sparteine as a probe drug. One hundred and ninety-five Caucasian individuals were genotyped for seven nonfunctional (CYP2D6 x 3, x 4, x 5, x 6, x 7, x 8, x 16) and eight functional alleles (CYP2D6 x 1, x 2, x 2 x 2, x 2B, x 2B x 2, x 9, x 10, x 17). The metabolic ratio distribution for sparteine showed trimodality, with 15 poor metabolizers, 21 intermediate metabolizers, and 1.59 extensive and ultrarapid metabolizers. All poor metabolizers were unambiguously identified as carriers of two nonfunctional alleles. In contrast, the most frequent functional genotypes extensively overlapped and, with few exceptions, genotype was not a useful predictor of function. Gene dose effects among homozygotes and heterozygotes of the major functional alleles were not significant and could not explain the wide variations. Only a minor fraction of phenotypical ultrarapid metabolizers, arbitrarily defined as individuals with a metabolic ratio < 0.2, could be identified as carriers of three functional gene copies, including duplicated CYP2D6 x 2 x 2 alleles. Similarly, only a minor fraction of the intermediate metabolizers had predictive genotypes involving alleles coding for enzyme with impaired function. Thus, genotyping correctly identifies poor metabolizers, but quantitative prediction of drug metabolism capacity among extensive metabolizers is not possible.

Loss of analgesic effect of morphine due to coadministration of rifampin

Methadone withdrawal symptoms have been reported in drug addicts treated with the tuberculostatic rifampin. Whereas this interaction can be explained by induction of phase I drug metabolism (CYP3A4), knowledge about induction of phase II metabolism (e.g., UDP-glucuronosyltransferases = UGTs) and its influence on drug effects in man, however, is very limited. The potent analgesic morphine is metabolized by more than one UGT to the active metabolite morphine-6-glucuronide and to morphine-3-glucuronide, which is devoid of analgesic activity. Thus, differential induction of UGTs involved in metabolism of morphine might lead to decreased or increased analgesic effects, depending on which UGT is preferentially induced. We therefore investigated the influence of the potent enzyme inducer rifampin on analgesic effects and pharmacokinetics of morphine, which is primarily eliminated by phase II metabolism. Ten healthy male volunteers participated in this double-blind, placebo-controlled study with double crossover design. Morphine (10 mg p.o.) and placebo were administered on two separate occasions before and near the end of 13 days of treatment with rifampin (600 mg/day). Blood samples were collected for 31 h. Morphine effects on pain sensation were determined using the cold pressor test. When morphine was given alone, the opioid elicited a significant increase in pain threshold and pain tolerance in comparison to placebo (P < or = 0.05). However, following administration of rifampin no analgesic effect of morphine was observed. In agreement, the area under the serum concentration-time curve (AUC) of morphine and the maximum serum concentration of morphine were considerably reduced during coadministration of rifampin (-27.7 +/- 19.3% and -40.7 +/- 27.1%; P < or = 0.01). Moreover, during treatment with rifampin a proportional reduction of AUCs of morphine-3-glucuronide (P < or = 0.01), morphine-6-glucuronide (P < or = 0.05) and morphine was observed. Since urinary recoveries of both morphine-3-glucuronide and morphine-6-glucuronide were also reduced during administration of rifampin, there is no evidence for a contribution of UGT induction to the observed interaction. In summary, a major drug interaction was observed between morphine and rifampin, which could not be attributed to induction of UGTs, but resulted in a complete loss of analgesic effects of the opioid.

