Pharmacokinetics of dextromethorphan and metabolites in humans: influence of the CYP2D6 phenotype and quinidine inhibition

Neuroprotective profile of dextromethorphan in an experimental model of penetrating ballistic-like brain injury

Dextromethorphan (DM) has been well-characterized as a neuroprotective agent in experimental models of CNS injury. The goal of this study was to determine the neuroprotective profile of DM in a military-relevant model of penetrating ballistic-like brain injury (PBBI). In an acute (3 day) dose-response study, anesthetized male Sprague-Dawley rats were exposed to a unilateral frontal PBBI with DM (0.156-10 mg/kg) or vehicle delivered as an i.v. bolus from 30 min to 48 h post-injury. In a follow-up (7 day) experiment, the 10-mg/kg bolus injections of DM were administered in conjunction with a 6-h infusion (5 mg/kg/h). DM bolus injections alone produced a dose-dependent improvement in motor recovery on a balance beam task at 3 days post-injury. However, more rapid recovery (24 h) was observed on this task when the bolus injections were combined with the 6-h infusion. Moreover, the DM bolus/infusion treatment regimen resulted in a significant (76%) improvement in cognitive performance in a novel object recognition (NOR) task at 7 days post-injury. Although post-injury administration of DM (all doses) failed to reduce core lesion size, the maximum dose of DM (10 mg/kg) was effective in reducing silver-stained axonal fiber degeneration in the cortical regions adjacent to the injury.

Dextromethorphan abuse: clinical effects and management

OBJECTIVE

To describe the epidemiology, patient presentation, and clinical management associated with dextromethorphan (DM) abuse.

DATA SOURCES

PubMed/Medline search using terms dextromethorphan and abuse through July 2008, bibliographies of selected publications, epidemiology tracking databases.

STUDY SELECTION

By the authors.

DATA EXTRACTION

English language-published review articles, clinical trials, and case reports that described the epidemiologic and toxicologic profile of DM were included.

DATA SYNTHESIS

DM is a relatively inexpensive and easily accessible over-the-counter (OTC) medication intended for use as an antitussive. Increasingly, illicit use of the drug has been reported. At clinical doses, the drug produces few adverse effects. However, when abused in large quantities (>2 mg/kg), the drug has been associated with a dissociative effect similar to those described by ketamine and phencyclidine abusers. Massive ingestions of the drug may be associated with untoward effects, including tachycardia, hypertension, and respiratory depression. Overdose symptoms may also be associated with coformulated products such as antihistamines and sympathomimetic amines. Management is primarily supportive. Naloxone has been used to manage DM toxicity but with conflicting reports of effectiveness.

CONCLUSION

Recent reports indicate that DM is often abused by individuals seeking its dissociative effects. Clinicians should be aware of the abuse potential of DM. Pharmacists might be particularly cognizant of the risks involved with DM abuse as they control OTC access to the drug.

Dose-response relationship for the pharmacokinetic interaction of grapefruit juice with dextromethorphan investigated by human urinary metabolite profiles

Grapefruit juice (GFJ) has been shown to affect the pharmacokinetics of a large number of drugs, essentially by inhibition of efflux transporters and CYP3A4 monooxygenase in the small intestine. The GFJ dose usually used in human studies was one glass single-strength (1x). Information on a respective dose-response relationship is not available. We investigated the effect of GFJ of different concentration (0.25 x, 0.5x, 1x, 2x) dosed in biweekly intervals in 19 volunteers. Components considered responsible for drug interactions, naringin, naringenin, bergamottin, and 6',7'-dihydroxybergamottin were determined by LC-tandem mass spectrometry. Immediately after ingestion of GFJ, participants took an aqueous solution of dextromethorphan (DEX) as probe drug. Urine was collected in two sampling periods, 0-2 and 2-4h, and excreted amounts of DEX and five metabolites associated with CYP3A4 and/or CYP2D6 enzyme activity were determined. Effects of GFJ were analyzed by the Wilcoxon matched-pairs signed-rank test against an average of four water control experiments. Two effects were highly significant: (i) a delay of total metabolite excretion in the first 2h and (ii) an inhibition of the CYP3A4-dependent metabolic pathways. Effect magnitude and significance levels were dose-dependent and indicated 200 ml 1x GFJ as "lowest observed effect level" LOEL.

Pharmacokinetic effect of AMD070, an Oral CXCR4 antagonist, on CYP3A4 and CYP2D6 substrates midazolam and dextromethorphan in healthy volunteers

BACKGROUND

Many antiretroviral drugs used in HIV care involve complex drug metabolism by CYP3A4 and CYP2D6 enzymes, and drug interactions are problematic clinically. AMD070, a novel entry inhibitor, is an inhibitor of X4-tropic HIV virus. In vitro data suggested that it is a CYP3A4 substrate and may inhibit CYP2D6 and CYP3A4.

METHODS

Twelve healthy subjects were given a single oral dose of 5 mg of midazolam and 30 mg of dextromethorphan on day 1 and 9, and 200 mg of AMD070 twice daily on days 2 through 9 (inclusive). Pharmacokinetic parameters of midazolam and dextromethorphan were assessed alone and in the presence of AMD070.

RESULTS

The mean AUC0-24 and Cmax of dextromethorphan increased 2.86-fold (2.20 to 5.10, 90% confidence interval [CI]) and 2.52-fold (1.99 to 4.24, 90% CI), respectively, in the presence of AMD070. Plasma AUC0-12 of midazolam increased 1.33-fold (1.15 to 1.61, 90% CI) without change in Cmax. The half-life did not change for both drugs, but significant, parallel decrease in apparent oral clearance and volume of distribution was observed.

