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

Permeability of blood-brain barrier in macaque model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinson disease

Brain bioavailability of drugs developed to address central nervous system diseases is classically documented through cerebrospinal fluid collected in normal animals, i.e., through an approximation as there are fundamental differences between cerebrospinal fluid and tissue contents. The fact that disease might affect brain availability of drugs is almost never considered at this stage although several conditions are associated with blood-brain barrier damage. Building upon our expertise in Parkinson's disease translational research, the present study addressed this gap comparing plasma and cerebrospinal fluid bioavailability of l-3,4-dihydroxyphenylalanine, carbamazepine, quinidine, lovastatin, and simvastatin, in healthy and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated macaques, the gold standard model of Parkinson's disease. The drugs were selected based upon their differential transport across the blood-brain barrier. Interestingly, brain bioavailability of quinidine was decreased while others were unaffected. Pharmacokinetics and pharmacodynamics experiments of drugs addressing Parkinson's disease might thus be performed in healthy animals unless the drugs are known to interact with the organic cation transporter.

Clinically significant psychotropic drug-drug interactions in the primary care setting

In recent years, the growing numbers of patients seeking care for a wide range of psychiatric illnesses in the primary care setting has resulted in an increase in the number of psychotropic medications prescribed. Along with the increased utilization of psychotropic medications, considerable variability is noted in the prescribing patterns of primary care providers and psychiatrists. Because psychiatric patients also suffer from a number of additional medical comorbidities, the increased utilization of psychotropic medications presents an elevated risk of clinically significant drug interactions in these patients. While life-threatening drug interactions are rare, clinically significant drug interactions impacting drug response or appearance of serious adverse drug reactions have been documented and can impact long-term outcomes. Additionally, the impact of genetic variability on the psychotropic drug's pharmacodynamics and/or pharmacokinetics may further complicate drug therapy. Increased awareness of clinically relevant psychotropic drug interactions can aid clinicians to achieve optimal therapeutic outcomes in patients in the primary care setting.

Drug-drug interactions that reduce the formation of pharmacologically active metabolites: a poorly understood problem in clinical practice

Drug-drug interactions can lead to reduced efficacy of medical treatment. Therapeutic failure may for instance result from combined treatment with an inhibitor of the specific pathway that is responsible for the generation of pharmacologically active drug metabolites. This problem may be overlooked in clinical practice. Several examples of drugs will be discussed -clopidogrel, losartan, tamoxifen and codeine - to illustrate differences in the potential impact on drug treatment in clinical practice. We conclude that the combined use of cytochrome P450-blocking serotonin reuptake inhibitors and tamoxifen or codeine should be avoided, whereas the situation is much more complex regarding the use of proton pump inhibitors together with clopidogrel, and the evidence regarding cytochrome P450 inhibitor-dependent activation of losartan is inconclusive.

Impact of the CYP2D6 genotype on post-operative intravenous oxycodone analgesia

BACKGROUND

Oxycodone is a semi-synthetic opioid with a mu-receptor agonist-mediated effect in several pain conditions, including post-operative pain. Oxycodone is metabolized to its active metabolite oxymorphone by O-demethylation via the polymorphic CYP2D6. The aim of this study was to investigate whether CYP2D6 poor metabolizers (PMs) yield the same analgesia post-operatively from intravenous oxycodone as extensive metabolizers (EMs).

METHODS

Two hundred and seventy patients undergoing primarily thyroid surgery or hysterectomy were included and followed for 24 h post-operatively. The CYP2D6 genotype was blinded until study procedures had been completed for all patients. All patients received intravenous oxycodone as pain treatment for 24 h post-operatively and morphine 5 mg was used as escape medication. A responder was characterized as a patient without the need for escape medication and a positive evaluation in a questionnaire 24 h post-operatively.

RESULTS

Twenty-four patients were PM (8.9%) and 246 were EM (91.1%). One PM (4.17%, CI=0.1-21.1) was a non-responder and 42 EM (17.07%, CI=12.6-22.4) were non-responders. The non-responder rate did not differ between the two genotypes (P=0.14). There was no difference in the total consumption of oxycodone between the two genotypes (EM=14.7 mg, CI=13.0-16.4 and PM=13.0 mg, CI=8.9-17.0, P=0.42). The mean oxymorphone/oxycodone ratios were 0.0031 and 0.00081 in the EMs and PMs, respectively (P<0.0001).

