氮烯乙茶在大鼠、犬、猴肝亚细胞成分中的生物转化

目的:利用肝亚细胞成分研究I类抗癌新药氮烯乙茶的生物转化及其种属差异。方法:以高效液相色谱法检测肝9 000 ×g 上清液(S-9) 和微粒体中氮烯乙茶及其代谢产物的浓度。结果:大鼠、犬、猴肝S-9中氮烯乙茶的固有清除率(CL_int) 分别为14-9,10-4,30-4 μL·min^-1·mg^-1 protein,代谢产物为7-乙基-8-氨基茶碱(7-ethyl-8-aminotheophylline,EAT),氮烯乙茶在肝的生物转化部位在微粒体。结论:预计氮烯乙茶在生物转化途径上的种属差异较小;肝亚细胞成分是研究药物代谢及种属差异的较好体外模型。     

重组α-L-鼠李糖苷酶制备普鲁宁

目的 利用重组α-L-鼠李糖苷酶,以柚皮苷为底物,通过生物转化法制备普鲁宁。方法 以HPLC检测生物转化反应产物,通过DNS法对酶促反应过程进行动态分析,分析酶促反应动力学,优化普鲁宁制备工艺。结果 重组α-L-鼠李糖苷酶水解柚皮苷反应完全后,可得到反应产物普鲁宁和鼠李糖;生物转化法制备普鲁宁的最适反应温度为60 ℃、pH值为4.0、加酶量为12 U·mL^-1、底物浓度为2.0 g·L^-1,在此最优工艺条件下,94%的柚皮苷转化为普鲁宁;高浓度的Mn²⁺、Fe²⁺能够促进柚皮苷转化生成普鲁宁;重组酶对底物亲和力强,酶促反应的米氏常数K_m为1.02 μmol·mL^-1,V_max为0.19 μmol·mL^-1·min^-1。结论 利用重组α-L-鼠李糖苷酶生物转化法制备普鲁宁,转化率高、重现性好、产物易于分离,可为普鲁宁功能研究、开发利用及改性等提供重要依据。     

五味子醇甲在大鼠肝微粒体内的代谢动力学和性别差异

体外研究五味子醇甲(schizandrin，SZ)在大鼠肝微粒体内的代谢动力学和性别差异。制备正常雌、雄大鼠肝微粒体，与SZ共同温孵，以高效液相色谱法测定SZ及其代谢产物。SZ在雄鼠肝微粒体内代谢反应的最大速率V_max、米氏常数K_m和清除率Cl_int分别为(21.88±2.30) μmol·L^-1·min^-1·mg^-1(protein)，(389.00±46.26) μmol·L^-1和(0.056 3±0.000 7) min·mg^-1(protein)；在雌鼠肝微粒体内代谢反应的最大速率V_max、米氏常数K_m和清除率Clint_{分别为(0.61±0.07) μmol·L^-1·min^-1·mg^-1(protein)，(72.64±13.61) μmol·L^-1和(0.008 4±0.000 8) min·mg^-1(protein)，雌、雄鼠肝微粒体内SZ的主要代谢物不同，分别为7,8-顺二羟基五味子醇甲(M1)和7,8-顺二羟基-2-去甲基五味子醇甲(M2b)。酮康唑、奎尼丁和奥芬得林对SZ的在雌、雄大鼠肝微粒体内代谢均有不同程度的抑制作用，西咪替丁对其在雄鼠肝微粒体内的代谢也有一定的抑制作用。SZ在雌、雄大鼠肝微粒体中代谢动力学及代谢产物存在明显的性别差异，这种差异可能主要是由CYP3A和CYP2C11在大鼠肝微粒体内的性别差异引起的。}     

