Codon optimization increases the sensitivity of stable human cell lines to the prodrug CB1954. (A) HEK-293, (B) SH-SY5Y and (C) SHP-77 cells stably transfected with pGNTR, pGNTRo or pEGFP-C1 (control). MTT assays were performed to determine relative cell viability after 48 hours of cultivation in presence of different CB1954 concentrations. One representative experiment out of three is shown. *: p < 0.05; n = 4. (D) The expression of GNTR was confirmed by immunodetection using anti-GFP and anti-actin (loading control) antibodies. Amount of lysate loaded to each lane: pEGFP-C1 (3 μg), pGNTR (20 μg) and pGNTRo (20 μg).