Pharmacokinetics and pharmacodynamics of the enantiomers of gallopamil

The pharmacokinetics and pharmacodynamics of the enantiomers of the calcium antagonist gallopamil have been investigated in six healthy volunteers. Each subject was studied on five occasions after receiving, in randomized order: placebo, 25 mg of (R)-gallopamil, 25 mg of (S)-gallopamil, 50 mg of pseudoracemic [25 mg of deuterated (S)-gallopamil and 25 mg of (R)-gallopamil] and 100 mg of (R)-gallopamil HCl orally. After separate administration, the apparent oral clearances of both enantiomers were similar [(R), 15.1 +/- 9.9 liters/min; (S), 11.0 +/- 6.0 liters/min], indicating that gallopamil first-pass metabolism is not stereoselective. After coadministration, the apparent oral clearance of each enantiomers decreased [(R), 5.9 +/- 2.8 liters/min; (S), 5.8 +/- 2.66 liters/min], suggesting that a partial saturation of first-pass metabolism occurs because the dose was twice as high than for the single enantiomers. Serum protein binding and renal elimination of gallopamil are stereoselective, favoring (S)-gallopamil. Analysis of urine samples revealed a marked degree of stereoselectivity in the formation of O- and N-dealkyl metabolites. Because these showed opposite stereoselectivity, canceling out each other, the net result was no or only marginal stereoselectivity. Twenty-five milligrams of (S)-gallopamil prolonged the PR interval in all subjects; however, a greater effect was elicited by 50 mg of (RS)-gallopamil. (R)-Gallopamil (100 mg) did not significantly alter the PR interval, although higher concentrations were attained than after the pseudoracemate. Based on a consideration of (S)-gallopamil serum concentrations, a comparable relationship between (S)-gallopamil level and effect occurred after (S)- and (RS)-gallopamil, indicating that the pharmacological effect produced by the racemate could be totally accounted for by the higher concentrations of (S)-gallopamil attained.

Effect of codeine on gastrointestinal motility in relation to CYP2D6 phenotype

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The influence of CYP2D6 polymorphism and quinidine on the disposition and antitussive effect of dextromethorphan in humans

OBJECTIVES

We studied the disposition of dextromethorphan in extensive and poor metabolizer subjects, as well as the effect of this polymorphism on the antitussive action of dextromethorphan.

METHODS

Six extensive metabolizers were studied on four occasions: (1) after 30 mg dextromethorphan, (2) after 30 mg dextromethorphan 1 hour before 50 mg quinidine, (3) after placebo, and (4) after 50 mg quinidine. Six poor metabolizers were studied on two occasions: (1) after 30 mg dextromethorphan and (2) after placebo. Blood and urine were collected over 168 hours and assayed for dextromethorphan, total (conjugated and unconjugated) dextrorphan, 3-methoxymorphinan, and total 3-hydroxymorphinan. On each occasion at each blood sampling time, capsaicin was administered as an aerosol to provoke cough.

RESULTS

Dextromethorphan area under the plasma concentration-time curve (AUC) was 150-fold greater in the poor metabolizers than in the extensive metabolizers, and quinidine increased the AUC in extensive metabolizers 43-fold. The median dextromethorphan half-life was 19.1 hours in poor metabolizers, 5.6 hours in extensive metabolizers given quinidine, and 2.4 hours in extensive metabolizers. For dextrorphan (as total), the AUC was reduced 8.6-fold in poor metabolizers; quinidine had no effect on the AUC. The median half-life was 10.1 hours in poor metabolizers, 6.6 hours in extensive metabolizers given quinidine, and 1.4 hours in extensive metabolizers. The apparent partial clearance of dextromethorphan to dextrorphan was 1.2 L/hr in poor metabolizers, 78.5 L/hr in extensive metabolizers given quinidine, and 970 L/hr in extensive metabolizers. There was a strong (r2 = 0.82) and significant (p < 0.01) positive correlation between the prestudy urinary metabolic ratios and the partial clearances of dextromethorphan to dextrorphan. There was very large intersubject variability in responsiveness to capsaicin. There was no difference in the capsaicin-induced cough frequency in the three groups. Dextromethorphan had no antitussive effect in this experimental cough model.

CONCLUSION

The disposition of dextromethorphan was substantially influenced by CYP2D6 status. Capsaicin may not be an ideal agent in experimental cough studies.

Impact of quinidine on plasma and cerebrospinal fluid concentrations of codeine and morphine after codeine intake

OBJECTIVE

The analgesic effect of codeine depends on its O-demethylation to morphine via sparteine oxygenase (CYP2D6) in the liver and presumably also via this enzyme in the CNS. We studied the ability of quinidine, which is a potent inhibitor of CYP2D6, to penetrate the blood brain barrier and its possible impact on codeine O-demethylation in CNS.

