2.4. In Vitro Drug Release Test

The cumulative drug release test was measured using a UV-1750 ultraviolet visible spectrophotometer (UV-1750, Shimadzu company, Kyoto, Japan). The characteristic absorption peak of the MPT in the UV spectrum was 222 nm, and the applicable concentration range of the standard curve for the determination of the MPT was 0.6–60 μg/mL. There was a good linear relationship between the UV absorbance and MPT concentration in this range (r > 0.999).

For each test, the samples were immersed in 10 mL of phosphate-buffered saline (PBS) solutions and incubated in an incubator (100 rpm, 37 °C). At each time point selected for measurement, the solution was totally exchanged with fresh PBS. All the experiments were performed in triplicate. The concentrations of MPT were calculated according to the standard curve equation.