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.