CONCLUSIONS

The data support an alteration in bioavailability due to an AMD070-mediated inhibition of presystemic metabolism, though an intestinal P-glycoprotein effect could also be contributing. Interactions between AMD070 with CYP3A4 and, especially, 2D6 substrates of clinical importance in HIV care should be further explored.

Brain levels of dextromethorphan and the intensity of opioid withdrawal in mice

Consistent with their antagonistic actions at N-methyl-D-aspartate type glutamate receptors, dextromethorphan (DXM) and its metabolite, dextrorphan (DXT) decrease the intensity of opioid withdrawal syndrome. Since quinidine (QND) affects CYP2D6-mediated metabolism and P-glycoprotein governed transport, we sought to determine whether co-treatment with QND would affect brain levels of DXM and DXT as well as the effect of these compounds on opioid withdrawal syndrome in mice. We found that DXM dose dependently inhibited the intensity of opioid withdrawal syndrome and that there was a tendency for a further decrease when QND was co-administered with DXM. Administration of 30 mg/kg of DXM resulted in higher brain levels of DXM and DXT than administration of 10 mg/kg of DXM, but much lower DXT levels than that produced by 30 mg/kg of DXT. Co-treatment with QND resulted in higher brain levels of DXM (but not DXT) suggesting that QND produces an increase in the brain availability of DXM. In summary, brain levels of DXM were inversely correlated with the intensity of opioid withdrawal syndrome. QND induced increased brain levels of DXM tend to attenuate the intensity of opioid withdrawal syndrome. We suggest that it is DXM, rather than DXT, that is responsible for the attenuating effect on the intensity of opioid withdrawal syndrome, and that the beneficial action of QND on the effect of DXM should be more pronounced in humans.

Review article: The prevalence and clinical relevance of cytochrome P450 polymorphisms

BACKGROUND

Most drugs currently used in clinical practice are effective in only 25% to 60% of patients, while adverse drug reactions (ADRs) as a consequence of treatment are estimated to cost billions of US dollars and tens of thousands of deaths.

AIM

To review the prevalence and clinical significance of cytochrome P450 polymorphisms.

RESULTS

The cytochrome P450 enzyme families 1-3 are responsible for 70 to 80% of all phase I dependent drug metabolisms. In 90% metabolic activity dependents on six enzymes: CYP1A2, CYP3A, CYP2C9, CYP2C19, CYP2D6 and CYP2E1. Polymorphisms in the CYP450 gene can influence metabolic activity of the subsequent enzymes. A poor metabolizer (PM) has no or very poor enzyme activity. A consequence of PM is drug toxicity if no other metabolic route is available, or when multiple drugs are metabolized by the same cytochrome. In that case dose reduction is an option to prevent toxic effects.

CONCLUSIONS

In the future genotyping should be considered to identify patients who might be at risk of severe toxic responses, in order to guide appropriate individual dosage. Medical therapy should be a close cooperation between clinicians, pharmacologists and laboratory specialists, leading to reduced therapeutic errors, ADRs and health care costs.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

The spinal anaesthetic effect of dextromethorphan, dextrorphan, and 3-methoxymorphinan

Dextromethorphan, an antitussive, has a complex pharmacologic profile and has not been well studied. Our aim was to evaluate whether dextromethorphan and its metabolites, dextrorphan and 3-methoxymorphinan, have a spinal anaesthetic effect. Using a method of spinal blockade in rats, we evaluated the potencies and durations of the effects of dextromethorphan and its metabolites on spinal blockades of motor function and nociception. Bupivacaine was the active control. We found that dextromethorphan and its metabolites produced a dose-related spinal blockade of motor function and nociception. On an ED(50) basis, the ranks of potencies were bupivacaine>dextrorphan>3-methoxymorphinan>dextromethorphan (p<0.05 for the differences). On an equipotent basis, dextrorphan and bupivacaine produced similarly longer nociceptive blockades than did dextromethorphan and 3-methoxymorphinan (p<0.05 for the differences). Co-administration of dextromethorphan or its metabolites with bupivacaine produced an additive effect. In conclusion, intrathecal injections of dextromethorphan or its metabolites, dextrorphan and 3-methoxymorphinan, produced dose-related spinal blockades of motor function and nociception. The suitability of these drugs as clinical spinal anaesthetics is worth further evaluation.

Dextromethorphan or dextrorphan have a local anesthetic effect on infiltrative cutaneous analgesia in rats

BACKGROUND

Dextromethorphan blocks sodium channels, the site of action of local anesthetics. In this study we evaluated whether dextromethorphan has a local anesthetic effect.

METHODS

We administered dextromethorphan and its active metabolite--dextrorphan, and lidocaine subcutaneously to rats and tested them for cutaneous anesthesia. Drug-drug interactions and systemic safety indices (LD50s/ED50s) were also evaluated.

RESULTS

Dextromethorphan and dextrorphan had a local anesthetic effect after cutaneous infiltration. The ranking of potencies was dextromethorphan > dextrorphan > lidocaine (P < 0.01 for each comparison). A combination of dextromethorphan or dextrorphan with lidocaine produced an additive effect. Dextromethorphan and dextrorphan had 2.4- and 1.9-fold higher system safety indices than did lidocaine.

CONCLUSION

Dextromethorphan and dextrorphan were more potent local anesthetics than lidocaine, but with higher systemic safety indices. Coadministration of dextromethorphan or dextrorphan with lidocaine produced an additive effect.

