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.

Pharmacokinetic and pharmacodynamic profiles of the antitussive principles of Glycyrrhizae radix (licorice), a main component of the Kampo preparation Bakumondo-to (Mai-men-dong-tang)

We examined the pharmacokinetic and pharmacodynamic properties of liquiritin apioside, a main antitussive component of Glycyrrhizae radix (licorice), with regard to its antitussive effect in guinea pigs. The peak plasma concentration of the unchanged compound was observed 15 min after the administration of liquiritin apiosaide. The plasma concentration then gradually decreased and was almost undetectable 4 h after administration. Liquiritigenin, a des-glycoside of liquiritin apioside, appeared in the plasma 2 h after the administration of liquiritin apioside and remained for more than 6 h after administration. The plasma concentration of unchanged liquiritigenin was observed 15 min after administration and then gradually increased for more than 6 h after administration. When the antitussive effects of liquiritin apioside, liquiritin and liquiritigenin, at respective doses of 30 mg/kg, p.o., were examined 1 h after administration, liquiritin apioside and liquiritigenin caused a significant reduction in the number of capsaicin-induced coughs. However, at the same dose, liquiritin had no significant effect on the number of capsaicin-induced coughs. On the other hand, when the antitussive effects of liquiritin apioside, liquiritin and liquiritigenin, at doses of 30 mg/kg, p.o., were examined 4 h after administration, each caused a more than 40% reduction in the number of capsaicin-induced coughs. The present results suggest that G. radix (licorice) may produce a persistent antitussive effect, and that liquiritin apioside plays an important role in the earlier phase, while liquiritigenin, which is a metabolite of liquiritin apioside and liquiritin, plays an important role in the late phase.

Increasing bioanalytical throughput using pcSFC-MS/MS: 10 minutes per 96-well plate

The utility of packed-column supercritical, subcritical, and enhanced fluidity liquid chromatographies (pcSFC) for high-throughput applications has increased during the past few years. In contrast to traditional reversed-phase liquid chromatography, the addition of a volatile component to the mobile phase, such as CO2, produces a lower mobile-phase viscosity. This allows the use of higher flow rates which can translate into faster analysis times. In addition, the resulting mobile phase is considerably more volatile than the aqueous-based mobile phases that are typically used with LC-MS, allowing the entire effluent to be directed into the MS interface. High-throughput bioanalytical quantitation using pcSFC-MS/MS for pharmacokinetics applications is demonstrated in this report using dextromethorphan as a model compound. Plasma samples were prepared by automated liquid/liquid extraction in the 96-well format prior to pcSFC-MS/MS analysis. Three days of validation data are provided along with study sample data from a patient dosed with commercially available Vicks 44. Using pcSFC and MS/MS, dextromethorphan was quantified in 96-well plates at a rate of approximately 10 min/plate with average intraday accuracy of 9% or better. Daily relative standard deviations (RSDs) were less than 10% for the 2.21 and 14.8 ng/mL quality control (QC) samples, while the RSDs were less than 15% at the 0.554 ng/mL QC level.

Rapid bioanalytical determination of dextromethorphan in canine plasma by dilute-and-shoot preparation combined with one minute per sample LC-MS/MS analysis to optimize formulations for drug delivery

The determination of dextromethorphan in canine plasma is used to demonstrate the high throughput bioanalytical approach of automated dilute-and-shoot (DAS) sample preparation followed by a 1 min isocratic liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis. Dilute-and-shoot preparation is commonly used for the determination of drugs in several biological matrices such as urine and saliva, but is not typically used with plasma samples because the amount of protein present in plasma can lead to a variety of problems including column failure. As a result, plasma sample preparation usually removes protein by precipitation, extraction or filtration; however, the dilute-and-shoot approach solubilizes proteins throughout the chromatographic portion of the assay. The attributes of this approach are compared with a previously validated liquid/liquid extraction procedure for determination of dextromethorphan in plasma. Accuracy and precision of both methods are similar. The lower limit of quantitation (LLOQ) of the dilute-and-shoot approach is much higher at 2 ng/ml versus 5 pg/ml with the liquid/liquid extraction; however, the sample throughput of the preparation portion of the dilute-and-shoot approach is more than 50-fold greater. The ruggedness of the dilute-and-shoot method was thoroughly investigated because of the problems traditionally associated with the direct injection of diluted plasma onto an LC-MS/MS instrument. With the optimal conditions, greater than 1,000 injections of diluted plasma have been successfully performed on a single column in less than 19 h making this technique an excellent approach for the rapid preparation and high throughput of plasma samples containing drug levels in the ng/ml range or higher. Application of this methodology to measure the levels of dextromethorphan in canine plasma to evaluate drug delivery from various formulations is also presented.

