On April 28, 2014, the U.S. Food and Drug Administration approved a 20 mg/mL oral suspension of mercaptopurine (Purixan) indicated for the treatment of patients with acute lymphoblastic leukemia (ALL) as part of a combination regimen. Successive clinical trials have demonstrated that mercaptopurine contributes to successful maintenance therapy and improved survival of patients with ALL.
This approval was based on a clinical pharmacology study that assessed the bioequivalence of mercaptopurine tablets with that of mercaptopurine oral suspension in a healthy adult population.
The drug was originally approved in 1953 and has been commercially available as a 50-mg tablet. However, body surface area dosing and dose adjustments are not easily accomplished with the 50-mg tablet, and tablets are not an ideal dosage form of medication for children less than 6 years old. Ad hoc local formulations compounded in pharmacies are commonly used, and 50-mg tablets are often split to provide children with the desired dose.
Compared to tablets, a suspension offers the advantage of more accurately delivering the desired dose to children with a wide range of weights using a consistent administration schedule and also allows more flexibility in adjusting the dose. A commercially produced suspension is more likely to provide a more consistent dose of mercaptopurine than ad hoc compounded formulations.
The starting dose of mercaptopurine in multiagent combination chemotherapy maintenance regimens is 1.5 to 2.5 mg/kg (50 to 75 mg/m2) as a single daily dose. After initiating mercaptopurine, continuation of appropriate dosing requires periodic monitoring of absolute neutrophil count and platelet count to ensure sufficient drug exposure and to adjust for excessive hematologic toxicity. ■