384 JOURNAL OF COSMETIC SCIENCE lOO I 80 60 40 20 0 5 10 15 20 25 Time (h) Figure 2. Cumulative percentages of TA released from various liposome formulations and solutions during 24 h at 37øC: ', 7:2:1 (5% TA,+) O, 7:2:1 (10% TA,+) /•, 7:2:1 (5% TA,-) 7:2:1 (10% TA,-) •,, 5% TA solution X, 10% TA solution. Symbols are the mean values of six determinations. data were fitted to the first-order equation, compared to those fitted to the zero order and Higuchi model equations. Thus, the first-order equation seemed to be the most accept- able approach to be employed in this stability study, and was used to estimate the leakage rate constant and shelf life of the entrapped TA in various liposome formula- tions. On the other hand, for the release study, the Higuchi model exhibited the best fit (r 2 -- 0.97) when the experimental data obtained were fitted to the proposed model equations (Table I). Thus, this model was used to estimate the release rate constant of TA from various liposome formulations and TA solutions (5% and 10%) in DI water. The leakage rates of entrapped TA in the positively charged liposomes (0.0030-0.0055 d -•) were higher than those in the negatively charged liposomes (0.0016-0.0039 d -•, Table VI). The negatively charged liposome with the entrapped 10% drug {7:2:1(10% TA,-)} demonstrated the slowest leakage rate (Table VI). This may be associated with the positive charges of TA, which may bind strongly with the negative charges of the liposomal membrane, thereby lowering the leakage rate of the entrapped drug from the liposomes (21). Thus, charges appeared to affect the leakage rate and shelf life of TA in liposomes. In contrast, the degradation rate constants of total TA in the positive and negative liposomes, and in the pellets of positive and negative liposomes, were broadly comparable (Table V). The leakage rate was accelerated as the storage temperature increased (Table VI). The best storage condition was 4øC, since the lipid becomes more flexible and fluid at high temperature (22). Thus, the entrapped drug can leak easily from the lipid bilayers of liposomes. In consideration of the degradation rate constants of total TA in liposomes (Table V), TA in all liposome formulations exhibited relatively high stability, as more than 90% of the drug remained following storage at 4 ø, 30 ø, and 45øC for 60 days. This

TRANEXAMIC ACID LIPOSOMES 385 indicated the stability of TA when entrapped in liposomes. Similar to the leakage rate constants, the degradation rate constants of total TA in liposomes and in the pellets of liposomes were accelerated as the storage temperature increased (Table V). In the in vitro release study, the release rate constants of TA from the 7:2:1(5% TA,+) and 7:2:1 (5% TA,-) liposomes (6.1-6.3 %/h •/2) were lower than the 7:2:1(10% TA,+) and 7:2:1 (10% TA,-) liposomes (8.1-8.6 %/h •/2, Table VI). Thus, the release rates of TA from liposome formulations were dependent on the initial TA concentrations and the hydrophilicity ofTA. TA is a hydrophilic drug with weak positive charge properties that can be incorporated in an aqueous layer of the multilamellar liposomes. In the negatively charged lip4somes {7:2:1 (5% TA,-) and 7:2:1 (10% TA,-)}, TA may interact with the surface charge of liposomal bilayers, resulting in the difficulty of TA to be released from liposomes (23,24). However, there were no significant differences in the release rates of TA from the positively and negatively charged liposomes (6.1-8.1%/h •/2 vs 6.3-8.6 %/h •/2, Table VI). Thus, the release rates of TA from liposomes were apparently inde- pendent of charges. The release rates of TA from the 7:2:1 (5% TA,+), 7:2:1 (10% TA,+), 7:2:1 (5% TA,-), and 7:2: ! (10% TA,-)varied between 6.1 and 8.6%/h •/2. In comparison, the respective release rates of TA from 5% and 10% solutions in DI water were 18.9% and 19.8%/h •/2. Thus, the release rates of TA from all liposome formulations were - 3 times slower than that from the solution. This suggests that all liposome formulations prolong and sustain the release of TA. CONCLUSION TA in all liposome formulations demonstrated relatively high chemical stability. The best formulation, evaluated from liposomal size, physical stability, leakage rate constant, shelf life, release rate, and the total amount of TA released, was found to be the negatively charged liposome with the 10% entrapped TA {7:2:1 (10% TA,-)}. In addition, TA entrapped in liposome formulations showed sustained and prolonged release, as their release rates were slower than that in the solution. We have previously found that the 7:2:1 (10% TA,-) liposome exhibited a relatively high percentage of entrapment of the drug (13%). The charges of liposomes appeared to affect the physical stability, leakage rate, and shelf life of TA in liposomes, whereas TA concentrations apparently affect the release rate of TA and the total amount of TA released from liposomes. It is a challenge for the future development of TA entrapped in liposomes, for parenteral and topical applications. The developed topical product of TA can function not only as a depot system but as a product with low irritation as well. ACKNOWLEDGMENTS The authors acknowledge Dr. Kuncoro Foe for his assistance in the preparation of the manuscript. We also acknowledge JJ-Degussa (T) Ltd., Thailand, for the gift of Emul- metik 950 ©, and Thistle Corp., Ltd. (Thailand) for the gift oftranexamic acid. We thank the Research and Development of Natural Products for Thai Traditional Medicines Research Unit, Pharmaceutical-Cosmetic Raw Materials and Natural Products Research