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.

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.

Multiple night-time doses of valerian (Valeriana officinalis) had minimal effects on CYP3A4 activity and no effect on CYP2D6 activity in healthy volunteers

Valerian (Valeriana officinalis) is a popular dietary supplement. The objective of this study was to assess the influence of a valerian extract on the activity of the drug-metabolizing enzymes cytochrome P450 2D6 (CYP2D6) and 3A4. Probe drugs dextromethorphan (30 mg; CYP2D6 activity) and alprazolam (2 mg; CYP3A4 activity) were administered orally to healthy volunteers (n = 12) at baseline and again after exposure to two 500-mg valerian tablets (1000 mg) nightly for 14 days. The valerian supplement contained a total valerenic acid content of 5.51 mg/tablet. Dextromethorphan to dextorphan metabolic ratios (DMRs) and alprazolam pharmacokinetics were determined at baseline and after valerian treatment. The DMR was 0.214 +/- 0.025 at baseline and 0.254 +/- 0.026 after valerian supplementation (p > 0.05). For alprazolam, the maximum concentration in plasma was significantly increased after treatment with valerian (25 +/- 7 ng/ml versus 31 +/- 8 ng/ml; p < 0.05). There were no significant differences in other pharmacokinetic parameters at baseline and after valerian exposure (all p values > or = 0.05; time to reach maximum concentration in plasma, 3.0 +/- 3.2 versus 3.1 +/- 2.1 h; area under the plasma concentration versus time curve, 471 +/- 183 versus 539 +/- 240 hx ng x ml(-1); half-life of elimination, 13.5 +/- 4.3 versus 12.2 +/- 5.6 h). Our results indicate that although a modest increase was observed in the alprazolam Cmax, typical doses of valerian are unlikely to produce clinically significant effects on the disposition of medications dependent on the CYP2D6 or CYP3A4 pathways of metabolism.

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.

Semi-automated liquid--liquid back-extraction in a 96-well format to decrease sample preparation time for the determination of dextromethorphan and dextrorphan in human plasma

A semi-automated, 96-well based liquid-liquid back-extraction (LLE) procedure was developed and used for sample preparation of dextromethorphan (DEX), an active ingredient in many over-the-counter cough formulations, and dextrorphan (DOR), an active metabolite of DEX, in human plasma. The plasma extracts were analyzed by liquid chromatography-tandem mass spectrometry (LC-MS-MS). The analytes were isolated from human plasma using an initial ether extraction, followed by a back extraction from the ether into a small volume of acidified water. The acidified water isolated from the back extraction was analyzed directly by LC-MS-MS, eliminating the need for a dry down step. A liquid handling system was utilized for all aspects of liquid transfers during the LLE procedure including the transfer of samples from individual tubes into a 96-well format, preparation of standards, addition of internal standard and the addition and transfer of the extraction solvents. The semi-automated, 96-well based LLE procedure reduced sample preparation time by a factor of four versus a comparable manually performed LLE procedure.

Characterization of cytochrome P-450 2D1 activity in rat brain: high-affinity kinetics for dextromethorphan

We investigated the enzymatic function, stability, and regional distribution of rat brain cytochrome P-450 (CYP) 2D1 activity. CYP2D1 is the homolog of human CYP2D6, a genetically variable enzyme that activates or inactivates many clinical drugs acting on the central nervous system (e.g., antidepressants, monoamine oxidase inhibitors, serotonin uptake inhibitors, and neuroleptics), drugs of abuse (e.g., amphetamine and codeine), neurotoxins (e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3, 4-tetrahydroquinoline), and endogenous neurochemicals (e.g., tryptamine). The CYP2D family has been identified in rodent, canine, and primate brain. Conversion of dextromethorphan to dextrorphan by rat brain membranes was assayed by HPLC and was dependent on NADPH, protein concentration, and incubation time. Significant loss of activity was observed in some homogenizing buffers and after freezing of whole tissues or membrane preparations. Dextromethorphan (0.5-640 microM) metabolism was mediated by high- and low-affinity enzyme systems; K(m1) was 2.7 +/- 2.6 and K(m2) was 757 +/- 156 microM (n = 3 rats, mean +/- S.E.). The enzyme activity was significantly (p <.01) and stereoselectively inhibited by CYP2D1 inhibitors quinine and quinidine (not by CYP2C or CYP3A inhibitors), and by anti-CYP2D6 peptide antiserum (not by anti-CYP2C, -CYP2B, or -CYP3A antibodies). The enzymatic activity demonstrated significant brain regional variation (n = 10 regions, p <.001). These data characterize CYP2D1-mediated dextromethorphan metabolism in rat brain and suggest that localized metabolism of other CYP2D1 substrates (drugs, neurotoxins, and possibly endogenous compounds) within the brain will occur. In humans, CYP2D6 is genetically polymorphic; the variable expression of brain CYP2D6 may result in interindividual differences in central drug and neurotoxin metabolism, possibly contributing to interindividual differences in drug effects and neurotoxicity.

