Highly Protein-Bound, Narrow Therapeutic Index Drugs: ADMET Fundamentals and Their Role in Clinical Pharmacokinetics and Toxicology

Drugs that are both highly bound to plasma proteins and characterized by a narrow margin between therapeutically effective and toxic exposure present a distinctive clinical pharmacokinetic challenge: small changes in the free, pharmacologically active fraction, whether from a displacement interaction, altered protein concentration in disease states, or organ impairment, can produce clinically significant changes in effect without any corresponding change in total measured drug concentration (Benet & Hoener, 2002). This article examines the ADMET fundamentals specific to this drug category, the assays used to characterize protein binding and free-fraction behavior, the clinical and nonclinical implications of narrow therapeutic index status, and the therapeutic drug monitoring strategies this combination of properties typically requires. The article closes with recommendations for regulatory affairs and clinical pharmacology professionals developing or managing a program in this category.Keywords: protein binding; narrow therapeutic index; free fraction; displacement interaction; therapeutic drug monitoring; equilibrium dialysis; regulatory artificial intelligence