Refining high-throughput in vitro-in vivo extrapolation modeling through incorporation of intestinal toxicokinetics

New approach methods (NAMs) that combine high-throughput toxicity and toxicokinetic data have gained prominence as federal entities attempt to evaluate tens of thousands of commercial chemicals for human health hazards. In vitro-in vivo extrapolation employing a generic high-throughput toxicokinetic (HTTK) model to convert in vitro points of departure (POD) to human equivalent doses (i.e. PODNAMs) has proven successful in translating in vitro data to real-world exposures; however, conservative assumptions, including consideration of only hepatic metabolism, have resulted in PODNAMs that are 10- to 100-fold more conservative when compared to available in vivo-based PODs. This effort evaluates the impact of incorporating intestinal metabolism through consideration of cytochrome P450 monooxygenase isozyme 3A4 (CYP3A4), a cytochrome P450 isozyme responsible for over 80% of intestinal clearance. For 11 chemicals, intrinsic clearance rates were derived in human liver and intestinal microsomes with and without inhibition of CYP3A4 to quantitate relative CYP3A4 contribution. Physiologically based TK simulations were conducted using Simcyp Simulator to (i) recapitulate the HTTK approach and (ii) incorporate CYP3A4 contribution into the elimination model, which by extension incorporates intestinal clearance occurring via CYP3A4. CYP3A4 contribution ranged from 0% to 71% across the chemicals tested, and estimates of oral bioavailability, steady-state concentration, and fraction escaping gut metabolism typically decreased with increasing CYP3A4 involvement. Further, incorporation of in vitro PODs with this refined model showed a concomitant increase in PODNAMs, indicating that incorporating such information into HTTK provides more predictive risk-based prioritization of the commercial chemical space.

The toxicokinetic model presented herein incorporates intestinal metabolism into an established generic toxicokinetic model designed for New Approach Method application. Considering CYP3A4 contribution as a surrogate for intestinal metabolism is sufficient and results in improved estimates of NAM-based POD for risk prioritization.