2.9. CPE Reduction Assay

The SARS-CoV-2 strain at a 0.001 multiplicity of infection (MOI) was inoculated onto near confluent Vero E6 cell monolayers (1 × 10⁵ cells/well) for 1 h with occasional rocking. The medium was removed and replaced by DMEM with each lipoprotein at different concentrations. The cultures were incubated for 72 h at 37 °C under a 5% CO₂ atmosphere until the cells in the infected, untreated control well showed a complete viral cytopathic effect (CPE), as observed by optical microscopy. Each lipoprotein was assayed for virus inhibition in triplicate.

After 72 h incubation in all antiviral assays, the cells were replaced with only media, and a 10 μL MTT solution was added to each well and incubated at 37 °C for 4 h. After removing the supernatant, 100 μL of a 0.04 M HCl–isopropanol solution was added to dissolve the formazan crystals. The absorbance was measured at 540 nm with a subtraction of the background measurement at 655 nm using a microplate reader (Bio-Rad model 680). The 50% inhibitory concentration (IC₅₀) was calculated by regression analysis. The selective index (SI) was calculated using the formula, SI = CC₅₀/IC₅₀, as described previously [29].