水胺硫磷在大鼠肝微粒体的生物转化和代谢动力学

目的 探讨水胺硫磷在大鼠肝微粒体的体外代谢活化产物及代谢动力学特征。方法 应用液相色谱-四级杆-飞行时间质谱(LC/QTOF MS)筛查并鉴定水胺硫磷在大鼠肝微粒体孵育液中的氧化产物。用乙酰胆碱酯酶抑制法考察水胺硫磷及其氧化产物水胺氧磷的抑酶活性。应用液相色谱-三重四级杆串联质谱(LC/Triple-Q MS/MS)定量检测肝微粒体中的水胺硫磷及其代谢产物水胺氧磷,研究水胺硫磷及其产物的消长动力学和氧化产物生成的酶动力学。结果 筛查并鉴定了水胺硫磷在大鼠肝微粒体的氧化脱硫产物水胺氧磷。水胺氧磷对乙酰胆碱酯酶的抑制活性远高于水胺硫磷,IC₅₀值比水胺硫磷低4个数量级,表明水胺硫磷的氧化脱硫反应是一个代谢活化过程。水胺硫磷在大鼠肝微粒体中的半衰期(t_1/2)为14.6 min,外推得到体内肝清除率Cl_H为43.8 ml·min^-1·kg^-1。水胺氧磷的生成符合双相酶动力学模型,K_m,app1为1.12 μmol·L^-1,产物生产最大速率(V_max1)为0.43 μmol·min^-1·g ^-1;蛋白K_m,app2为67.92 μmol·L^-1,V_max2为1.28 μmol·min^-1·g ^-1蛋白。结论 水胺硫磷在大鼠肝微粒体中能快速代谢消除,生成抑酶活性更高的产物水胺氧磷而产生毒性。     

HPLC同时测定护肝片中5种成分的含量

摘 要 目的：建立HPLC法同时测定护肝片中腺苷、五味子醇甲、五味子酯甲、五味子甲素和五味子乙素的含量。方法: 采用Agilent Eclipse XDB色谱柱（250 mm×4.6 mm,5 μm）,流动相为乙腈-0.7%磷酸溶液,梯度洗脱,流速为0.80 ml·min^-1,0～14 min检测波长为260 nm,14～58 min检测波长为250 nm,柱温为30℃,进样量为10 μl。结果: 腺苷、五味子醇甲、五味子酯甲、五味子甲素和五味子乙素的线性范围分别为2.35～47.00 μg·mL^-1（r=0.999 8）、14.90～297.00 μg·mL^-1（r=1.000 0)、3.46～69.20 μg·mL^-1（r=0.999 9）、4.00～80.10 μg·mL^-1（r=1.000 0）、3.42～68.30 μg·mL^-1（r=0.999 9）。5种成分的平均回收率均不低于98％。结论:本方法简便、准确、重复性好,可用于护肝片中腺苷、五味子醇甲、五味子酯甲、五味子甲素和五味子乙素的含量测定。     

细胞色素P450介导的氯沙坦钾对瑞格列奈在大鼠肝微粒体中代谢的影响

目的 研究瑞格列奈在大鼠肝微粒体中的酶促反应动力学，并考察氯沙坦钾对其在大鼠肝微粒体中代谢的影响。方法 建立大鼠肝微粒体体外孵育体系对瑞格列奈的代谢进行研究；以洛伐他汀为内标，应用UPLC测定大鼠肝微粒体中瑞格列奈的浓度。采用底物减少法，通过GraphPad Prism 5.0软件计算瑞格列奈的酶促反应动力学常数V_max和K_m；分别以系列浓度氯沙坦钾（2.5~50μmol·L^-1）与瑞格列奈（44 μmol·L^-1）于37℃水浴中共同孵育，并测定肝微粒体中瑞格列奈的减少量，考察氯沙坦钾对瑞格列奈的抑制作用。结果 瑞格列奈在大鼠肝微粒体的最佳孵育时间为40 min，最佳蛋白质量浓度为1 mg·mL^-1；瑞格列奈酶促反应动力学参数V_max=47.29μmol·min^-1·（mg·protein）^-1，K_m=51.41 μmol·L^-1；氯沙坦钾对瑞格列奈在体外肝微粒体抑制作用的IC₅₀值为17.89 μmol·L^-1。结论 氯沙坦钾对瑞格列奈在大鼠肝微粒体中的代谢具有较强的抑制作用，两药联合应用可能发生相互作用，具有诱发低血糖的风险。     

广藿香醇及广藿香油中广藿香醇在大鼠体内药代动力学比较

目的建立毛细管气相色谱法测定大鼠血浆中广藿香醇的方法；比较广藿香醇和广藿香油单次静脉注射给药后，广藿香醇在大鼠体内的药代动力学差异。方法以丁香酚为内标；用毛细管气相色谱法，色谱柱为HP-5MS 毛细管气相色谱柱 (30 m×0.32 mm×0.25 μm)；氢火焰离子化检测器；用程序升温法，初始温度80 ℃，保持1 min；15 ℃·min^-1升温至200 ℃，保持1 min；60 ℃·min^-1升温至290 ℃，保持1 min。结果广藿香醇在25～5 000 μg·L^-1线性关系良好（r=0.999 0），定量限为25 μg·L^-1，检测限为10 μg·L^-1，方法精密度（RSD％）小于10％，方法准确度为90%～110％。结论该法可用于广藿香醇的药代动力学研究；广藿香醇单体与广藿香油中广藿香醇在大鼠体内的主要药代动力学参数（AUC，t_1/2β，MRT等）有显著性差异。     

