Relation between transfection efficiency and cytotoxicity of poly(2-(dimethylamino)ethyl methacrylate)/plasmid complexes

Abstract

Poly(2-(dimethylamino)ethyl methacrylate), p(DMAEMA), with varying molecular weight was synthesized, complexed with pCMV-LacZ plasmid and evaluated as transfection agent in COS-7 and OVCAR-3 cells. Both the transfection efficiency and cell viability were monitored after two days culturing. The transfection efficiency of p(DMAEMA)/plasmid complexes was studied as a function of polymer/plasmid (w/w) ratio and compared with two known carriers: poly( l-lysine) and DEAE dextran. For all polymeric carriers a maximum in transfection efficiency was found when cell viability was 40–70%. This maximum was found at polymer/plasmid ratio 5/1, 6/1–13/1 and 50/1–200/1 (w/w) for poly( l-lysine), p(DMAEMA) and DEAE dextran respectively. The transfection efficiency of p(DMAEMA)/plasmid complexes was two-fold higher than DEAE dextran/plasmid complexes and eight times higher than poly( l-lysine)/plasmid complexes, although the carrier/plasmid complexes had about the same physicochemical characteristics. This indicates that differences in the transfection efficiency can be ascribed to differences in structural properties of the transfectant. At a sub-optimum p(DMAEMA)/plasmid ratio (5/1, w/w) a tenfold increase in number of transfected cells (after 48 h culturing) was found with increasing incubation time of the cells with plasmid (from 5 to 240 min) and was associated with a high number of viable cells (75–100%). On the other hand, at high polymer/plasmid ratio (20/1, w/w) the transfection efficiency passed through a maximum after 30 min incubation. Longer incubation times resulted in a substantial reduction of living cells. The same trends were observed in the presence of chloroquine and serum. However, the viability of the cells was substantially reduced, probably due to the toxicity of chloroquine. Transfection efficiency was also studied as a function of p(DMAEMA) molecular weight. Polymers with molecular weight >300 kDa were better transfection agents in both COS-7 and OVCAR-3 cells than low molecular weight polymers. Dynamic light scattering measurements showed that high molecular weight polymers were able to condense DNA effectively resulting in particles of 0.17–0.21 μm. In contrast, when plasmid was incubated with low molecular weight p(DMAEMA), large complexes were formed (size of approximately 1.0 μm). Obviously, smaller complexes have an advantage over larger polymer/plasmid complexes in cell entry. Taken together the results of transfection efficiency and cytotoxicity, we hypothesize that complexes enter cells by membrane destabilization, either at the cell surface or within vesicles.