Title:Optimisation of Synthetic Vector Systems for Cancer Gene Therapy – The Role of the Excess of Cationic Dendrimer Under Physiological Conditions
Volume: 14
Issue: 9
Author(s): M.J. Santander-Ortega, M. de la Fuente, M.V. Lozano, M.L. Tsui, K. Bolton, I.F. Uchegbu and A.G. Schatzlein
Affiliation:
Keywords:
Dendrimers, gene therapy, depletion forces, proteins, electrolytes, cell culture.
Abstract: We have previously demonstrated in a therapeutic study that a single systemic course of DAB-Am16 dendriplexes
loaded with plasmid expressing TNFα over a period of time of 10 days led to a regression of 100% of tumours and
to long term cures of up to 80% of animals. However, the formulation had a relatively low colloidal stability requiring
administration soon after nanoparticle preparation. Similar to other cationic polyplex and dendrimer DNA delivery systems,
DAB-AM16 dendrimer formulations contained a substantial proportion of free polymer; this free polymer is present
independently of the specific polymer:DNA ratio and increases with increasing proportion of polymer (N:P charge ratio)
in the formulation. It has previously been shown for this and other systems that the excess of polymer plays a role in promoting
the transfection efficiency of synthetic vectors. This has been linked to effects of the polymer on the efficiency of
intracellular processing, e.g. endosomal release. However, the free polymer may have additional effects that are relevant
to the efficiency of the formulation. This study therefore considered the effect of free dendrimer on the colloidal stability
of the complexes, the interaction of the complex with the formulation medium, and with biological components, i.e. electrolytes
and serum proteins after administration. Analysis of the total potential of interaction shows that, even at high N:P
ratios, the excess of free dendrimer in the medium is not enough to induce the aggregation of the formulation due to depletion
forces. This finding is unusual and can be attributed to the particularly low Mw of these dendrimers (1.6 kDa). On
the other hand, formulations are highly sensitive to the strength of the dendrimer:DNA interactions. These can be controlled
by the degree of protonation (α) of the dendrimer which is strongly dependent on bulk pH. Modulation of the protonation
level to α≥0.4 allows reproducible production of colloidally stable particles. Finally, we have demonstrated that
electrolytes and proteins present in physiological media play a crucial role to favour the efficiency of these synthetic vectors
reducing the toxicity associated with their cationic groups.