Development of the “Inje Cocktail” for High-throughput Evaluation of Five Human Cytochrome P450 Isoforms in vivo

To develop and validate an in vivo cocktail method for high-throughput phenotyping of CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A, 12 healthy subjects received five probe drugs alone or simultaneously. The in vivo phenotyping index of CYP2C9, the ratio of 8 h urine concentration of losartan to its metabolite after a single administration of losartan, was not significantly different from that obtained using the five-drug cocktail. Similarly, the ratios of [omeprazole]/[5-hydroxyomeprazole] (CYP2C19) and [paraxanthine]/[caffeine] (CYP1A2) in 4 h plasma samples and the log ratio of [dextromethorphan]/[dextrorphan] (CYP2D6) in 8 h urine samples and the 4 h plasma concentrations of midazolam (CYP3A) after single administration or well-established three-drug cocktail of caffeine, omeprazole, and dextromethorphan were not significantly different from those after the new five-drug cocktail. In conclusion, the new five-drug cocktail regimen, named the "Inje cocktail," can be used as a tool to phenotype in vivo enzyme activities of CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A with only 4 h blood sampling and 8 h urine collection following simultaneous administration of the five probe drugs.

Potential interactions of methylphenidate and atomoxetine with dextromethorphan

OBJECTIVE

To examine the potential for drug-drug interactions to influence drug metabolism between the attention-deficit/hyperactivity disorder (ADHD) dl-methylphenidate and atomoxetine with dextromethorphan, a probe for interactions involving cytochrome P450 (CYP) 2D6 isoenzyme.

DESIGN

In vitro and ex vivo analysis of changes in metabolism of study drugs.

SETTING

Laboratory.

PATIENTS

Not applicable.

INTERVENTIONS

Pooled human liver microsomal fractions prepared at CEDRA Corporation (now CellzDirect, Austin, Tex.) by the standard differential centrifugation method (lot 821-1). Human liver microsomes were pooled from 15 donors. Recombinant CYP 2D6-containing microsomes (Supersomes; lots 20 and 24 BD Gentest; Woburn, Mass.) were prepared from a baculovirus-infected insect cell line that expressed only the human CYP 2D6 isoform. Dextromethorphan, with and without effector, was incubated with pooled human liver and recombinant CYP 2D6-containing microsomes. Atomoxetine and dl-methylphenidate were tested at 0.1x, 1x, and 10x their reported therapeutic concentrations. Paroxetine, a known inhibitor of CYP 2D6, was used as a reference agent, and quinidine was used as a positive control inhibitor of CYP 2D6.

MAIN OUTCOME MEASURES

Changes in substrate metabolism indicative of CYP 2D6-mediated interactions.

RESULTS

Atomoxetine and paroxetine inhibited the formation of dextrorphan by about 50% in human liver microsomes and by more than 80% in recombinant microsomes; the profiles of atomoxetine and the known 2D6 inhibitor paroxetine were similar. High concentrations of dextromethorphan reversed the inhibition of its metabolism, indicating a competitive mechanism of the interaction. Conversely, dextromethorphan and dextrorphan only modestly inhibited atomoxetine and paroxetine metabolism. dl-Methylphenidate did not inhibit dextrorphan formation in either microsome preparation, and dl-methylphenidate metabolism was unaffected by dextromethorphan or dextrorphan.

CONCLUSION

These results demonstrate the potential for in vivo interactions between dextromethorphan and atomoxetine in patients with ADHD. However, they do not support the plausibility of an in vivo interaction between dextromethorphan and dl-methylphenidate.

Prediction of in vivo drug clearance from in vitro data. I: impact of inter-individual variability

The Simcyp Population-Based ADME Simulator was used to predict median drug clearances and their associated variance from in vitro data. Fifteen drugs satisfied the entry criteria for the study and the relevant information (in vitro metabolism data and in vivo human clearance values) were collated from the literature. Predicted values of median clearances fell within 2-fold of observed values for 73% of the drugs (oral route) and 78% of the drugs (intravenous route) when microsomal binding was disregarded, and for 93% (oral) and 100% (intravenous) when it was considered. Irrespective of whether microsomal binding was considered, the predicted fold variability fell within 2-fold of the observed variability for 80% (oral) and 67% (intravenous) of the drugs.

Dextromethorphan, 3-methoxymorphinan, and dextrorphan have local anaesthetic effect on sciatic nerve blockade in rats

Dextromethorphan has been used as an antitussive for more than 40 years and is considered a drug with a good margin of safety. The aim of the study was to evaluate whether dextromethorphan and its metabolites--3-methoxymorphinan and dextrorphan--had local anaesthetic effects. Using a method of sciatic nerve blockade in rats, the potencies and durations of actions of dextromethorphan and its metabolites on sciatic nerve blockades of motor function, proprioception, and nociception were evaluated. Lidocaine was used as control. We found that dextromethorphan and its metabolites produced dose-related local anaesthetic effects on sciatic nerve blockades of motor function, proprioception, and nociception. The ranks of potencies were lidocaine>dextromethorphan>3-methoxymorphinan>dextrorphan (P<0.01 for each comparison). Under an equi-potent basis, dextrorphan and 3-methoxymorphinan had durations of actions longer than that of lidocaine (P<0.05 for each comparison). Co-administration of dextromethorphan or its metabolites with lidocaine produced an additive effect on sciatic nerve blockades. In conclusion, dextromethorphan and its metabolites - 3-methoxymorphinan and dextrorphan- had a local anaesthetic effect on sciatic nerve blockades of motor function, proprioception and nociception with durations of actions longer than that of lidocaine. Co-administration of dextromethorphan and its metabolites produced an additive effect on sciatic nerve blockades.

