There are two primary factors that can be manipulated to effect emulsion stability, liposome concentration, and percentage of oil in the emulsion. Emulsions made with 25-mM liposomal phospholipids are very unstable, breaking within a few hours. By increasing the phospholipid concentration of the liposomes, emulsions can be made to break in a few hours or a few days to being stable for long periods of times (Fig. 6). The volume of the liposomes can also be changed. Similarly, the stability of emulsion can be varied by changing the percentage of light mineral oil. Emulsions with low percentages of oil are unstable breaking within two days, while emulsions with high percentage of oil (42.5%) are stable for long periods of time (Fig. 7). In addition, a combination of these factors can be varied to change the stability of the emulsion. The emulsion that we used in clinical trials with prostate cancer patients contained 1 mL of PSA liposomes and 0.1 mL of light mineral oil (2,20). This emulsion started to break after eight hours at 4°C or room temperature. By changing the emulsion to 150 mM phospholipid liposomes (final concentration in the emulsion) and 40% light mineral oil, we were able to obtain an emulsion that was stable for three years at both 4°C and room temperature.
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