A 34-year-old female with bipolar I disorder has been stable on valproate 1000 mg daily for 18 months. Due to persistent weight gain and tremor, the PMHNP plans a cross-titration to lamotrigine. The current valproate level is 85 mcg/mL. As lamotrigine is initiated, the PMHNP uses a slower titration schedule than the standard monotherapy titration. Which of the following best explains the pharmacokinetic basis for this modified titration approach?