Enhancing the uptake of dextromethorphan in the CNS of rats by concomitant administration of the P-gp inhibitor verapamil

Clinical trials evaluating high doses of dextromethorphan hydrobromide (DM) for the treatment of neurological disorders have resulted in numerous adverse events due to the presence of its active metabolite dextrorphan (DX). Since the uptake of drugs in the CNS can be modulated by P-glycoprotein (P-gp) inhibition at the blood-brain barrier (BBB), we propose to determine whether the P-gp inhibitor verapamil can enhance the uptake of DM in the CNS. Rats (n=42) received an oral dose of DM (20 mg/kg) alone or 15 min after an intravenous dose of verapamil (1 mg/kg). Rats were euthanized at different time points over 12 h, and concentrations of DM and DX (conjugated and unconjugated) were assessed in plasma, brain and spinal cord using a LC-ESI/MS/MS method. Pharmacokinetic parameters were calculated using noncompartmental methods. Verapamil treatments did not affect the biodisposition of DM in plasma. On the other hand, verapamil treatments increased the area under curve of DM in the brain (from 1221 to 2393 ng h/g) and spinal cord (from 1753 to 3221 ng h/g) by approximately 2-fold. The uptake of DX in brain and spinal cord were markedly lower than those of DM and increased by only 15% and 22% following verapamil treatments, respectively. These results suggest that the P-gp inhibitor verapamil can enhance the uptake of DM in the CNS without affecting that of DX. This change is most likely related to an inhibition of P-gp or other transporters located in the BBB since the biodisposition of DM in plasma remained unaffected by verapamil treatments.

Intravenous dextromethorphan to human volunteers: relationship between pharmacokinetics and anti-hyperalgesic effect

The aim of this study was to investigate the effect of dextromethorphan (DM) 0.5 mg/kg administered intravenously (i.v.) on hyperalgesia and pain after a tissue injury in human volunteers, and to describe the relationship between pharmacokinetic and pharmacodynamic data. The heat-capsaicin sensitisation model, a well-established experimental hyperalgesia model was induced in 24 healthy, male volunteers aged 21-35 years. The subjects received i.v. DM 0.5 mg/kg or isotonic saline on two separate study sessions. The primary outcome measure from 0 to 3 h was reduction in area of established secondary hyperalgesia. Secondary outcome measures were reduction in area of secondary hyperalgesia in response to brief thermal stimulation, heat pain detection thresholds and painfulness after tonic heat pain. Blood samples were collected throughout the study to describe the relationship between pharmacokinetic and pharmacodynamic data. Intravenous DM 0.5 mg/kg significantly reduced areas of established secondary hyperalgesia with an average of 39% (P<0.05). Development of secondary hyperalgesia was substantially prevented by DM (P<0.05). No significant effect was seen on either heat pain detection thresholds or after tonic heat pain. The pharmacokinetic-pharmacodynamic relationship showed a large inter-subject variation with a mean delay in effect of nearly 2 h in relation to peak serum concentration. The results strongly indicate that DM is an anti-hyperalgesic drug. The delay in effect may be explained by several mechanisms and suggests that timing of DM administration is an essential factor for using the drug in clinical settings.

Dextromethorphan-induced delirium and possible methadone interaction

INTRODUCTION

Dextromethorphan is a commonly used antitussive agent that can be purchased over the counter. It is metabolized primarily by the cytochrome P450 (CYP) 2D6 isozyme. Methadone has been found to inhibit CYP2D6, indicating a potential for interaction with dextromethorphan.

CASE SUMMARY

An 83-year-old woman was evaluated for delirium, hypersomnia, confusion, lethargy, impaired concentration, and poor food intake. Symptoms resolved soon after discontinuing dextromethorphan.

DISCUSSION

Vulnerability to delirium was potentially caused by coadministration of methadone, which can inhibit the CYP2D6 isozyme.

CONCLUSION

Evaluation of delirium should include close investigation of the patient's medications for potential interactions with dextromethorphan.

Pharmacokinetics of dextromethorphan after single or multiple dosing in combination with quinidine in extensive and poor metabolizers

Dextromethorphan (DM) pharmacological properties predict that the widely used cough suppressant could be used to treat several neuronal disorders, but it is rapidly metabolized after oral dosing. To find out whether quinidine (Q), a CYP2D6 inhibitor, could elevate and prolong DM plasma profiles, 2 multiple-dose studies identified the lowest oral dose of Q that could be used in a fixed combination with 3 doses of DM. A multiple-dose study in healthy subjects with an extensive or a poor enzyme metabolizer phenotype evaluated the safety and pharmacokinetic profile of a selected fixed-dose combination (AVP-923). Study 1 randomized 46 healthy subjects, who were extensive CYP2D6 metabolizers, to receive 0, 2.5, 10, 25, 50, or 75 mg Q twice daily in combination with 30 mg DM for 7 days. Plasma and urine samples were collected after the first and last doses for the assay of DM, dextrorphan (DX), and Q. Study 2 randomized 65 healthy extensive CYP2D6 metabolizers to 8 groups given twice-daily 45- or 60-mg DM doses combined with 0, 30, 45, or 60 mg Q for 7 days. The effects of increasing Q were not different with doses greater than 25 mg, whereas lower doses showed a dose-related increase in plasma DM concentrations. Urinary ratios of DM/DX showed a Q dose- and time-related increase in the number of subjects converted to the poor metabolizer phenotype that reached 100% on day 3 of dosing with 25 mg Q. Results from both studies indicated that 25 to 30 mg Q is adequate to maximally suppress O-demethylation of DM. Study 3 evaluated 7 extensive metabolizers and 2 poor metabolizers given an oral capsule every 12 hours containing 30 mg Q combined with 30 mg DM. DM plasma AUC values increased in both groups of subjects during the 8-day study. The mean urinary metabolic ratio (DM/DX) increased at least 27-fold in extensive metabolizers by day 8. There was no effect of Q on urinary metabolic ratios in poor metabolizers. Safety evaluations, including electrocardiograms, indicated that the combination was well tolerated, with no difference between extensive and poor metabolizer phenotypes.

