Takahiro Rikako, Nakamura Saki, Kohno Hiroyuki, Yoshimura Naoki, Nakamura Tsuneyuki, Ozawa Sayaka, Hirono Keiichi, Ichida Fukiko, Taguchi Masato
Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama.
Biol Pharm Bull. 2015;38(1):58-65. doi: 10.1248/bpb.b14-00566.
The aim of this study was to characterize the kinetics of metabolite formation of the phosphodiesterase type-5 (PDE5) inhibitors sildenafil and tadalafil by CYP3A4, CYP3A5, and CYP3A7 isoforms. The formations of N-desmethyl sildenafil and desmethylene tadalafil were examined using CYP3A supersomes co-expressing human P450 oxidoreductase and cytochrome b5. Both sildenafil N-demethylation and tadalafil demethylenation were catalyzed by CYP3A4, CYP3A5, and to a lesser extent by CYP3A7. The kinetics of desalkyl metabolite formation of the two drugs were well fitted to the Hill equation; however, the Hill coefficients (n) suggested CYP3A-mediated negative cooperativity. Next, we analyzed the kinetics with a two binding sites model assuming two reaction steps: reaction 1 with high-affinity and low-capacity metabolism and reaction 2 with low-affinity and high-capacity metabolism. The kinetics of desalkyl metabolite formation were also fitted to the two binding sites model. The intrinsic clearance (CLint) values of reactions 1 and 2 for sildenafil N-demethylation were 0.733 and 0.033 µL/min/pmol P450 for CYP3A4, 0.788 and 0.019 µL/min/pmol P450 for CYP3A5, and 0.079 and 0.004 µL/min/pmol P450 for CYP3A7, respectively. The CLint values of reactions 1 and 2 for tadalafil demethylenation were 0.187 and 0.014 µL/min/pmol P450 for CYP3A4, 0.050 and <0.001 µL/min/pmol P450 for CYP3A5, and 0.004 and <0.001 µL/min/pmol P450 for CYP3A7, respectively. These results may provide the basis not only for understanding the metabolic properties of the two PDE5 inhibitors, but also for one possible explanation of the mechanisms of CYP3A-mediated negative cooperativity.
本研究的目的是表征5型磷酸二酯酶（PDE5）抑制剂西地那非和他达拉非经CYP3A4、CYP3A5和CYP3A7同工酶形成代谢物的动力学。使用共表达人P450氧化还原酶和细胞色素b5的CYP3A微粒体研究了N-去甲基西地那非和去亚甲基他达拉非的形成。西地那非的N-去甲基化和他达拉非的去亚甲基化均由CYP3A4、CYP3A5催化，CYP3A7的催化作用较小。两种药物去烷基代谢物形成的动力学很好地符合希尔方程；然而，希尔系数（n）表明CYP3A介导负协同作用。接下来，我们用双结合位点模型分析动力学，该模型假设两个反应步骤：反应1为高亲和力和低容量代谢，反应2为低亲和力和高容量代谢。去烷基代谢物形成的动力学也符合双结合位点模型。西地那非N-去甲基化反应1和反应2的内在清除率（CLint）值，对于CYP3A4分别为0.733和0.033μL/min/pmol P450，对于CYP3A5分别为0.788和0.019μL/min/pmol P450，对于CYP3A7分别为0.079和0.004μL/min/pmol P450。他达拉非去亚甲基化反应1和反应2的CLint值，对于CYP3A4分别为0.187和0.014μL/min/pmol P450，对于CYP3A5分别为0.050和<0.001μL/min/pmol P450，对于CYP3A7分别为0.004和<0.001μL/min/pmol P450。这些结果不仅可为理解这两种PDE5抑制剂的代谢特性提供依据，也可为CYP3A介导负协同作用机制的一种可能解释提供依据。
