Mitotane is currently employed as adjuvant
therapy as well as in the medical treatment of
adrenocortical carcinoma (ACC), alone or in combination with chemotherapeutic agents. It was previously demonstrated that
mitotane potentiates chemotherapeutic drugs cytotoxicity in
cancer cells displaying chemoresistance due to
P-glycoprotein (P-gp), an efflux pump involved in
cancer multidrug resistance. The majority of ACC expresses high levels of P-gp and is highly chemoresistent. The aim of our study was to explore in vitro whether
mitotane, at concentrations lower than those currently reached in vivo, may sensitize ACC cells to the cytotoxic effects of
doxorubicin and whether this effect is due to a direct action on P-gp. NCI-H295 and SW13 cell lines as well as 4 adrenocortical
neoplasia primary cultures were treated with
mitotane and
doxorubicin, and cell viability was measured by MTT assay. P-gp activity was measured by
calcein and P-gp-Glo assays. P-gp expression was evaluated by Western blot. We found that very low
mitotane concentrations sensitize ACC cells to the cytotoxic effects of
doxorubicin, depending on P-gp expression. In addition,
mitotane directly inhibits P-gp detoxifying function, allowing
doxorubicin cytotoxic activity. These data provide the basis for the greater efficacy of combination
therapy (
mitotane plus chemotherapeutic drugs) on ACC patients. Shedding light on
mitotane mechanisms of action could result in an improved design of
drug therapy for patients with ACC.