METHODS

The study comprised 16 extensive and one poor metaboliser of sparteine, who underwent spinal anaesthesia for urinary tract surgery or examination. Eight patients were given an oral dose of 125 mg codeine and 9 patients (including the poor metaboliser) were given 200 mg quinidine 2 h before the same dose of codeine. Plasma and spinal fluid samples were collected 2 h after codeine intake.

RESULTS

Free concentrations of quinidine were 11-times lower in cerebrospinal fluid than in plasma, and ranged from 9-15 nmol.l-1. Morphine concentrations were significantly lower in patients pre-treated with quinidine, both in plasma (median 1.45 nmol.l-1, range 0.74-1.95 nmol.l-1 vs 9.86 nmol.l-1, range 4.59-28.4 nmol.l-1) and in cerebrospinal fluid (0.23, 0.16-0.61 nmol.l-1 vs 3.63, 0.6-8.09 nmol.l-1). The morphine/codeine concentration ratio in plasma (3.07 x 10 (-3), 1.68-3.68 x 10 (-3) vs 19.87 x 10 (-3), 9.87-66.22 x 10 (-3) and in cerebrospinal fluid (0.83 d 10 (-3), 0.58-1.45 x 10 (-3) vs 7.19 x 10 (-3), 2.03-17.7 x 10 (-3) was also lower. The morphine/codeine concentration ratios were significantly lower in cerebrospinal fluid both without and with quinidine, but the difference between the plasma and spinal fluid ratio was significantly smaller with quinidine than without (p = 0.0002).

CONCLUSION

Quinidine penetrates the blood brain barrier poorly, but quinidine pre-treatment leads to pronounced lowering of the cerebrospinal fluid concentration of morphine after codeine intake. However, the O-demethylation of codeine in CNS may not be totally blocked by quinidine.

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.

Simultaneous determination of dihydrocodeine and dihydromorphine in serum by gas chromatography-tandem mass spectrometry

A sensitive and specific method was developed for the determination of dihydrocodeine and its metabolite dihydromorphine in human serum using codeine and morphine as internal standards. Measurement is performed with GC-tandem MS after one simple extraction step and derivatization to the pentafluoropropionic esters. Sensitivity of the method is excellent and allows for the reproducible quantification of dihydrocodeine and dihydromorphine with limits of quantification of 2 ng/ml and 40 pg/ml serum, respectively. The method is therefore well suited for investigation of the pharmacokinetics and the metabolism of dihydrocodeine.

Stereoselectivity in cardiovascular and biochemical action of calcium antagonists: studies with the enantiomers of the dihydropyridine nitrendipine

OBJECTIVES

The cardiovascular and biochemical effects of R- and S-nitrendipine were studied in six healthy subjects in a single-blind placebo-controlled study.

METHODS

After received oral doses of placebo, 20 mg R-, 80 mg R- (n = 5), 20 mg S-, and 20 mg racemic nitrendipine, heart rate, systolic, diastolic, and mean arterial blood pressure, leg blood flow, peripheral vascular resistance, plasma renin activity, norepinephrine, epinephrine, dopamine, and aldosterone plasma levels were measured before and up to 3 hours after administration.

RESULTS

Neither placebo nor 20 or 80 mg R-nitrendipine caused significant changes of cardiovascular and biochemical parameters. After 20 mg S-nitrendipine and 20 mg racemic nitrendipine, significant changes in diastolic blood pressure (-9.1/-7.4 mm Hg), heart rate (+21.9/+17.3 beats/min), leg blood flow (+6.8 ml.min-1.gm tissue-1), peripheral vascular resistance (-16.9 mm Hg.min.gm tissue.ml-1), norepinephrine (+476/+281 ng.L-1), and plasma renin activity (+9.5/+3.6 ng.ml-1.hr-1) were observed. The changes in cardiovascular and biochemical parameters were closely related to the serum S-nitrendipine concentrations.

CONCLUSIONS

It can be concluded that, after administration of the racemate, the S-enantiomer is responsible for the cardiovascular and biochemical effects observed and that S-nitrendipine is at least an order of magnitude more potent than the R-enantiomer.