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  • Cy5.5 NHS ester From the relative risk model an estimate of the relative


    From the relative risk model, an estimate of the relative cancer risk coefficient (β) is obtained, expressed as risk per internal dose. This coefficient is assumed to be approximately independent of tumor site, sex, and Cy5.5 NHS ester (Granath et al., 1999). The relative risk could also be expressed as the doubling dose (1/β) of the carcinogen, which is the dose that doubles the lifetime cumulative hazard; this implies that it approximately leads to a doubling of lifetime risk if the background tumor incidence is low. The applicability of the relative risk model to data from carcinogenicity studies of genotoxic compounds has so far been validated for a few compounds. In these evaluations the number of tumors in exposed animals predicted with the relative risk model was shown to correlate well with observed tumor incidence in responding sites, irrespectively of tumor site or species (Fred et al., 2008; Törnqvist et al., 2008; Granath et al., 1999). The model has also been successfully evaluated and validated with data from mutagenicity tests in vitro (expressed as relative mutagenic potency) for two of these compounds (Fred et al., 2008; Granath et al., 1999). Thus, the evaluations support that the cancer risk coefficient obtained with this model is a measure of the genotoxic potency of the studied compound. These evaluations strongly indicate that application of the relative risk model for cancer risk assessment of genotoxic chemicals also facilitates the use of in vitro genotoxicity data for assessment of the relative cancer risk coefficients for such chemicals.
    In the present work we have further evaluated the relative risk model for its applicability to genotoxic chemicals using data from published carcinogenicity studies of glycidol in mice and rats (Irwin et al., 1996; NTP, 1990). To obtain accurate estimates of the risk coefficient, the AUCs per exposure dose of glycidol in mouse and rat are obtained from short-term exposures performed at similar conditions as in the carcinogenicity studies. Hemoglobin (Hb) adduct levels measured in the exposed animals are used for calculation of the AUC's. Finally, the risk coefficient is transferred to human exposures.
    2. Materials and methods
    2.1. In vivo dosimetry of glycidol
    Male and female B6C3F1 mice (approximately 10 weeks) and Sprague Dawley rats (approximately 8 weeks) were obtained from Envigo, Venray (Netherlands). The animals were housed in a controlled facility with standard diet and tap water ad libitum. The ethical  Food and Chemical Toxicology 128 (2019) 54–60
    application was approved by the Ethical committee on animal experi-ments, Swedish Board of Agriculture, license number S7-15. Three animals per dose group were administered glycidol (96%; Acros Organics, Geel, Belgium; CAS 556-52-5; 74.08 g/mol) (10 mL/kg), dis-solved in water, at identical dose levels as in the published 2-year carcinogenicity studies (Irwin et al., 1996; NTP, 1990): 25 and 50 mg/ kg (mice) and 37.5 and 75 mg/kg (rats), by gavage once daily for five consecutive days. Blood collection from each animal was performed at one occasion in EDTA-coated tubes three days after the final dosing day (eight days after first exposure) by sampling from the tail vein (rats) or from the orbital plexus during anesthesia with isoflurane and oxygen (mice). The blood samples were stored at −20 °C until preparation and analysis of Hb adducts, used as a biomarker for internal dose.
    2.2. Quantification of hemoglobin adducts
    Blood samples from the treated mice and rats were prepared ac-cording to the FIRE procedure to measure the adduct levels from gly-cidol to the N-terminal valine in Hb (Aasa et al., 2017; von Stedingk et al., 2010). This procedure enables the detachment from Hb of the formed adduct, N-(2,3-dihydroxypropyl)valine (diHOPrVal), through derivatization with the Edman reagent fluorescein isothiocyanate (FITC), giving the corresponding fluorescein isothiohydantoin (FTH). Quantification of diHOPrVal-FTH and the corresponding (13C5)-sub-stituted internal standard was performed by ultra-pressure liquid chromatography with high resolution mass spectrometry (UPLC/ HRMS) after clean-up of the blood samples. The calibration curve, prepared by adding known amounts of diHOPrVal-FTH to derivatized human blood samples, was processed in parallel. See Supplementary Data and Fig. S1 for further details.