Metabolism of Dextrorphan by CYP2D6 in Different Recombinantly Expressed Systems and its Implications for the In Vitro Assessment of Dextromethorphan Metabolism

Effects of Saikosaponin D on CYP1A2 and CYP2D6 in HepaRG Cells

Background

Bupleurum is one of the most important traditional Chinese medicines and an ingredient in many compound preparations. It is widely used together with other drugs in clinical practice, and thus there is great potential for drug–drug interactions. Saikosaponin D (SsD) is a major bioactive triterpenoid saponin extracted from Bupleurum with anti-inflammatory, anticancer, antioxidative, and antihepatic fibrosis effects. Effects of the main components of Bupleurum on cytochromes P450 (CYPs) need to be clarified in the clinical application of combination therapies of formulations containing SsD or Bupleurum.

Purpose

This study aimed to investigate the effects of SsD on the CYP1A2 and CYP2D6 mRNAs, protein expression, and relative enzyme activities in HepaRG cells.

Methods

HepaRG cells were cultured with SsD at concentrations of 0.5, 1, 5 and 10 μM for 72 hours. mRNA and protein expression of CYP1A2 and CYP2D6 were analyzed with real-time PCR and Western blot analysis. Relative enzyme activities were analyzed with HPLC based on consumption of the specific probe substrate.

Results

SsD significantly induced expression of mRNA and increased relative activity of CYP1A2 in HepaRG cells after the cells had been treated with SsD at concentrations of 1, 5 and 10 μM. SsD also induced protein expression of CYP1A2 at concentrations of 5 and 10 μM. SsD exhibited an inductive effect on CYP2D6 mRNA and protein expression, while increasing the relative activity of CYP2D6 at concentrations of 5 and 10 μM.

Conclusion

This study is the first to investigate the effect of SsD on CYP1A2 and CYP2D6 in HepaRG cells, and the results may provide some useful information on potential drug–drug interactions related to clinical preparations containing SsD or Bupleurum.

CYP450-Mediated Metabolism of Mitragynine and Investigation of Metabolites in Human Urine

Mitragyna speciosa (Kratom) has emerged as a recreational drug and a substance of medicinal intrigue. Although the drug was initially used recreationally for its sedating and euphoric effects, more recently its use has been associated with the non-medically supervised treatment of opioid abstinence syndrome. Mitragynine is the principal pharmacologically active alkaloid in kratom. Although metabolites of mitragynine have been identified, the cytochrome P450 (CYP450) enzymes responsible for its biotransformation are still under investigation. The goal of this study was to contribute further knowledge regarding CYP450 activity as it relates to mitragynine. Recombinant cytochrome P450 enzymes (rCYPs) were used to investigate the isoforms involved in its metabolism. Biotransformational products were identified using liquid chromatography-quadrupole/time of flight-mass spectrometry. Four rCYP enzymes (2C18, 2C19, 2D6 and 3A4) were found to contribute to the metabolism of mitragynine. 7-Hydroxymitragynine (which has an affinity for the mu-opioid receptor >10-folds that of morphine) was produced exclusively by 3A4. 9-O-demethylmitragynine, the most abundant metabolite in vitro (and the most prevalent metabolite in urine among kratom users) was produced by 2C19, 3A4 and 2D6. 16-Carboxymitragynine was produced by rCYPs 2D6, 2C19 and 2C18. 2C19 was solely responsible for the formation of 9-O-demethyl-16-carboxymitragynine. In vitro rCYP studies were compared with phase I metabolites in urine from cases involving mitragynine.

Heterologous expression of human cytochromes P450 2D6 and CYP3A4 in Escherichia coli and their functional characterization