CONCLUSION

This study showed for the first time in patients that the oxymorphone formation depends on CYP2D6, but we found no difference in the post-operative analgesic effect of intravenous oxycodone between the two CYP2D6 genotypes.

Effect of metabolic blockade on the psychoactive effects of dextromethorphan

OBJECTIVE

Variation in the activity of cytochrome P450 2D6 (CYP2D6) affects the pharmacokinetics and effectiveness of dextromethorphan (DM), because it controls the production of dextrorphan, an active metabolite, with higher affinity for the NMDA receptor than the parent compound. This study examined whether pharmacological inhibition of CYP2D6 activity with quinidine would mimic the genetic mutation and thus also alter the psychoactive effects of DM.

METHODS

In a single-blind, within-subjects study, eight healthy volunteers (all homozygous for the wild type allele for CYP2D6) received placebo and varying doses of DM, both with and without quinidine pre-treatment. Pharmacokinetic and pharmacodynamic measures were assessed at baseline and every hour post-drug for 6 h.

RESULTS

Compared to the no quinidine condition, quinidine pre-treatment decreased the area under the dose-response curve on subjective measures of positively reinforcing effects (e.g., euphoria, p < 0.04; drug liking, p < 0.05), and was significantly greater for measures of dysphoria (e.g., unpleasantness, p < 0.02). These changes corresponded to increased DM and decreased dextrorphan plasma concentrations.

CONCLUSIONS

Compared to DM alone, quinidine pre-treatment inhibited DM metabolism and changed its subjective effects, demonstrating that the psychoactive properties of DM are a function of drug metabolism. These results demonstrate the relationship between CYP2D6 activity, plasma drug levels, and psychoactive drug effects, and have implications for both the abuse liability and therapeutic utility of DM.

The hypoalgesic effect of oxycodone in human experimental pain models in relation to the CYP2D6 oxidation polymorphism

Oxycodone is O-demethylated by CYP2D6 to oxymorphone which is a potent micro-receptor agonist. The CYP2D6 oxidation polymorphism divides the Caucasian population in two phenotypes: approximately 8% with no enzyme activity, poor metabolizers (PM) and the remainder with preserved CYP2D6 activity, extensive metabolizers (EM). The objective of the study was to determine if the analgesic effect of oxycodone in human experimental pain depends on its metabolism to oxymorphone. The analgesic effect of oxycodone was evaluated in a randomized, placebo-controlled, double-blinded, crossover experiment including 33 (16 EM and 17 PM) healthy volunteers. Pain tests were performed before and 1, 2, 3 and 4 hr after medication and included pain detection and tolerance thresholds to single electrical sural nerve stimulation, pain summation threshold to repetitive electrical sural nerve stimulation and the cold pressor test with rating of discomfort and pain-time area under curve (AUC(0-2 min.)). For single sural nerve stimulation, there was a less pronounced increase in thresholds on oxycodone in pain detection (9% vs. 20%, P = 0.02, a difference of 11%, CI: 2%-20%) and pain tolerance thresholds (15% vs. 26%, P = 0.037, a difference of 10%, CI: 1%-20%) for PM compared with EM. In the cold pressor test, there was less reduction in pain AUC on oxycodone for PM compared with EM (14% vs. 26%, P = 0.012, a difference of 12%, CI: 3%-22%). The plasma oxymorphone/oxycodone ratio was significantly lower in PM compared with EM (P < 0.001). Oxycodone analgesia seems to depend both on oxycodone itself and its metabolite oxymorphone.

Co-prescription of cytochrome P450 2D6/3A4 inhibitor-substrate pairs in clinical practice. A retrospective analysis of data from Norwegian primary pharmacies

OBJECTIVE

Inhibition of cytochrome P (P450) (CYP) enzymes, in particular CYP3A4 and CYP2D6, is an important drug-interacting mechanism. The objective of our study was to assess how frequently CYP3A4 and CYP2D6 inhibitors are co-prescribed with substrates of the respective enzymes.

METHODS

Included inhibitors were clarithromycin, erythromycin, fluconazole, itraconazole, ketoconazole and nefazodone (CYP3A4 inhibitors) and bupropion, fluoxetine, paroxetine and terbinafine (CYP2D6 inhibitors). The inhibitors were combined with substrates shown to be pharmacokinetically sensitive towards inhibition (190 drug pairs in total). Lists of patients receiving inhibitors and substrates were drawn from prescription databases (approximately 43,500 patients) of three Norwegian primary pharmacies during a 6-month period (July 2002 to January 2003). The lists were matched on name and date of birth to identify patients using drug pairs. Concurrent use was made probable from dates of purchase and drug profiles.