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.

Metabolism of dextromethorphan in human liver microsomes: a rapid HPCL assay to monitor cytochrome P450 2D6 activity

A new HPLC assay was developed to study dextromethorphan O-demethylation to dextrorphan in vitro using human liver microsomes to investigate the activity of the polymorphic monooxygenase cytochrome P450 2D6 (CYP 2D6). The separation of dextromethorphan and its main metabolite dextrorphan was performed on a polymeric C18 reversed-phase column with UV-detection using levallorphan as an internal standard. Liver samples from ten subjects were screened for dextrorphan formation whereby three groups with different abilities to metabolize dextromethorphan could be found. Seven microsomal preparations from extensive metabolizers showed an average dextrorphan formation rate of 298 +/- 68 pmol/mg protein.min, one sample was classified to belong to an intermediate dextromethorphan metabolizer (79 pmol/mg protein.min), whereas two samples of poor metabolizers exhibited significantly lower rates of dextromethorphan metabolism with values of 11 and 27 pmol/mg protein.min, respectively. This assay permits not only a fast in vitro screening for cytochrome P450 2D6 monooxygenase activity but is also an excellent tool to determine potential drug-drug interactions with this important metabolizing enzyme.

Pharmacokinetics of dextromethorphan and dextrorphan in epileptic patients

The present report describes the pharmacokinetic characteristics of dextromethorphan (DM) and its main active metabolite dextrorphan (DX) in a group of epileptic patients receiving comedication. Patients were sequentially dosed with DM 40 mg/6 h (8 weeks) and 50 mg/6 h (8 weeks) while concurrent antiepileptic drugs were kept stable. During baseline period, patients were phenotyped with regard to their drug metabolizing capacity. At the end of each treatment period, timed plasma DM and DX levels were determined post-dose by HPLC. Urine and cerebrospinal fluid f1p4) samples were also collected. The pharmacokinetic parameters of DM showed a wide intersubject variation. The genetic polymorphism of DM metabolism was identified as the possible cause of the observed variability. For both DM and DX mean values for Cmax and AUC increased in a linear fashion with dose, while the mean values of tmax and t 1/2 were not dependent on dose. The mean values of CL/F and Vss/F for DM were also dose-dependent. 3-Methoxymorphinan, an N-demethylated metabolite of DM was detected in plasma and CSF of some patients and warrants further investigation as to its possible CNS effects. In conclusion, DM given in doses up to 50 mg/6 h can produce plasma and brain concentrations similar to the in vitro antiepileptic levels, without causing significant adverse effects.

Safety, tolerability and pharmacokinetics of the N-methyl-D-aspartate antagonist Ro-01-6794/706 in patients with acute ischemic stroke. The Dextrorphan Study Group and Hoffmann-La Roche

Dextrorphan HCl (Ro 01-6794/706) is an NMDA receptor antagonist with clinical potential for administration in an elderly population of acute ischemic stroke patients. In vivo experience with such patients demonstrated a consistent pharmacologic effect/adverse experience profile that is typical of an NMDA receptor antagonist (e.g., nystagmus, nausea, vomiting, agitation, somnolence, hallucinations and hypertension). For the most part, these pharmacologic effects were mild to moderate in severity; short-lived; reversible; not life-threatening and subjectively tolerated. The most serious pharmacologic effect produced by dextrorphan administration was hypotension, which occurred within a well-defined window of 90 minutes from the start of the loading dose infusion in patients who received 200 mg/hr or greater loading dose infusions. In all cases it was reversible without neurologic sequelae. Careful review of demographic and pharmacokinetic parameters did not demonstrate any overriding factor(s) to the production of hypotension other than the rate of the loading dose infusion. Severe hypotension, severe decreased levels of consciousness and respiratory depression should not be generally expected at loading doses less than 200 mg/hr. In summary, dextrorphan can be safely given to an elderly population of ischemic stroke patients as a loading dose rate below 200 mg/hr and as a maintenance dose rate between 50-90 mg/hr for 24 hours when patients are monitored carefully for pharmacologic effects.

