Omission of the deconjugation step in urine analysis and the unaltered outcome of CYP2D6 phenotyping with dextromethorphan

Fluoxetine- and norfluoxetine-mediated complex drug-drug interactions: in vitro to in vivo correlation of effects on CYP2D6, CYP2C19, and CYP3A4

Fluoxetine and its circulating metabolite norfluoxetine comprise a complex multiple-inhibitor system that causes reversible or time-dependent inhibition of the cytochrome P450 (CYP) family members CYP2D6, CYP3A4, and CYP2C19 in vitro. Although significant inhibition of all three enzymes in vivo was predicted, the areas under the concentration-time curve (AUCs) for midazolam and lovastatin were unaffected by 2-week dosing of fluoxetine, whereas the AUCs of dextromethorphan and omeprazole were increased by 27- and 7.1-fold, respectively. This observed discrepancy between in vitro risk assessment and in vivo drug-drug interaction (DDI) profile was rationalized by time-varying dynamic pharmacokinetic models that incorporated circulating concentrations of fluoxetine and norfluoxetine enantiomers, mutual inhibitor-inhibitor interactions, and CYP3A4 induction. The dynamic models predicted all DDIs with less than twofold error. This study demonstrates that complex DDIs that involve multiple mechanisms, pathways, and inhibitors with their metabolites can be predicted and rationalized via characterization of all the inhibitory species in vitro.

Effect of cinacalcet hydrochloride, a new calcimimetic agent, on the pharmacokinetics of dextromethorphan: in vitro and clinical studies

Cinacalcet hydrochloride (cinacalcet) is a positive allosteric modulator of the calcium-sensing receptor indicated for the treatment of secondary hyperparathyroidism in dialysis patients. In vitro study has demonstrated that cinacalcet is a potent inhibitor of cytochrome P450 (CYP) 2D6 with a K(i) value of 0.087 micromol/L, which is comparable to the well-known potent CYP2D6 inhibitor, quinidine (0.064 micromol/L). A clinical study was conducted to assess the inhibitory effect of cinacalcet on CYP2D6 substrates in healthy volunteers. Each subject received 50 mg of cinacalcet or a matched placebo orally once daily for 8 days with 30 mg of dextromethorphan coadministered on day 8. The mean AUC(0-infinity) and C(max) of dextromethorphan increased 11- and 7-fold, respectively, in extensive metabolizers when coadministered with cinacalcet versus placebo. Therefore, during concomitant treatment with cinacalcet, it may be necessary to consider making dose adjustments for drugs with a narrow therapeutic index that are mainly metabolized by CYP2D6.

Ontogeny of dextromethorphan O- and N-demethylation in the first year of life

The exponential increase in the number of drugs used to treat infant and childhood illnesses necessitates an understanding of the ontogeny of drug biotransformation for the development of safe and effective therapies. Healthy infants received an oral dose (0.3 mg/kg) of dextromethorphan (DM) at 0.5, 1, 2, 4, 6, and 12 months of age. DM and its major metabolites were measured in urine. CYP2D6 genotype was determined by polymerase chain reaction-restriction fragment length polymorphism. Genotyping data indicated a strong correlation between CYP2D6 genotype and DM O-demethylation (beta=-0.638; 95% CI: -0.745, -0.532; P<0.001). CYP2D6 activity was detectable and concordant with genotype by 2 weeks of age, showed no relationship with gestational age, and did not change with post natal age up to 1 year. In contrast, DM N-demethylation developed significantly more slowly over the first year of life. Genotype and the temporal acquisition of drug biotransformation are critical determinants of a drug response in infants.

Evaluation of probe drugs and pharmacokinetic metrics for CYP2D6 phenotyping

INTRODUCTION

Cytochrome P450 2D6 (CYP2D6) is one of the most important enzymes catalyzing biotransformation of xenobiotics in the human liver. This enzyme's activity shows a high degree of interindividual variability caused in part by its genetic polymorphism, the so-called debrisoquine/sparteine polymorphism. The genetic component influencing CYP2D6 activity can be determined by genotyping. However, genotyping alone is not sufficient to accurately predict an individual's actual CYP2D6 activity, as this is also influenced by other factors. For the determination of the exact actual enzymatic activity ("phenotyping"), adequate probe drugs have to be administered prior to measurements of these compounds and/or their metabolites in body fluids. PROBE DRUGS: Debrisoquine, sparteine, metoprolol or dextromethorphan represent well-established probe drugs while tramadol has been recently investigated for this purpose. The enzymatic activity is reflected by various pharmacokinetic metrics such as the partial clearance of a parent compound to the respective CYP2D6-mediated metabolite or metabolic ratios. Appropriate metrics need to fulfill pre-defined validation criteria.

METHODS

In this review, we have compiled a list of such criteria useful to select the best metrics to reflect CYP2D6 activity. A comprehensive Medline search for reports on CYP2D6 phenotyping trials with the above mentioned probe drugs was carried out.

CONCLUSION

Application of the validation criteria suggests that dextromethorphan and debrisoquine are the best CYP2D6 phenotyping drugs, with debrisoquine having the problem of very limited availability as a therapeutic drug. However, the assessment of the best dextromethorphan CYP2D6 phenotyping metric/procedure is still ongoing.