Dextromethorphan abuse

Dextromethorphan is an over-the-counter dissociative agent of increasing popularity as a drug of abuse among younger adolescents. The drug produces a range of toxicities depending upon either the dose or the components of the specific formulation that was ingested. Most cases improve with supportive care alone, but severely intoxicated patients may require significant attention. Because dextromethorphan is misused primarily by younger adolescents, the early identification and treatment of dextromethorphan abuse may be important in preventing broader substance abuse in these children. Consequently, it is important for clinicians to recognize, treat, and appropriately refer those who present with intoxication from this drug.

How useful is the "cocktail approach" for evaluating human hepatic drug metabolizing capacity using cytochrome P450 phenotyping probes in vivo?

Relatively selective in vivo substrate probes have been developed for several major CYP isoforms involved in oxidative drug metabolism. There are basically two in vivo methods for identifying the phenotype. One method, the selective (CYP-specific) phenotyping method, involves administering one single probe drug, whereas the other is a mixed phenotyping or "cocktail" method involving the simultaneous administration of multiple probe drugs, specific for the individual P450. At present, caffeine and chlorzoxazone are used most often as probe drugs for CYP1A2 and CYP2E1, respectively, but these are not necessarily the best probe drugs. Of the potential probe drugs for CYP2C9, CYP2C19, CYP2D6 and CYP3A4, none is really useful. Despite current limitations, the cocktail method for obtaining information about multiple CYP activities in a single experimental session is likely to be more widely used as a screening or phenotyping method for humans in the future.

Mirtazapine in combination with amitriptyline: a drug-drug interaction study in healthy subjects

OBJECTIVE

To assess the steady-state pharmacokinetics of mirtazapine (30 mg/day orally) and amitriptyline (75 mg/day orally) during combined administration compared with that of either drug administered alone. To evaluate the tolerability and effects on psychometric tests of acute and subchronic administration of both drugs combined and alone.

METHODS

In a single-blind, three-way cross-over study, 24 (12 male and 12 female) healthy subjects were randomly assigned to six different sequences of three 9-day treatments, i.e. racemic mirtazapine (30 mg/day), amitriptyline (75 mg/day) or the combination of these drugs. To control for acute pharmacodynamic assessments, during the first treatment period, a placebo group (n = 8; 4 females and 4 males) was added. Serial blood samples were drawn for plasma level measurements that were subsequently subjected to pharmacokinetic analysis. Psychometric tests assessed attentional performance, and a computer-assisted telephone questionnaire assessed self-ratings of drowsiness/alertness and sleep quality.

RESULTS

Amitriptyline increased the C(max) of mirtazapine (+ 36%, p < 0.05) in male subjects only. Mirtazapine altered the C(max) of amitriptyline in both male (+ 23%, p < 0.05) and female (- 23%, p < 0.05) subjects. No changes were observed for other pharmacokinetic parameters. Metabolite parameters were not affected. Changes in parent compound levels mainly resulted from effects on absorption. The psychometric test results did not reveal significant changes between combined and single drug treatments. The telephone registrations of VAMRS and LSEQ did not show clinically relevant differences between the active treatments.

CONCLUSION

Combined administration of mirtazapine (30 mg/day) and amitriptyline (75 mg/day) alters the pharmacokinetics of either compound to a minor extent. Adding one drug to the other and substituting one drug by the other had no major effects on tolerability. Nevertheless, caution is warranted when combining amitriptyline and mirtazapine.

The molecular and enzyme kinetic basis for the diminished activity of the cytochrome P450 2D6.17 (CYP2D6.17) variant. Potential implications for CYP2D6 phenotyping studies and the clinical use of CYP2D6 substrate drugs in some African populations

In this study, the basis for the diminished capacity of CYP2D6.17 to metabolise CYP2D6 substrate drugs and the possible implications this might have for CYP2D6 phenotyping studies and clinical use of substrate drugs were investigated in vitro. Enzyme kinetic analyses were performed with recombinant CYP2D6.1, CYP2D6.2, CYP2D6.17 and CYP2D6.T107I using bufuralol, debrisoquine, metoprolol and dextromethorphan as substrates. In addition, the intrinsic clearance of 10 CYP2D6 substrate drugs by CYP2D6.1 and CYP2D6.17 was determined by monitoring substrate disappearance. CYP2D6.17 exhibited generally higher K(m) values compared to CYP2D6.1. The V(max) values were generally not different except for metoprolol alpha-hydroxylation with the V(max) value for CYP2D6.17 being half that of CYP2D6.1. CYP2D6.1 and CYP2D6.2 displayed similar kinetics with all probe drugs except for dextromethorphan O-demethylation with the intrinsic clearance value of CYP2D6.2 being half that of CYP2D6.1. CYP2D6.17 exhibited substrate-dependent reduced clearances for the 10 substrates studied. In a clinical setting, the clearance of some drugs could be affected more than others in individuals with the CYP2D6(*)17 variant. The CYP2D6(*)17 allele might, therefore, contribute towards the poor correlation of phenotyping results when using different probe drugs in African populations. To investigate effects of CYP2D6(*)17 mutations on the structure of the enzyme, a homology model of CYP2D6 was built using the CYP2C5 crystal structure as a template. The results suggest an alteration in position of active-site residues in CYP2D6.17 as a possible explanation for the reduced activity of the enzyme.

