CJ � · ·s:CJ COSMETIC FORMULATIONS WITH ARGIRELINE® 163 80000 .---------------�������:::-, _,._ 25 °C 24 h -e---25 °C 7 days ---- 25 °C 30 days 60000 40000 20000 0 -+----+---+----+-------1----+----+-----+■---+----1 0.5 2.5 10 100 Speed (rpm) Figure 1. Viscosity vs speed of Argireline® creams maintained at 25 ° C as a function of storage time. viscosity, and shear rates were the same in both samples at all assay times, a result that suggests that the cream formulation can be safely stored at room temperature. Figures 3 and 4 show the findings for the gel formulation after storage for up to 30 days at the two temperatures. Viscosity was slightly lower in refrigerated samples than in samples kept at room temperature, as a result of thermal gelling (seen at low shear rate) (21). In samples tested after 30 days of storage, viscosity was the same at both tem­ peratures. At both storage temperatures, viscosity was lower in the gel than in the cream formu­ lation. However, in general, temperature did not affect either formulation under our study conditions. No significant changes in rheological characteristics were seen in either formulation during the 30-day period in which viscosity was studied. STABILITY The chromatographic data are shown in Table V. Figures 5 to 7 are chromatograms of acetyl hexapeptide-8 at room temperature (25°C) and after being heated to 40°C and 60°C for 24 h. The presence of the active principle decreased to 58.8% and 41 %, respectively, making extreme temperatures a factor to take into consideration in efforts to improve the stability of the active ingredient during storage and during heating, if this is required in the process of formulation.

164 JOURNAL OF COSMETIC SCIENCE 80000 --a- 4 ° C 24 h --e- 4 ° C 7 days �4 ° C 30 days 60000 � · 40000 ·- 20000 0 0.5 2.5 10 100 Speed (rpm) Figure 2. Viscosity vs speed of Argireline® creams maintained at 4°C as a function of storage time. IN VITRO RELEASE Release assays with no membrane. Figure 8 shows the percentage of acetyl hexapeptide-8 released from the cream and gel excipient into the medium with time in samples stored at 4 ° C and 25 ° C. In the cream formulation, release was greater from samples stored at room temperature than from refrigerated samples. The viscosity of the cream formula­ tion at 25 ° C was lower than at 4 ° C hence the faster release of the active principle. However, in the gel formulation, percentage release was lower from samples stored at 25 ° C than from refrigerated samples, because of gelling as noted above in the rheological assays (22). The data showed an increase in release from both excipients with time at both tem­ peratures, with maximal release after 90 min. The rate of release of the active principle was considered suitable for use in topical preparations since it did not interfere with other processes that take place when the active principle is placed in contact with the skin. Diffusion across the membrane. We selected as the most suitable membrane that which offered the least resistance to diffusion of the active principle, in order to minimize the