In vivo characterization of CYP2D6*12, *29 and *84 using dextromethorphan as a probe drug: a case report.

CYP2D6*84 was first described in a Black South African subject, however, its function remains unknown. Astrolabe, a probabilistic scoring tool developed in our laboratory to call genotypes from whole genome sequence, identified CYP2D6*84 in a trio. The father presented with intermediate metabolism when challenged with the CYP2D6 probe drug dextromethorphan (DM/dextrorphan [DX] = 0.0839). Since his second allele, CYP2D6*12, is nonfunctional, the observed activity is derived by CYP2D6*84. This finding suggests that the allele's hallmark P267H causes decreased activity toward DM and that this allele should receive a value of 0.5 for Activity Score calculations. The mother's DM/DX of 0.0543 was consistent with the decreased activity classification of CYP2D6*29. The child, a critically ill neonate, was not phenotyped, but predicted to be a normal metabolizer.

Ranolazine-induced severe bladder hypotonia.

OBJECTIVE: To describe a case of acute urinary retention due to bladder hypotonia during ranolazine treatment. CASE SUMMARY: An 81-year-old male with multiple cardiovascular diseases was hospitalized for worsening angina and heart failure symptoms. Ranolazine 375 mg twice daily was started, in addition to ongoing therapy (clopidogrel 75 mg once daily, diltiazem 60 mg 3 times daily, isosorbide mononitrate 40 mg 3 times daily, carvedilol 6.25 mg twice daily, rosuvastatin 20 mg once daily, enoxaparin 5000 IU once daily, pentoxifylline 600 mg twice daily, pantoprazole 40 mg twice daily, enalapril 20 mg twice daily, furosemide 150 mg once daily, and spironolactone 37 mg once daily). Two months later, the ranolazine dose was increased to 500 mg twice daily; shortly thereafter, acute urinary retention occurred and persisted despite institution of α-lytic (alfuzosin) and antiandrogenic (dutasteride) therapy. A urodynamic study revealed that urinary retention was caused by severe hypocontractility of the detrusor muscle. Ranolazine was withdrawn and, within 2 days, the patient recovered his ability to void spontaneously; a second urodynamic study confirmed that detrusor contractility was substantially improved. Drug rechallenge was not performed due to the patient's clinical condition. Nevertheless, a phenotyping test to assess the activity of the cytochrome isoenzymes CYP3A4 and CYP2D6 (responsible for ranolazine metabolism) was performed, with dextromethorphan used as the probe drug. The urinary metabolic ratios indicated relatively low activity for CYP3A4 and intermediate activity for CYP2D6. DISCUSSION: The causal role of ranolazine in our case of bladder hypotonia is probable according to the Naranjo criteria. The mechanism of bladder dysfunction is tentatively ascribed to blockage of late sodium current in smooth muscle cells. Although drug plasma concentrations were not measured, they were probably elevated, since the metabolic activity of CYP3A4 was at the lower end of the reference range. Enzyme inhibition produced by diltiazem may have contributed to decreasing CYP3A4 activity. CONCLUSIONS: Acute urinary retention in elderly men taking ranolazine may be due to drug-induced bladder hypotonia.

Cytochrome P450 2D6 phenotyping in an elderly population with dementia and response to galantamine in dementia: a pilot study.

BACKGROUND: The cytochrome P450 (CYP) 2D6 enzyme is involved in the metabolism of many drugs used by the elderly population. Variations in its activity can lead to altered drug response. However, few studies on the activity of this enzyme system have enrolled the elderly population. OBJECTIVE: The goal of this pilot study was to assess the feasibility of in vivo phenotyping of CYP2D6 in an elderly population with dementia and to determine if part of the variability in response to treatment with galantamine is attributable to CYP2D6 phenotype. METHODS: Patients with dementia attending geriatric clinics and receiving galantamine treatment for at least 6 months were enrolled in this case-control study. CYP2D6 phenotype was determined by analysis of the urinary concentrations of the probe drug dextromethorphan and its primary metabolite dextrorphan after ingestion of 30 mg of dextromethorphan. Patients were classified as robust responders to galantamine if their cognitive testing, as measured by using scores on the Mini-Mental State Examination or Alzheimer's Disease Assessment Scale-Cognitive subscale, had not changed or had improved after 6 months of treatment. RESULTS: Forty-three patients (23 men, 20 women; mean age, 78.4 years; 98% white) underwent phenotyping. The mean number of concomitantly prescribed medications was 5.7, and 16 patients (37%) were receiving other CYP2D6 substrate or inhibitor drugs. The distribution of CYP2D6 phenotype was similar to that seen in other white populations. There was no correlation between the phenotypic metabolic ratio and age, the number of routinely taken medications, whether patients were receiving other prescribed substrate or inhibitor drugs of CYP2D6 (P = 0.63), or whether they were a robust responder (P = 0.47). CONCLUSIONS: Urinary assays of CYP2D6 phenotype are technically feasible in older individuals with dementia who are taking multiple medications, and may be a useful clinical tool in this population. However, the study was unable to make inferences about an association between CYP2D6 phenotype and galantamine responsiveness.

