Atovaquone enhances doxorubicin's efficacy via inhibiting mitochondrial respiration and STAT3 in aggressive thyroid cancer.

The clinical management of anaplastic thyroid carcinoma and follicular thyroid carcinoma is challenging and requires an alternative therapeutic strategy. Although atovaquone is an FDA-approved anti-malarial drug, studies has recently demonstrated its anti-cancer activities. In line with these efforts, our study shows that atovaquone is an attractive candidate for thyroid cancer treatment. We show that atovaquone significantly inhibits growth, migration and survival in a concentration-dependent manner in 8505C and FTC113 cells. Mechanistically, atovaquone inhibits mitochondrial complex III activity, leading to mitochondrial respiration inhibition and reduction of ATP production in thyroid cancer cells. The inhibitory effects of atovaquone is reversed in mitochondrial respiration-deficient 8505C ρ0 cells, confirming mitochondrial respiration as the mechanism of atovaquone's action in thyroid cancer. In addition, atovaquone suppresses phosphorylation of STAT3 in thyroid cancer wildype but not ρ0 cells, demonstrating that STAT3 phosphorylation inhibition by atovaquone is a consequence of mitochondrial respiration inhibition. Notably, we further demonstrate that atovaquone significantly augments doxorubicin's inhibitory effects via suppressing mitochondrial respiration and STAT3. Our findings suggest that atovaquone can be repurposed for thyroid cancer treatment. Our work also highlights that targeting mitochondrial respiration may represent potential therapeutic strategy in thyroid cancer.