Effect of terbinafine on the pharmacokinetics and pharmacodynamics of desipramine in healthy volunteers identified as cytochrome P450 2D6 (CYP2D6) extensive metabolizers

Terbinafine-CYP2D6 inhibition was evaluated by assessing 48-hour concentration-time profiles of the tricyclic antidepressant desipramine in 12 healthy volunteers identified as extensive cytochrome P450 2D6 (CYP2D6) metabolizers by genotyping and phenotyping. Pharmacokinetics was evaluated at baseline (50 mg oral desipramine given alone), steady state (after 250 mg oral terbinafine for 21 days), and 2 and 4 weeks after terbinafine discontinuation. Pharmacodynamics was evaluated before and 2 hours after each desipramine administration, using Mini-Mental Status Examination (MMSE) and EGG. Terbinafine administration inhibited CYP2D6 metabolism, as indicated by the significant increase in desipramine C(max) (19 ng/ml vs. 36 ng/ml) and AUC0-infinity (482 ng.h/ml vs. 2383 ng.h/ml) and decrease in AUC0-24 and C(max) of the CYP2D6-mediated metabolite, 2-hydroxydesipramine. In addition, the C(max) and AGUC0-infinity of desipramine and metabolite were still elevated 4 weeks after terbinafine discontinuation. Caution should be exercised when coprescribing terbinafine and drugs metabolized by CYP2D6, particularly those with a narrow therapeutic index.

The effect of grapefruit juice and seville orange juice on the pharmacokinetics of dextromethorphan: the role of gut CYP3A and P-glycoprotein

The objective of this study was to determine the effects of grapefruit juice and seville orange juice on dextromethorphan (DM) pharmacokinetics. Eleven healthy volunteers were studied over a 3-week period consisting of 5 study days each separated by a three-day washout. All subjects refrained from drinking caffeine containing beverages (coffee, soda, etc.) 8 h before orally taking DM (30 mg) with 200 ml water, 200 ml grapefruit juice, 200 ml water, 200 ml seville orange juice, and 200 ml water on Study Days 1 to 5. Aliquots of urine samples were assayed and analysed for DM, and the DM metabolites dextrorphan, 3-methoxymorphinan and 3-hydroxymorphinan using a validated HPLC method employing a phenyl column and a fluorescence detection. Results suggests that DM could provide some useful information on P-glycoprotein or related membrane efflux protein activity in the human gastro-intestinal tract. Bioavailability (F) of DM increased significantly with grapefruit and seville orange juice, but only returned to half the baseline value after three days of washout. This confirms that grapefruit and seville orange juice are long-lasting and perhaps irreversible inhibitors of gut CYP3A/P-glycoprotein. Grapefruit and seville orange juice appeared to have the same overall effect on DM pharmacokinetics. In addition, this paper presents a novel method of phenotyping for CYP2D6, CYP3A and P-glycoprotein using DM as a probe.

The role of pharmacogenetically-variable cytochrome P450 enzymes in drug abuse and dependence

The risk of drug dependence is determined by the interaction of drug, individual and environment. 'Pharmacogenetics' is the study of the influence of heredity on the response to drugs and their fate in the body; these studies aim to improve the understanding of inter-individual variability in drug response. The authors have applied this research approach to the study of drug metabolism and dependence. Specifically the interaction of genetically variable hepatic cytochrome P450 (CYP) enzymes and their effect on self-administration of drugs has been examined. Many drugs of abuse are substrates (e.g., amphetamines, codeine, nicotine) or inhibitors (e.g., (-)-cocaine) of polymorphic CYPs. Drug metabolism by genetically polymorphic enzymes can have significant clinical implications relating to drug toxicity, therapeutic failure, drug-drug interactions, disease susceptibility and abuse liability. There is good evidence that drug metabolism by genetically variable CYPs can influence the risk of drug dependence, the amount of drug consumed by dependent individuals and some of the toxicities associated with drug-taking behavior. It is anticipated that pharmacogenetics will be used to identify individuals at a greater risk for specific drug dependencies, provide information that can lead to novel treatment and prevention approaches as well as provide guidance for individualization of treatment choice.

Summary of information on human CYP enzymes: human P450 metabolism data

This chapter is an update of the data on substrates, reactions, inducers, and inhibitors of human CYP enzymes published previously by Rendic and DiCarlo (1), now covering selection of the literature through 2001 in the reference section. The data are presented in a tabular form (Table 1) to provide a framework for predicting and interpreting the new P450 metabolic data. The data are formatted in an Excel format as most suitable for off-line searching and management of the Web-database. The data are presented as stated by the author(s) and in the case when several references are cited the data are presented according to the latest published information. The searchable database is available either as an Excel file (for information contact the author), or as a Web-searchable database (Human P450 Metabolism Database, www.gentest.com) enabling the readers easy and quick approach to the latest updates on human CYP metabolic reactions.

Mirtazapine and paroxetine: a drug-drug interaction study in healthy subjects

Paroxetine inhibits cytochrome P(450) 2D6, which is involved in the metabolism of mirtazapine. The possible drug-drug interaction between two pharmacologically distinct antidepressants, mirtazapine and paroxetine, has been investigated in a randomized, three-way crossover study in 24 healthy male and female subjects. After a titration phase of 3 days, each subject received single daily doses of 30 mg mirtazapine, 40 mg paroxetine or the combination for 6 days. Assessments included serial blood sampling for pharmacokinetics at steady state, cognitive testing using the test battery of CDR Ltd, a visual analogue mood rating scale (Bond and Lader) and the Leeds Sleep Evaluation Questionnaire. Paroxetine inhibits the metabolism of mirtazapine, as shown by increases of approximately 17% and 25% of the 24 h AUC's of mirtazapine and its demethyl metabolite, respectively. Mirtazapine did not alter the pharmacokinetics of paroxetine. The combined administration of mirtazapine and paroxetine probably does not alter cognitive functioning or result in major changes on the visual analogue mood rating scale and Sleep Evaluation Questionnaire, compared with the administration of either drug alone. The incidence of adverse events was lower during combined administration of mirtazapine and paroxetine than during administration of either drug alone. Fatigue, dizziness, headache, nausea, anxiety and somnolence were the most common adverse events during combined administration. These data suggest that the combination of mirtazapine and paroxetine is unlikely to lead to clinically relevant drug-drug interactions and can be used without dose adjustment of either drug. The combination may even be better tolerated than either drug alone. Copyright 2001 John Wiley & Sons, Ltd.