Effects of multiple-dose pegylated interferon alfa-2b on the activity of drug-metabolizing enzymes in persons with chronic hepatitis C.

PURPOSE: To examine the effect of pegylated interferon (PEG-IFN) alfa-2b on the activity of major drug-metabolizing enzymes. METHODS: This nonrandomized, open-label, multiple-dose study examined the effects of PEG-IFN alfa-2b on the activity of CYP450 1A2, 2 C8/9, 2D6, and 3A4 enzymes and N-acetyltransferase in subjects with chronic hepatitis C. Eligible subjects received PEG-IFN alfa-2b 1.5 µg/kg subcutaneously once weekly for 4 weeks (days 3, 10, 17, and 24). Oral probe substrates (dextromethorphan hydrobromide 45 mg, caffeine 200 mg, tolbutamide 500 mg, and dapsone 100 mg) were administered after a 10-h fast on days 1 and 25. Midazolam 4 mg was administered orally on days 2 and 26. Enzyme activity for each CYP450 isozyme and for N-acetyltransferase was estimated based on the ratios of the observed concentrations of the substrates and metabolites in plasma or urine samples. RESULTS: Twenty-six subjects enrolled in the study. Mean age was 44.3 years, mean weight was 78.9 kg, and mean body mass index was 26.3 kg/m(2). Multiple doses of PEG-IFN alfa-2b inhibited CYP1A2 activity to a limited extent (point estimate = 84.2%, 90% confidence interval [CI] 79-90), increased CYP2C8/9 activity to a limited extent (point estimate = 127.6%, 90% CI 115-142), increased CYP2D6 activity (point estimate = 167%, 90% CI 125-223), and had no effect on the activity of CYP3A4 or N-acetyltransferase. CONCLUSION: Weekly administration of PEG-IFN alfa-2b to subjects with chronic hepatitis C increased CYP2C8/9 and CYP2D6 activity in some individuals.

Activities of cytochrome P450 1A2, N-acetyltransferase 2, xanthine oxidase, and cytochrome P450 2D6 are unaltered in children with cystic fibrosis.

The activities of hepatic cytochrome P450 (CYP) 1A2, N-acetyltransferase 2 (NAT-2), xanthine oxidase (XO), and CYP2D6 were evaluated in 12 young children (aged 3-8 years) with mild cystic fibrosis (CF) and 12 age-matched healthy control subjects by use of standard caffeine and dextromethorphan phenotyping methods. Subjects were given 4 oz of Coca-Cola (approximately 35 mg caffeine) (The Coca-Cola Company, Atlanta, Ga) and a single 0.5-mg/kg dose of dextromethorphan. Urine was collected for 8 hours after biomarker administration, and enzyme activity was assessed by use of previously validated caffeine and dextromethorphan molar ratios. CYP2D6 genotyping was also performed in 10 of 12 subjects with CF and 11 of 12 control subjects. There were no significant differences in the urinary molar ratios for any of the enzyme systems evaluated. These data suggest that CF does not alter the activities of CYP1A2, NAT-2, XO, and CYP2D6. Altered biotransformation of drugs in this patient population is likely enzyme- and isoform-specific and thus is apparent for only selected compounds that are substrates for enzymes other than CYP1A2, NAT-2, XO, and CYP2D6.

CYP2D6 genotype and phenotyping by determination of dextromethorphan and metabolites in serum of healthy controls and of patients under psychotropic medication.

Fourteen drug free healthy volunteers and 22 psychiatric patients under psychotropic medication were phenotyped for their individual CYP2D6 activity using dextromethorphan as a probe drug. A solution containing 20 mg dextromethorphan was administered and blood was taken 60 min later for determination of dextromethorphan and metabolites in serum. For comparison, urine was collected over 8 h after ingestion of 20 mg dextromethorphan in a separate test. The CYP2D6 phenotype was determined from the ratio of dextromethorphan to dextrorphan. For genotyping, mutant alleles of the CYP2D6 gene were identified using allele-specific polymerase chain reactions. Genotyping revealed five poor metabolizers of CYP2D6. The others were extensive metabolizers. The ratio of dextromethorphan to dextrorphan ranged from 0.01-2.53 in serum and from 0.0007-4.252 in urine. Probit analysis of serum ratios revealed a bimodal distribution with an antimode at 0.126. According to this antimode, control subjects exhibited identical phenotypes and genotypes, whereas patients under paroxetine, moclobemide or metoprolol who had been genotyped as extensive metabolizers were poor metabolizer phenotypes. Administration of tricyclic antidepressants did not change the CYP2D6 phenotype. The serum assay was more rapid and more accurate than the standard urine approach. Therefore the determination of dextromethorphan and metabolites in serum could be advantageous to measure individual CYP2D6 activities in vivo and thus optimize the dosing of drugs metabolized by CYP2D6.