顶空进样气相色谱法；有机溶剂残留；蚕砂提取物

摘 要 目的：建立测定蚕砂提取物中有机溶剂残留量的气相色谱法。方法: 采用顶空进样气相色谱法,色谱柱：DB-5MS石英毛细管色谱柱（30 m×0.25 mm×0.25 μm）;载气：氮气;流速：0.6 ml·min^-1;进样口温度：200℃;检测器类型及温度：氢焰离子化检测器(FID),250℃;柱升温程序：40℃保持10 min,然后以5 ℃·min^-1的速度升至200℃,保持5 min;顶空条件：平衡温度： 95℃;平衡时间：30 min;传输管温度：110℃;进样环温度：125℃。结果: 丙酮,2,3-二甲基戊烷,3-甲基己烷,正庚烷,2,2-二甲基己烷,对二甲苯,间二甲苯,邻二甲苯,2,4,6-三甲基吡啶的线性范围分别为：101~3 034 μg·mL^-1,100~2 995 μg·mL^-1, 107~3 197 μg·mL^-1,101~3 019 μg·mL^-1,99~2 962 μg·mL^-1,45~1 358 μg·mL^-1,44~1 325 μg·mL^-1, 47~1 411 μg·mL^-1 和104~3 130 μg·mL^-1,r值均在0.992以上,回收率符合要求,空白溶剂对样品的测定没有影响,8批样品检验均符合限度要求。结论:顶空进样气相色谱法测定蚕砂提取物中有机溶剂,方法简单,结果准确。     

和厚朴酚、厚朴酚、栀子苷、绿原酸和黄芪甲苷对人和大鼠体外CYP1A2、CYP3A和CYP2D的抑制作用

目的 观察和厚朴酚、厚朴酚、栀子苷、绿原酸和黄芪甲苷5种中药成分体外对人和大鼠肝CYP1A2、CYP3A和CYP2D的抑制作用。方法 在人和大鼠肝微粒体孵育体系中,分别以非那西丁、咪达唑仑和右美沙芬为探针,应用HPLC检测受试物对探针代谢产物生成量的影响,评估5种中药成分对CYP1A2、CYP3A和CYP2D在该体系中的活性影响,并计算得到抑制率和IC₅₀。结果 和厚朴酚对人和大鼠CYP1A2、CYP2D的IC₅₀值分别为5.5、3.9、35.3和46.7 μmol·L^-1;厚朴酚对人CYP1A2、大鼠CYP1A2和CYP2D的IC50值分别为23.8,29.1和39.9 μmol·L^-1;栀子苷、绿原酸和黄芪甲苷对3种CYP酶亚型的IC₅₀均>100 μmol·L^-1;和厚朴酚对人和大鼠CYP3A的IC₅₀均>100 μmol·L^-1;厚朴酚对人CYP3A、CYP2D和大鼠CYP3A的IC₅₀均>100 μmol·L^-1。结论 和厚朴酚体外对人和大鼠CYP1A2和CYP2D有抑制作用,厚朴酚体外对人CYP1A2、大鼠CYP1A2和CYP2D有抑制作用,均呈浓度依赖性。     

代谢综合征MSG肥胖大鼠胰岛β 细胞功能紊乱机制的初步研究

评价代谢综合征动物模型MSG肥胖大鼠的胰岛β细胞功能，并对其功能紊乱机制进行初步研究。采用高葡萄糖钳夹技术证实肥胖性胰岛素抵抗MSG大鼠存在β细胞功能缺陷，GIR值较正常动物降低33.8%［MSG (23.1±3.2) mg·kg^{-1·min-1， NOR (34.9±8.4) mg·kg-1·min-1， P<0.05］；考察病理与生化指标显示MSG大鼠胰岛β细胞数目减少，胰腺甘油三酯含量、NO含量显著增加，SOD水平、胰腺线粒体中总ATP酶和Na+-K+-ATP酶活性均显著降低。结果提示，肥胖性胰岛素抵抗MSG大鼠具有典型的代谢综合征特征，同时伴有胰岛β细胞功能缺陷。这可能与其肥胖导致的炎症因子增加，机体抗氧化能力及膜结构中ATP酶活性下降有关。}     

