6.4.7　Biodistribution

The nano-drug will enter the tumor and various healthy organs with the blood circulation.Therefore its biodistribution will influence the therapeutic effect and toxic side effects.By testing the in vivo biodistribution of nano-drug，we will obtain the distribution in healthy organs such as liver and spleen，the peak concentration in tumors or blood，the time to reach peak concentration，and the circulation time of nano-drug，etc.Its measuring method contains two aspects：animal sampling analysis and in vivo imaging analysis.The animal sampling analysis measures the blood parameters of the nano-drug.The concentrations of the nano-drug in the blood are determined by the physical and chemical properties of nano-drug carriers or the loaded drugs after collected blood samples of animals at specific time points.Unfortunately，the number of blood collections and the amount of blood taken each time are limited since one mice has only about 2 mL of blood.For the measurement of the distribution of the nano-drug in various organs or tumors，it is necessary to sacrifice mice at specific time points and take samples of organs and tumor tissues for analysis.Therefore，each mouse can only provide drug distribution information for one time point.In comparison，the in vivo imaging method is a non-invasive method，which enables us to detect the real-time distribution of nano-drug at different time points.The imaging techniques include fluorescence imaging，photoacoustic imaging，magnetic resonance imaging（MRI），positron emission tomography（PET），single photon emission computed tomography（SPECT），etc.Among them，fluorescence imaging，photoacoustic imaging，and MRI can semi-quantitatively measure the tumor enrichment and organ distribution of nano-drug by the signal intensity.Fluorescence imaging and photoacoustic imaging provides fast imaging but low penetration depth，while MRI can penetrate deeply，but take more time for imaging.In comparison，PET and SPECT have a high imaging penetration depth and a quantified distribution of nano-drug in tumors and organs accurately.However，these two technologies require to use radioactive isotope-labeled nano-drug and cost a lot.Appropriate technologies should be selected according to the specific requirements of each experiment since every imaging technology has its advantages and disadvantages.Most in vivo imaging methods need to introduce functional groups with corresponding functions into the polymer nano-drug carriers.Therefore，if the functional groups are cleaved by the enzymes or ions in vivo，it may cause errors in results.Besides，if the drug is released before entering the diseased area，it may also cause errors.Thus，these factors should be considered during the measurement in vivo.(https://www.daowen.com)