How does multiple substrate binding lead to substrate inhibition of CYP2D6 metabolizing dextromethorphan? A theoretical study.

CYP2D6 is one of the most important metalloenzymes involved in the biodegradation of many drug molecules in the human body. It has been found that multiple substrate binding can lead to substrate inhibition of CYP2D6 metabolizing dextromethorphan (DM), but the corresponding theoretical mechanism is rarely reported. Therefore, we chose DM as the probe and performed molecular dynamics simulations and quantum mechanical calculations on CYP2D6-DM systems to investigate the mechanism of how the multiple substrate binding leads to the substrate inhibition of CYP2D6 metabolizing substrates. According to our results, three gate residues (Arg221, Val374, and Phe483) for the catalytic pocket are determined. We also found that the multiple substrate binding can lead to substrate inhibition by reducing the stability of CYP2D6 binding DM and increasing the reactive activation energy of the rate-determining step. Our findings would help to understand the substrate inhibition of CYP2D6 metabolizing the DM and enrich the knowledge of the drug-drug interactions for the cytochrome P450 superfamily.

The regioselectivity of the interaction between dextromethorphan and CYP2D6.

CYP2D6 is an important enzyme of the cytochrome P450 superfamily, and catalyzes nearly 25% of the drugs sold in the market. For decades, the interactions and metabolism between CYP2D6 and substrates have been a hot topic. However, the key factors of the catalytic regioselectivity for CYP2D6 still remain controversial. Here, we construct four systems to explore the interaction between dextromethorphan (DM) and CYP2D6. A new binding mode of CYP2D6 is defined, and two key residues (residue Asp301 and residue Glu216) are discovered working simultaneously to stabilize the DM at the reactive site by forming water bridge hydrogen bonds when CYP2D6 binds DM. Our results also indicate that the substrate concentration could mediate the binding mode between the substrate and CYP2D6 by decreasing the volume of the catalytic pocket, which is not conducive to the O-demethylation of DM but benefits the N-demethylation of DM. These results could shed light on the process of CYP2D6 binding to the substrate, and help to better understand the regioselectivity of CYP2D6 catalyzing the substrates.

Pharmacokinetics and pharmacodynamics of dextromethorphan: clinical and forensic aspects.

Dextromethorphan (DXM) is a safe and effective antitussive agent present in several over the counter cough and cold medications. At higher doses, it causes psychoactive effects, making it appealing for abuse. In this work, the pharmacokinetics and pharmacodynamics of DXM with clinical and forensic relevance were extensively reviewed. DXM and related known metabolizing enzymes and metabolites were searched in books and in PubMed (U.S. National Library of Medicine) without a limiting period. Major metabolic pathways include sequential O-demethylation and N-demethylation of DXM, yielding dextrorphan (DXO), the major active metabolite, and 3-hydroxymorphinan, the bi-demethylated product, respectively. The demethylation order described may reverse being the resultant mid product 3-methoxymorphinan. UDP-glucuronosyltranferase produces glucuronide conjugates. Genotypic variations in enzymes and interactions with other drugs can result in large inter-individual variability in the pharmacological and toxicological effects produced. Knowing the metabolism of DXM may help to better understand the inter-individual variability in the pharmacokinetics and pharmacodynamics and to avoid adverse effects.

Unraveling the Mechanisms Behind the Complete Suppression of Cocaine Electrochemical Signals by Chlorpromazine, Promethazine, Procaine, and Dextromethorphan.

The present work investigates the challenges accompanied by the electrochemical cocaine detection in physiological conditions (pH 7) in the presence of chlorpromazine, promethazine, procaine, and dextromethorphan, frequently used cutting agents in cocaine street samples. The problem translates into the absence of the cocaine oxidation signal (signal suppression) when in a mixture with one of these compounds, leading to false negative results. Although a solution to this problem was provided through earlier experiments of our group, the mechanisms behind the suppression are now fundamentally investigated via electrochemical and liquid chromatography quadrupole-time-of-flight mass spectrometry (LC-QTOF-MS) strategies. The latter was used to confirm the passivation of the electrodes due to their interaction with promethazine and chlorpromazine. Electron transfer mechanisms were further identified via linear sweep voltammetry. Next, adsorption experiments were performed on the graphite screen printed electrodes both with and without potential assistance in order to confirm if the suppression of the cocaine signals is due to passivation induced by the cutting agents or their oxidized products. The proposed strategies allowed us to identify the mechanisms of cocaine suppression for each cutting agent mentioned. Suppression due to procaine and dextromethorphan is caused by fouling of the electrode surface by their oxidized forms, while for chlorpromazine and promethazine the suppression of the cocaine signal is related to the strong adsorption of these (nonoxidized) cutting agents onto the graphite electrode surface. These findings provide fundamental insights in possible suppression and other interfering mechanisms using electrochemistry in general not only in the drug detection sector.

