Covalent binding of 1,1,1,2-tetrachloroethane to nucleic acids as evidence of genotoxic activity

Correlation of In Vivo Versus In Vitro Benchmark Doses (BMDs) Derived From Micronucleus Test Data: A Proof of Concept Study

In this study, we explored the applicability of using in vitro micronucleus (MN) data from human lymphoblastoid TK6 cells to derive in vivo genotoxicity potency information. Nineteen chemicals covering a broad spectrum of genotoxic modes of action were tested in an in vitro MN test using TK6 cells using the same study protocol. Several of these chemicals were considered to need metabolic activation, and these were administered in the presence of S9. The Benchmark dose (BMD) approach was applied using the dose-response modeling program PROAST to estimate the genotoxic potency from the in vitro data. The resulting in vitro BMDs were compared with previously derived BMDs from in vivo MN and carcinogenicity studies. A proportional correlation was observed between the BMDs from the in vitro MN and the BMDs from the in vivo MN assays. Further, a clear correlation was found between the BMDs from in vitro MN and the associated BMDs for malignant tumors. Although these results are based on only 19 compounds, they show that genotoxicity potencies estimated from in vitro tests may result in useful information regarding in vivo genotoxic potency, as well as expected cancer potency. Extension of the number of compounds and further investigation of metabolic activation (S9) and of other toxicokinetic factors would be needed to validate our initial conclusions. However, this initial work suggests that this approach could be used for in vitro to in vivo extrapolations which would support the reduction of animals used in research (3Rs: replacement, reduction, and refinement).

Genotoxicity, carcinogenicity and acid-suppressing medications

With the availability of increasingly potent acid-suppressing medications, questions continue to rise concerning the safety of these compounds in regards to carcinogenetic potential. In this review, we examine current concepts and procedures relating to genotoxicity, the potential for a chemical agent to interact with and alter the genomic information of the cell, and carcinogenesis. A description and discussion of commonly utilized techniques for the determination of (a) in vitro mutagenicity, (b) in vitro and in vivo DNA damage and repair, (c) in vitro and in vivo chromosomal damage and (d) chronically dosed animal tumorigenesis development is presented. Observations from these procedures as they have been applied to a review of the safety of acid-suppressing medications will be discussed. An evaluation of reports relating to potential genotoxic and carcinogenic hazards of therapeutically relevant acid-suppressing medications (cimetidine, ranitidine, omeprazole) is presented. Information related to the effect of prolonged administration of acid-suppressing medications, alterations of serum gastrin levels, and the potential for tumor promotion is discussed.

Defluorination of 1,1,1,2-tetrafluoroethane (R-134a) by rat hepatocytes

As part of its toxicological evaluation we assessed the in vitro metabolism of 1,1,1,2-tetrafluoroethane (R-134a), a non-ozone-depleting chemical likely to replace dichlorodifluoromethane (R-12) as an air-conditioning refrigerant. Hepatocyte suspensions in sealed flasks produced increasing quantities of F- (detected in the liquid media) as the headspace concentration of R-134a increased from 1% to 50% (balance of atmosphere 95% O2-5% CO2); the kinetics of defluorination suggested substrate-saturation. Little F- was detected in cultures without R-134a or in cell suspensions heated prior to addition of R-134a. Halothane (1,1,1-trichloro-2-bromo-2-chloro-ethane), although not defluorinated by hepatocytes maintained with 95% O2, inhibited defluorination of R-134a. Hepatocytes from phenobarbital-treated rats dehalogenated high (greater than or equal to 25%) concentrations of R-134a at greater rates than cells from untreated rats. These findings are consistent with the hypothesis that oxidative metabolism of R-134a by cytochrome P-450 can occur in vivo.