Purpose: Mitotane is the only approved treatment for metastatic adrenocortical carcinoma (ACC). Monitoring plasma levels is recommended, but its predictive value is insufficient.

Methods: This prospective study of the French ENDOCAN-COMETE network aimed to investigate the prognostic role of plasma mitotane levels pharmacokinetics and free or bound to lipoprotein fraction measurements during six consecutive months. Lipoprotein fractions were isolated by ultracentrifugation, and mitotane level was determined by HPLC-UV. Total, free, and lipoprotein fraction bound plasma mitotane were monitored every two months for six months with morphological assessment. The primary endpoint was overall survival (OS).

Results: 21 patients with metastatic ACC were included. Median overall survival was 23 months. The median free mitotane level per patient was 12% (± 7%), and the majority (88%) was bound to lipoprotein fractions. Several pharmacokinetics measures of total mitotane were related to OS: first level at one month (p = 0.026), mean level (p = 0.055), and area under the curve (AUC) (p = 0.048), with higher exposure associated to longer OS. Free mitotane (not bounded) and mitotane bounded to lipoprotein subfraction added no prognostic values. The relationship between the mitotane level and OS suggested a minimum "effective" threshold of 10-15 mg/L or an area under the curve above 100 mg/L/month with no individualized maximum value.

Conclusion: This prospective study did not identify any added prognostic value of free mitotane level over the total level. Early total mitotane level measurements (before 3-6 months) were related to OS with a higher and faster exposure related to more prolonged survival.