The methyl-thiazolyl-tetrazolium (MTT) reagent was used to assess cell viability by detecting functional mitochondria in living cells. HL-1 cells (1.5 × 10⁴ cells per well) were seeded into 96-well plates, starved in serum-free medium for 6 hours, and subsequently incubated with 2-5 μM DOX for 4 hours. Control cells were treated with 0.1% DMSO. Next, 5 μL of MTT reagent (5 mg/mL) was added to each well, followed by incubation at 37 °C for 4 hours. Formazan salts produced were dissolved by adding 100 μL of a solution containing 50% (v/v) N,N-dimethylformamide and 20% (w/v) sodium dodecyl sulfate (pH 4.5). After 24 hours of incubation at 37 °C, absorbance was measured at 550 nm. Viability variations were determined by normalizing the optical density (OD) values of treated cells to those of the controls.