Constan Alexander A, Wong Brian A, Everitt Jeffrey I, Butterworth Byron E
CIIT Centers for Health Research, 6 Davis Drive, P.O. Box 12137, Research Triangle Park, North Carolina 27709-2137, USA.
Toxicol Sci. 2002 Apr;66(2):201-8. doi: 10.1093/toxsci/66.2.201.
Chloroform is a nongenotoxic-cytotoxic carcinogen in rodent liver and kidney, including the female B6C3F1 mouse liver. Because tumors are secondary to events associated with cytolethality and regenerative cell proliferation, these end points are valid surrogates for tumor formation in cancer risk assessments. The purpose of the experiments presented here was to more clearly define the combinations of atmospheric concentration and duration of exposure necessary to induce cytolethality and regenerative cell proliferation in the sensitive female B6C3F1 mouse liver. Female B6C3F1 mice were exposed to chloroform by inhalation for 7 consecutive days using atmospheres of 10, 30, or 90 ppm and selected exposure times of 2, 6, 12, or 18 h/day. Bromodeoxyuridine (BrdU) was given the last 3.5 days via an implanted osmotic pump to label cells in S-phase. Labeled hepatocytes were visualized immunohistochemically, and the labeling index (LI) was determined as the percentage of cells in S-phase. LI was a more sensitive indicator of cellular damage than histopathological examination and is the more conservative end point for use in risk assessments. Significant concentration and exposure time related increases in LI were observed at 30 and 90 ppm but not at any 10-ppm exposure. These data defined an empirical relationship for the combinations of airborne exposure concentration and duration needed to induce cytolethality. These results suggest that concentrations of about 10 ppm or below will not induce hepatotoxicity in these mice regardless of exposure duration. Thus, the rate of production of toxic metabolites and the subsequent rate of cellular damage produced by a continual exposure of approximately 10 ppm chloroform are less than the maximum rates at which hepatocytes can detoxify those metabolites and repair any induced cellular damage. A physiologically based pharmacokinetic (PBPK) dosimetry model was used to compare anticipated responses in mice and humans and predicted that chloroform concentrations of approximately an order of magnitude greater than 10 ppm would be required to induce human liver toxicity. Thus, no safety factor to account for species to species extrapolation should be required in formulating a chloroform inhalation cancer risk assessment based on the dose x time inhalation data presented here.
氯仿是啮齿动物肝脏和肾脏中的一种非基因毒性细胞毒性致癌物，包括雌性B6C3F1小鼠肝脏。由于肿瘤是细胞致死率和再生细胞增殖相关事件的继发结果，因此在癌症风险评估中，这些终点是肿瘤形成的有效替代指标。本文所述实验的目的是更明确地确定在敏感的雌性B6C3F1小鼠肝脏中诱导细胞致死率和再生细胞增殖所需的大气浓度和暴露持续时间的组合。雌性B6C3F1小鼠通过吸入暴露于氯仿中，连续7天，使用10、30或90 ppm的大气浓度以及每天2、6、12或18小时的选定暴露时间。在最后3.5天通过植入的渗透泵给予溴脱氧尿苷（BrdU）以标记处于S期的细胞。通过免疫组织化学观察标记的肝细胞，并将标记指数（LI）确定为处于S期的细胞百分比。LI是比组织病理学检查更敏感的细胞损伤指标，并且是用于风险评估的更保守的终点。在30和90 ppm时观察到LI有与浓度和暴露时间相关的显著增加，但在任何10 ppm暴露时均未观察到。这些数据确定了诱导细胞致死率所需的空气暴露浓度和持续时间组合的经验关系。这些结果表明，无论暴露持续时间如何，约10 ppm或更低的浓度不会在这些小鼠中诱导肝毒性。因此，持续暴露于约10 ppm氯仿产生的有毒代谢物的产生速率和随后的细胞损伤速率小于肝细胞解毒这些代谢物并修复任何诱导的细胞损伤的最大速率。基于生理学的药代动力学（PBPK）剂量测定模型用于比较小鼠和人类的预期反应，并预测诱导人类肝脏毒性需要比10 ppm大约一个数量级的氯仿浓度。因此，根据此处给出的剂量×时间吸入数据制定氯仿吸入癌症风险评估时，不应需要考虑种间外推的安全系数。
