HP661, a selective inhibitor of mitochondrial complex I (NADH dehydrogenase), impedes complex I activity by 77.6% at 1 µM, demonstrating lesser inhibition towards complex III (28.1%) and no inhibitory effects on complexes II and IV. It notably decreases the viability of human lung cancer cells—H460, NCI H441, and trametinib-resistant A549 cells (IC50s = 10.6, 29.7, and 15.1 nM, respectively)—which exhibit high levels of oxidative phosphorylation. In contrast, it shows minimal activity against NCI H358 lung cancer cells and non-cancerous HPNE and MRC-5 cells (IC50s = >10,000 nM for both), which have lower oxidative phosphorylation levels. Furthermore, in an H460 mouse xenograft model, HP661 at a dose of 30 mg/kg twice daily significantly reduces tumor volume and enhances the tumor-growth inhibitory effects of trametinib.