6.4.5　In vitro drug release

The efficiency and rate of drugs release affect the metabolism，ability to pass through the physiological barrier，residence time at the disease site，and targeting performance of drugs.A key issue in improving the therapeutic effect of drugs is to achieve the controlled release of drugs.The in vitro drug release performance of polymer nano-drug carriers reflects the in vivo release to a certain extent.Therefore，it is an essential parameter for designing and optimizing carrier systems.The drug release curve refers to the relationship of the ratio of released drugs to the total amount of drugs over time.A typical drug release curve consists of a rapid release phase，a sustained release phase，and a plateau phase（Figure 6-8（b））.Some special responsive nano-drug carriers，such as light-responsive nanodrug carriers，could accelerate the drug release in response to the specific stimuli.To measure the drug release curve，we need to remove the free drug in the solution，and separate the released drug from the polymer nano-drug carrier material and the unreleased drug in the release process.In vitro drug release is usually performed in phosphate buffer solution（PBS）at 37℃ to simulate the internal environment.The PBS that dissolves the released drug is regularly replaced to simulate the loss of the drug from the disease site.Specific solutions such as acidic，oxidizing（e.g.，hydrogen peroxide），and reducing（e.g.，glutathione）solutions are also used to simulate the characteristics of tumor or skin microenvironment or to verify the responsibility of polymer nano-drug carriers.The in vitro drug release methods include dialysis method，Franz diffusion method，and high-speed or ultrafiltration centrifugation.Both the dialysis method and the Franz diffusion method separate the released drug from the nano-drug by the concentration difference of the drug on both sides of the semi-permeable membrane.In the dialysis method，nano-drug is put in a dialysis bag and dialysis towards large amounts of solution（Figure 6-8（c））.The drug release curve of the polymer nano-drug carrier is obtained by recording the amount of drug increased in the solution outside the dialysis bag.The Franz diffusion method，which composed of a drug supply pool，a receiving pool，and a semi-permeable membrane（Figure 6-8（d）），is employed to measure the transdermal drug release.The semi-permeable membrane is used for the simulation of skin，and the released drug is collected from the receiving pool.Except for the above two methods，ultrafiltration centrifugation and high-speed centrifugation are also used for in vitro drug release.In this method，nano-drug is mixed with the release solution with shaking and stirring to simulate the in vivo environment.Nano-drug and the released drug are separated by centrifugation at specific time points.The ultrafiltration centrifuge tube consists of an inner concentrating tube with an ultrafiltration membrane at the bottom and a collection tube.After centrifugation，the nano-drug and unreleased drug remain in the concentrating tube，and the released small molecule drugs pass through the ultrafiltration membrane and are collected in the collection tube.The amount of drug released can be determined by measuring the amount of the drug in the collection tube.Highspeed centrifugation separates the mixed solution of nano-drug and released drug by direct centrifugation.After centrifugation，the nano-drug is precipitated on the bottom of the centrifuge tube.The in vitro drug release curve can be calculated by measuring the drug content in the supernatant.The adsorption of drugs on the wall of dialysis bags and centrifuge tubes may result in errors in the in vitro drug release.Besides，it is difficult to completely simulate the drug release environment in vivo by the above in vitro experiments.As a result，the actual in vivo drug release are still required.(https://www.daowen.com)