Fig. 3. Assessing antiviral activities of compounds 9 and 12 in PRRSV-infected MARC-145 cells by virus titer and PCR. Cells grown in 6-well plates were infected with PRRSV GD-XH (0.05 MOI) for 2 h at 37 °C and then cultured in fresh medium containing various concentrations of 9 or 12. At 48 h ((A) and (B)) or indicated time-points (C) post infection, the samples were subjected to viral titer, or RT-PCR analysis. ((A) and (C)) The PRRSV titer was determined after treatment with compounds for 48 h (A) or indicated time-points (C) using the end point dilution assay and expressed as log₁₀ TCID₅₀/1 mL. (B) Relative PRRSV NSP9 mRNA level was analyzed using real-time RT-PCR after 48 h compound treatment. Expression of GAPDH was shown as a loading control, and samples with solvent DMSO addition were used as no inhibition control (set as 1). Statistical significances are denoted by *P < 0.05, **P < 0.01, and ***P < 0.001 compared to DMSO-treated control.