A 2-year rat carcinogenicity study may add limited value when existing evidence already provides a robust assessment of human carcinogenic risk. ICH S1B(R1) uses a weight-of-evidence (WoE) approach to determine whether human carcinogenic potential is likely, unlikely, or uncertain, and whether a 2-year rat study would meaningfully reduce remaining uncertainty [1]. Model-Informed Drug Development (MIDD) can strengthen this assessment by quantitatively linking exposure, biological effects, and human relevance.

  • Understand metabolism and exposure (discovery through Phase 1): Integrate rat/human in vitro metabolism with preclinical and clinical PK. PBPK/PK models can characterize parent and major metabolites, tissue and unbound exposures, accumulation, and rat-to-human exposure margins, providing an exposure anchor for the WoE.
  • Understand pharmacology and mechanism (discovery through early clinical development): Compare target expression, tissue distribution, potency, target engagement, downstream signaling, and relevant off-target pharmacology across rats and humans. Mechanistic PK–PD/QSP/QST models can link exposure → biological response and assess whether potential carcinogenic mechanisms are relevant at human therapeutic exposures.
  • Build the WoE throughout (before Phase 3): Integrate genotoxicity, secondary pharmacology, endocrine/immune effects, mechanistic in vitro assays, and repeat-dose toxicity. The 6-month rat study is a key integration point, particularly for exposure-related hypertrophy, hyperplasia, persistent injury/inflammation, preneoplastic changes, or tumors [1,3,4].
  • Quantify human relevance and remaining uncertainty: Integrate nonclinical findings with clinical exposure, PD, safety, and patient variability. MIDD can characterize exposure–response relationships and potential biological thresholds, assess species differences, and determine whether mechanisms observed in rats are likely to be engaged at anticipated human exposures [2].
  • Integrate and engage regulators: Following the 6-month rat study, integrate the evidence to determine whether human carcinogenic potential is likely, unlikely, or uncertain and whether a 2-year rat study would provide additional information [1].

A strong case that a 2-year rat study is unlikely to add value would combine:

Adequate exposure coverage + no concerning chronic histopathology/genotoxicity/endocrine or immune signals + well-characterized pharmacology + quantitative evidence that clinically relevant human exposure does not engage a plausible carcinogenic mechanism.

The role of MIDD is therefore not to predict cancer directly, but to quantify and reduce uncertainty and determine whether a 2-year rat study is likely to materially change the human carcinogenicity risk assessment [2].