Publications (21) View all
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Article: The relative impact of incorporating pharmacokinetics on predicting in vivo hazard and mode-of-action from high-throughput in vitro toxicity assays.
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ABSTRACT: The use of high-throughput in vitro assays has been proposed to play a significant role in the future of toxicity testing. In this study, rat hepatic metabolic clearance and plasma protein binding were measured for 59 ToxCast Phase I chemicals. Computational in vitro-to-in vivo extrapolation (IVIVE) was used to estimate the daily dose in a rat, called the oral equivalent dose, which would result in steady-state in vivo blood concentrations equivalent to AC(50) or lowest effective concentration (LEC) across more than 600 ToxCast Phase I in vitro assays. Statistical classification analysis was performed using either oral equivalent doses or unadjusted AC(50)/LEC values for the in vitro assays to predict the in vivo effects of the 59 chemicals. Adjusting the in vitro assays for pharmacokinetics did not improve the ability to predict in vivo effects as either a discrete (yes or no) response or as a low effect level (LEL) on a continuous dose scale. Interestingly, a comparison of the in vitro assay with the lowest oral equivalent dose with the in vivo endpoint with the lowest LEL suggested that the lowest oral equivalent dose may provide a conservative estimate of the point-of-departure for a chemical in a dose-response assessment. Further, comparing the oral equivalent doses for the in vitro assays with the in vivo dose range that resulted in adverse effects identified more coincident in vitro assays across chemicals than expected by chance suggesting that the approach may be used to identify potential molecular initiating events leading to adversity.Toxicological Sciences 01/2013; · 4.65 Impact Factor -
Article: Influence of toluene co-exposure on the metabolism and genotoxicity of benzene in mice using continuous and intermittent exposures.
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ABSTRACT: Benzene exposure in occupational settings often occurs with concurrent exposure to toluene, the methyl-substituted derivative of benzene. Toluene is also readily metabolized by CYP450 isozymes although oxidation primarily occurs in the methyl group. While earlier mouse studies addressing co-exposure to benzene and toluene at high concentrations demonstrated a reduction in benzene-induced genotoxicity, we have previously found, using an intermittent exposure regimen with lower concentrations of benzene (50 ppm) and toluene (100 ppm), that toluene enhances benzene-induced clastogenic or aneugenic bone marrow injury in male CD-1 mice with significantly increased CYP2E1, and depleted GSH and GSSG levels. The follow-up study reported here also used the same daily and total co-exposures but over consecutive days and compared the effects of co-exposure on genotoxicity and metabolism in CD-1 mice both with and without buthionine sulfoximine (BSO) treatment to deplete GSH. In this study the toluene co-exposure doubled the genotoxic response (as determined by the erythrocyte micronucleus test) to benzene alone. Further, GSH depletion caused a reduction in this genotoxicity in both benzene exposed and benzene/toluene co-exposed mice. The results are discussed in terms of the analyses of urinary metabolites from this consecutive day study and the intermittent exposure study as well as levels of CYP2E1, epoxide hydrolase, quinone reductase, alcohol dehydrogenase, and aldehyde dehydrogenase activities. The results suggest that the presence of glutathione is necessary for benzene genotoxicity either as a metabolite conjugate or through an indirect mechanism such as TNF-induced apoptosis.Chemico-biological interactions 03/2010; 184(1-2):233-9. · 2.46 Impact Factor -
Article: Response to "accurate risk-based chemical screening * relies on robust exposure estimates".
Toxicological Sciences 04/2012; 128(1):297-9. · 4.65 Impact Factor -
Article: Integration of dosimetry, exposure, and high-throughput screening data in chemical toxicity assessment.
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ABSTRACT: High-throughput in vitro toxicity screening can provide an efficient way to identify potential biological targets for chemicals. However, relying on nominal assay concentrations may misrepresent potential in vivo effects of these chemicals due to differences in bioavailability, clearance, and exposure. Hepatic metabolic clearance and plasma protein binding were experimentally measured for 239 ToxCast Phase I chemicals. The experimental data were used in a population-based in vitro-to-in vivo extrapolation model to estimate the daily human oral dose, called the oral equivalent dose, necessary to produce steady-state in vivo blood concentrations equivalent to in vitro AC(50) (concentration at 50% of maximum activity) or lowest effective concentration values across more than 500 in vitro assays. The estimated steady-state oral equivalent doses associated with the in vitro assays were compared with chronic aggregate human oral exposure estimates to assess whether in vitro bioactivity would be expected at the dose-equivalent level of human exposure. A total of 18 (9.9%) chemicals for which human oral exposure estimates were available had oral equivalent doses at levels equal to or less than the highest estimated U.S. population exposures. Ranking the chemicals by nominal assay concentrations would have resulted in different chemicals being prioritized. The in vitro assay endpoints with oral equivalent doses lower than the human exposure estimates included cell growth kinetics, cytokine and cytochrome P450 expression, and cytochrome P450 inhibition. The incorporation of dosimetry and exposure provide necessary context for interpretation of in vitro toxicity screening data and are important considerations in determining chemical testing priorities.Toxicological Sciences 09/2011; 125(1):157-74. · 4.65 Impact Factor -
Article: Systems pharmacology assessment of the 5-fluorouracil pathway.
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ABSTRACT: To assess the impact of the 5-fluorouracil (5-FU) drug-pathway genes on cytotoxicity, and determine whether loss-of-function analyses coupled with functional assays can help prioritize pharmacogenomic candidate genes. Dose-response experiments were used to quantify the phenotype of sensitivity to 5-FU following the specific knockdown of genes selected from the 5-FU PharmGKB drug pathway in three human colorectal cell lines. Changes in sensitivity were considered significant if the IC(50) for shRNA-exposed cells were three standard deviations outside the mean IC(50) for control-treated cells. Of the 24 genes analyzed, 13 produced significant changes on the phenotype of sensitivity to 5-FU (DHFR, DPYS, DTYMK, DUT, FPGS, GGH, NME1, NT5C, RRM1, TYMS, UCK2, UNG and UMPS). The RNAi screening strategy enabled prioritization of the genes from the 5-FU drug pathway. Further validation of the genes credentialed in this study should include gene activity or expression and mutation analyses of clinical samples.Pharmacogenomics 03/2011; 12(3):341-50. · 3.97 Impact Factor