Safety, tolerability, and pharmacokinetics of the N-methyl-D-aspartate antagonist dextrorphan in patients with acute stroke. Dextrorphan Study Group

BACKGROUND AND PURPOSE

Dextrorphan hydrochloride is a noncompetitive N-methyl-D-aspartate antagonist that is neuroprotective in experimental models of focal brain ischemia. The purpose of this study was to determine the maximum loading dose and maintenance infusion of dextrorphan hydrochloride that are well tolerated in patients with an acute stroke.

METHODS

An intravenous infusion of dextrorphan or placebo was begun within 48 hours of onset of a mild-to-moderate hemispheric stroke. Initially, patients were treated with either placebo (n = 15) or dextrorphan (n = 22) using a 1-hour loading dose (60 to 150 mg) followed by a 23-hour ascending-dose maintenance infusion (maximum total dose, 3310 mg). Subsequently, 29 patients were treated with dextrorphan in an open trial using a 1-hour loading dose (145 to 260 mg) followed by an 11-hour constant rate (30 to 70 mg/h) infusion.

RESULTS

Transient and reversible adverse effects, including nystagmus, nausea, vomiting, somnolence, hallucinations, and agitation, commonly occurred in dextrorphan-treated patients. Loading-dose escalation was stopped because of rapid-onset, reversible, symptomatic hypotension in 7 of 21 patients treated with doses of 200 to 260 mg/h. At the highest rates of maintenance infusion (> 90 mg/h), 3 patients developed deep stupor or apnea. The maximum tolerated loading dose was 180 mg/h, and the maximum tolerated maintenance infusion was 70 mg/h. Maximum plasma levels of 750 to 1000 ng/mL were obtained in 9 patients. There was no difference in neurological outcome at 48 hours between the dextrorphan-treated and placebo-treated patients.

CONCLUSIONS

The highest doses of dextrorphan administered were associated with serious adverse experiences in some patients. Lower doses (loading doses of 145 to 180 mg, maintenance infusions of 50 to 70 mg/h) were better tolerated and rapidly produced potentially neuroprotective plasma concentrations of dextrorphan. These doses were associated with well-defined pharmacological effects compatible with N-methyl-D-aspartate receptor antagonism.

Comparative pharmacokinetics of oral dextromethorphan and dextrorphan in the rabbit

The pharmacokinetics of dextromethorphan (CAS 125-71-3) and its metabolite dextrorphan (CAS 125-73-5) was compared. The drugs were administered orally at the same molar dose of 0.085 mmol/kg. Plasma levels of dextromethorphan, dextrorphan, and metabolites 3-hydroxymorphinan and 3-methoxymorphinan were determined by HPLC with fluorimetric detection. Dextromethorphan was rapidly and extensively metabolized and the plasma profiles of dextrorphan, administered directly or as metabolite of dextromethorphan, were similar. The concentrations of 3-hydroxymorphinan were higher after dextromethorphan than dextrorphan. 3-Methoxymorphinan was detectable only 60 and 120 min after dextromethorphan. This work proposes the therapeutic use of dextrorphan instead of its precursor dextromethorphan.

High affinity [3H]dextrorphan binding in rat brain is localized to a noncompetitive antagonist site of the activated N-methyl-D-aspartate receptor-cation channel

[3H]Dextrorphan recognition sites were characterized in rat brain membranes. The pharmacological profile and regional distribution of [3H]dextrorphan binding sites appear to distinguish these sites from those labeled either by [3H]dextromethorphan or by putative sigma receptor radioligands. Data from thoroughly washed forebrain membranes suggest that [3H]dextrorphan predominantly labels a high affinity site defined by the activated state of the N-methyl-D-aspartate (NMDA) receptor-channel complex. Regulation of [3H]dextrorphan binding by specific modulators of NMDA receptor function suggests that [3H]dextrorphan binding is predominantly localized to a domain of the receptor-channel complex also recognized by the prototypical noncompetitive antagonist radioligands (+)-[3H]5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imi ne (MK-801) and [3H]1-[1-(2-thienyl)cyclohexyl]piperidine (TCP). The critical relationship between [3H]dextrorphan binding and activation of the NMDA receptor-complex is suggested by the profound dependence of [3H]dextrorphan binding on glutamate in well washed membranes. Basal specific [3H]dextrorphan binding is nearly totally suppressed by the specific competitive NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), in a glutamate- but not glycine-surmountable manner. Glutamate and glycine each stimulate [3H]dextrorphan binding in a concentration-dependent manner, effecting maximal increases from control of up to 30- and 14-fold, respectively. The NMDA receptor specificity of the modulation of [3H]dextrorphan binding by glutamate and glycine is indicated by the sensitivity of their effects to competitive antagonism by D-AP5 and 3-amino-1-hydroxy-2-pyrrolidone (HA-966), respectively, and by the accordant rank orders of potency of glycine analogs as modulators of [3H]dextrorphan binding and as ligands at the strychnine-insensitive glycine site. The divalent cations Mg2+ and Zn2+ and the polyamines spermine and spermidine regulate [3H]dextrorphan binding in a manner consistent with radioligand interaction at the noncompetitive NMDA antagonist domain. Mg2+ and spermidine regulate [3H]dextrorphan binding biphasically in well washed forebrain membranes, whereas Zn2+ monotonically inhibits [3H]dextrorphan binding. Mg2+ and spermidine regulate [3H]dextrorphan binding with qualitative similarity and in a contrasting fashion to their regulation of [3H]MK-801 and [3H]TCP binding. First, spermidine and Mg2+ are significantly more potent modulators of [3H]dextrorphan binding than of [3H]MK-801 and [3H]TCP binding in well washed membranes; second, whereas the potencies of spermidine and Mg2+ as modulators of [3H]MK-801 and [3H]TCP binding are significantly increased by glutamate and glycine in well washed membranes, their potencies as regulators of [3H]dextrorphan binding appear to be unaffected by glutamate and glycine.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose-dependent and time-dependent pharmacokinetics in the dog after intravenous administration of dextrorphan

