Respiratory complex I null cancer cells and molecular docking reveal specificity and mode of action inhibitors with anticancer activity

Respiratory complex I (NADH:ubiquinone oxidoreductase; EC 1.6.5.3; CI) derangement or inhibition have shown to slow down tumor growth, confining aggressive tumors into a low proliferative state and opening a possible window for additional therapeutic intervention. Hence, this enzyme has emerged as an attractive druggable target for cancer treatment and several inhibitors have been proposed as possible therapeutic agents in different experimental preclinical settings. However, none of them is currently used in clinical practice and several open issues remain, including their mechanism of action, efficacy and specificity. We here investigated the specificity and the antiproliferative activity of three inhibitors (metformin, BAY 87-2243 and EVP 4593) using unique models lacking CI generated in different cancer cell background, namely colon cancer, ovarian cancer and melanoma. In these models, the antiproliferative effect of metformin resulted independent from CI inhibition, while both BAY 87-2243 and EVP 4593 were highly selective at optimized concentrations fully inhibiting mitochondrial respiration. Molecular docking predictions indicated such high efficiency may derive from the tight network of interactions in the quinone binding site, where EVP 4593 was foreseen to interact with amino acids from nuclear subunits forming the deep site, while BAY 87-2243 is predicted to bind in the shallow site. Our data prompt for caution when referring to metformin as CI targeting compound and highlight the need of dosage optimization and careful evaluation of molecular interactions between inhibitors and CI. Amino acids involved in the interactions with EVP 4593 and BAY 87-2243 may be subjected to polymorphic variants, both inherited or somatic, although with low allele frequency. Hence, aiming at a personalized medicine approach, genotyping is suggested before treatment with these compounds, especially those binding the shallow quinone binding site where ND1 plays a major contribution.