Effect of venlafaxine versus fluoxetine on metabolism of dextromethorphan, a CYP2D6 probe

Two antidepressants, venlafaxine and fluoxetine, were evaluated in vivo for their effect on cytochrome P450 2D6 (CYP2D6) activity, measured by the ratio of dextromethorphan, a sensitive CYP2D6 marker, to its metabolite dextrorphan (i.e., DM:DT) excreted in urine after DM coadministration. Twenty-eight healthy extensive metabolizers of CYP2D6 received either venlafaxine (37.5 mg bid for 7 days, then 75 mg bid until Day 28) or fluoxetine (20 mg daily for 28 days); 26 completed the study. Plasma concentrations of both drugs and their active metabolites were determined. DM:DTs were evaluated at baseline (Day 0), on Days 7 and 28 of dosing, and 2 weeks after drug discontinuation (Day 42). Steady-state drug and metabolite levels were achieved in both groups by Day 28. Mean DM:DTs for venlafaxine and fluoxetine differed statistically significantly (p < 0.001) on Days 7, 28, and 42. Comparisons of DM:DT as a percentage of baseline values showed that DM:DT increased 1.2-fold for venlafaxine and 9.1-fold for fluoxetine on Day 7 (p < 0.001) and increased 2.1-fold for venlafaxine and 17.1-fold for fluoxetine on Day 28 (p < 0.001). Inhibition of CYP2D6 metabolism persisted for 2 weeks after discontinuation of fluoxetine, unlike the case with venlafaxine. These in vivo results confirm in vitro data demonstrating significantly weaker inhibition of CYP2D6 with venlafaxine than with fluoxetine. This suggests that clinically significant interactions involving CYP2D6 inhibition could occur between fluoxetine and drugs metabolized by CYP2D6 but may be less likely to occur with venlafaxine.

Mimicking gene defects to treat drug dependence

The genetic basis for drug dependence has focused on genes that encode receptors involved in the reinforcing properties of drugs of abuse or that determine drug-taking behavior (e.g. impulsivity, etc.). Pharmacogenetic variations in the patterns of metabolism among individuals can also importantly modulate the risk of drug dependence. Cytochrome P450 drug metabolizing enzymes (CYPs), can activate (e.g. codeine to morphine) or deactivate (e.g. nicotine to cotinine) drugs of abuse. Some CYPs are polymorphic, that is, there are gene mutations which result in individuals with no (null mutations) or decreased enzyme activity (e.g. CYP2D6*10). Individuals with two null mutations appear in the population as phenotypic poor metabolizers. Using in vitro studies, we have identified drugs of abuse that are substrates of the polymorphic enzymes CYP2D6 (codeine, amphetamines, dextromethorphan), CYP2A6 (nicotine) and CYP2C19 (flunitrazepam). In human experimental studies, we have shown that CYP phenotype and genotype affect abuse liability of CYP2D6 metabolized drugs of abuse. In addition, we inhibited CYP2D6 and decreased individuals' risk of dependence experimentally (codeine, dextromethorphan) and treated codeine dependence. In epidemiologic studies CYP2D6 and CYP2A6 null mutations protect individuals from becoming codeine and tobacco dependent, respectively. With respect to CYP2A6, individuals with mutations, smoke fewer cigarettes and can quit more easily. Inhibiting CYP2A6 (e.g. tranylcypromine, methoxsalen) decreases smoking and the activation of procarcinogens. By mimicking these gene defects the risk of dependence can be decreased in individuals and new treatments developed.

Phenotyping of drug-metabolizing enzymes in adults: a review of in-vivo cytochrome P450 phenotyping probes

Cytochrome P450 phenotyping provides valuable information about real-time activity of these important drug-metabolizing enzymes through the use of specific probe drugs. Despite more than 20 years of research, few conclusions regarding optimal phenotyping methods have been reached. Caffeine offers many advantages for CYP1A2 phenotyping, but the widely used caffeine urinary metabolic ratios may not be the optimal method of measuring CYP1A2 activity. Several probes of CYP2C9 activity have been suggested, but little information exists regarding their use, largely due to the narrow therapeutic index of most CYP2C9 probes. Mephenytoin has long been considered the standard CYP2C19 phenotyping probe, but problems such as sample stability and adverse effects have prompted the investigation of potential alternatives, such as omeprazole. Several well-validated CYP2D6 probes are available, including dextromethorphan, debrisoquin and sparteine, but, in most cases, dextromethorphan may be preferred due to its wide safety margin and availability. Chlorzoxazone remains the only CYP2E1 probe that has received much study. However, questions concerning phenotyping method and involvement of other enzymes have impaired its acceptance as a suitable CYP2E1 phenotyping probe. CYP3A phenotyping has been the subject of numerous investigations, reviews and commentaries. Nevertheless, much controversy regarding the selection of an ideal CYP3A probe remains. Of all the proposed methods, midazolam plasma clearance and the erythromycin breath test have been the most rigorously studied and appear to be the most reliable of the available methods. Despite the limitations of many currently available probes, with continued research, phenotyping will become an even more valuable research and clinical resource.

