What F_PEN represents
F_PEN is a fraction rather than a concentration. It represents the fraction of the population receiving the active substance and can be refined using reliable prevalence data and, where relevant, treatment-regimen information.
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ERA calculations
PECSW and FPEN in Pharmaceutical Environmental Risk Assessment
Phase I environmental exposure assessment uses the predicted environmental concentration in surface water (PECsw). The population fraction term, commonly represented as F_PEN in the current EMA guideline, links exposure to the proportion of a population receiving the active substance.
Key considerations
What PECsw represents
PECsw is the predicted concentration of the active substance in surface water under the Phase I exposure scenario. It is an exposure estimate used to determine whether further environmental assessment is needed.
Why prevalence data matter
The current EMA guideline allows F_PEN to be refined using reliable, independent prevalence data for the relevant indication(s). The guideline describes how regional or member-state differences and data recency should be considered.
Avoiding a misleading calculation
The Phase I framework retains other default realistic-worst-case parameters unless the guideline specifically allows refinement. A project-specific ERA should document the source and rationale for any refinement.
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