Human biomonitoring and toxicokinetics as key building blocks for next generation risk assessment

Human health risk assessment is historically built upon animal testing, often following Organisation for Economic Co-operation and Development (OECD) test guidelines and exposure assessments. Using combinations of human relevant in vitro models, chemical analysis and computational (in silico) approaches bring advantages compared to animal studies. These include a greater focus on the human species and on molecular mechanisms and kinetics, identification of Adverse Outcome Pathways and downstream Key Events as well as the possibility of addressing susceptible populations and additional endpoints. Much of the advancement and progress made in the Next Generation Risk Assessment (NGRA) have been primarily focused on new approach methodologies (NAMs) and physiologically based kinetic (PBK) modelling without incorporating human biomonitoring (HBM). The integration of toxicokinetics (TK) and PBK modelling is an essential component of NGRA. PBK models are essential for describing in quantitative terms the TK processes with a focus on the effective dose at the expected target site. Furthermore, the need for PBK models is amplified by the increasing scientific and regulatory interest in aggregate and cumulative exposure as well as interactions of chemicals in mixtures. Since incorporating HBM data strengthens approaches and reduces uncertainties in risk assessment, here we elaborate on the integrated use of TK, PBK modelling and HBM in chemical risk assessment highlighting opportunities as well as challenges and limitations. Examples are provided where HBM and TK/PBK modelling can be used in both exposure assessment and hazard characterization shifting from external exposure and animal dose/response assays to animal-free, internal exposure-based NGRA.

Acute inhalation toxicity of carbon monoxide and hydrogen cyanide revisited: Comparison of models to disentangle the concentration × time conundrum of lethality and incapacitation

Contemporary emergency response planning guidelines are stratified to consider the threshold for serious toxicity and/or impairment of escape, relative to the potentially lethal level above this threshold and the lower level at which individuals should not experience or develop effects more serious than mild irritation. While harmonized testing guidelines and risk assessment paradigms are available for the quantification of thresholds for lethality or establishing no adverse effect levels, the quantification of 'impairment of escape' appears to be a more elusive goal. Approaches were explored in context with CO and HCN in past experimental combustion toxicology studies to estimate the time available for escape. This point of departure (POD) was compared with the non-lethal threshold (LC01) and one third thereof from published recent acute inhalation studies in rats examining the Cxt-matrix of both CO and HCN. The findings from this analysis suggest that the rat delivers the most consistent data. However, it remains challenging yet to bridge the behavioral variables of human behavior typical of escape to any surrogate animal model. For the asphyxiant gases examined, the PODs characterizing 'impairment of escape' were difficult to distinguish from those indicative of impending death. No specific modeled carboxyhemoglobin (COHb) level could be linked to onset of incapacitation. In summary, the higher ventilation of rats (kg body weight adjusted) renders this species even more susceptible than heavy breathing humans. LCt01 × 1/3 values derived from the comprehensive Cxt matrix of rat inhalation studies are considered to be most suitable and robust to estimate the human equivalent threshold (POD) of 'impairment of escape'.