Ribociclib Cytotoxicity Alone Or Combined With Progesterone And/Or Mitotane In In Vitro Adrenocortical Carcinoma Cells

: Mitotane is the only approved drug for the treatment of adrenocortical carcinoma (ACC). The regimen to be added to mitotane is a chemotherapy with etoposide, doxorubicin, and cisplatin. This pharmacological approach, however, has a limited efficacy and significant toxicity. Target-therapy agents represent a new promising approach to cancer therapy. Among these, a preeminent role is played by agents that interfere with cell cycle progression, such as CDK4/6-inhibitors. Here, we investigated whether ribociclib could induce a cytotoxic effect both in ACC cell line and patient-derived primary cell cultures, alone or in combined settings. Cell viability was determined by MTT assay while cell proliferation was evaluated by direct count. Binary combination experiments were performed using Chou and Talalay method. Gene expression was analyzed by qRT-PCR while protein expression was evaluated by immunofluorescence. A double staining assay revealed that ribociclib induced a prevalent apoptotic cell death. Cell cycle analysis was performed to evaluate the effect of ribociclib treatment on cell cycle progression in ACC cell models. Our results indicated that ribociclib was cytotoxic and reduced the cell proliferation rate. The effect on cell viability was enhanced when ribociclib was combined with progesterone and/or mitotane. The effect of ribociclib on cell cycle progression revealed a drug-induced cell accumulation in G2 phase. The positive relationship underlined by our results between ribociclib, progesterone and mitotane strengthen the clinical potential of this combination.