Physiologically Based Pharmacokinetic Modeling for 1-Bromopropane in F344 Rats Using Gas Uptake Inhalation Experiments

1-Bromopropane (1-BP), a halogenated alkane, was introduced into the workplace as an alternative to ozone-layer depleting solvents (ODA) after the discovery of the reproductive and hematopoietic toxicities of 2-brompropane (2-BP) in workers (Kim et al., 1997; Yu et al., 1999a, c, 2001a). 2-BP was first used as an ODA in Korea and Japan and discovered to cause reproductive disorders in workers (Kim et al., 1997; Park et al., 1997). Animal studies confirmed its adverse effects, which included reductions in testes weight and sperm counts, atrophy of the seminiferous tubules, a decrease in the number of ovarian follicles and an increase in irregular estrous cycles (Ichihara et al., 1997, 1999, 2000a,b, 2004a, 2005; Kamijima et al., 1997a, b; Nakajima et al., 1997a, 1997b; Wang et al., 2002, 2003; Yu et al., 1999a, b, 2001a, b; Yamada et al., 2003). The isomer 1-BP was thereafter introduced and approved as an ODA by Environmental Protection Agency’s (EPA, 2007). Subsequent animal studies identified the potential for 1-BP-mediated reproductive and neurotoxicity (Yu et al., 1998a, 2001a). 1-BP is categorized as a high-production volume chemical (Eisenberg and Ramsey, 2010). Its usage has increased dramatically, rising to around 20 million pounds/year, which could result in a widespread occupational exposure (Anderson et al., 2010; NTP, 2013; OSHA, 1999). 1-BP is demonstrated to be a potent neurotoxic compound compared with 2-BP. In addition, 1-BP has reproductive toxicity, but the target cells are different from those of 2-BP. For example, exposure to 1-BP inhibits spermiation in male rats and disrupts the development of follicles in female rats, in contrast to 2-BP, which targets spermatogonia and oocytes in primordial follicles. Because the first animal study revealing the neurotoxicity of 1-BP (Yu et al., 1998a), over a dozen human cases of neurotoxicity have been reported with manifestations of 1-BP toxicity. Among 1-BP exposed workers, dose-dependent prolongation of distal latency in the tibial nerve with decreased vibratory sensation in the lower extremities has been shown (Ichihara et al., 2004b). Species and strain-specific effects of 1-BP were observed in rats and mice (Ichihara et al., 2012; Liu et al., 2009). The majority of toxicological studies were conducted in rats. In a comparative study between the 2 inbred strains of rats, F344 and Wistar, neurotoxicity revealed by distal latency was more significant in F344 than in Wistar rats. Furthermore, mice were reported to be more susceptible than rats to 1-BP-mediated hepatotoxicity and reproductive toxicity (Liu et al., 2009). Hepatotoxicity and male reproductive toxicity were compared among the 3 strains of mice (C57BL/6J, DBA/2J, and BALB/cA) exposed to 1-BP at 0, 50, 110, and 250 ppm for 8 h/day for 28 days by inhalation. Histopathological evaluation of the liver damage showed a significantly larger area of necrosis and more degenerative lobules in BALB/cA in the order of BALB/cA > C57BL/6J > DBA/2J. BALB/cA showed higher CYP2E1 protein level and lower total glutathione (GSH) content and glutathione-S-transferases (GST) activity in the liver than DBA/2J. These results indicate that BALB/cA mice are the most susceptible to the hepatotoxicity of 1-BP among the 3 strains tested, and that CYP2E1 and GSH level/GST activity may contribute to the susceptibility to 1-BP hepatotoxicity. Exposure to ≥ 50 ppm of 1-BP also decreased sperm count and sperm motility and increased sperms with abnormal heads in all 3 strain mice, whereas rats exposed to 200 ppm of 1-BP for 12 weeks showed no changes in sperm count and sperm motility (Ichihara et al., 2000b), suggesting mice are far more susceptible than rats. Most recently, a long-term inhalation animal study from the National Toxicological Program (NTP) found that exposure of male and female F344/N rats to 1-BP significantly increased the incidences of adenomas of the large intestine and skin neoplasms (Morgan et al., 2011; NTP, 2013). In male rats, the incidence of malignant mesothelioma was statistically significantly increased at 500 ppm. There was no evidence of carcinogenic activity of 1-BP in male B6C3F1 mice; however, significantly increased incidences of alveolar/bronchiolar neoplasms of the lung were present in female mice. The mechanism of the different incidence of the carcinogenesis observed between rat and mice and male and female is still unclear, which makes it uncertain how to translate these results from animals to humans. Proposed Occupational Exposure limits (OELs) for 1-BP are diverse in both the selection of critical effects and judgments of remaining uncertainty because its toxicological mechanisms are still poorly known. The OEL values differ by ∼10-fold. Toxicology Excellence for Risk Assessment evaluated the underlying basis of existing OELs through critical effect, benchmark dose, and uncertainty factor, and concluded that the critical effect has decreased the live litter size with a benchmark dose lower confidence limit of 190 ppm (Maier et al., 2004). The OEL of 20 ppm is derived using an uncertainty factor of 10-fold, which is composed of 3-fold for extrapolation from an experimental animal study to humans for expected toxicodynamic differences and 3-fold for expected human variability in toxicokinetics and toxicodynamics within the worker population. The American Conference of Industrial Hygienists (ACGIH) recommends an 8-h time weighted average threshold limit values of 0.10 ppm to provide protection against the potential for neurotoxicity, hepatotoxicity, and reproductive and developmental toxicity in 1-BP exposed workers (ACGIH, 2014). Other professional organizations and manufacturers have recommended exposure limits (RELs) ranging from 20 to 100 ppm (Stelljes and Wood, 2004). The acceptable exposure limit of 1-BP from the U.S. EPA is 25 ppm (8-h time-weighted average). Currently, the National Institute for Occupational Safety and Health does not have a REL, nor does the Occupational Safety and Health Administration have a permissible exposure limit for 1-BP. The potential for human exposure to 1-BP and its adverse effects related with occupation requires understanding of the potential mechanism of these adverse effects in rats and mice as a means of understanding risk in workers. Risk assessment for 1-BP exposure is limited due to lack of animal or human toxicokinetic study. Gas uptake studies analyzed by physiologically based pharmacokinetic (PBPK) models have been used to estimate metabolic parameters for many volatiles (Dobrev et al., 2003, 2008; Lilly et al., 1997). The metabolic constants for a saturable pathway (Vmax and Km) and for a first-order process are typically inferred from the decline in chemical concentration observed in closed chamber exposures. The objective of the current study was to develop a physiologically based PBPK model to simulate the concentration of 1-BP in a closed chamber from a gas-uptake experiment in the F-344 rats. The PBPK model tested the hypotheses, including sex-specific metabolism of 1-BP, and investigated the role of the 2 major metabolic pathways, cytochrome P450 CYP2E1 and GSH conjugation, in the metabolism of 1-BP. The results showed that these 2 metabolic pathways adequately simulated the concentration of 1-BP in the closed chamber. Furthermore, the above model was further tested by simulating the gas-uptake data from a female rat pre-treated with a general P450 suicide inhibitor 1-aminobenzotrizole (ABT) or GSH biosynthesis inhibitor d,l-buthionine (S,R)-sulfoximine (BSO) prior to the exposure of 800 ppm 1-BP. The comparative investigation on the metabolic pathways of 1-BP through the PBPK modeling in both sexes provides critical information in understanding the role of P450 and GSH conjugation in the metabolism of 1-BP and eventually helps to quantitatively extrapolate animal studies to human.