For decades we have known that technically we can fix imbalances in the production of proteins simply by delivering plasmid DNA or mRNA encoding for the target proteins in the cells (Cutlar et al. 2015). Unfortunately, there are still no effective ways to deliver exogenous DNA/RNA inside human cells without using a virus (Meng et al. 2017). The pioneering studies used large amounts of naked DNA in the hope that eventually some would get in the cells (Liu et al. 2016; Mehier-Humbert and Guy 2005). But, the cells membrane is negatively charged and so is also the DNA, thus naked DNA is not going to enter cells spontaneously. To circumvent this issue, scientists attempted to use physical methods to temporarily disrupt the integrity of the cell membrane like for example the gene gun, electroporation, sonoporation or magnetofection (Mehier-Humbert and Guy 2005). The issue with this approach is that it can only be performed ex vivo in a lab. Handling cells from patients is prohibitively expensive and it would also require several surgeries, not the most feasible approach if you want to know my opinion.

We made so much progress in exploiting the efficacy of viral vectors. We learned to engineer viruses so that they can’t replicate if we don’t want them to. To be fair much effort is spent in optimizing viral-based gene therapy so that 70% of all the gene therapy clinical trials use modified viruses (Yin et al. 2014). Then why don’t we see much viral therapies applied in the clinic? Well, despite the progress there are still several drawbacks. One limitation is the size of the genes we can insert in a virus. With the current procedures and technologies, we can only insert small sequences. Another big issue is that producing viruses is seriously expensive because of all the precautions and the special facilities required for their manufacturing. Despite all the modifications, we are still talking about immunogenic entities, they could activate the host’s immune system creating havoc. Moreover, some viruses were found to be carcinogenic (Yin et al. 2014), not sure many patients would be happy to receive such a therapy.

Compared to viral vectors, this is a much safer alternative, so safe that it’s hard to make it work. Anyway, it’s relatively easy to design and modify plasmids and they are also cheap to produce. So why there isn’t a larger use of plasmids in the clinic? Well, earlier I’ve hinted that on its own, plasmid DNA will hardly get inside the cells and the options to make it work are: we either temporarily disrupt the cell membrane to enable the entering of DNA or we complex the DNA with cationic vectors making positively charged particles. These vectors can be either made using lipidic or polymeric building blocks, either way the downside is that they are toxic. So you can guess now what is the catch, you can get a cell to express a therapeutic protein by delivering complexed plasmids but you may eventually kill the cell you targeted. This could be ok in tissues with high regenerative potential but what about tissues where cells do not regenerate like the brain or cartilage? Besides, there is another issue here. In order to work, plasmids need to enter the nucleus of a cell and they can only do so when cells divide. So, to have effective transfection using plasmids we need to have actively proliferating cells. This may be difficult to obtain in diseased tissues (in many cases you have a disease because cells don’t proliferate enough). So generally, for the frustration of scientists, you can have the situation in which transfection experiments work in a laboratory under controlled conditions, where you can induce cell proliferation, but they don’t work in vivo where you most need them to.