New strategy to quantify hepatic cytochrome P450 3A activity with deoxyschizandrin as an in vivo probe: A V_{max, app}...

Interindividual variability in drug response stems largely from metabolic enzyme activity differences, with cytochrome P450 3A (CYP3A) being the predominant enzyme whose fluctuations critically impact exposure to narrow-therapeutic-index drugs (eg, tacrolimus) and anticancer agents. Current clinical probes such as midazolam (suboptimal hepatic uptake, sedation) and the erythromycin breath test (poor selectivity) remain inadequate. This study proposes deoxyschizandrin (DS), derived from Schisandra chinensis, as a novel in vivo CYP3A probe substrate. Using integrated rat in situ liver perfusion, whole-animal studies, physiologically based pharmacokinetic modeling, and proteomics, we demonstrated DS's advantageous properties: ideal biliary disposition (>80% hepatic accumulation, negligible parent/metabolite biliary excretion <0.02%); superior hepatic influx/uptake rate constants (0.24 s^-1 vs midazolam's 0.13 s^-1); robust metabolic kinetics (consistent in situ maximum metabolic rate [V_max] vs in vitro V_max: 3.30 vs 3.40 nmol/min/g liver); achievable in vivo enzyme saturation (physiologically based pharmacokinetic-predicted hepatic unbound concentrations > 5 × K_m) for ∼20 minutes after a bolus dose of 25 mg/kg, enabling linear metabolite generation and apparent V_max determination; and precise activity quantification (in vivo V_max strongly correlated with rat hepatic CYP3A2 protein content, R² = 0.89, P = .0047). Collectively, these findings demonstrate that DS enables accurate quantification of CYP3A activity through the V_max approach, thereby providing a promising candidate for the development of a clinically viable in vivo CYP3A probe. SIGNIFICANCE STATEMENT: This study identifies deoxyschizandrin as a novel, safe, and liver-specific in vivo probe substrate for cytochrome P450 3A (CYP3A). It enables the quantitative determination of hepatic CYP3A maximum metabolic capacity through a bolus dosing protocol, offering a promising candidate for the development of a clinically viable in vivo CYP3A probe.