Pharmacokinetics as a Biomarker.

Pharmacokinetics (PK) has long been differentiated from pharmacodynamics (PD) and biomarkers, yet this distinction undervalues PK's translational relevance. In this Perspective, we propose that PK itself functions as a biomarker that bridges dose, exposure, and response. Using examples from target-mediated drug disposition, antidrug antibodies, cerebrospinal fluid PK, high-dose methotrexate therapy, and anti-infective pharmacology, we illustrate how PK serves as a measurable, predictive, and actionable biomarker that informs drug development, guides decisions, and advances precision medicine.