Application of a quality-by-design approach for utilizing sodium stearyl fumarate as a taste-masking agent in dextromethorphan hydrobromide orally disintegrating tablets.

Orally dissolving tablets (ODTs) represent one of the recent advances in drug delivery. The foremost objective of this study was to optimize the utilization of lubricant sodium stearyl fumarate in the preparation of dextromethorphan hydrobromide ODTs with enhanced taste-masking properties. The simple blending of sodium stearyl fumarate with the powder bed would result in taste-masking through physical adsorption of the lubricant particles on the drug particles. A randomized 32 full factorial experimental design was used to characterize the relationship between lubricant ratio (X1), mixing time (X2), and the tablet properties. The tablets were assessed for friability, hardness, disintegration time, and in-vitro dissolution. All tablets showed hardness within the range of 3.0-3.7 kp, and the % loss in friability test was less than 1.1%. The in-vitro disintegration time ranged between 9 and 25 s. An in-vitro drug release study of the prepared ODTs showed that more than 90% of the drug was released within 30 min. A palatability test of the optimized formula conducted in human volunteers showed acceptable taste and mouthfeel with in-vivo disintegration time of 17 s. Thus, results obtained convincingly showed successful fast disintegration of the prepared tablets and acceptable palatability when using sodium stearyl fumarate as a taste masking agent.

Effects of Drug-Polymer Interactions on Tablet Properties During the Development of Abuse-Deterrent Dosage Forms.

The objective of the present study is to understand the effects of drug-PEO interactions during the thermal treatment of polyethylene oxide (PEO)-based, directly compressed, abuse-deterrent formulations (ADFs). The drugs studied were dextromethorphan HBr monohydrate, ketoprofen, promethazine HCl, and anhydrous theophylline. Thermal treatment above the melting point of PEO resulted in tablets with higher crushing strength (> 500 N). It was observed that drug-PEO interactions during thermal treatment (80°C) led to solubilization of the incorporated drug. Drugs with higher solubility in the molten PEO, when added at higher weight fractions, interfered with the process of tablet densification which led to an increase in tablet dimensions and created defects in the fused matrix. These changes resulted in the formation of a more porous matrix. Thermal treatment led to a decrease in PEO crystallinity. The decreased crystallinity led to differences in the hydration and dissolution properties of the PEO. The change in dissolution properties of PEO accompanied with the dimensional and microstructural changes resulted in a greater drug release for some of the studied drugs. In conclusion, although thermal treatment above the melting point of PEO is an efficient manufacturing process in imparting crush-resistant features, drug-PEO interactions during the thermal treatment and the impact of thermal treatment on the properties of formulation components may impact tablet properties and lead to potential performance differences.

Reversible and Selective Encapsulation of Dextromethorphan and ß-Estradiol Using an Asymmetric Molecular Capsule Assembled via the Weak-Link Approach.

An allosterically regulated, asymmetric receptor featuring a binding cavity large enough to accommodate three-dimensional pharmaceutical guest molecules as opposed to planar, rigid aromatics, was synthesized via the Weak-Link Approach. This architecture is capable of switching between an expanded, flexible "open" configuration and a collapsed, rigid "closed" one. The structure of the molecular receptor can be completely modulated in situ through the use of simple ionic effectors, which reversibly control the coordination state of the Pt(II) metal hinges to open and close the molecular receptor. The substantial change in binding cavity size and electrostatic charge between the two configurations is used to explore the capture and release of two guest molecules, dextromethorphan and ß-estradiol, which are widely found as pollutants in groundwater.