Identification of cytochrome P4503A as the major subfamily responsible for the metabolism of roquinimex in man

1. Roquinimex, a novel immunomodulator, is metabolized in liver microsomes from mouse and rat via cytochrome P450s to four hydroxylated and two demethylated metabolites (R1?6). The study investigated which cytochrome P450 enzyme(s) is responsible for the metabolism of roquinimex in man. 2. Enzyme kinetic analysis demonstrated an apparent K_m = 1.28-7.00?mm and V_max = 50-159 pmol·mg^?1 microsomal protein·min^?1 for the primary metabolites in human liver microsomes. The sum of Cl_int for the primary pathways was 0.167 μl·mg^?1 microsomal protein·min^?1. 3. A correlation between the formation rate of R1-6 and 6β-hydroxylation of testosterone was obtained within a panel of liver microsomes from 11 individuals (r² = 0.72-0.97). Furthermore, significant inhibition (<90%) of roquinimex primary metabolism was demonstrated by ketoconazole and troleandomycin, specific inhibitors of CYP3A4 as well as with anti-CYP3A4 antibodies. Moreover, a similar metabolite pattern was produced from roquinimex by incubation with cDNA-expressed CYP3A4 as by human liver microsomes. 4. In conclusion, these data indicate a major role for CYP3A4 in the formation of roquinimex primary metabolites in human liver microsomes.     

Biotransformation and pharmacokinetics of the antiplasmodial naphthylisoquinoline alkaloid dioncophylline A

The biotransformation of the antiplasmodial naphthylisoquinoline alkaloid dioncophylline A by rat liver microsomes and its pharmacokinetics in male rats were studied. Incubation of dioncophylline A with rat liver microsomes resulted in the formation of the major metabolite 5′-O-demethyldioncophylline A, and a second minor metabolite, corresponding to the mass of an as yet unknown 4-hydroxydioncophylline A. Kinetic constants of the formation of 5′-O-demethyldioncophylline A were K_m?=?32?nmol and V_max?=?20?pmol?min^?1?mg^?1). Administration of dioncophylline A at a dose of 6.67?mg?kg^?1 body weight to rats intravenously and orally (n?=?4 per group) resulted in peak plasma levels of 0.84 and 0.11?µg?ml^?1, respectively. Levels of metabolites were below the limit of quantitation (LOQ). The following pharmacokinetic parameters of dioncophylline A were determined: oral bioavailability of 25%, plasma half-life of 2.5?h and partition volume of 8?l?kg^?1 body weight. Concentrations of dioncophylline A metabolites in all plasma and urine samples were below the limit of detection (LOD) and recovery of dioncophylline A in urine was very low, suggesting distribution into lipid rich tissues.     

The disposition of nifurtimox in the rat isolated perfused liver: effect of dose size

Abstract— The disposition of nifurtimox was studied in the rat isolated perfused liver using a recirculating system. The drug was administered as a bolus (5·0, 15·0 or 30·0 μg mL^?1), and its disappearance was monitored by analysing perfusate samples. In all experiments perfusate disappearance was monoexponential, and no significant difference was found between the three doses for the elimination constant (0·016, 0·011 and 0·012 min^?1, respectively), half-life (46·6, 65·8 and 66·8 min, respectively), extraction rate (0·128, 0·091 and 0·099, respectively) and distribution volume (41·1, 47·3 and 30·7 mL g^?1, respectively). At 30 μg mL^?1 the hepatic clearance was lower than the other concentrations of nifurtimox (0·66, 0·51 and 0·34 mL min^?1 g^?1, respectively). Relatively little parent drug was recovered from the liver at the end of the perfusions. In summary, nifurtimox is cleared slowly from the rat isolated perfused liver, is poorly extracted by hepatocyte cells and is completely metabolized from 2 to 4 h after perfusion.     

