RÉSUMÉ
Radix Isatidis (Banlangen in Chinese) is a traditional Chinese medicinal (TCM) herb, and is frequently used for treating influenza. However, the current quality control method for Radix Isatidis should be developed since it has little correlation to the pharmacodynamic action. In this paper, the in vitro inhibitory action of Radix Isatidis on neuraminidase (NA) was investigated by fluorometric assay with 4-methylumbelliferyl-D-N-acetylneuraminate (FL-MU-NANA) method. Based on the method, the experimental condition was optimized and a bioassay statistic method was established according to the reaction type and the regularity of "parallel lines of qualitative effect". Then the bioassay method of Radix Isatidis was established. This study indicated that Radix Isatidis had obvious in vitro inhibitory activity on NA with IC50 = (0.90 +/- 0.20) mg x mL(-1) (herb). The correlation between logarithmic dose and reaction rate showed an "S" shape--is quite similar to Tamiflu's reaction curve, which hinted that Radix Isatidis had the same inhibitory function on NA as Tamiflu. The established bioassay method of "parallel lines of qualitative effect" had a good reproducibility (RSD = 5.78%). The results of potency determination of Radix Isatidis were reliable (reliability test: deviation from straight line P > 0.05, deviation from parallel line P > 0.05) and well regular. As a conclusion, this bioassay method is suitable to control and evaluate the quality of Radix Isatidis.