Fluorescent bovine serum albumin interacting with the antitussive quencher dextromethorphan: a spectroscopic insight.

The interaction of dextromethorphan hydrobromide (DXM) with bovine serum albumin (BSA) is studied by using fluorescence spectra, UV-vis absorption, synchronous fluorescence spectra (SFS), 3D fluorescence spectra, Fourier transform infrared (FTIR) spectroscopy and circular dichroism under simulated physiological conditions. DXM effectively quenched the intrinsic fluorescence of BSA. Values of the binding constant, K(A), are 7.159 × 10(3), 9.398 × 10(3) and 16.101 × 10(3) L/mol; the number of binding sites, n, and the corresponding thermodynamic parameters ΔG°, ΔH° and ΔS° between DXM and BSA were calculated at different temperatures. The interaction between DXM and BSA occurs through dynamic quenching and the effect of DXM on the conformation of BSA was analyzed using SFS. The average binding distance, r, between the donor (BSA) and acceptor (DXM) was determined based on Förster's theory. The results of fluorescence spectra, UV-vis absorption spectra and SFS show that the secondary structure of the protein has been changed in the presence of DXM.

Development and validation of a sensitive UHPLC-MS/MS method for the simultaneous analysis of tramadol, dextromethorphan chlorpheniramine and their major metabolites in human plasma in forensic context: application to pharmacokinetics.

The prerequisites for forensic confirmatory analysis by LC/MS/MS with respect to European Union guidelines are chromatographic separation, a minimum number of two MS/MS transitions to obtain the required identification points and predefined thresholds for the variability of the relative intensities of the MS/MS transitions (MRM transitions) in samples and reference standards. In the present study, a fast, sensitive and robust method to quantify tramadol, chlorpheniramine, dextromethorphan and their major metabolites, O-desmethyltramadol, dsmethyl-chlorpheniramine and dextrophan, respectively, in human plasma using ibuprofen as internal standard (IS) is described. The analytes and the IS were extracted from plasma by a liquid-liquid extraction method using ethyl acetate-diethyl-ether (1:1). Extracted samples were analyzed by ultra-high-performance liquid chromatography coupled to electrospray ionization tandem mass spectrometry (UHPLC-ESI-MS/MS). Chromatographic separation was performed by pumping the mobile phase containing acetonitrile, water and formic acid (89.2:11.7:0.1) for 2.0 min at a flow rate of 0.25 µL/min into a Hypersil-Gold C18 column, 20 × 2.0 mm (1.9 µm) from Thermoscientific, New York, USA. The calibration curve was linear for the six analytes. The intraday precision (RSD) and accuracy (RE) of the method were 3-9.8 and -1.7-4.5%, respectively. The analytical procedure herein described was used to assess the pharmacokinetics of the analytes in 24 healthy volunteers after a single oral dose containing 50 mg of tramadol hydrochloride, 3 mg chlorpheniramine maleate and 15 mg of dextromethorphan hydrobromide.

Deuterated Drugs.

Many drugs are carbon-based, and carbon-hydrogen bonding is particularly relevant for understanding important properties of drug molecules. Deuteration refers to the selective replacement of protium hydrogen isotope atoms in small-molecule drugs with deuterium hydrogen isotope atoms. Deuteration of a drug is most likely to affect pharmacokinetic properties, such as metabolism, rather than its pharmacodynamic effects. For this reason, the metabolism of certain drugs may be favorably influenced when deuterium is substituted for protium, resulting in improved safety, tolerability, or efficacy. Examples of deuterated drugs that have been evaluated in clinical studies include paroxetine, tetrabenazine, and dextromethorphan.

Synthesis, in vitro and in vivo studies, and molecular modeling of N-alkylated dextromethorphan derivatives as non-competitive inhibitors of α3ß4 nicotinic acetylcholine receptor.

