Abstract: Aim To study the most stable pH value of endomorphin-1 solution and to estimate the shelf-life of EM-1 solution. Methods The content of EM-1 in its solution was determined by HPLC, and stability was studied by accelerated tests. Results The most stable pH value of EM-1 solution was observed. By the accelerated tests we obtained the Arrhenius relationship and the pharmacokinetic parameters. Conclusion The most stable pH value of EM-1 solution is 4.5-4.8. The t1/2 (25 °C) is 45 d.

Key words: endomorphin 1, endomorphin 1, most stable pH value, most stable pH value, accelerated tests, accelerated tests, HPLC, HPLC