Simple chromatography method for simultaneous determination of dextromethorphan and its main metabolites in human plasma with fluorimetric detection

Dextromethorphan, the innocuous non-narcotic antitussive agent, is the most widely used probe drug to assess CYP2D6 function both in vivo and in vitro. For this reason a simple and selective high performance liquid chromatography method with fluorimetric detection for simultaneous quantitation of dextromethorphan, and its main metabolites in human plasma was developed and validated. The method involved a simple and rapid protein precipitation protocol, using a mixture of ZnSO(4) and methanol. The analysis was performed on a 3 microm, C(18) Tracer Excel 15 cm x 0.4 cm i.d. column by gradient elution in which Mobile phase A consisted of potassium dihydrogen phosphate buffer (pH = 3, 0.01 M):methanol:tetrahydrofuran (68.5:31:0.5), and mobile phase B consisted of methanol:tetrahydrofuran (93.25:6.75). Linear calibration curves were obtained in the range of 10-500 ng/ml for dextromethorphan, dextrorphan and hydroxymorphinan. The limit of quantitation (LOQ) was 10 ng/ml for each compound. The maximum within and between days precisions were 7.4 and 7.8%, respectively. The accuracies at four different concentration levels ranged from 88.2 to 111.5%. The recoveries were between 88.0 and 108.6%. The assay method was successfully applied to determine dextromethorphan metabolic ratio after an oral dose of 30 mg of dextromethorphan hydrobromide.

In-vivo indices of CYP2D6 activity: comparison of dextromethorphan metabolic ratios in 4-h urine and 3-h plasma

OBJECTIVES

Dextromethorpan (DM) is widely used as a probe drug to assess in vivo the activity of the cytochrome P450 2D6 (CYP2D6). The aim of the study was to compare metabolic ratios (MRs) of DM to dextrorphan (DEX) in plasma and urine. We examined, separately in urine and plasma, the relationships between the MRs which are based on DEX and those involving the sum of DEX and a secondary metabolite hydroxymorphinan (HM). Furthermore, we compared the MRs in plasma obtained with and without hydrolysis of DEX glucuronides.

METHODS

Concentrations of DM and metabolites in urine and plasma were determined by HPLC after a single oral dose of 30 mg DM hydrobromide to 101 healthy Caucasian subjects. Urine was collected over the time interval 0-4 h after the dose and plasma was obtained from 95 subjects 3 h after administration.

RESULTS

Six subjects (5.9%) were of poor metaboliser (PM) phenotype (urinary DM:DEX ratio >0.3). A good correlation (r2 = 0.777, P < 0.00001) was observed between the metabolic ratios of DM:DEX in plasma and urine. There was an excellent correlation, both in plasma and urine, between the log-transformed ratios of DM:DEX and of DM to the sum of molar concentrations of DEX and HM (r2 > 0.996, P < 0.00001). Plasma samples of 89 subjects (83 EM and 6 PM) were analyzed without deconjugation of DEX glucuronide also. The correlation between the plasma ratios of DM:DEX based on unconjugated DEX and those involving glucuronide (r2 = 0.793, P < 0.00001) was comparable to that reported by other authors on urine.

CONCLUSION

In healthy Caucasian subjects, the MRs of DM to DEX in plasma obtained at 3 h correlated reasonably well with those in urine collected over the time interval 0-4 h after the dose. Nevertheless, repeatability of this plasma index should be determined before its wide use can be recommended. Finally, the interindividual variation in DEX metabolism to HM (catalyzed by CYP3A) contributes only minimally to the interindividual variability of the MRs.

