A serendipitous observation was made in two of nine patients who volunteered in a sunitinib pharmacokinetic study. Both patients were diagnosed with adrenocortical carcinoma (ACC) and were exposed to mitotane. The sunitinib PK study was designed to determine the relationship between CYP3A4 activity and sunitinib exposure using 7.5 mg midazolam orally as a phenotypic probe. Patient and methods Serial blood samples for PK analysis of midazolam, 1-hydroxy-midazolam, and sunitinib were collected at steady-state sunitinib PK (between days 14 and 20). To confirm this observation in the mitotane-exposed patients, midazolam PK was evaluated in two additional patients with ACC and mitotane treatment.