Dextromethorphan can be N-demethylated to 3-methoxymorphinan by CYP3A4, CYP2D6, and CYP2C9 or O-demethylated to dextrorphan by CYP2D6 and CYP2C9. Dextrorphan is N-demethylated by CYP3A4 and CYP2D6, while 3-methoxymorphinan is O-demethylated by CYP2D6. Both are metabolized to form 3-hydroxymorphinan. Dextrorphan and 3-hydroxymorphinan are both O-glucuronidated or O-sulfated.
Genetic polymorphism has profound effects on its metabolism. Dextromethorphan undergoes polymorphic metabolism depending on variation in cytochrome P-450 enzyme phenotype. The specific cytochrome P-450 enzyme is P450 2D6(CYP2D6). Fast metabolizers constitute about 84% of the population. After a 30 mg dose plasma levels are less than 5 ng/mL four hours postingestion. Intermediate metabolizers constitute about 6.8% of the population. After an oral dose of 30 mg plasma levels are 10 to 20 ng/mL at 4 hours and less than 5 ng/mL at 24 hours postingestion. Poor metabolizers constitute 5% to 10% of the Caucasian population. The ratio of metabolite to parent drug in 8 hour urine sample is less than 10 to 1 after a 15 mg dose. After an oral dose of 30 mg plasma levels are greater than 10 ng/mL at 4 hours and greater than 5 ng/mL at 24 hours.
There is a clear first pass metabolism and it is generally assumed that the therapeutic activity is primarily due to its active metabolite, dextrophan.
It is metabolized in the liver by extensive metabolizers to dextrorphan. Dextrorphan is itself an active antitussive compound. Only small amounts are formed in poor metabolizers. Less than 15% of the dose form minor metabolites including D-methoxymorphinane.
The pentose phosphate pathway (PPP) is involved in the activity of glucose-6-phosphate dehydrogenase (G6PD) and generation of NADPH, which plays a key role in drug metabolism. The aim of this study was to investigate the effects of modulation of the PPP on drug metabolism capacity in vitro. A pair of hepatic cell lines, ie, the cancerous HepG2 cells and normal L02 cells, was used. The expression of CYP450 enzymes, p53 and G6PD in the cells were analyzed. The metabolism of testosterone (TEST, 10 umol/L) and dextromethorphan (DEM, 1 umol/L), the two typical substrates for CYP3A4 and CYP2D6, in the cells was examined in the presence of different agents. Both the expression and metabolic activities of CYP3A4 and CYP2D6 were considerably higher in HepG2 cells than in L02 cells. The metabolism of TEST and DEM in HepG2 cells was dose-dependently inhibited by the specific CYP3A4 inhibitor ketoconazole and CYP2D6 inhibitor quinidine. Addition of the p53 inhibitor cyclic PFT-alpha (5, 25 umol/L) in HepG2 cells dose-dependently enhanced the metabolism of DEM and TEST, whereas addition of the p53 activator NSC 66811 (3, 10, 25 umol/L) dose-dependently inhibited the metabolism. Furthermore, addition of the G6PD inhibitor 6-aminonicotinamide (5, 15 umol/L) in HepG2 cells dose-dependently inhibited the metabolism of DEM and TEST, whereas addition of the PPP activity stimulator menadione (1, 5, 15 umol/L) dose-dependently enhanced the metabolism. Modulation of p53 and the PPP alters the metabolism of DEM and TEST, suggesting that the metabolic flux pattern of PPP may be closely involved in drug metabolism and the individual variance.
IDENTIFICATION AND USE: Dextromethorphan is a white odorless solid crystal or powder. It is an opium alkaloid derivative, which is found in cough and cold preparations. HUMAN STUDIES: Dextromethorphan is a drug of abuse. The main risks associated with dextromethorphan are ataxia, central nervous system (CNS) stimulation, dizziness, lethargy and psychotic behavior. Less frequently with large doses seizures and respiratory depression can occur. Nausea, vomiting, constipation and tachycardia may also occur. CNS effects include ataxia, drowsiness, vertigo and rarely coma. CNS stimulation may be observed. Restlessness, increased muscle tone with body rigidity have been reported. With extremely large ingestions respiratory depression can occur. Gastrointestinal effects include nausea, vomiting, constipation and dry mouth. Urinary retention may be seen. Dextromethorphan abuse has been described and produces euphoria, CNS stimulation, visual and/or auditory hallucinations. Dextromethorphan hydrobromide is the drug form, thus the possibility of bromide poisoning should be considered in the long term abuser. Dextromethorphan enhances serotonin activity by inhibiting the reuptake of serotonin. Specific non-opioid dextromethorphan binding sites are present in the CNS which mediate the antitussive effects, separate from codeine and other opioids. Death has been reported after overdose in two cases but quantity was uncertain. In adults, it can cause intoxication with hyperexcitability, visual and/or auditory hallucinations. It has been reported that sniffing two to three times a day over 2 to 3 months produced euphoria and restlessness for up to 2 hours followed by dizziness, nausea, depression and fatigue. Toxicity may be variable in children. Some children have shown no symptoms while others have shown ataxia, stupor, transient fever, lethargy or nystagmus. Seizures have also been reported. Long-acting products may be more toxic in children, producing prolonged CNS depression. There was no association between dextromethorphan and malformations. Dextromethorphan is generally considered safe to use during pregnancy. Urticaria was reported in a child after acute overdose of a long-acting preparation. A cutaneous lesion consistent with a fixed-drug reaction was reported after ingestion over 2 to 3 weeks in therapeutic doses. ANIMAL STUDIES: Dextromethorphan is a weak noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist. It is metabolized in vivo to dextrorphan, a more potent noncompetitive NMDA antagonist that is the dextrorotatory enantiomer of the opioid agonist levorphanol. Although in rats both dextromethorphan and dextrorphan increased plasma levels of adrenocorticotropic hormone (ACTH) and corticosterone, the dextromethorphan-induced responses occurred more rapidly than the dextrorphan-induced responses. Three sets of zebrafish embryos/larvae were exposed to dextromethorphan at 24, 48 and 72 hr 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. After treatment with a high-dose of dextromethorphan (40 mg/kg/day) for 2 weeks, rats showed increased depression-like behavior when compared to the control animals. Neurogenesis in the hippocampus was suppressed by dextromethorphan treatment, which was indicated by decreases in number of proliferative cells and immature neurons. Dextromethorphan was not genotoxic in mice.
参考文献：M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. 美国食品药品监督管理局批准的药物标签用于研究药物诱导的肝损伤，《药物发现今日》，16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank：按人类发展药物诱导肝损伤风险排名的最大参考药物清单。《药物发现今日》2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
References:M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. FDA-Approved Drug Labeling for the Study of Drug-Induced Liver Injury, Drug Discovery Today, 16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank: the largest reference drug list ranked by the risk for developing drug-induced liver injury in humans. Drug Discov Today 2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
Dextromethorphan is rapidly absorbed from the GI tract and exerts its antitussive effect in 15-30 minutes after oral administration. The duration of action is approximately 3-6 hours with conventional dosage forms.
Dextromethorphan and its metabolites are excreted via the kidney. Depending on the metabolism phenotype up to 11% may be excreted unchanged or up to 100% as demethylated conjugated morphinan compounds. In the first 24 hours after dosing, less than 0.1% is eliminated in the feces.
Dextromethorphan is well absorbed from the gastrointestinal tract with maximum serum level occurring at 2.5 hours. Peak concentration of the major metabolite dextrorphan) was 1.6 to 1.7 hours.