9 N-alkylated derivatives of dextromethorphan are synthesized and studied as non-competitive inhibitors of α3ß4 nicotinic acetylcholine receptors (nAChRs). In vitro activity towards α3ß4 nicotinic acetylcholine receptor is determined using a patch-clamp technique and is in the micromolar range. Homology modeling, molecular docking and molecular dynamics of ligand-receptor complexes in POPC membrane are used to find the mode of interactions of N-alkylated dextromethorphan derivatives with α3ß4 nAChR. The compounds, similarly as dextromethorphan, interact with the middle portion of α3ß4 nAChR ion channel. Finally, behavioral tests confirmed potential application of the studied compounds for the treatment of addiction.

Nonaqueous capillary electrophoresis of dextromethorphan and its metabolites.

This study deals with the nonaqueous capillary electrophoretic separation of dextromethorphan and its metabolites using a methanolic background electrolyte. The optimization of separation conditions was performed in terms of the resolution of dextromethorphan and dextrorphan and the effect of separation temperature, voltage, and the characteristics of the background electrolyte were studied. Complete separation of all analytes was achieved in 40 mM ammonium acetate dissolved in methanol. Hydrodynamic injection was performed at 3 kPa for 4 s. The separation voltage was 20 kV accompanied by a low electric current. The ultraviolet detection was performed at 214 nm, the temperature of the capillary was 25°C. These conditions enabled the separation of four analytes plus the internal standard within 9 min. Further, the developed method was validated in terms of linearity, sensitivity, and repeatability. Rat liver perfusate samples were subjected to the nonaqueous capillary electrophoretic method to illustrate its applicability.

Understanding the determinants of selectivity in drug metabolism through modeling of dextromethorphan oxidation by cytochrome P450.

Cytochrome P450 enzymes play key roles in the metabolism of the majority of drugs. Improved models for prediction of likely metabolites will contribute to drug development. In this work, two possible metabolic routes (aromatic carbon oxidation and O-demethylation) of dextromethorphan are compared using molecular dynamics (MD) simulations and density functional theory (DFT). The DFT results on a small active site model suggest that both reactions might occur competitively. Docking and MD studies of dextromethorphan in the active site of P450 2D6 show that the dextromethorphan is located close to heme oxygen in a geometry apparently consistent with competitive metabolism. In contrast, calculations of the reaction path in a large protein model [using a hybrid quantum mechanical-molecular mechanics (QM/MM) method] show a very strong preference for O-demethylation, in accordance with experimental results. The aromatic carbon oxidation reaction is predicted to have a high activation energy, due to the active site preventing formation of a favorable transition-state structure. Hence, the QM/MM calculations demonstrate a crucial role of many active site residues in determining reactivity of dextromethorphan in P450 2D6. Beyond substrate binding orientation and reactivity of Compound I, successful metabolite predictions must take into account the detailed mechanism of oxidation in the protein. These results demonstrate the potential of QM/MM methods to investigate specificity in drug metabolism.

Simultaneous quantitative analysis of dextromethorphan, dextrorphan and chlorphenamine in human plasma by liquid chromatography-electrospray tandem mass spectrometry.

A sensitive and accurate HPLC-MS/MS method was developed for the simultaneous determination of dextromethorphan, dextrorphan and chlorphenamine in human plasma. Three analytes were extracted from plasma by liquid-liquid extraction using ethyl acetate and separated on a Kromasil 60-5CN column (3 µm, 2.1 × 150 mm) with mobile phase of acetonitrile-water (containing 0.1% formic acid; 50:50, v/v) at a flow rate of 0.2 mL/min. Quantification was performed on a triple quadrupole tandem mass spectrometer in multiple reaction monitoring mode using positive electrospray ionization. The calibration curve was linear over the range of 0.01-5 ng/mL for dextromethorphan, 0.02-5 ng/mL for dextrorphan and 0.025-20 ng/mL for chlorphenamine. The lower limits of quantification for dextromethorphan, dextrorphan and chlorphenamine were 0.01, 0.02 and 0.025 ng/mL, respectively. The intra- and inter-day precisions were within 11% and accuracies were in the range of 92.9-102.5%. All analytes were proved to be stable during sample storage, preparation and analytic procedures. This method was first applied to the pharmacokinetic study in healthy Chinese volunteers after a single oral dose of the formulation containing dextromethorphan hydrobromide (18 mg) and chlorpheniramine malaeate (8 mg).

Engineered Imine Reductase for Asymmetric Synthesis of Dextromethorphan Key Intermediate.

