Protein Binding Determination in In Vitro DDI Studies

in vitro drug-drug interaction (DDI) studies are performed to investigate whether a drug candidate inhibits or induces the cytochrome P450 enzymes most commonly involved in drug metabolism. EMA Guidelines recommend the use of estimated or determined unbound drug concentrations in the in vitro system. In situations where a precise unbound microsomal free fraction value is critical (such as estimation of inhibition or induction potential not followed by a clinical study), determining the fraction experimentally is recommended. In our study, we sought to determine in vitro protein binding in human plasma and in the incubation medium used for inhibition and induction studies.

In general, we observed a loose correlation in that drugs that are highly bound to human plasma also tend to bind to microsomal cell culture incubation medium, though to a much lesser extent. We also observed many exceptions.

Based on this study, we concluded that experimentally-determined unbound fractions would certainly be beneficial and sometimes critical in projecting in vivo drug‐drug interaction potential.

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