Worboys P D, Bradbury A, Houston J B
Department of Pharmacy, University of Manchester, UK.
Drug Metab Dispos. 1997 Apr;25(4):460-7.
The time course for distribution of five compounds (caffeine, tolbutamide, phenytoin, ondansetron, and diazepam) was studied in precision-cut rat liver slices. Transport of these compounds differed greatly, with caffeine being distributed rapidly, but not accumulating above the media concentration. Although tolbutamide similarly was not accumulated within the tissue, its uptake rate was slower. The rate of phenytoin, ondansetron, and diazepam distribution (with appropriate corrections for metabolism) was slower still; yet, these drugs were accumulated within the cells of the slice to a concentration approximately 15-fold that of the media. Examination of the physicochemical properties of these compounds demonstrated that the extent of accumulation positively correlated with lipophilicity, whereas the rate of uptake was not statistically correlated with log D. The extent of accumulation within the slice was assessed by an apparent volume of distribution parameter (ranging from 26 to 195 microliters/slice). Using cell:media partition coefficients determined in hepatocytes and the intra- and extracellular spaces within the slice (as measured with the markers tritiated water and sucrose), it was possible to predict apparent volumes of distribution for each drug in the liver slice. Comparison of observed and predicted apparent volumes of distribution gave ratios of 0.34-1. Intrinsic clearance values for these five drugs are available for slices and cells (slice: cell intrinsic clearance ratios 0.05-0.43; Worboys et al., Drug Metab. Dispos. 24, 676-681, 1996). Drugs that demonstrate low intrinsic clearance ratios also have low apparent volume ratios, thus indicating that reduced drug uptake and clearance in slices, relative to hepatocytes, are interdependent. Both phenomena may be rationalized by the existence of a drug concentration gradient within the slice. At very high drug concentrations, Vmax operates, and the consequence of the gradient is minimal. Therefore, it is possible to speculate upon the fraction of hepatocytes within the slice contributing to clearance by considering Vmax values. For six pathways of metabolism, Vmax in slices averages 35% of the corresponding parameter in isolated hepatocytes. This is most likely due to limited oxygen and compromised metabolic function of the core cells. These distribution phenomena severely complicate the possibility of using a scaling factor based on the theoretical number of slices obtainable from a liver to predict in vivo intrinsic clearance.
在精密切割的大鼠肝切片中研究了五种化合物（咖啡因、甲苯磺丁脲、苯妥英、昂丹司琼和地西泮）的分布时程。这些化合物的转运差异很大，咖啡因分布迅速，但不会在介质浓度之上蓄积。虽然甲苯磺丁脲同样不在组织内蓄积，但其摄取速率较慢。苯妥英、昂丹司琼和地西泮的分布速率（经适当的代谢校正）仍然较慢；然而，这些药物在切片细胞内蓄积至约为介质浓度15倍的浓度。对这些化合物理化性质的研究表明，蓄积程度与亲脂性呈正相关，而摄取速率与log D无统计学相关性。切片内的蓄积程度通过分布容积参数（范围为26至195微升/切片）进行评估。利用在肝细胞以及切片内细胞内和细胞外空间（用标记物氚水和蔗糖测量）测定的细胞：介质分配系数，可以预测每种药物在肝切片中的分布容积。观察到的和预测的分布容积的比较给出了0.34 - 1的比率。这五种药物的内在清除率值可用于切片和细胞（切片：细胞内在清除率比率为0.05 - 0.43；Worboys等人，《药物代谢与处置》24，676 - 681，1996）。显示出低内在清除率比率的药物也具有低表观容积比率，因此表明相对于肝细胞，切片中药物摄取和清除的降低是相互依赖的。这两种现象都可以通过切片内存在药物浓度梯度来解释。在非常高的药物浓度下，Vmax起作用，梯度的影响最小。因此，通过考虑Vmax值，可以推测切片内对清除有贡献的肝细胞比例。对于六种代谢途径，切片中的Vmax平均为分离肝细胞中相应参数的35%。这很可能是由于核心细胞的氧气有限和代谢功能受损。这些分布现象严重复杂化了使用基于从肝脏可获得的理论切片数量的比例因子来预测体内内在清除率的可能性。