(S)-1-(4-Methoxybenzyl)-1,2,3,4,5,6,7,8-octahydroisoquinoline ((S)-1-(4-methoxybenzyl)-OHIQ) is the key intermediate of the nonopioid antitussive dextromethorphan. In this study, (S)-IR61-V69Y/P123A/W179G/F182I/L212V (M4) was identified with a 766-fold improvement in catalytic efficiency compared with wide-type IR61 through enzyme engineering. M4 could completely convert 200 mM of 1-(4-methoxybenzyl)-3,4,5,6,7,8-hexahydroisoquinoline into (S)-1-(4-methoxybenzyl)-OHIQ in 77% isolated yield, with >99% enantiomeric excess and a high space-time yield of 542 g L-1 day-1, demonstrating a great potential for the synthesis of dextromethorphan intermediate in industrial applications.

A novel electrochemical sensor based on a Cu-coordinated molecularly imprinted polymer and MoS2 modified chitin-derived carbon for selective detection of dextromethorphan.

Dextromethorphan (DXM) is a widely utilized central antitussive agent, which is frequently abused by individuals seeking its recreational effect. But DXM overdose can cause some adverse effects, including brain damage, loss of consciousness, and cardiac arrhythmias, and hence its detection is significant. Herein, an electrochemical sensor based on a Cu-coordinated molecularly imprinted polymer (Cu-MIP) was fabricated for its detection. For constructing the sensor, nitrogen-doped carbon nanosheets (CCNs) were prepared through calcining chitin under an argon atmosphere, and molybdenum disulfide (MoS2) was allowed to grow on their surface. Subsequently, the obtained MoS2/CCNs composite was employed to modify a glassy carbon electrode (GCE), and the Cu-MIP was electrodeposited on the electrode in a Cu-1,10-phenanthroline (Cu-Phen) solution containing DXM, where Cu2+ played a role in facilitating electron transfer and binding DXM. Due to the large specific surface area, good electrocatalytic properties and recognition of the resulting composite, the resulting Cu-MIP/MoS2/CCNs/GCE showed high selectivity and sensitivity. Under optimized experimental conditions, the peak current of DXM and its concentration exhibited a good linear relationship over the concentration range of 0.1-100 µM, and the limit of detection (S/N = 3) was 0.02 µM. Furthermore, the electrochemical sensor presented good stability, and it was successfully used for the determination of DXM in pharmaceutical, human serum and urine samples.

Influence of Carbopol 71G-NF on the release of dextromethorphan hydrobromide from extended-release matrix tablets.

The objective of this study was to evaluate the potential of Carbopol(®) 71G-NF on the release of dextromethorphan hydrobromide (DM) from matrix tablets in comparison with hydroxypropyl methylcellulose (HPMC(®) K15M) and Eudragit(®) L100-55 polymers. Controlled release DM matrix tablets were prepared using Carbopol 71G-NF, HPMC K15M, and Eudragit L100-55 at different drug to polymer ratios by direct compression technique. The mechanical properties of the tablets as tested by crushing strength and friability tests were improved as the concentration of Carbopol, HPMC, and Eudragit increased. However, Carbopol-based tablets showed a significantly (P<0.05) higher crushing strength and a lower friability than HPMC and Eudragit tablets. No significant differences in weight uniformity and thickness values were observed between the different formulations. It was also found that Carbopol significantly (P<0.05) delayed the release of DM in comparison with HPMC K15M and Eudragit L100-55. A combination of HPMC K15M and Eudragit L100-55 in a 1:1 ratio at 20 and 30% significantly (P<0.05) delayed the release of DM than Eudragit L100-55 alone. Moreover, blends of Carbopol and HPMC at a 1:1 ratio at the 10, 20, and 30% total polymer concentration were investigated. The blend of Carbopol and HPMC at 10% level significantly (P<0.05) slowed the release of DM than Carbopol or HPMC alone, whereas blends at 20 and 30% level significantly (P<0.05) delayed the release of DM compared with HPMC or Carbopol alone. The results with these polymer blends showed that it was possible to reduce the total amount of polymers when used as a combination in formulation.

Development and optimization of dextromethorphan hydrobromide oral disintegrating tablets: effect of formulation and process variables.

