Thermal degradation kinetics of cynatratoside-C and its antiparasitic efficacy after being stored in sterile water and aquaculture water

Abstract

Cynatratoside-C (CyC), a C-21 steroidal saponin isolated from Cynanchum atratum, is a promising candidate for treating freshwater fish infected with the ciliated parasite Ichthyophthirius multifiliis. To promote the drug development process of CyC, the present study investigated the stability of CyC under heating, sterile water, and aquaculture water treatments. An accelerated stability test demonstrated that the thermal degradation kinetics followed a first-order reaction with a predicted half-life (t0.5) of 3472 days (9.5 years) at a temperature of 25 °C. According to the Arrhenius model, the activation energy Ea of CyC was calculated as 91.7 kJ/mol. When CyC was stored in sterile water for 3 years, the mortality rates of theronts and nonencysted tomonts were decreased significantly in the low CyC concentration treatment group, and the minimum concentration that inhibited the production of encysted tomonts increased from 0.06 mg/L to 0.125 mg/L. When CyC was stored in sterile water for 6 years, it showed a nearly complete loss of antiparasitic efficacy, indicating that water may promote the degradation of CyC. When CyC was stored in aquaculture water, the antiparasitic efficacy of CyC against theronts and tomonts was decreased significantly at 2 days post-storage, indicating that microorganisms in aquaculture water could absorb and degrade CyC. The present study provides important information concerning the stability of CyC and the drug approval process.