Radiolabeled Reaction Phenotyping

Quantitative In vitro Radiolabeled Reaction Phenotyping Studies... 

If human metabolism studies with radiolabeled drug have not been performed prior to the conduct of reaction phenotyping studies, the initial experiments should use as complete an in vitro test system as possible, depending on the drug (e.g., tissue homogenates, liver slices, hepatocytes, etc.). 

Ideally, definitive CYP reaction phenotyping should be available before the initiation of clinical development. Unfortunately, more accurate data can only be obtained once clearance pathways are identified in human subjects, and human radiolabeled studies are generally not conducted as the first set of clinical studies for NCEs. In this context, CYP reaction phenotyping, performed using various human in vitro systems is expected to be as complete as possible (Bjornsson, 2003). 

For the purposes of definition, nonradiolabeled reaction phenotyping is that conducted ahead of data from radiolabel studies in humans, and is typically conducted before first dosing of humans (Williams et ah, 2003). This differs from reaction phenotyping later in development, which in the presence of radiolabel or metabolite standards is more definitive in nature (Bjornsson et ah,... 

With radiolabeled drug in hand much more detailed reaction phenotyping studies can be preformed. This is especially the case with compounds that are poorly metabolized in vitro and where there is difficulty in accurately measuring parent disappearance. Radiolabeling allows quantitation of all metabolites formed even in small quantity. The same in vitro techniques are used as described above, but this time with accurate quantitation of all components. There are methods that allow more rapid phenotyping studies, either through the use of authentic metabolite standards and LC—MS or a combination of radiolabeling and LC—MS. 

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