How are interspecies extrapolation and intraspecies variability addressed in risk assessment?

• A. Interspecies extrapolation is ignored and not addressed.
• B. Intraspecies variability is assumed nonexistent.
• C. Interspecies extrapolation and intraspecies variability are addressed solely by selecting lower dose levels.
• D. Interspecies extrapolation accounts for differences between animals and humans; intraspecies variability accounts for differences among humans; handled via uncertainty factors and PK/PD modeling.

Answer: (D)

Understanding risk assessment involves translating animal toxicity data to humans and accounting for variability among people. Interspecies extrapolation captures differences in physiology, metabolism, and sensitivity between animals and humans. Intraspecies variability reflects how responses differ among people due to genetics, age, sex, health, and other factors. The standard way to address both is through uncertainty factors and pharmacokinetic/pharmacodynamic (PK/PD) modeling. Uncertainty factors provide conservative buffers, typically a default 10-fold factor for species differences and another 10-fold for human variability, with the total margin reflecting data gaps and uncertainties. PK/PD modeling, including physiologically based PK models and allometric scaling, translates animal exposure metrics to human equivalents and describes how dose relates to response across species and within human populations. This integrated approach yields reference doses or safe exposure levels that protect most of the population while incorporating mechanistic insight and remaining knowledge gaps. Simply ignoring interspecies differences or assuming no human variability would remove essential safety margins, and choosing lower doses without a formal quantitative framework cannot reliably capture variability or bridge species differences.