Pharmacokinetics of mercaptopurine in children with acute lymphocytic leukemia

To the Editor: The study reported by Koren et al. (July 5 issue) ¹ purported to show a relation between plasma concentrations of mercaptopurine and disease outcome in children with acute lymphocytic leukemia, which would be an important observation considering the high degree of variability between patients in plasma drug concentrations measured after oral administration of this agent. Unfortunately, we believe that the authors based their correlations on an invalid pharmacokinetic index. For most anticancer drugs, the area under the concentration—time curve (the AUC) — a measure of systemic drug exposure —is the variable that best correlates with response or toxicity.