Orally disintegrating tablets (ODTs), which disintegrate rapidly (<1 min) in the mouth and do not require water for administration, have become a very popular dosage form. The study aims to develop a simple and inexpensive method of manufacturing ODTs of a sparingly water-soluble drug, Dextromethorphan hydrobromide. Two factors, three levels (3(2)) full factorial design was used to optimize the diluent, microcrystalline cellulose (X(1)) and superdisintegrant, croscarmellose sodium (X(2)) concentrations. Disintegration time, hardness and T(50) values for all the formulations varied from 12.5 to 152.6 s, 3.58 to 4.92 kp and 0.8 to 2.8 min, respectively. The results indicated that the selected variables have a strong influence on disintegration time, hardness and T(50) of the ODTs. The manufactured ODTs formula composed of 30% microcrystalline cellulose in combination with 3% croscarmellose sodium was chosen as optimized formula, as it showed the lowest disintegration time (12.5 ± 1.22 s), low T(50) (0.8 min.) and hard tablets (4.92 ± 0.28 kp) amongst other tested ODTs formulations. Hardness of DM ODTs was not affected by changing the type of superdisintegrant and lubricant. The disintegration time was significantly (p < 0.05) increased by using sodium starch glycolate instead of croscarmellose sodium.

Development of an enantioselective high-performance liquid chromatography-tandem mass spectrometry method for the quantitative determination of methorphan and its O-demethylated metabolite in human blood and its application to post-mortem samples.

In the present work an isocratic enantioselective high-performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed for the separation and quantitative determination of dextro - and levo -methorphan and their pharmacologically relevant metabolites, dextrorphan and levorphanol, respectively, in human blood samples. The separation of enantiomers of methorphan and metabolites was performed on the polysaccharide-based chiral column Lux AMP in combination with acetonitrile and 5 mM aqueous ammonium bicarbonate pH 11 in the ratio 50:50 (%, v/v) as mobile phase with the flow rate 1 mL/min. The mass spectrometer was operated in scheduled multiple reaction monitoring (MRM) mode, with four transitions for each dextromethorpan, levomethorphan, dextrorphan and dextromethorphan-d3 and two transitions for each levorphanol, levorphanol-d3 and dextrorphan-d3. Application of this method to human post-mortem blood samples confirmed cases of severe overdosing with dextromethorphan, levomethorphan, and less commonly with both.

Prediction of relative in vivo metabolite exposure from in vitro data using two model drugs: dextromethorphan and omeprazole.

Metabolites can have pharmacological or toxicological effects, inhibit metabolic enzymes, and be used as probes of drug-drug interactions or specific cytochrome P450 (P450) phenotypes. Thus, better understanding and prediction methods are needed to characterize metabolite exposures in vivo. This study aimed to test whether in vitro data could be used to predict and rationalize in vivo metabolite exposures using two model drugs and P450 probes: dextromethorphan and omeprazole with their primary metabolites dextrorphan, 5-hydroxyomeprazole (5OH-omeprazole), and omeprazole sulfone. Relative metabolite exposures were predicted using metabolite formation and elimination clearances. For dextrorphan, the formation clearances of dextrorphan glucuronide and 3-hydroxymorphinan from dextrorphan in human liver microsomes were used to predict metabolite (dextrorphan) clearance. For 5OH-omeprazole and omeprazole sulfone, the depletion rates of the metabolites in human hepatocytes were used to predict metabolite clearance. Dextrorphan/dextromethorphan in vivo metabolite/parent area under the plasma concentration versus time curve ratio (AUC(m)/AUC(p)) was overpredicted by 2.1-fold, whereas 5OH-omeprazole/omeprazole and omeprazole sulfone/omeprazole were predicted within 0.75- and 1.1-fold, respectively. The effect of inhibition or induction of the metabolite's formation and elimination on the AUC(m)/AUC(p) ratio was simulated. The simulations showed that unless metabolite clearance pathways are characterized, interpretation of the metabolic ratios is exceedingly difficult. This study shows that relative in vivo metabolite exposure can be predicted from in vitro data and characterization of secondary metabolism of probe metabolites is critical for interpretation of phenotypic data.