Due to the metabolism of drugs within the body before they reach their target location, in vivo and in vitro bioassays can give contradictory information regarding the effectiveness of a certain molecule. Hence, it is important to keep both perspectives in mind while optimizing the design of a drug molecule.
During the study of a lead compound containing a methyl ester which could be hydrolyzed to a carboxylic acid, the in vivo and in vitro bioassays provided contradictory results regarding its activity. The in vivo assay suggested that the acid was inactive and the ester was active. However, the in vitro assay suggested that the ester was inactive and the acid was active.
These observations can be explained as a consequence of the differential absorption of certain molecules in the body. In vivo, drug molecules are need to be absorbed into the bloodstream from the gastrointestinal tract in order to function effectively. Given this phenomenon, the contradictory results can be made sense of.
Going by the in vitro assay results, the active compound that binds to the desired target is indeed the carboxylic acid. However, this carboxylic acid may be poorly absorbed from the gastrointestinal tract due to its polar nature, resulting in a loss of drug activity.
However, although the methyl ester does not directly bind to the desired target, it is better absorbed from the gastrointestinal tract before being hydrolyzed to the active carboxylic acid within the body. This active molecule then successfully reaches its desired target, resulting in successful drug activity.