Kimble B, Li K M, Valtchev P, Higgins D P, Krockenberger M B, Govendir M
Faculty of Veterinary Science, The University of Sydney, Sydney, NSW, Australia.
Discipline of Pharmacology, Sydney Medical School, The University of Sydney, Sydney, NSW, Australia.
Comp Biochem Physiol C Toxicol Pharmacol. 2014 Apr;161:7-14. doi: 10.1016/j.cbpc.2013.12.002. Epub 2013 Dec 15.
Quantitative and qualitative aspects of in vitro metabolism of the non-steroidal anti-inflammatory drug meloxicam, mediated via hepatic microsomes of specialized foliage (Eucalyptus) eating marsupials (koalas and ringtail possums), a generalized foliage eating marsupial (brushtail possum), rats, and dogs, are described. Using a substrate depletion method, intrinsic hepatic clearance (in vitro Clint) was determined. Significantly, rates of oxidative transformation of meloxicam, likely mediated via cytochromes P450 (CYP), were higher in marsupials compared to rats or dogs. The rank order of apparent in vitro Clint was brushtail possums (n=3) (mean: 394μL/min/mg protein), >koalas (n=6) (50), >ringtail possums (n=2) (36) (with no significant difference between koalas and ringtail possums), >pooled rats (3.2)>pooled dogs (in which the rate of depletion, as calculated by the ratio of the substrate remaining was <20% and too slow to determine). During the depletion of meloxicam, at a first-order rate constant, 5-hydroxymethyl metabolite (M1) was identified in the brushtail possums and the rat as the major metabolite. However, multiple hydroxyl metabolites were observed in the koala (M1, M2, and M3) and the ringtail possum (M1 and M3) indicating that these specialized foliage-eating marsupials have diverse oxidation capacity to metabolize meloxicam. Using a well-stirred model, the apparent in vitro Clint of meloxicam for koalas and the rat was further scaled to compare with published in vivo Cl. The closest in vivo Cl prediction from in vitro data of koalas was demonstrated with scaled hepatic Cl(total) (average fold error=1.9) excluding unbound fractions in the blood and microsome values; whereas for rats, the in-vitro scaled hepatic Cl fu(blood, mic), corrected with unbound fractions in the blood and microsome values, provided the best prediction (fold error=1.86). This study indicates that eutherians such as rats or dogs serve as inadequate models for dosage extrapolation of this drug to marsupials due to differences in hepatic turnover rate. Furthermore, as in vivo Cl is one of the pharmacokinetic indexes for determining therapeutic drug dosages, this study demonstrates the utility of in vitro to in vivo scaling as an alternative prediction method of drug Cl in koalas.
本文描述了非甾体抗炎药美洛昔康在专门食用树叶的有袋动物（考拉和环尾袋貂）、一般食用树叶的有袋动物（帚尾袋貂）、大鼠和犬的肝脏微粒体介导下的体外代谢的定量和定性方面。采用底物消耗法测定肝脏内在清除率（体外Clint）。值得注意的是，与大鼠或犬相比，有袋动物中美洛昔康的氧化转化速率可能通过细胞色素P450（CYP）介导，更高。体外表观Clint的排序为：帚尾袋貂（n = 3）（平均值：394μL/min/mg蛋白质）＞考拉（n = 6）（50）＞环尾袋貂（n = 2）（36）（考拉和环尾袋貂之间无显著差异）＞合并大鼠（3.2）＞合并犬（其底物剩余比例计算的消耗速率＜20%且太慢无法测定）。在美洛昔康消耗过程中，以一级速率常数，在帚尾袋貂和大鼠中鉴定出5-羟甲基代谢物（M1）为主要代谢物。然而，在考拉（M1、M2和M3）和环尾袋貂（M1和M3）中观察到多种羟基代谢物，表明这些专门食用树叶的有袋动物具有不同的氧化能力来代谢美洛昔康。使用充分搅拌模型，美洛昔康对考拉和大鼠的体外表观Clint进一步进行缩放以与已发表的体内Cl进行比较。从考拉的体外数据得出的最接近的体内Cl预测是通过缩放肝脏Cl（总量）（平均倍数误差 = 1.9），不包括血液和微粒体值中的未结合分数；而对于大鼠，用血液和微粒体值中的未结合分数校正的体外缩放肝脏Cl fu（血液，微粒体）提供了最佳预测（倍数误差 = 1.86）。本研究表明，由于肝脏周转率的差异，大鼠或犬等真兽类动物作为将该药物剂量外推至有袋动物的不充分模型。此外，由于体内Cl是确定治疗药物剂量的药代动力学指标之一，本研究证明了体外到体内缩放作为考拉药物Cl的替代预测方法的实用性。