Simultaneous determination of dextromethorphan, dextrorphan, and guaifenesin in human plasma using semi-automated liquid/liquid extraction and gradient liquid chromatography tandem mass spectrometry

A method for the simultaneous determination of dextromethorphan (DEX), dextrorphan (DET), and guaifenesin (GG) in human plasma was developed, validated, and applied to determine plasma concentrations of these compounds in samples from six clinical pharmacokinetic (PK) studies. Semi-automated liquid handling systems were used to perform the majority of the sample manipulation including liquid/liquid extraction (LLE) of the analytes from human plasma. Stable-isotope-labeled analogues were utilized as internal standards (ISTDs) for each analyte to facilitate accurate and precise quantification. Extracts were analyzed using gradient liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Use of semi-automated LLE with LC-MS/MS proved to be a very rugged and reliable approach for analysis of more than 6200 clinical study samples. The lower limit of quantification was validated at 0.010, 0.010, and 1.0 ng/mL of plasma for DEX, DET, and GG, respectively. Accuracy and precision of quality control (QC) samples for all three analytes met FDA Guidance criteria of +/-15% for average QC accuracy with coefficients of variation less than 15%. Data from the thorough evaluation of the method during development, validation, and application are presented to characterize selectivity, linearity, over-range sample analysis, accuracy, precision, autosampler carry-over, ruggedness, extraction efficiency, ionization suppression, and stability. Pharmacokinetic data are also provided to illustrate improvements in systemic drug and metabolite concentration-time profiles that were achieved by formulation optimization.

Incorporating In Vitro Information on Drug Metabolism Into Clinical Trial Simulations to Assess the Effect of CYP2D6 Polymorphism on Pharmacokinetics and Pharmacodynamics: Dextromethorphan as a Model Application

In vitro-in vivo extrapolation of clearance, embedded in a clinical trial simulation, was used to investigate differences in the pharmacokinetics and pharmacodynamics of dextromethorphan between CYP2D6 poor and extensive metabolizer phenotypes. Information on the genetic variation of CYP2D6, as well as the in vitro metabolism and pharmacodynamics of dextromethorphan and its active metabolite dextrorphan, was integrated to assess the power of studies to detect differences between phenotypes. Whereas 6 subjects of each phenotype were adequate to achieve 80% power in showing pharmacokinetic differences, the power required to detect a difference in antitussive response was less than 80% with 500 subjects in each study arm. Combining in vitro-in vivo extrapolation with a clinical trial simulation is useful in assessing different elements of study design and could be used a priori to avoid inconclusive pharmacogenetic studies.

Automated determination of dextromethorphan and its main metabolites in human plasma by high-performance liquid chromatography and column switching

An automated column-switching technique coupled to isocratic high-performance liquid chromatography (HPLC) with fluorescence detection was developed for simultaneous determination of dextromethorphan and its three major metabolites, dextrorphan, hydroxymorphinan, and methoxymorphinan. After cleavage of conjugates by incubation with glucuronidasearylsulfatase at 37 degrees C for 15 h, plasma samples were injected directly into the HPLC system. Dextromethorphan and metabolites were retained on a cleanup column (10 x 4.6 mm internal diameter [ID]) filled with cyanopropyl (CN) material (Hypersil CPS, 10-microns article size) while interfering proteins and lipids were washed to waste. After column switching, the drugs were eluted from the cleanup column and separated on Spherisorb CN material (5-microns particle size, column size 250 x 4.6 mm ID). Fluorescence detection was carried out with an excitation wavelength of 220 nm and an emission wavelength of 305 nm. Sample cleanup and HPLC separation were completed within 20 min. Regression analyses found linearity (r > 0.99) between drug concentration and detector response over a wide range-5-220 ng/ml for dextromethorphan, 5-550 ng/ml for dextrorphan, 5-500 ng/ml for hydroxymorphinan, and 5-200 ng/ml for methoxymorphinan. The limit of quantification was approximately 5 ng/ml, and the recovery was > 90% for all compounds. At concentrations of 20-500 ng/ml, the intra- and interassay coefficients of variation ranged from 3.5 to 14.6% and from 7.0 to 14.0%, respectively. The method is suitable for in vivo phenotyping of CYP2D6 activity, which catalyzes the O-demethylation of dextromethorphan to dextrorphan, and is also applicable to pharmacokinetic studies in man.

