Fuhr U, Kober S, Zaigler M, Mutschler E, Spahn-Langguth H
Institute for Pharmacology, Clinical Pharmacology, University of Cologne, Germany.
Int J Clin Pharmacol Ther. 2005 Jul;43(7):327-34. doi: 10.5414/cpp43327.
Triamterene (TA), a potassium-sparing diuretic, is extensively metabolized by hydroxylation in 4'-position and subsequent conjugation by cytosolic sulfotransferases. To identify the cytochrome P450 enzyme(s) catalyzing hydroxylation of triamterene (the rate-limiting step in the formation of the sulfate ester (STA)), in vitro incubation studies were performed with human liver microsomes.
Initial rates of TA hydroxylation (0 - 300 microM) were determined during a ten-minute-incubation period with liver microsomes of two donors. The role of individual CYP enzymes was determined by pre-incubation with selective inhibitors/alternative substrates. Vice versa, the effect of TA (0 - 500 microM) on 3-demethylation of caffeine (0 - 1,000 microM) was assessed. Metabolite concentrations were estimated by reversed-phase HPLC methods.
TA Km values without inhibitors were 60 and 142 microM, Vmax was 177 and 220 pmol/min/mg protein, respectively. Mean inhibitor induced changes of 4'-hydroxy-TA formation were as follows: Furafylline 25 microM (CYP1A2), complete inhibition (-100%); omeprazole 250 microM (CYP1A2 inhibitor/CYP2C 19 substrate), -30%; coumarin 25 microM (CYP2A6), -11%; quinidine 25 microM (CYP2D6), -9%; ketoconazole 25 microM (CYP3A), -18%; and erythromycin 250 microM (CYP3A), -8%. In the reverse inhibition studies, TA competitively inhibited caffeine 3-demethylation with Ki values of 65 and 111 microM, respectively.
4'-hydroxylation of TA in humans appears to be mediated exclusively by CYP1A2. Inhibition or induction of CYP1A2 will change the time course of both TA and its active phase-II metabolite. The net pharmacodynamic effect of such changes is difficult to predict and needs to be evaluated in clinical studies.
氨苯蝶啶（TA）是一种保钾利尿剂，通过4'-位羟基化及随后胞质磺基转移酶的结合作用而广泛代谢。为鉴定催化氨苯蝶啶羟基化反应（硫酸酯（STA）形成的限速步骤）的细胞色素P450酶，用人肝微粒体进行了体外温育研究。
在两名供体的肝微粒体10分钟温育期间测定TA羟基化的初始速率（0 - 300μM）。通过与选择性抑制剂/替代底物预温育来确定各个CYP酶的作用。反之，评估TA（0 - 500μM）对咖啡因（0 - 1,000μM）3-去甲基化的影响。代谢物浓度通过反相HPLC方法估算。
无抑制剂时TA的Km值分别为60和142μM，Vmax分别为177和220 pmol/分钟/毫克蛋白。平均抑制剂诱导的4'-羟基-TA形成变化如下：25μM呋拉茶碱（CYP1A2），完全抑制（-100%）；250μM奥美拉唑（CYP1A2抑制剂/CYP2C19底物），-30%；25μM香豆素（CYP2A6），-11%；25μM奎尼丁（CYP2D6），-9%；25μM酮康唑（CYP3A），-18%；250μM红霉素（CYP3A）,-8%。在反向抑制研究中，TA竞争性抑制咖啡因3-去甲基化，Ki值分别为65和111μM。
人类中TA的4'-羟基化似乎仅由CYP1A2介导。CYP1A2的抑制或诱导将改变TA及其活性II期代谢物的时间进程。这种变化的净药效学效应难以预测，需要在临床研究中进行评估。
