(172a) Characterising the Particle Specificity of Drug Delivery Systems to Melanoma Cells by the Atomic Force Microscope

Proficient delivery of anti-cancer drugs to melanoma cells requires that only the targeted organ receives the drugs at a pre-programmed rate, and at concentrations required for effective treatment. In this way, the negative side-effects of the drugs affecting other cells can be minimized or eliminated. This may be achieved by embedding the drugs in a carrier with functionalities specific only to the cell in question. The problem is, though, that we still do not know the surface properties of a melanoma cell, the carrier surface functionality groups to which the cell is specific, and the optimum conditions for administering the drugs. In this study, we investigated these subjects with the atomic force microscope (AFM) by using functionalised cantilever colloid probes, which modelled the drug carrier, and living malignant melanoma B16F10 cells, which were used as the substrate.

Previous studies have suggested that the success of a drug delivery system (DDS) may be influenced by the force used in administrating the drug, the position where the drug is administrated on a cell, and the residential time of a drug at a cell. There appears to be no study, however, that investigates the single importance of each of these points ? a process necessary to improve the DDS. In order to explore the importance of the force strength, we varied the pushing force of the colloid probe to the cell, and noticed its effect on the adhesion force. The result was that the magnitude of the pushing force did not appear to affect the adhesion, as long as the force was small enough not to damage the cell. Next, we investigated the difference in the adhesion force between the cell and colloid probe, when we measured at the cell nucleus and near the edge of the cell. We found no relation between the cell surface position and the adhesion force. Finally, the influence of the adhesion time of the probe at the cell surface was examined, by varying the adhesion time from 10 to 60 min. A large dependence was seen, where the largest adhesion was noted for longer times.

Next, keeping the adhesion time constant at 10 min, we varied the type of carrier, in order to find the functionality with the highest specificity to the malignant cell. As carriers containing hydrophobic, polyethylene glycol (PEG), or positive surface groups appear to show specificity to non-malignant cells, it was thought that a malignant cell may respond in a similar manner. However, as cancer cells differ from normal cells and may therefore acquire altered differentiated functions, it was unclear as to which DDS functional group a malignant cell is most specific. Thus, we tested the effect of charge, hydrophobicity, and polymer presence on a DDS carrier on its degree of specificity to the melanoma cell. This was possible, as the maximum in the adhesive force upon decompression in the AFM force curves is related to the strength of the specificity of the DDS carrier to the malignant cell. We surprisingly found that negatively charged surfaces, hydrophobic, and PEG modified surfaces all have similar low adhesive force values. Additionally, there was no observable dependence on the degree of hydrophobicity of the probe surface to a B16F10 cell ? this was in opposition to the current beliefs. Only the particle that was modified to give a positive charge was seen to give strong adhesive forces with the B16F10 cell. Thus, DDS carriers with positive charges appear to have the highest specificity to malignant melanoma cells under these conditions.

In conclusion, positively charged DDS particles are most specific to malignant melanoma cells, and their specificity increases with their time of contact with the cell.