EXTRAHEPATIC FIRST-PASS METABOLISM OF NIFEDIPINE IN THE RAT

The peroral (po) bioavailability of nifedipine is reported to range from about 45 to 58% in the rat; this compares favourably to human beings. The metabolism of nifedipine is similar in rats and humans (oxidation of the dihydropyridine ring), with the liver believed to be solely responsible for the systemic clearance of the drug and the observed first-pass effect after po dosing. The purpose of this study was to determine whether intestinal metabolism also contributes to the first-pass elimination of nifedipine in the rat. The systemic availabilities of nifedipine doses given by po, intracolonic (ic), and intraperitoneal (ip) routes of administration were compared to that for an intravenous (iv) dose (in each case a dose of 6 mg kg⁻¹ was given) using adult male Sprague–Dawley rats (249–311 g, n =6 or 7/group). The geometric mean of systemic nifedipine plasma clearance after iv dosing was 10·3 mL min⁻¹ kg⁻¹. The nifedipine blood-to-plasma ratio was found to be about 0·59. Therefore, the systemic blood clearance of nifedipine was about 17·5 mL min⁻¹ kg⁻¹; which, compared to the hepatic blood flow of rats (55 to 80 mL min⁻¹ kg⁻¹) showed that nifedipine is poorly extracted by the liver (0·22≤E_H≤0·32). The mean absolute bioavailabilities of the po, ip, and ic doses were 61, 90, and 100%, respectively. Assuming complete absorption of the extravascular nifedipine doses these results indicate that, in addition to hepatic extraction, substantial first-pass elimination of nifedipine occurs within the wall of the small intestine but not the colon of the rat. © 1997 John Wiley & Sons, Ltd.     

Metabolism of Roxithromycin in the Isolated Perfused Rat Liver

Roxithromycin is a macrolide antibiotic with high clinical potency. N-Demethylation is considered to be one of the main pathways of roxithromycin metabolism in rats. We have studied the hepatic metabolism of roxithromycin in the isolated perfused rat liver. After addition of roxithromycin (30 μM) to the perfusion medium the parent compound and one major metabolite were detected in bile by high-performance liquid chromatography. The metabolite was identified as monodesmethylated roxithromycin by mass spectrometric analysis. Onset of biliary excretion of native roxithromycin was fast, reaching a maximum (130.52 ±43.88 pmol g^?1 min^?1) after only 10 min, whereas excretion of the metabolite was delayed (maximum 75.83 ± 11.92 pmol g^?1 min^?1 at 30 min). The cumulative excretion of roxithromycin and its metabolite into bile during the 60 min of application amounted to only 1.09 ± 0.30 and 0.64 ± 0.22% of the roxithromycin cleared from the perfusate during the same time. The liver content was 0.48 μmol (g liver)^?1, indicating high retention within the organ. No release of the metabolite into the perfusate was detected. In conclusion, this study has demonstrated the importance of phase-I metabolism for the biliary excretion of roxithromycin in rat liver. These findings might be predictive of roxithromycin biotransformation and biliary excretion in man.     

Identification of human cytochrome P450 enzymes involved in the formation of 4-hydroxyestazolam from estazolam

To predict drug interactions with estazolam, the biotransformation of estazolam to its major hydoxylated metabolite, 4-hydroxyestazolam was studied in vitro using pooled human liver microsomes and individual expressed human cytochrome P450 (CYP) enzymes. Estazolam was metabolized to 4-hydroxyestazolam according to the Hill kinetic model in pooled human liver microsomes. The K_m value for the 4-hydroxylation of estazolam was 24.1?µM, and the V_max value was 52.6?pmol?min^?1?mg^?1 protein. The formation of 4-hydroxyestazolam from estazolam in pooled human liver microsomes was significantly inhibited by itraconazole and erythromycin, specific CYP3A4 inhibitors, in a dose-dependent manner, with IC₅₀ values of 1.1 and 12.8?µM, respectively. When estazolam was incubated with expressed human CYP enzymes (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4), it was metabolized only by CYP3A4. In conclusion, the biotransformation of estazolam to 4-hydroxyestazolam was catalyzed by CYP3A4.     

血浆中去甲地西泮及代谢物奥沙西泮的HPLC测定方法和大鼠口服药代动力学

采用高效液相色谱法测定去甲地西泮(去甲安定)及其代谢产物奥沙西泮。以RP-C₁₈为固定相,乙腈—0.01mol·L^-1醋酸钠(pH3.8,33.3:66.6)为流动相,地西泮为内标物,紫外波长240nm处定量测定。去甲地西泮、奥沙西泮和内标物的保留时间分别为2.8min,4.85min和8.5min;绝对回收率分别为74%,86%和86%。奥沙西泮在35.3～2260ng·ml^-1,去甲地西泮在20～2560ng·ml^-1血浆浓度范围内线性关系良好,r=0.9997和r=0.9998。二药的最低检测浓度分别为10ng·ml、和7ng·ml^-1;日内和日间相对标准偏差(RSD)均分别小于6%和10%(n=5)。多种常用药物对样品的色谱峰无干扰。并用此法研究了大鼠单次口服去甲地西泮的药代动力学。     