The disposition of dextrorphan after single ascending iv doses and multiple iv dosing regimens was studied in Marshall beagle dogs. A dose-dependent decrease in plasma clearance was observed after the administration of single iv doses of 0.88 mg/kg, 2.64 mg/kg, and 8.8 mg/kg of dextrorphan (i.e. mean plasma clearance values +/- SD were 100 +/- 25 vs. 68 +/- 28 vs. 48 +/- 20 ml/min.kg, respectively; p less than 0.001). Upon multiple dosing, the plasma clearance of dextrorphan increased in a time-dependent fashion for the two highest doses, approaching values observed for the 0.88 mg/kg/day iv dosing regimen. Female dogs exhibited a greater increase in plasma clearance with time. For all dogs, however, dextrorphan plasma clearance approached or exceeded hepatic plasma flow rate, suggesting the possibility of extrahepatic metabolism or elimination. Modest dose- and time-dependent changes in the steady-state volume of distribution of dextrorphan also were observed. The AUC of the conjugated metabolites of dextrorphan decreased in a time-dependent manner for the 8.8 mg/kg/day dosing regimen. The nonlinear kinetics of dextrorphan after iv administration appeared to occur only after potentially toxic dosing regimens of dextrorphan hydrochloride. We postulate mechanisms to explain the dose- and time-dependent kinetics of dextrorphan observed in the beagle dog.

Simultaneous determination of dextromethorphan and three metabolites in plasma and urine using high-performance liquid chromatography with application to their disposition in man

A simple, sensitive, and reproducible high-performance liquid chromatrography assay is described for the simultaneous determination of dextromethophan, dextrorphan, 3-hydroxymorphinan, and 3-methoxymorphinan in plasma and urine. A conventional solvent-solvent extraction procedure was used for the isolation of the analytes from plasma and urine samples. The compounds were separated on a cyano column (150 x 4.6 mm, 5-micron particle size) using a mobile phase of acetonitrile/triethylamine/distilled water (17:0.06:82.94, vol/vol), pH 3.0, and then were measured by fluorescence detection. Calibration curves in the range 2-200 ng/ml for plasma and 0.05-10 micrograms/ml for urine were linear and passed through the origin. The precision and accuracy were greater than 90% and the lowest detectable concentrations were 0.5 ng/ml for 3-hydroxymorphinan and 3-methoxymorphinan and 1 ng/ml for dextromethorphan and dextrophan in plasma. The utility of this method is demonstrated in a preliminary study of dextromethorphan metabolism and pharmacokinetics in man.

Pharmacokinetics of dextromethorphan and dextrorphan: a single dose comparison of three preparations in human volunteers

We have carried out a single-dose comparison of three different dextromethorphan cough mixtures in 10 healthy human volunteers. Dextromethorphan was administered in a single dose of 60 mg in random order. The concentrations of dextromethorphan and its main metabolite, dextrorphan, were determined from the plasma samples using high performance liquid chromatography. The concentrations of dextrorphan were 170 times higher than the concentrations of dextromethorphan. No therapeutically significant differences were detected between the three preparations tested, and there were no great differences between the pharmacokinetic profiles of dextromethorphan and dextrorphan. The three test preparations were Resilar and Redol comp. (Remeda Pharmaceutical Co., Finland), and Extuson (Ferrosan Ab, Sweden).