Abstract B36: Antimalarial agent, atovaquone, inhibits cancer cell proliferation by targeting oxidative phosphorylation and is a candidate for chemoprevention and chemotherapy of ovarian cancer
Authors: Kapur Arvinder , Felder Mildred , Alharbi Yousef , Cappuccini Allegra , Pattnaik Bikash , Barroilhet Lisa , Patankar Manish 
Published: September 2020

Abstract
Atovaquone is a FDA-approved antimalarial agent that has also shown potent anticancer properties. In the current study, we demonstrate that atovaquone is a prime candidate for treatment of high-grade serous ovarian cancer. We have tested atovaquone against a panel of ovarian cancer cells lines and shown that this drug inhibits proliferation and induces apoptosis at IC50 concentrations between 10-20 microM. Exposure to atovaquone results in increased apoptosis as indicated by an increase in cleaved caspase 3 and other relevant apoptotic markers. Additionally, our data also suggest that atovaquone is able to reactivate p53. Inhibition of p53 attenuates the cytotoxicity of atovaquone. However, the primary event that occurs in atovaquone-treated cells is the rapid surge in intracellular oxygen radicals. Imaging cytometry experiments conclusively demonstrate that the oxygen radical surge originates in the mitochondria. The oxidative stress caused by atovaquone is responsible for DNA damage and p53 activation. The atovaquone-induced oxygen radical surge also leads to damage to the Na+/K+-ATPase, perturbing the ability of the cancer cells to maintain an optimal membrane potential. This increase in oxygen radicals occurs because atovaquone targets the oxidative phosphorylation pathway in the mitochondria and blocks electron transport. Measurements conducted on the Seahorse analyzer show decreased consumption of oxygen and lower levels of ATP in the atovaquone-treated cells. In the malarial parasite, atovaquone targets mitochondrial complex III. In ovarian cancer cells, supplying succinate did not reverse atovaquone-induced inhibition of oxidative phosphorylation. On the other hand, when cells were supplemented with the substrate for complex IV, the oxygen consumption rate normalized in atovaquone-exposed cells. These experiments suggest that atovaquone is likely inhibiting complex I-III of mitochondria. Atovaquone is a naphthoquinone with significant structural similarity with the electron transporter ubiquinone. In silico modeling experiments indicate that atovaquone can bind to the ubiquinone binding pocket on complexes I-III. Based on these experiments, we propose that atovaquone is competing with ubiquinone to inhibit efficient electron transport and thereby increasing the intracellular oxygen radicals. Ovarian tumor-bearing mice tolerated prolonged treatment with Mepron®, the clinical formulation of atovaquone. Daily treatment of ID8 tumor-bearing C57BL/6 mice with atovaquone for 3-4 weeks resulted in a significant decrease in tumor growth. Additionally, we also show decreased proliferation of human ovarian tumors treated ex vivo with atovaquone. These experiments indicate that atovaquone can be used as a safe and effective drug for chemoprevention and chemotherapy of ovarian tumors.
Citation Format: Arvinder Kapur, Mildred Felder, Yousef Alharbi, Allegra Cappuccini, Bikash Pattnaik, Lisa Barroilhet, Manish Patankar. Antimalarial agent, atovaquone, inhibits cancer cell proliferation by targeting oxidative phosphorylation and is a candidate for chemoprevention and chemotherapy of ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research; 2019 Sep 13-16, 2019; Atlanta, GA. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(13_Suppl):Abstract nr B36.