Evaluation of the methemoglobinemia associated with sulofenur. 
A new class of antineoplastic agents, the diarylsulfonylureas entered clinical trials with the testing of Sulofenur (LY186641).  Phase I trials and preclinical studies showed the dose limiting toxicity to be methemoglobinemia.  We studied the incidence of methemoglobinemia, sulfhemoglobinemia and cytochrome b5 reductase deficiency in nine consecutive patients enrolled in a phase II trials using Sulofenur.  The specific Malloy method as well as clinically standard co-oximeter measurements were used to determine methemoglobin levels and marked discrepancies were noted.  One patient with symptomatic methemoglobinemia had enzyme levels and family history consistent with a heterozygous state for a cytochrome b5 reductase deficiency.  We conclude that the clinical incidence of methemoglobinemia will be overestimated by co-oximeter measurements but that Sulofenur does produce clinically significant methemoglobinemia in cytochrome b5 reductase deficient patients.