Historically, toxicity testing has been conducted by giving lab animals high doses of chemicals and observing them for adverse events. But quantifying the risks chemicals pose to humans based on animal studies is problematic as the chemical doses are often orders of magnitude higher than environmental levels. Moreover, this process is slow, expensive, and ethically questionable.
High-throughput in vitro screening (HTS) is gaining acceptance as an approach to toxicity testing that is efficient, economical, and humane. To fully leverage HTS to assess the risks chemicals pose to human health requires also considering the variability in responses to chemicals due to pharmacokinetic (PK) or pharmacodynamic (PD) differences among sub-populations. In this blog post, I’ll discuss how researchers at the U.S. Environmental Protection Agency (EPA) and The Hamner Institutes for Health Sciences developed a new toxicity paradigm that estimates chemical-specific PK variability across multiple sub-populations.
People vary in their sensitivity to chemicals
In performing toxicological risk assessment, it’s important to consider that xenobiotic (a foreign chemical substance found within an organism that is not normally naturally produced) exposure can vary by age, gender, ethnicity, and renal/hepatic function. For example, children have higher food ingestion and inhalation rates for their weight compared to adults and are more likely to come into contact with contaminants during play. Likewise, the PK of xenobiotics is known to vary by age due to changes in multiple factors including protein binding, tissue blood flows, and the expression of xenobiotic-metabolizing enzymes (primarily CYPs—cytochrome P450-dependent monooxygenases—and UGTs—uridine 5′-diphospho-glucuronysyltransferases). Genetic polymorphisms in CYPs and UGTs also impact their drug metabolizing capability. The relative frequency of these genetic polymorphisms can vary between ethnic populations.
Linking in vitro HTS results with subpopulation-specific dosimetry and exposure estimates
In a previous study, in vitro hepatic metabolic clearance and plasma protein binding data for 239 ToxCast chemicals were combined with in vitro-in vivo extrapolation (IVIVE) modeling. This enabled calculation of steady-state blood concentrations (CSS) resulting from repeated, daily chemical exposure. This information was used to estimate the oral equivalent dose values for healthy adults. To quantify population variability in pharmacokinetics, they used the following approach:
- Measured isozyme-specific clearance for a subset of nine ToxCast chemicals for 18 recombinantly-expressed CYPs and UGTs.
- Entered the isozyme-specific clearance rates into an IVIVE model, which used the Simcyp Simulator to account for known differences in isozyme expression for several different age groups and ethnicities.
- Predicted the population-specific CSS values using the model.
- Performed correlated Monte Carlo simulations to quantify the variability for each population.
- Used reverse dosimetry to estimate population-specific oral equivalents from population-specific CSS values and chemical in vitro bioactivity concentrations.
- Compared these values against population-specific exposure estimates.
A new framework for high-throughput chemical risk assessment
The US EPA uses default uncertainty factors in its risk assessments to ensure the protection of public health. In recent years, there has been a growing movement away from default uncertainty factors to data-driven chemical-specific human toxicokinetic adjustment factors (HKAFs) that consider the variability in toxicological responses between the general population and more sensitive populations. In this study, the population-specific IVIVE models were used to quantify the range in variability in CSS values. This variability was then used to estimate the HKAFs. Evaluation of the nine compounds revealed differences between 1.3- and 4.3-fold in the median CSS for a healthy population versus more sensitive populations. Alarmingly, these results suggest that the current default uncertainty value does not provide sufficient protection for sensitive populations—generally children under six months of age. This emerging approach for estimating chemical-specific PK variability across multiple sub-populations will be a valuable tool in implementing high-throughput chemical risk assessment.
All information presented derive from public source materials.
Ready to learn more?
For more information, read the Toxicological Sciences article, “Incorporating Population Variability and Susceptible Subpopulations into Dosimetry for High-throughput Toxicity Testing.” This paper was selected by the Society of Toxicology as the Outstanding Paper Published in 2014 for Advancing the Science of Risk Assessment. Congratulations to our collaborators at the EPA and The Hamner Institutes for Health Sciences and to my colleague, Dr. Lisa Almond!
You can also learn more by watching this webinar by Dr. Barbara Wetmore of the Hamner Institute where she discussed how she led a team of Hamner, EPA, and Certara researchers to develop a new toxicity paradigm that estimates chemical-specific PK variability across multiple life-stages and sub-populations.