This study aimed to express two major drug-metabolizing human hepatic cytochromes P450 (CYPs), CYP2D6 and CYP3A4, together with NADPH-cytochrome P450 oxidoreductase (OxR) in Escherichia coli and to evaluate their catalytic activities. Full length cDNA clones of both isoforms in which the N-terminus was modified to incorporate bovine CYP17α sequence were inserted into a pCWori(+) vector. The modified CYP cDNAs were subsequently expressed individually, each together with OxR by means of separate, compatible plasmids with different antibiotic selection markers. The expressed proteins were evaluated by immunoblotting and reduced CO difference spectral scanning. Enzyme activities were examined using high performance liquid chromatography (HPLC) assays with probe substrates dextromethorphan and testosterone for CYP2D6 and CYP3A4, respectively. Results from immunoblotting demonstrated the presence of both CYP proteins in bacterial membranes and reduced CO difference spectra of the cell preparations exhibited the characteristic absorbance peak at 450 nm. Co-expressed OxR also demonstrated an activity level comparable to literature values. Kinetic parameters, K(m) and V(max) values determined from the HPLC assays also agreed well with literature values. As a conclusion, the procedures described in this study provide a relatively convenient and reliable means of producing catalytically active CYP isoforms suitable for drug metabolism and interaction studies.

Developmental toxicity of dextromethorphan in zebrafish embryos/larvae

Dextromethorphan is widely used in over-the-counter cough and cold medications. Its efficacy and safety for infants and young children remains to be clarified. The present study was designed to use zebrafish as a model to investigate the potential toxicity of dextromethorphan during embryonic and larval development. Three sets of zebrafish embryos/larvae were exposed to dextromethorphan at 24, 48 and 72 h post fertilization (hpf), respectively, during the embryonic/larval development. Compared with the 48 and 72 hpf exposure sets, the embryos/larvae in the 24 hpf exposure set showed much higher mortality rates which increased in a dose-dependent manner. Bradycardia and reduced blood flow were observed for the embryos/larvae treated with increasing concentrations of dextromethorphan. Morphological effects of dextromethorphan exposure, including yolk sac and cardiac edema, craniofacial malformation, lordosis, non-inflated swim bladder and missing gill, were also more frequent and severe among zebrafish embryos/larvae exposed to dextromethorphan at 24 hpf. Whether the more frequent and severe developmental toxicity of dextromethorphan observed among the embryos/larvae in the 24 hpf exposure set, as compared with the 48 and 72 hpf exposure sets, is due to the developmental expression of the phase I and phase II enzymes involved in the metabolism of dextromethorphan remains to be clarified. A reverse transcription-polymerase chain reaction analysis, nevertheless, revealed developmental stage-dependent expression of mRNAs encoding SULT3 ST1 and SULT3 ST3, two enzymes previously shown to be capable of sulfating dextrorphan, an active metabolite of dextromethorphan.

New insights into the structural characteristics and functional relevance of the human cytochrome P450 2D6 enzyme

To date, the crystal structures of at least 12 human CYPs (1A2, 2A6, 2A13, 2C8, 2C9, 2D6, 2E1, 2R1, 3A4, 7A1, 8A1, and 46A1) have been determined. CYP2D6 accounts for only a small percentage of all hepatic CYPs (< 2%), but it metabolizes approximately 25% of clinically used drugs with significant polymorphisms. CYP2D6 also metabolizes procarcinogens and neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3,4-tetrahydroquinoline, and indolealkylamines. Moreover, the enzyme utilizes hydroxytryptamines and neurosteroids as endogenous substrates. Typical CYP2D6 substrates are usually lipophilic bases with an aromatic ring and a nitrogen atom, which can be protonated at physiological pH. Substrate binding is generally followed by oxidation (5-7 A) from the proposed nitrogen-Asp301 interaction. A number of homology models have been constructed to explore the structural features of CYP2D6, while antibody studies also provide useful structural information. Site-directed mutagenesis studies have demonstrated that Glu216, Asp301, Phe120, Phe481, and Phe483 play important roles in determining the binding of ligands to CYP2D6. The structure of human CYP2D6 has been recently determined and shows the characteristic CYP fold observed for other members of the CYP superfamily. The lengths and orientations of the individual secondary structural elements in the CYP2D6 structure are similar to those seen in other human CYP2 members, such as CYP2C9 and 2C8. The 2D6 structure has a well-defined active-site cavity located above the heme group with a volume of approximately 540 A(3), which is larger than equivalent cavities in CYP2A6 (260 A(3)), 1A2 (375 A(3)), and 2E1 (190 A(3)), but smaller than those in CYP3A4 (1385 A(3)) and 2C8 (1438 A(3)). Further studies are required to delineate the molecular mechanisms involved in CYP2D6 ligand interactions and their implications for drug development and clinical practice.