RESULTS

Inhibitors were prescribed to 2,062 patients. Altogether, 369 events of substrate co-prescription were registered. The highest frequencies of co-prescribed substrates were found for paroxetine (101 events per 267 patients, 38%), fluoxetine (36 events per 110 patients, 33%) and clarithromycin (59 events per 242 patients, 24%). The drugs most often detected in combination with inhibitors were codeine (116 events) and metoprolol (38 events) for CYP2D6 and zopiclone (45 events) and simvastatin (26 events) for CYP3A4.

CONCLUSION

Several commonly used CYP2D6 and CYP3A4 inhibitors are frequently co-prescribed with substrates in Norwegian clinical practice. Alertness when inhibitors are prescribed would aid physicians and pharmacists to detect many drug combinations with potential interaction risk.

Antidepressant Drug-Drug Interaction Profile Update

Drug-drug interactions continue to be underappreciated and misunderstood by most clinicians. Although life-threatening drug interactions are rare, serious clinical consequences, including altered drug response, poor tolerability with reduced medication adherence, and increased costs for care tied to the increased complexity of therapy, are fairly commonplace. Drug interactions may be further complicated by genetic differences in metabolic capacity. Patients who routinely require long-term treatment for depression have an increased likelihood of experiencing a drug-drug interaction since they will take over-the-counter and prescription medications for intercurrent and/or co-morbid illness. Antidepressants can be the object of drug interactions when their metabolic pathways are affected by other substances, or they can precipitate interactions by inhibiting enzyme pathways. Clinicians can improve the short- and long-term outcomes of patients with a depressive disorder by considering the possibility of drug-drug interactions both before prescribing a specific antidepressant and while monitoring for response, adverse effects and patient compliance.

A review of the efficacy and safety of opioid analgesics post-craniotomy*

--Codeine phosphate is the most commonly used analgesic post-craniotomy. --It is argued, in this paper, that codeine phosphate is an unpredictable pro-drug that does not equate to a safe and effective method of providing analgesia post-craniotomy. --Lack of evidence supporting tramadol's usage and concerns over its interactions and side effects mean its use cannot be advocated. --The traditional justification for withholding morphine in post-craniotomy pain appears to be largely based on anecdotal evidence. --Raising awareness among critical care nurses of the pharmacological properties of the analgesics used is imperative, if post-craniotomy pain is to be adequately treated. --There is an explicit challenge to the neurosurgical community to re-evaluate their pain-management strategies in the post-craniotomy patient.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

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

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

Inhibition of P-glycoprotein-mediated drug transport: A unifying mechanism to explain the interaction between digoxin and quinidine [seecomments]

BACKGROUND

Although quinidine is known to elevate plasma digoxin concentrations, the mechanism underlying this interaction is not fully understood. Digoxin is not extensively metabolized, but it is known to be transported by the drug efflux pump P-glycoprotein, which is expressed in excretory tissues (kidney, liver, intestine) and at the blood-brain barrier. Accordingly, we tested the hypothesis that inhibition of P-glycoprotein-mediated digoxin transport by quinidine contributes to the digoxin-quinidine interaction.

METHODS AND RESULTS

First, we demonstrated active transcellular transport of both digoxin and quinidine in cultured cell lines that express P-glycoprotein in a polarized fashion. In addition, 5 micromol/L quinidine inhibited P-glycoprotein-mediated digoxin transport by 57%. Second, the effect of quinidine on digoxin disposition was studied in wild-type and in mdr1a(-/-) mice, in which the gene expressing the major digoxin-transporting P-glycoprotein has been disrupted. Because the in vitro data showed that quinidine itself is a P-glycoprotein substrate, quinidine doses were reduced in mdr1a(-/-) mice to produce plasma concentrations similar to those in wild-type control animals. Quinidine increased plasma digoxin concentrations by 73.0% (P=0.05) in wild-type animals, compared with 19.5% (P=NS) in mdr1a(-/-) mice. Moreover, quinidine increased digoxin brain concentrations by 73.2% (P=0.05) in wild-type animals; by contrast, quinidine did not increase digoxin brain concentrations in mdr1a(-/-) mice but rather decreased them (-30.7%, P<0.01).