Plasma profile and pharmacokinetics of dextromethorphan after intravenous and oral administration in healthy dogs

Dextromethorphan is an N-methyl-D-aspartate (NMDA) noncompetitive antagonist which has been used as an antitussive, analgesic adjunct, probe drug, experimentally to attenuate acute opiate and ethanol withdrawal, and as an anticonvulsant. A metabolite of dextromethorphan, dextrorphan, has been shown to behave pharmacodynamically in a similar manner to dextromethorphan. The pharmacokinetics of dextromethorphan were examined in six healthy dogs following intravenous (2.2 mg/kg) and oral (5 mg/kg) administration in a randomized crossover design. Dextromethorphan behaved in a similar manner to other NMDA antagonists upon injection causing muscle rigidity, ataxia to recumbency, sedation, urination, and ptyalism which resolved within 90 min. One dog repeatedly vomited upon oral administration and was excluded from oral analysis. Mean +/- SD values for half-life, apparent volume of distribution, and clearance after i.v. administration were 2.0 +/-0.6 h, 5.1 +/- 2.6 L/kg, and 33.8 +/- 16.5 mL/min/kg. Oral bioavailability was 11% as calculated from naive pooled data. Free dextrorphan was not detected in any plasma sample, however enzymatic treatment of plasma with glucuronidase released both dextromethorphan and dextrorphan indicating that conjugation is a metabolic route. The short half-life, rapid clearance, and poor bioavailability of dextromethorphan limit its potential use as a chronic orally administered therapeutic.

Determination of dextromethorphan and dextrorphan in human plasma by liquid chromatography/tandem mass spectrometry

Rapid, sensitive and selective methods were developed for the determination of dextromethorphan and its major metabolite, dextrorphan, in human plasma using liquid chromatography/tandem mass spectrometry (LC/MS/MS). Plasma samples spiked with stable-isotope internal standards were prepared for analysis by a liquid-liquid back-extraction procedure. Dextromethorphan and dextrorphan were chromatographed on a short reversed-phase column, using separate isocratic mobile phase conditions optimized to elute each compound in approximately 1.1 min. For both analytes, calibration curves were obtained over four orders of magnitude and the limit of quantitation was 5 pg ml-1 using a 1 ml plasma sample volume. The accuracy across the entire range of spiked DEX and DOR concentrations was, in general, within 10% of the spiked value. The precision was generally better than 6% for replicate sample preparations at levels of 50 pg ml-1 or higher and typically better than 12% at levels below 50 pg ml-1. The method was applied for the evaluation of the pharmacokinetic profiles of dextromethorphan and dextrorphan in a human volunteer following peroral administration of a commercially available cough formulation.

Determination of dextromethorphan and its metabolites in rat serum by liquid-liquid extraction and liquid chromatography with fluorescence detection

Dextromethorphan is an effective and safe antitussive, but has liabilities with respect to its abuse potential at doses above the therapeutic dose. At these higher doses, people report phencyclidine-like effects from the drug. A number of animal models have suggested that dextrorphan, an active metabolite of dextromethorphan, is responsible for the abuse liability of the parent compound when dextromethorphan is taken at high doses. Full pharmacokinetic profiles in single animals have not been demonstrated in these studies due to a lack of analytical sensitivity and/or selectivity for dextromethorphan and its metabolites. We have developed a low-cost liquid chromatographic method capable of characterizing the concentration-time profile for dextromethorphan and dextrorphan for 8 h in rats following an 18 mg/kg i.p. dose of dextromethorphan. Limits of quantitation (S/N=10) in 100 microL of serum were 0.25, 0.19, 0.27, and 0.22 nmol/mL for 3-hydroxymorphinan, dextrorphan, 3-methoxymorphinan, and dextromethorphan, respectively. Inter-day precision was better than 11% across the dynamic range of the method.

