簡介
Cytochrome P450 (CYP450) 3A酵素在人體及大白鼠中主要分布於肝臟細胞及腸道上皮細胞，在肝臟酵素中佔最大比例，並參與許多物質和藥品的代謝。由於許多藥品會受到CYP3A的影響且酵素本身變異性大，造成相關藥品調整劑量上的困難，因此開發快速檢測且方便使用的探針性試藥來評估酵素在體內代謝活性也慢慢受到許多藥政相關單位的重視，像是美國FDA、歐盟EUFEPS及製藥工業等等。目前最廣為應用之探針性試藥為midazolam，但在來源取得及使用上有其限制。 Mosapride為新一代胃腸蠕動劑，主要經由CYP3A代謝。而在之前本實驗室的相關研究中發現，在大白鼠動物模式中，證實mosapride可作為一可行的CYP3A體內探針性試藥。
研究目的
為了進一步強化mosapride的適用性，本研究利用現行最常使用的midazolam和本實驗室先前已開發之探針性試藥mosapride作體內清除率的相關性比較，並同時驗證先前開發之有限採樣法的可行性。
研究方法
先以靜脈輸注給予大白鼠midazolam (輸注速率：0.02 mg/min)，等達到穩定狀態血中濃度(Css)後，停止輸注並靜待2小時以待midazolam由體內洗除，接著於同隻大白鼠以靜脈注射給予mosapride (5 mg/kg)，推求其AUC和CL，並分析兩藥間相關性。另外以ketoconazole進行CYP3A酵素的抑制；或dexamethasone進行CYP3A酵素的誘導，同樣也觀察兩藥物在酵素活性調節後之藥動參數相關性的變化。並於動物實驗結束後取其肝臟以西方點墨法分析其CYP3A總含量。
研究結果
本實驗結果再次確認：mosapride在大白鼠體內可反應出肝臟酵素的活性變化，且可利用well-stirred model來加以描述，大白鼠體內清除率與肝臟微粒體中CYP3A2含量呈良好相關（R=0.7929，P<0.001， N=40）。兩CYP3A探針性試藥mosapride與midazolam在同隻大白鼠體內之清除率之間呈統計有顯著意義的正向相關（R = 0.794，P<0.001，N=23），而以有限採樣法所預估之清除率也和midazolam具有良好的一致性（R = 0.854，P<0.001，N=23）。同時當以血漿內主要代謝物des-4-fluoro-benzyl mosapride與mosapride在各個時間點之血中濃度分率對於CYP3A活性的評估，在給藥120分鐘後，此血中濃度分率與肝臟微粒體中CYP3A2含量有顯著相關性（R=0.884，P<0.001，N=17）。
研究結論
以靜脈注射給予mosapride後，mosapride之體內清除率可適當的反應大白鼠體內肝臟CYP3A活性，且其清除率與現行廣為使用之探針性試藥midazolam具有顯著的一致性。本次研究並驗證了本實驗室先前開發之有限採樣法之應用性。結果證明以mosapride作為大白鼠體內CYP3A活性探針性試藥為一個具有來原易取得，易操作且相對節省時間的評估方式。確可在大白鼠上用來評估CYP3A相關的藥物交互作用。

Introduction
Cytochrome P450 3A is one of the most important CYP450 subfamilies because of its large number of xenobiotics and endogenous substrates. Considerable interindividual variability in the expression and activity of CYP3A was proved to be responsible for variability in drug response. The use of selected drugs as “probes” to assess in vivo CYP activity has been the subject of intense interest for over a decade.This approach is suggested by numerous organizations, including the US Food and Drug Administration, European Federation of Pharmaceutical Sciences, the American Association of Pharmaceutical Sciences, and the pharmaceutical industry. Among many CYP3A probes, midazolam is the most accepted CYP3A probe used in human, but it’s use is limited by being a control substance, and it’s assay difficulties. Other probes have similar problems. A more convenient, easy-used, and time-saving CYP3A probe is still required.
Purpose
In this project, mosapride, a new prokinetic agent, was evaluated as an in vivo probe for measuring hepatic CYP3A activity in SD rats to determine whether by the developed limited sampling strategies, it’s clearance could be used to reflect in vivo CYP3A activity. Furthermore, the relationship between the clearance of mosapride and the clearance of midazolam was examined to determine the applicability of mosapride in the CYP3A-related drug-drug interactions, such as under CYP3A induction or inhibition conditions.
Methods
Each male SD rats was first introduced midazolam IV infusion (infusion rate：0.02 mg/min), followed 2-hour midazolam washout period, mosapride was administered to the same rats (5 mg/kg); its plasma concentrations were followed up to 360 minutes. In the CYP3A modulation group, rats received midazolam and mosapride after pretreatment with ketoconazole or dexamethasone. All plasma samples were analyzed by a validated HPLC method. The liver was excised afer in vivo pharmacokinetic experiment for CYP3A2 content measurement.
Results
Mosapride could reflect CYP3A activity through it’s pharmacokinetic parameters, the clearance（CL）decreased from 58.9 to 23.1 mL/min/kg when pretreatment with ketoconazole.On the other hand, the CL increased from 58.9 to 77.4 mL/min/kg when pretreatment with dexamethasone. The same trend were also observed in the CL of midazolam. There was significant concordance between mosapride and midazolam CL（R = 0.794, P<0.001, N=23）. Based on the data from limited sampling strategies, mosapride CL still showed strong correlation with midazolam CL.
Conclusion
Strong correlation between the clearances of mosapride and midazolam supports the applicability of mosapride as a probe to assess hepatic CYP3A4 activity in vivo. Mosapride plasma concentration at 120 min after a single IV mosapride dose was proved useful as a single-point determination of plasma clearance which can be reflected the total CYP3A4 activity in vivo

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