CONCLUSIONS

Quinidine and digoxin are both substrates for P-glycoprotein, and quinidine is a potent inhibitor of digoxin transport in vitro. The in vivo data strongly support the hypothesis that inhibition of P-glycoprotein-mediated digoxin elimination plays an important role in the increase of plasma digoxin concentration occurring with quinidine coadministration in wild-type mice and thus support a similar mechanism in humans.

Effects of blocking CYP2D6 on the pharmacokinetics and pharmacodynamics of oxycodone

BACKGROUND

Oxycodone is metabolized in the liver by means of O-demethylation to form oxymorphone in a reaction catalyzed by the enzyme cytochrome P450 2D6 (CYP2D6). This enzyme is expressed as 2 phenotypes (extensive and poor metabolizers). Several drugs are metabolized by CYP2D6, and clinically relevant drug interactions may occur. The aim of this study was to evaluate the role of oxymorphone in mediating the opioid effects of oxycodone by means of blocking CYP2D6 with quinidine.

METHODS

Ten healthy extensive metabolizers were administered 20 mg controlled-release oxycodone after premedication with placebo or 200 mg quinidine in this randomized, double-blind crossover study. A dose of 100 mg quinidine was administered 6 hours later. Plasma opioid concentrations, subjective pharmacodynamic ratings, and psychomotor function were assessed for 24 hours after drug administration.

RESULTS

No oxymorphone was detected at any time after quinidine premedication in 8 of 10 subjects. Plasma oxycodone (difference not significant) and noroxycodone (P < .01) concentrations were greater after quinidine pretreatment. Prevention of the production of oxymorphone by quinidine did not affect the psychomotor or subjective drug effects of oxycodone. No difference in number of adverse effects was observed after the 2 pretreatments.

CONCLUSIONS

A significant reduction in plasma oxymorphone levels did not substantially alter the pharmacodynamic effects of oxycodone. Analgesia was not evaluated because pain was not present.

The visceral and somatic antinociceptive effects of dihydrocodeine and its metabolite, dihydromorphine. A cross-over study with extensive and quinidine-induced poor metabolizers

AIMS

Dihydrocodeine is metabolized to dihydromorphine via the isoenzyme cytochrome P450 2D6, whose activity is determined by genetic polymorphism. The importance of the dihydromorphine metabolites for analgesia in poor metabolizers is unclear. The aim of this study was to assess the importance of the dihydromorphine metabolites of dihydrocodeine in analgesia by investigating the effects of dihydrocodeine on somatic and visceral pain thresholds in extensive and quinidine-induced poor metabolizers.

METHODS

Eleven healthy subjects participated in a double-blind, randomized, placebo-controlled, four-way cross-over study comparing the effects of single doses of placebo and slow-release dihydrocodeine 60 mg with and without premedication with quinidine sulphate 50 mg on electrical, heat and rectal distension pain tolerance thresholds. Plasma concentrations and urinary excretion of dihydrocodeine and dihydromorphine were measured.

RESULTS

In quinidine-induced poor metabolizers the plasma concentrations of dihydromorphine were reduced between 3 and 4 fold from 1.5 h to 13.5 h after dosing (P < 0.005) and urinary excretion of dihydromorphine in the first 12 h was decreased from 0.91% to 0.28% of the dihydrocodeine dose (P < 0.001). Dihydrocodeine significantly raised the heat pain tolerance thresholds (at 3.3 h and 5 h postdosing, P < 0.05) and the rectal distension defaecatory urge (at 3.3 h and 10 h postdosing, P < 0.02) and pain tolerance thresholds (at 3.3 h and 5 h postdosing, P < 0.05) compared with placebo. Premedication with quinidine did not change the effects of dihydrocodeine on pain thresholds, but decreased the effect of dihydrocodeine on defaecatory urge thresholds (at 1.5 h, 3.3 h and 10 h postdosing, P < 0.05).

CONCLUSIONS

In quinidine-induced poor metabolizers significant reduction in dihydromorphine metabolite production did not result in diminished analgesic effects of a single dose of dihydrocodeine. The metabolism of dihydrocodeine to dihydromorphine may therefore not be of clinical importance for analgesia. This conclusion must however, be confirmed with repeated dosing in patients with pain.