Anti-cancer drug-mediated increase in mitochondrial mass limits the application of metabolic viability-based MTT assay in cytotoxicity screening

The high-throughput metabolic viability-based colorimetric MTT test is commonly employed to screen the cytotoxicity of different chemotherapeutic drugs. The assay assumes a cell density-dependent linear correlation with the MTT spectral absorbance. Therefore, the present study aimed to compare the cytotoxicity assessment between the MTT assay and gold standard cell number enumeration. The cytotoxicity was induced by Cisplatin, Etoposide, and Doxorubicin in human lung epithelial adenocarcinoma cells (A549) and cervix carcinoma (HeLa) cell lines. The mitochondrial mass was estimated, and immunoblotting of succinate dehydrogenase (SDH-A) was performed following drug treatment in both cell lines. Student’s t-test paired analysis was employed to calculate the significance of the results, where the value p < 0.05 was considered statistically significant. The drug-induced cytotoxic response estimated by MTT absorbance did not show any significant difference with respect to control, and no correlation was observed with the enumerated cell number in both A549 and HeLa cells. Interestingly, per-cell metabolic viability was found to be increased by 1.18 to 3.26-fold (p < 0.05) following drug treatment. Further, mechanistic investigation revealed a drug concentration-dependent significant increase in mitochondrial mass (1.21 to 4.2-fold) and upregulation of SDH protein (50–70%) as well as enzymatic activity with respect to control in both A549 and Hela cells. The limitation of the MTT assay for drug-induced cytotoxicity assessment is due to increased mitochondrial mass and SDH upregulation in surviving cells, leading to enhanced formazan formation. This leads to a lack of correlation between cell number and MTT spectral absorbance, suggesting that the MTT assay may provide an erroneous conclusion for cytotoxicity assessment.