Influence of oil droplet surface charge on the performance of antibody–emulsion conjugates

Targeted drug delivery requires binding of (and subsequent uptake by) the carrier to target cells. The purpose of our present study is to compare the binding and uptake of emulsions with different electric surface properties to SK-BR3 cell line, which over-expresses the HER2 receptor. Cationic emulsion was prepared by incorporating 0.25% w/w of the cationic lipid, stearylamine in the formulation while the anionic emulsion formulation was identical but lacking stearylamine. Immunoemulsions were prepared by conjugating the 2-iminothiolane derivative of the monoclonal antibody trastuzumab (Herceptin®) through the reactive maleimide group of the octadecyl-4-(maleimidomethyl)cyclohexane-carboxylic amide linker which was incorporated in the oil phase of the anionic and cationic emulsions. Cationic emulsion exhibited a droplet size of ∼130 nm and a zeta potential of +50 mV compared to anionic emulsion with a droplet size of ∼140 nm and a zeta potential of –30 mV which decreased to –5 mV following antibody coupling. There was no significant difference in the coupling efficiency of trastuzumab to anionic and cationic emulsions which was in the range of 60–70%. The cationic emulsion and immunoemulsion appeared to be physically stable over a long period of time, as indicated by particle-size measurements while the droplets of the anionic immunoemulsion coalesced with time resulting in phase separation within 20 days storage at 4 °C. The results of binding and uptake to cells showed that both cationic and anionic immunoemulsions bind and internalized to cells much more than the respective blank emulsions. The enhanced penetration of the probe coumarin-6 with both immunoemulsions clearly indicated that the internalization process was mainly controlled by a cell-receptor endocytosis mechanism mediated by the binding affinity of trastuzumab to the cell surface receptor since the uptake of the cationic immunoemulsion was not significantly different from the uptake of the anionic immunoemulsion. However, only the cationic immunoemulsion might be considered for further investigation in view of its long standing physical stability.