Comparative disposition of codeine and pholcodine in man after single oral doses

Four healthy male subjects received single oral doses of 15, 30 and 60 mg of codeine and pholcodine according to a balanced cross-over design with an interval of 7 days between the six treatments. Blood samples were collected for 8 h after each drug administration. In phase 2 of the study six different male volunteers received single oral doses of 60 mg of codeine and pholcodine with a 14 day interval between successive drug treatments. Blood was sampled for 12 h after codeine and 121 h after pholcodine administration. Plasma concentrations of free (unconjugated) and total (unconjugated plus conjugated) codeine, pholcodine and morphine were determined by radioimmunoassay and selected pharmacokinetic parameters were derived from these data. Pharmacokinetics of both drugs were independent of dose. Codeine was absorbed and eliminated relatively rapidly [elimination t1/2 = 2.3 +/- 0.4 h (mean +/- s.d.)]. While codeine kinetics were adequately described by a one-compartment open model with first-order absorption, a two-compartment model was required to describe pholcodine elimination from plasma (t1/2,z = 37.0 +/- 4.2 h). Plasma concentrations of conjugated codeine were much greater than those of the unconjugated alkaloid. By contrast, pholcodine appeared to undergo little conjugation. Biotransformation of codeine to morphine was evident in all subjects, although the extent of this metabolic conversion varied considerably between subjects. Morphine was not detectable in the plasma of any subject after pholcodine administration.

Dextromethorphan in Wisconsin Drivers

Dextromethorphan is a synthetic analogue of codeine used in hundreds of over-the-counter medications for its antitussive effects. There have been numerous reports of dextromethorphan abuse by young adults. Dextromethorphan can produce psychoactive effects similar to that of marijuana, and higher doses will produce dissociative effects, including sensory enhancement and hallucinations. The Wisconsin State Laboratory of Hygiene examined data from blood samples submitted from January 1999 through December 2004 to determine the incidence of dextromethorphan in suspected impaired drivers. A total of 108 samples were found to be positive for dextromethorphan during this time. Dextromethorphan concentrations in these cases ranged from less than 5 to 1800 ng/mL (mean 207 ng/mL), compared to an expected therapeutic concentration range of 0.5-5.9 ng/mL. Overall, the highest dextromethorphan concentrations observed were in males aged 16-20 years. Ninety-six percent of the specimens included in this study were also found to be positive for drugs other than dextromethorphan. A review of police and drug recognition expert reports from several of these cases showed that dextromethorphan-impaired drivers exhibited poor psychomotor performance on standardized field sobriety tests, horizontal gaze nystagmus, vertical gaze nystagmus, and overall signs of central nervous system depression.

Simultaneous determination of dextrorphan and guaifenesin in human plasma by liquid chromatography with fluorescence detection

A sensitive liquid chromatographic (LC) method was developed and validated for the simultaneous determination of dextrorphan and guaifenesin in human plasma using fluorescence detection. Dextrorphan and guaifenesin were extracted from plasma by a liquid-liquid extraction procedure using chloroform containing laudanosine as the internal standard. A cyano column (15 cm x 46 mm i.d., Spherisorb 5-CN) and a mobile phase containing acetonitrile-triethylamine-distilled water (10:1:89, v/v/v) (pH 6) were used. The concentration-response relationship for dextrorphan was found to be linear over a concentration range of 23-515 ng ml-1 with a lower limit of detection of 20 ng ml-1; the accuracy of the method would fall (95% confidence limit) within 9.53% and 11.07% of the true value for the inter-and intra-day, respectively; the inter- and intra-day precision, as measured by RSD, ranged from 1.88% to 30.07% (mean 2.28%) and from 4.69% to 7.51% (mean 5.67%) over the dynamic concentration range of the method (33-326 ng ml-1). The concentration-response relationship for guaifenesin was found to be linear over a concentration range of 181-8136 ng ml-1 with a lower detection limit of 30 ng ml-1; the accuracy of the method would fall (95% confidence limit) within 9.78% and 8.04% of the true value for the inter- and intra-day, respectively; the inter- and intra-day precision, as measured by the RSD, ranged from 2.55 to 6.07% (mean 3.90%) and from 3.12 to 3.90% (mean 3.52%) over the dynamic concentration range of the method (435-6430 ng ml-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of pharmacokinetic parameters for assessment of dextromethorphan metabolic phenotypes

