447 ePBAE was combined with 2 different PEG-PBAEs to generate 18 different PEG-PBAE NP formulations. Cell viability of (a) GBM1A and (b) BTIC375 cells from incubation with 18 and 12 PEG-PBAE NP formulations, respectively, normalized to untreated control determined by MTS assay (n = 4, mean ± s.d., *: p < 0.05 compared to untreated control). pEGFP transfection efficacy of 18 PEG-PBAE NP formulations in (c) GBM1A and (d) BTIC375 cells determined by flow cytometry (n = 4, mean ± s.d.).