Cytochrome P450 2D6 and treatment of codeine dependence

Oral opioid analgesics such as codeine are used extensively worldwide and are frequently misused. Codeine is a substrate of CYP2D6, a genetically polymorphic P450 enzyme, and is metabolized to the more potent drug morphine. CYP2D6 activity can be inhibited by fluoxetine, and the inhibition of morphine formation may help individuals reduce their use of codeine. Fourteen long-term users of oral opiates (principally codeine) were assessed for an open-label pilot treatment study of fluoxetine 20 mg/day combined with a brief behavioral intervention and structured tapering of the opiate. Eight subjects entered and completed the 8-week treatment. Opiate use decreased by 30% to 100% of baseline use (p < 0.0001) in parallel with a decrease in CYP2D6 activity. Fluoxetine may have a role in the treatment of opiate dependence by decreasing opiate-reinforcing properties.

Simultaneous determination of dextromethorphan and its metabolites in human plasma by capillary electrophoresis

A sensitive capillary electrophoretic method was developed and validated for the simultaneous determination of dextromethorphan and its metabolites, dextrorphan, 3-hydroxymorphinan, and 3-methoxymorphinan, in human plasma. After cleavage of conjugates by enzymatic hydrolysis with beta-glucuronidase, dextromethorphan and its metabolites were extracted from 1.5 ml of plasma by a liquid liquid extraction procedure using heptane-ethylacetate (50:50, v/v) and re-extracted to aqueous phase. The compounds were separated within 8 min on a fused silica capillary, 75 microm internal diameter using sodium borate (pH 9.4; 50 mM) as running buffer, and measured by UV-detection at 195 nm using a detection cell with a path length of 1.2 mm. The method was accurate and precise. Linear relationships were observed between the peak response and the concentration in the range of 1-400 ng ml(-1) plasma with correlation coefficients above 0.998. The limit of detection was 0.5-1 ng ml(-1) plasma for all compounds. The method was used for determination of plasma levels of dextromethorphan and its metabolites after transdermal and oral administration of dextromethorphan.

Multiple human cytochromes contribute to biotransformation of dextromethorphan in-vitro: role of CYP2C9, CYP2C19, CYP2D6, and CYP3A

Cytochromes mediating the biotransformation of dextromethorphan to dextrorphan and 3-methoxymorphinan, its principal metabolites in man, have been studied by use of liver microsomes and microsomes containing individual cytochromes expressed by cDNA-transfected human lymphoblastoid cells. In-vitro formation of dextrorphan from dextromethorphan by liver microsomes was mediated principally by a high-affinity enzyme (Km (substrate concentration producing maximum reaction velocity) 3-13 microM). Formation of dextrorphan from 25 microM dextromethorphan was strongly inhibited by quinidine (IC50 (concentration resulting in 50% inhibition) = 0.37 microM); inhibition by sulphaphenazole was approximately 18% and omeprazole and ketoconazole had minimal effect. Dextrorphan was formed from dextromethorphan by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, and -2D6 but not by those expressing CYP1A2, -2E1 or -3A4. Despite the low in-vivo abundance of CYP2D6, this cytochrome was identified as the dominant enzyme mediating dextrorphan formation at substrate concentrations below 10 microM. Formation of 3-methoxy-morphinan from dextromethorphan in liver microsomes proceeded with a mean Km of 259 microM. For formation of 3-methoxymorphinan from 25 microM dextromethorphan the IC50 for ketoconazole was 1.15 microM; sulphaphenazole, omeprazole and quinidine had little effect. 3-Methoxymorphinan was formed by microsomes from cDNA-transfected lymphoblastoid cells expressing CYP2C9, -2C19, -2D6, and -3A4, but not by those expressing CYP1A2 or -2E1. CYP2C19 had the highest affinity (Km = 49 microM) whereas CYP3A4 had the lowest (Km = 1155 microM). Relative abundances of the four cytochromes were determined in liver microsomes by use of the relative activity factor approach. After adjustment for relative abundance, CYP3A4 was identified as the dominant enzyme mediating 3-methoxymorphinan formation from dextromethorphan, although CYP2C9 and -2C19 were estimated to contribute to 3-methoxymorphinan formation, particularly at low substrate concentrations. Although formation of dextrorphan from dextromethorphan appears to be sufficiently specific to be used as an in-vitro or in-vivo index reaction for profiling of CYP2D6 activity, the findings raise questions about the specificity of 3-methoxymorphinan formation as an index of CYP3A activity.

Psychotropic effects of dextromethorphan are altered by the CYP2D6 polymorphism: a pilot study

Dextromethorphan is a nonopioid antitussive metabolized by cytochrome P450 2D6 (CYP2D6) to an active metabolite, dextrorphan. CYP2D6 is polymorphically expressed in humans, with 5 to 10% of Caucasians being homozygous deficient for the active form of the enzyme. In a pilot study, the authors investigated the pharmacologic effects of dextromethorphan in individuals phenotyped and genotyped as extensive metabolizers (EMs, N = 4) and poor metabolizers (PMs, N = 2) of CYP2D6 substrates. Dextromethorphan doses ranged from 0 to 6 mg/kg based on individual subject tolerance. All EMs tolerated 3 to 6 mg/kg dextromethorphan, whereas PMs barely tolerated 3 mg/kg dextromethorphan and therefore received lower doses. As shown in previous studies, plasma kinetics show profound differences in dextromethorphan metabolism between EMs and PMs. Dextromethorphan produced qualitatively and quantitatively different objective and subjective effects in the two groups. Objectively, PMs had greater psychomotor impairment, as measured by a joystick tracking task, compared with EMs on 3 mg/kg dextromethorphan (mean performance +/- SE, 95+/-0.5% for EMs vs. 86+/-6% for PMs; p < 0.05). At this dose, EMs also reported greater abuse potential compared with PMs (p < 0.05), and PMs reported greater sedation and dysphoria compared with EMs (p < 0.01). These data provide preliminary evidence that dextrorphan contributes to dextromethorphan abuse liability, and therefore PMs may be less likely to abuse dextromethorphan.