In this study, the metabolic ratios of dextromethorphan to dextrorphan (DM/DX) in plasma were calculated at steady state after administering 2 dosage forms (Medicon) and Detusiv) of DM with different release rates. The urinary metabolic ratio for each subject was also determined based on the total drug concentration in the urine. An analysis of pharmacokinetic parameters for determining the DM metabolic phenotype was conducted. Results demonstrate that double logarithmic correlations between the metabolic ratios based on pharmacokinetic parameters of either AUC(0-tau,ss), C(max,ss), C(min,ss), or C(ave,ss) for Medicon and Detusiv and the urinary metabolic ratios were all significant. Probit plots of the metabolic ratios based on these pharmacokinetic parameters revealed 2 clusters of distribution, representing extensive and intermediate metabolizers. An antimode of 2.0 for total drug based on these pharmacokinetic parameters was determined and correspondingly referred to an antimode of 0.02 for the urinary metabolic ratio to delineate extensive and intermediate metabolizers. This model was also verified to be appropriate when using total plasma concentrations of DM and DX at any time during the period of the dosing interval at steady state to calculate the metabolic ratio for identifying extensive and intermediate metabolizers. Therefore, the metabolic ratio based on the pharmacokinetic parameters of either AUC(0-tau,ss), C(max,ss), C(min,ss), or C(ave,ss) and plasma concentrations of DM and DX in a single blood sample at steady state are proposed as an alternative way to identify phenotypes of CYP2D6.

Assessment of CYP2D6 and CYP2C19 activity in vivo in humans: a cocktail study with dextromethorphan and chloroguanide alone and in combination

OBJECTIVES

Dextromethorphan and chloroguanide (INN, proguanil) are used as prototypic phenotyping substrates of polymorphically expressed CYP2D6 and CYP2C19 in humans. We determined whether the dextromethorphan/dextrorphan and chloroguanide/cycloguanil metabolic ratios, obtained after administration of the parent drugs either alone or in combination, are equivalent.

METHODS

Thirty-six healthy male volunteers received single oral doses of 80 mg dextromethorphan and 200 mg chloroguanide during a three-period, randomized crossover study. Plasma and urine were collected to calculate metabolic ratios and analyze the disposition kinetics of the probe drugs.

RESULTS

All subjects were extensive metabolizers for both CYP2D6 and CYP2C19. Chloroguanide kinetics and urinary metabolic ratio were not altered after dextromethorphan administration. Dextromethorphan urinary metabolic ratio increased from -2.52 +/- 0.67 to -2.03 +/- 0.58 (P < .001) in the presence of chloroguanide. This was caused by an increase of dextromethorphan without a significant change of dextrorphan in both urine and plasma. Inhibition of CYP3A-dependent biotransformation of dextromethorphan to methoxymorphinan did not appear to be responsible for this change because the log(dextromethorphan/methoxymorphinan) urinary ratio, an index of CYP3A activity, did not significantly change during chloroguanide coadministration. The chloroguanide and dextromethorphan metabolic ratio determined from urine collection correlated with the corresponding metabolic ratio determined from plasma obtained 3 hours after oral administration.

CONCLUSION

When CYP2D6 and CYP2C19 activity are assessed, dextromethorphan and chloroguanide cannot be associated in a cocktail because chloroguanide increases the dextromethorphan metabolic ratio. CYP2D6 and CYP2C19 activity can be determined from a blood sample drawn 3 hours after oral administration of dextromethorphan and chloroguanide, respectively.

Fatal zipeprol and dextromethorphan poisonings in Korea

Zipeprol and dextromethorphan are abused together by young people in Korea to obtain a stronger hallucinogenic effect. Because large amounts of these drugs are taken for this reason, nine fatal poisonings due to zipeprol and dextromethorphan have been reported since 1993. In this paper, the concentration of drugs in the postmortem blood and gastric contents of these victims is examined. The determination and identification of the drugs in biological fluids were conducted by gas chromatography (GC)-thermionic specific detection and GC-mass spectrometry. Linear calibration curves and high recoveries were obtained. The blood concentrations of zipeprol varied from 1.3 to 28.6 micrograms/mL, and the concentrations of dextromethorphan ranged from 1.1 to 18.3 micrograms/mL. The concentration of zipeprol in the gastric contents ranged from 26.8 to 1384.8 micrograms/g, and dextromethorphan concentrations varied from 2.1 to 243.7 micrograms/g.

Highly sensitive high-performance liquid chromatographic-tandem mass spectrometric method for the analysis of dextromethorphan in human plasma

A stable-isotope-dilution HPLC-tandem mass spectrometry-based method was developed for the determination of dextromethorphan in human plasma. Plasma samples were prepared for analysis by solid-phase extraction on octadecylsilane extraction cartridges. Dextromethorphan and the deuterium-labeled dextromethorphan internal standard were chromatographed on a short reversed-phase column and detected by a selected-reaction-monitoring scheme. Linear standard curves were obtained over three orders of magnitude and the limit of quantitation for dextromethorphan was 50 pg/ml, using a 1-ml plasma sample. The combination of HPLC and electrospray tandem mass spectrometry resulted in a rapid, selective and sensitive method for the analysis of dextromethorphan in plasma. The method was applied for the evaluation of the pharmacokinetic profile of dextromethorphan in human volunteers following peroral administration.

