Influence of light and heat on the stability of rotundine sulfate injection

The influence of both light and heat on the stability of rotundine sulfate injection was studied. Results show that in experiments with either isothermal heating or exposure to light at high temperatures, the drug coloration rate obeys zero-order kinetics. The total rate constant, k(total), caused by both light and heat can be divided into two parts: k(total)=k(dark)+k(light), where k(dark) and k(light) are the rate constants caused by heat and light, respectively. The k(light) can be expressed as k(light)=A(light)exp(-E(a,light)/RT)E, where E is the illuminance of light, A(light) is an experimental constant related to light sources, and E(a,light) is an experiment constant. Because the form of k(light) is similar to the Arrhenius equation, it is suggested that E(a,light) might be the observed activation energy of the rate-determining step of the subsequent processes of the photochemical reaction. This viewpoint is supported by the fact that E(a,light) is independent of light sources.