A 58-year-old man with major depressive disorder has been taking fluoxetine 20 mg daily for six weeks with minimal clinical improvement and no notable side effects. Pharmacogenomic testing reveals he is a CYP2D6 ultrarapid metabolizer with three functional gene copies. His PMHNP is reviewing the pharmacokinetic implications of this finding. Which of the following best explains the clinical significance of this patient's CYP2D6 ultrarapid metabolizer status in the context of his poor treatment response?