代谢
在最近的一项药物代谢动力学研究中,六名人类志愿者通过吸入暴露于10 ppm (14)C-D4的环境中,持续1小时,期间交替休息和运动。测定了呼出气体和血液中的八甲基环四硅氧烷(D4)浓度。估计了血液中的总代谢物浓度,同时测量了尿液中个别代谢物的量。在这里,我们使用这些数据来开发一个基于生理学的药物代谢动力学(PBPK)模型,用于人类的D4。与大鼠D4的PBPK建模努力一致,一个假设限流组织摄取的传统吸入PBPK模型未能充分描述这些数据。一个改进的模型,包括血液中隔离的D4,扩散限制的组织摄取,以及D4代谢到短链线性硅氧烷的明确途径,成功描述了所有数据。根据模型参数计算,这些志愿者的肝脏提取率为0.65至0.8,即肝脏清除几乎达到限流。在运动期间,吸入D4的人类保留减少可以通过运动期间改变的通气血流特征以及迅速达到稳态条件来解释。代谢物随时间的尿液排泄模式与一种代谢方案一致,该方案包括线性硅氧烷的顺序水解,随后是氧化去甲基化和环开启。D4的不寻常特性(高脂溶性与高肝脏和呼出清除率相结合)导致血液中自由D4的迅速下降。在人类和大鼠中,具有相似生理结构的D4 PBPK模型的成功,增加了我们对模型在预测吸入暴露期间人类组织中D4及其代谢物浓度的信心。
In a recent pharmacokinetic study, six human volunteers were exposed by inhalation to 10 ppm (14)C-D4 for 1 hr during alternating periods of rest and exercise. Octamethylcyclotetrasiloxane (D4) concentrations were determined in exhaled breath and blood. Total metabolite concentrations were estimated in blood, while the amounts of individual metabolites were measured in urine. Here, we use these data to develop a physiologically based pharmacokinetic (PBPK) model for D4 in humans. Consistent with PBPK modeling efforts for D4 in the rat, a conventional inhalation PBPK model assuming flow-limited tissue uptake failed to adequately describe these data. A refined model with sequestered D4 in blood, diffusion-limited tissue uptake, and an explicit pathway for D4 metabolism to short-chain linear siloxanes successfully described all data. Hepatic extraction in these volunteers, calculated from model parameters, was 0.65 to 0.8, i.e., hepatic clearance was nearly flow-limited. The decreased retention of inhaled D4 seen in humans during periods of exercise was explained by altered ventilation/perfusion characteristics during exercise and a rapid approach to steady-state conditions. The urinary time course excretion of metabolites was consistent with a metabolic scheme in which sequential hydrolysis of linear siloxanes followed oxidative demethylation and ring opening. The unusual properties of D4 (high lipophilicity coupled with high hepatic and exhalation clearance) lead to rapid decreases in free D4 in blood. The success of D4 PBPK models with a similar physiological structure in both humans and rats increases confidence in the utility of the model for predicting human tissue concentrations of D4 and metabolites during inhalation exposures.
来源:Hazardous Substances Data Bank (HSDB)