In Vivo Prediction of DDIs for Inducers

The magnitude of a DDI involving an inductor is based on the equation below:

sf — a scaling factor that is empirically derived Emax — maximum induction response [I] — concentration of inducer

EC50 — concentration of inductor that results in half Emax

If a compound is a competitive inhibitor, a mechanism-based inhibitor, and an inducer, then the magnitude of DDI is expressed below:

DDI magnitude = 1

Ritonavir is a competitive inhibitor, mechanism-based inhibitor, and inducer of CYP3A4.

See Tables 5.11 and 5.12.

Table 5.11. Comparison of different drug interactions

Reversible Mechanism-based Mechanism inhibition inhibition Induction

Onset Immediate Depends on the rate of Slow (days)

enzyme inactivation Is prior exposure to Not needed Needed Needed inhibitor needed?

AUC (for victim) Increased Increased Decreased

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