Application of Mass Balance Models and the Chemical Activity Concept To Facilitate the Use of in Vitro Toxicity Data for Risk Assessment

Abstract

Practical, financial, and ethical considerations related to conducting extensive animal testing have resulted in various initiatives to promote and expand the use of in vitro testing data for chemical evaluations. Nominal concentrations in the aqueous phase corresponding to an effect (or biological activity) are commonly reported and used to characterize toxicity (or biological response). However, the true concentration in the aqueous phase can be substantially different from the nominal. To support in vitro test design and aid the interpretation of in vitro toxicity data, we developed a mass balance model that can be parametrized and applied to represent typical in vitro test systems. The model calculates the mass distribution, freely dissolved concentrations, and cell/tissue concentrations corresponding to the initial nominal concentration and experimental conditions specified by the user. Chemical activity, a metric which can be used to assess the potential for baseline toxicity to occur, is also calculated. The model is first applied to a set of hypothetical chemicals to illustrate the degree to which test conditions (e.g., presence or absence of serum) influence the distribution of the chemical in the test system. The model is then applied to set of 1194 real substances (predominantly from the ToxCast chemical database) to calculate the potential range of concentrations and chemical activities under assumed test conditions. The model demonstrates how both concentrations and chemical activities can vary by orders of magnitude for the same nominal concentration.

Thorough QT/QTc in a Dish: An In Vitro Human Model That Accurately Predicts Clinical Concentration‐QTc Relationships

“…In all cases, f media > f plasma , as expected, due to the media containing less protein. Measured media values correlated well with values calculated using the Armitage et al . model, although for citalopram (Sigma Aldrich, St. Louis, MO) and lamotrigine (APExBIO, Houston, TX), measured f media > 100%.…”

“…Sources are abbreviated as SA (Sigma Aldrich, St. Louis, MO), A (APExBIO, Houston, TX), and ME (MedChem Express, Monmouth Junction, NJ). Values reported in the literature ( Table S2) or calculated from the Armitage et al . (2014) model ( Tables S3 and S4) are expressed as a range.…”

“…Chromatography and the single reaction monitoring parameters for 13 drugs are detailed in Table S1. Free fraction was calculated by comparing the response ratios to the concentration of internal standard (IS) within both chambers, sample and buffer, as the following formula: f sample = % free = (chemical response/IS response) buffer ÷ (chemical response/IS response) sample , where “sample” = “plasma” or “media.” For plasma, results were compared to f plasma reported in the literature (see Table S2); for cardiomyocyte media, results were compared to f media calculated using the mass‐balance model of Armitage et al . (see Tables S3 and S4).…”

An exposure:activity profiling method for interpreting high-throughput screening data for estrogenic activity—Proof of concept

“…To address such challenges, considerable efforts have been devoted over the last few years in exploring high throughput exposure modeling approaches, including the ExpoCast and ExpoDat (Armitage et al, 2014) research programs. Integral to these approaches is research that translates bioactivity concentrations in HTS in vitro assays to concentrations of the chemical in the target tissue or blood in vivo (Yoon et al, 2014).…”

Incipient cytotoxicity: A time-independent measure of cytotoxic potency in vitro

“…The problem of using nominal concentrations which may differ considerably from the truly available free concentrations the cells are exposed to has been discussed previously (see e.g. Groothuis et al, 2015;Armitage et al, 2014; also for references).…”