Diastereomers are stereoisomers in which molecules have two or more chiral centers and the relationship between molecules is not mirrored. In PROTAC drug development, in order to evaluate the pharmacokinetics of a single diastereomer or mixture of diastereomers, manufacturers should quantitatively analyze samples from the early stages of drug development for a single diastereomer in vivo. This will allow the assessment of the possibility of mutual transformation as well as the distribution of absorption, distribution, biotransformation and excretion (ADBE) of individual isomers. When the drug is a raceme and the pharmacokinetic characteristics of the isomers are different, the manufacturer should monitor the diastereomer separately to determine properties such as dose linearity and the effects of metabolic or excretory changes and drug-drug interactions.