Gary W. Jepson

A physiological pharmacokinetic model for dermal absorption of vapors in the rat

Absorption of chemical vapors through the skin is a passive process that is not easily quantitated, but may be important in the assessment of health hazards in some occupational circumstances. Physiological modeling is a quantitative technique which may provide insight into the system being modeled and can be used for interspecies extrapolation. We developed a physiological model for the penetration of organic vapors through skin in vivo which allows the prediction of blood concentrations, after dermal vapor exposures in the rat, when chemical distribution coefficients, physiological and metabolic parameters, and skin permeability constants are known. We used the model in two distinct ways. First, permeability constants for dibromomethane (DBM), bromochloromethane (BCM), and methylene chloride (DCM) were calculated by using a physiologically based pharmacokinetic model for dihalomethanes to relate blood concentrations during dermal vapor exposures to the total amount of chemical which was absorbed through the skin. Second, a skin compartment was added to the model which had input based on the permeability-area-concentration product. This predictive model adequately described blood concentrations after DBM, BCM, and DCM dermal vapor exposures over a wide range of concentrations. This model could easily be modified for use with other organic vapors, and could be used to extrapolate to human vapor exposure conditions by substituting human physiological parameters for the animal values, providing permeability constants are known or can be determined.

Subcellular Distribution and Protein Binding of Perfluorooctanoic Acid in Rat Liver and Kidney

Perfluorooctanoic acid (PFOA) is an organic fluorochemical, and its elimination in rats is markedly sex-dependent. Liver and kidney are two primary tissues of distribution of PFOA in rats. In this study, the subcellular distribution of PFOA in male and female rat liver and kidney was examined. The results demonstrated that PFOA content in the liver cytosol of the female rat was significantly higher (49 ± 6% of total radioactive residues, TRR) than in the male liver (26 ± 5% TRR), whereas PFOA distribution in the heavier subcellular fractions, especially the nuclei and cell debris fraction, was marginally higher in male rat liver. In rat kidney, more than 70% of PFOA was distributed in the cytosolic fraction, with no significant difference between sexes. The degree of protein binding of PFOA in rat liver and kidney cytosol was analyzed by two different chromatographic methods. The percentage of protein-bound PFOA in the liver cytosol was found to be approximately 55% in both male and female rats. In contrast, significantly more PFOA was bound to cytosolic proteins in the kidney of male rats (42 ± 6% TRR) than in females (17 ± 5% TRR). Ligand blotting analysis revealed that multiple proteins from the liver cytosol, nuclei, and mitochondria fractions were capable of specific binding to PFOA.

Biotransformation of 2,3,3,3-tetrafluoropropene (HFO-1234yf)

2,3,3,3-Tetrafluoropropene (HFO-1234yf) is a non-ozone-depleting fluorocarbon replacement with a low global warming potential which has been developed as refrigerant. The biotransformation of HFO-1234yf was investigated after inhalation exposure. Male Sprague-Dawley rats were exposed to air containing 2000, 10,000, or 50,000 ppm HFO-1234yf for 6 h and male B6C3F1 mice were exposed to 50,000 ppm HFO-1234yf for 3.5 h in a dynamic exposure chamber (n=5/concentration). After the end of the exposure, animals were individually housed in metabolic cages and urines were collected at 6 or 12-hour intervals for 48 h. For metabolite identification, urine samples were analyzed by (1)H-coupled and decoupled (19)F-NMR and by LC/MS-MS or GC/MS. Metabolites were identified by (19)F-NMR chemical shifts, signal multiplicity, (1)H-(19)F coupling constants and by comparison with synthetic reference compounds. In all urine samples, the predominant metabolites were two diastereomers of N-acetyl-S-(3,3,3-trifluoro-2-hydroxy-propyl)-l-cysteine. In (19)F-NMR, the signal intensity of these metabolites represented more than 85% (50,000 ppm) of total (19)F related signals in the urine samples. Trifluoroacetic acid, 3,3,3-trifluorolactic acid, 3,3,3-trifluoro-1-hydroxyacetone, 3,3,3-trifluoroacetone and 3,3,3-trifluoro-1,2-dihydroxypropane were present as minor metabolites. Quantification of N-acetyl-S-(3,3,3-trifluoro-2-hydroxy-propyl)-l-cysteine by LC/MS-MS showed that most of this metabolite (90%) was excreted within 18 h after the end of exposure (t(1/2) app. 6 h). In rats, the recovery of N-acetyl-S-(3,3,3-trifluoro-2-hydroxy-propyl)-l-cysteine excreted within 48 h in urine was determined as 0.30+/-0.03, 0.63+/-0.16, and 2.43+/-0.86 micromol at 2000, 10,000 and 50,000 ppm, respectively suggesting only a low extent (<<1% of dose received) of biotransformation of HFO-1234yf. In mice, the recovery of this metabolite was 1.774+/-0.4 mumol. Metabolites identified after in vitro incubations of HFO-1234yf in liver microsomes from rat, rabbit, and human support the metabolic pathways of HFO-1234yf revealed in vivo. The obtained results suggest that HFO-1234yf is subjected to a typical biotransformation reaction for haloolefins, likely by a cytochrome P450 2E1-catalyzed formation of 2,3,3,3-tetrafluoroepoxypropane at low rates, followed by glutathione conjugation or hydrolytic ring opening.

Binding of Perfluorooctanoic Acid to Rat Liver‐form and Kidney‐form α2u‐Globulins

Perfluorooctanoic acid (PFOA) is an organic fluorochemical and is reported to have a long half‐life in human blood. Its urinary elimination in rats is markedly sex‐dependent, and characterized by significantly longer plasma half‐life of PFOA in male rats than in females. It has been postulated that male‐specific PFOA binding protein(s) is responsible for the long half‐life of PFOA in male rats. In this paper, two male rat specific proteins, liver‐ and kidney‐form α2u‐globulins (A2UL and A2UK), were purified from male rat urine and kidney, respectively. The binding of these two proteins to PFOA was investigated using ligand blotting, electrospray ionization mass spectrometry and fluorescence competitive binding assay. The results revealed that both A2UL and A2UK were able to bind PFOA in vitro under physiological conditions, and that PFOA and a fluorescent‐labeled fatty acid shared the same binding site on both A2UL and A2UK. The binding affinities, however, are relatively weak. The estimated dissociation constants are in the 10− 3 M range, indicating that bindings of PFOA to either A2UL or A2UK cannot adequately explain the sex‐dependent elimination of PFOA in rats, and it is unlikely that PFOA‐A2Uk binding would induce A2U nephropathy as seen with, for example, 1,4‐dichlorobenzene.