Development of drug carrier systems: Electrical field induced effects in cell membranes*

Abstract Mouse thymocytes and erythrocytes are loaded electrically with drugs and dyes in isotonic solution. The loaded cells are used for targeting the drugs to specific sites in the organism in order to achieve a controlled drug release in time and space. Erythrocytes are directed to the liver by changing the volume and the shape. Using erythrocytes as drug carrier systems, methotrexate, the most widely studied agent in chemotherapeutic cancer treatment, could be directed exclusively to the liver. Directing to other organs is obtained by either using electrically fused cells or by loading cells with magnetic particles (about 10 nm in diameter) and the drug simultaneously and by guiding the cells to any selected site of the organism. The field technique used for the loading of the cells is based on the electrical breakdown of the cell membrane which is observed when cell suspensions are subjected to external field pulses of 2 to 20 kV/cm for short time intervals (ns to ms). When an apparent membrane potential of about 1 V (pulse length in the range of μs) is reached in response to the external field the membrane breaks down reversibly. The breakdown of the membrane is associated with a remarkable and reversible permeability increase of the cell membrane. The increase in permeability depends on the strength and the duration of the field pulse. The duration of the high conductance state of the cell membrane induced by the electric field is sufficiently long to entrap large quantities of drugs or dyes inside the cells. Electron-micrographs of thymocytes subjected to field pulses of various strengths and durations show that the pulse length is a critical factor for the reversibility of the field induced effects in the cell. When loading thymocytes the field pulse length has to be less than 1 μs to avoid irreversible changes in the ultrastructure of the cell. On the other hand, erythrocytes can be subjected to field pulses of 40 μs duration without any irreversible changes in the membrane structure. The life-span of the loaded erythrocytes in the blood circulation can be considerably prolonged if loss of haemoglobin and intracellular enzymes is avoided during field application and the resealing process. The loss of intracellular proteins can be minimized if the erythrocytes are immobilized in a polymeric network of calcium-alginate during the field application. The matrix is permeable to low molecular weight compounds but impermeable towards larger molecules such as proteins. Chelation of the Ca2+-ions in the alginate network by addition of sodium citrate leads to the release of the loaded erythrocytes from the calcium-alginate matrix.