New Approach Methodology for Assessing Inhalation Risks of a Contact Respiratory Cytotoxicant: Computational Fluid Dynamics-Based Aerosol Dosimetry Modeling for Cross-Species and In Vitro Comparisons

Abstract

Regulatory agencies are considering alternative approaches to assessing inhalation toxicity that utilizes in vitro studies with human cells and in silico modeling in lieu of additional animal studies. In support of this goal, computational fluid-particle dynamics (CFPD) models were developed to estimate site-specific deposition of inhaled aerosols containing the fungicide, chlorothalonil, in the rat and human for comparisons to prior rat inhalation studies and new human in vitro studies. Under bioassay conditions, the deposition was predicted to be greatest at the front of the rat nose followed by the anterior transitional epithelium and larynx corresponding to regions most sensitive to local contact irritation and cytotoxicity. For humans, simulations of aerosol deposition covering potential occupational or residential exposures (1—50 µm diameter) were conducted using nasal and oral breathing. Aerosols in the 1-5 µm range readily penetrated the deep region of the human lung following both oral and nasal breathing. Under actual use conditions (aerosol formulations >10 µm), the majority of deposited doses were in the upper conducting airways. Beyond the nose or mouth, the greatest deposition in the pharynx, larynx, trachea, and bronchi was predicted for aerosols in the 10–20 µm size range. Only small amounts of aerosols >20 µm penetrated past the pharyngeal region. Using the ICRP clearance model, local retained tissue dose metrics including maximal concentrations (Cmaxs) and areas under the curve (AUCs) were calculated for each airway region following repeated occupational exposures. These results are directly comparable with benchmark doses from in vitro toxicity studies in human cells leading to estimated human equivalent concentrations (HEC) that reduce the reliance on animals for risk assessments.

Use of new approach methodologies (NAMs) to meet regulatory requirements for the assessment of industrial chemicals and pesticides for effects on human health

“…In addition, as part of a registration review, a NAM approach was used to evaluate inhalation exposures for the fungicide chlorothalonil, which is a respiratory contact irritant ( EPA, 2021c ). The approach utilizes an in vitro assay to derive an inhalation point of departure in conjunction with in silico dosimetry modeling to calculate human equivalent concentrations for risk assessment ( Corley et al, 2021 ; McGee Hargrove et al, 2021 ). The approach, which was reviewed and supported by a FIFRA Scientific Advisory Panel ( EPA, 2018a ), provided an opportunity to overcome challenges associated with testing respiratory irritants, while also incorporating human relevant information.…”

Advancing New Approach Methodologies (NAMs) for Tobacco Harm Reduction: Synopsis from the 2021 CORESTA SSPT—NAMs Symposium

“…simple binary approach (i.e., toxic vs. non-toxic), the concordance was below 85% [28-32 Thus, it is unrealistic to expect NAMs to achieve a higher concordance level than the in trinsic concordance exhibited by the in vivo test. Ongoing work for skin and eye irritation skin sensitization, and acute inhalation now incorporates human biological relevance int NAM assessment rather than a direct comparison with in vivo toxicology study resu [5,33,34].…”

“…The first used CFD and PBPK to model localized cell-and tissue-specific internal doses of reactive aldehydes, allowing hazard ranking of tobacco smoke constituents [64]. The second case study focused on reducing and replacing sub chronic (90-day) animal inhalation studies for pesticide aerosols with realistic human exposure characterizations, in vitro toxicity studies with human cells, and kinetic modeling of aerosol dosimetry as a follow-on for short-term toxicity studies [5,68]. Here, CFD with muco-ciliary clearance modeling was used to calculate region-specific retained doses and risk assessment, enabling a waiver for a 90-day inhalation study of chlorothalonil required for the pesticide registration renewal.…”

“…Traditionally, these toxicity findings would have triggered the requirement of a follow-up 90-day inhalation study. With the U.S. EPA's focus on a reduction in animal testing, data from in vitro studies with human respiratory cells coupled with enhanced characterization of the exposure and target doses were submitted in support of risk assessments as shown in Figure 6 [5,70]. For the in vitro cytotoxicity testing, the MucilAir™ system (Epithelix SA, Geneva, Switzerland) was used to derive an in vitro BMD for cytotoxicity as a point of departure (POD) for use in CFD-PBPK modeling.…”

“…Since the seminal 2007 National Research Council's report on 21st century toxicology testing [3] was released, there have been significant advancements to using NAMs in toxicological assessments of chemicals ("Alternative Methods Accepted by US Agencies", accessed May 2022 [4]). NAMs pursue more human-relevant, reliable, and efficient means to assess chemicals and are already being applied to replace some traditional in vivo animal tests [5][6][7].…”