Benzonatate overdose associated with seizures and arrhythmias

BACKGROUND

Benzonatate is an antitussive with a unique chemical structure. It can contain as many as 8 structural analogs. Therefore, laboratory analysis of benzonatate is difficult. We report 2 cases of benzonatate poisoning with seizures and cardiac arrest and an analytical method to identify and quantify benzonatate in human plasma.

CASE REPORTS

Case 1: A 12-month-old male presented to the emergency department of a rural hospital following ingestion of an unknown amount of benzonatate. Upon arrival, the child was seizing and in full cardiac arrest. Resuscitative measures were unsuccessful and the child died shortly after arriving at the emergency department. Case 2: A 39-year-old male ingested 36 benzonatate capsules in a suicide attempt. Enroute to the health care facility, the patient experienced a seizure, had a cardiac arrest, and was cardioverted. Upon arrival at the emergency department, the patient was acidotic with a pH of 6.8. Gastric lavage was performed followed by the administration of activated charcoal. Six hours after arrival at the emergency department, the patient was alert, oriented, and hemodynamically stable. The patient was observed for 24 hours and subsequently discharged. Laboratory confirmation of benzonatate in the plasma of the patient was performed using high-pressure liquid chromatography with tandem mass spectrometry (MS/MS). The benzonatate concentration was estimated to be 2.5 micrograms/mL.

CONCLUSION

Seizures and cardiac arrest are possible following an acute ingestion. Quantitative analysis of benzonatate is possible using high-pressure liquid chromatography with tandem mass spectrometry. Routine analysis for benzonatate is not common.

Solid phase extraction and high performance liquid chromatographic determination of doxophylline in plasma

A sensitive and selective high performance liquid chromatographic doxophylline assay with ultraviolet detection has been developed for plasma samples. The drug is isolated from biological samples with a reversed phase C18 disposable extraction column. Plasma standard curves are linear for concentrations of doxophylline from 0.03 to 10 mg/L. At the therapeutic range concentrations, the recoveries are better than 96.9%. The coefficients of variation for the procedure are 4.1% and 2.7% for the concentrations 0.03 mg/L and 10 mg/L, respectively. By this method, pharmacokinetic profiles are obtained for six adult volunteers.

Effect of activated charcoal on the pharmacokinetics of pholcodine, with special reference to delayed charcoal ingestion

We conducted a randomized study with four parallel groups to investigate the effect of single and multiple doses of activated charcoal on the absorption and elimination of pholcodine administered in a cough syrup. The first group received 100 mg of pholcodine on an empty stomach with water only (control); the second group took 25 g of activated charcoal immediately after pholcodine; the third group received 25 g of activated charcoal 2 h and the fourth group 5 h after ingestion of the 100-mg dose of pholcodine. In addition, the fourth group received multiple doses (10 g each) of charcoal every 12 h for 84 h. Blood samples were collected for 96 h and urine for 72 h. Pholcodine concentrations were measured by high-performance liquid chromatography. A significant reduction in absorption was found when charcoal was administered immediately after pholcodine; the AUC0-96h was reduced by 91% (p < 0.0005), the Cmax by 77% (p < 0.0005), and the amount of pholcodine excreted into urine by 85% (p < 0.0005). When charcoal was administered 2 h after pholcodine, the AUC0-96h was reduced by 26% (p = 0.002), the Cmax by 23% (p = NS), and the urinary excretion by 28% (p = 0.004). When administered 5 h after pholcodine, charcoal produced only a 17% reduction in the AUC0-96h (p = 0.06), but reduced the further absorption of pholcodine still present in the gastrointestinal tract at the time of charcoal administration, as measured by AUC5-96h (p = 0.006). Repeated administration of charcoal failed to accelerate the elimination of pholcodine. We conclude that activated charcoal is effective in preventing the absorption of pholcodine, and its administration can be beneficial even several hours after pholcodine ingestion.