TISSUE-SELECTIVE UPTAKE OF PRAVASTATIN IN RATS: CONTRIBUTION OF A SPECIFIC CARRIER-MEDIATED UPTAKE SYSTEM

Previously we demonstrated that a hydrophilic HMG-CoA reductase inhibitor, pravastatin, was actively taken up by the liver via the ‘multispecific anion transporter’ using isolated rat hepatocytes (M. Yamazaki, H. Suzuki, M. Hanano, T. Tokui, T. Komai, and Y. Sugiyama, Am. J. Physiol., 264 , G36–G44 (1993)). Such a carrier-mediated uptake of pravastatin may contribute to the liver selective inhibition of the cholesterol synthesis in vivo. To examine the early-phase tissue distribution of this drug, we carried out a pharmacokinetic and tissue distribution analysis of pravastatin in rats. After i.v. bolus administration of [^;14C]pravastatin, the time profiles of [¹⁴C]-radioactivity in plasma and several tissues were determined to calculate the tissue uptake clearance (CL_uptake). Among the tissues examined, liver accounted for the major uptake (CL_{uptake, liver}=22·8 mL min⁻¹ kg⁻¹), followed by kidney (CL_{uptake, kidney} (GFR corrected)=2·36 mL min⁻¹ kg⁻¹). Other tissues showed no significant uptake (less than 0·2 mL min⁻¹ kg⁻¹). After portal vein administration, the distribution to the liver became much larger than that to the kidney due to the extensive first-pass removal by the liver. The first-pass hepatic uptake ratio was estimated as 0·66. Administering a range of doses (0·4–400 μmol kg⁻¹) intravenously, an increase in early-phase half-life and a decrease in CL_{uptake, liver} were observed simultaneously at doses over 40 μmol kg⁻¹. In addition, CL_{uptake, kidney} decreased at doses over 4 μmol kg⁻¹. The effect of DBSP or PAH co-infusion (i.e. typical substrates for the transport system for organic anions in liver and kidney, respectively) on the initial uptake of pravastatin was also examined. DBSP clearly inhibited both the hepatic and renal uptake; however, PAH did not reduce the hepatic uptake of pravastatin although it inhibited the renal uptake. The transport systems in liver and kidney are thus considered different, based on the different saturability and inhibitory effect of organic anions.     

The oxidative metabolism of metoprolol in human liver microsomes: inhibition by the selective serotonin reuptake inhibitors

Objective: Biotransformation of metoprolol to α-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) is mediated by CYP2D6. The selective serotonin reuptake inhibitors (SSRIs) are known to inhibit CYP2D6. The aim was to study in vitro the potential inhibitory effect of SSRIs on metoprolol biotransformation. Methods: Using microsomes from two human livers, biotransformation of metoprolol to α-hydroxymetoprolol (HM) and O-demethylmetoprolol (ODM) as a function of the concentrations of the SSRIs and of some of their metabolites was studied. Results: The kinetics of the formation of both metabolites are best described by a biphasic enzyme model. The estimated values of V_max and k_M for the high affinity site are for the α-hydroxylation in human liver HL-1 32 pmol mg⁻¹ min⁻¹ and 75 μmol · l⁻¹ respectively, and in human liver HL-9 39 pmol mg⁻¹ · min⁻¹ and 70 μmol · l⁻¹ respectively; for the O-demethylation in HL-1 131 pmol mg⁻¹ min⁻¹ and 95 μmol · l⁻¹ respectively, and in HL-9 145 pmol mg⁻¹ min⁻¹ and 94 μmol · l⁻¹ respectively. Quinidine is for both pathways a potent inhibitor of the high-affinity site, with K_i values ranging from 0.03 to 0.18 μmol · l⁻¹. Fluoxetine, norfluoxetine and paroxetine are likewise potent inhibitors, with K_i values ranging from 0.30 to 2.1 μmol · l⁻¹ fluvoxamine, sertraline, desmethylsertraline, citalopram and desmethylcitalopram are less potent inhibitors, with K_i values above 10 μmol · l⁻¹. Conclusion: The rank order of the SSRIs for inhibition of metoprolol metabolism is comparable to that reported in the literature for other CYP2D6 substrates, with fluoxetine, norfluoxetine and paroxetine being the most potent. These findings need further investigation to determine their clinical relevance. Received: 24 October 1997 / Accepted: 17 January 1998