Hepatic clearance is a complex process which is influenced by multiple competing factors including influx into the hepatocyte, metabolism within the hepatocyte and efflux out of the hepatocyte. Failing to understand these interdependent processes can result in inaccuracies in the prediction of hepatic clearance.
The ability of a compound to cross the hepatocyte cell membrane is a critical prerequisite for drug metabolism due to the intracellular location of drug metabolising enzymes. This can be achieved by a passive process or facilitated by drug transporters. The predominant human uptake transporters involved in this process include the OATP, OCT, OAT and NTCP transporters, with OATP being identified as the most significant transporter in this respect.
The conventional hepatocyte suspension assay coupled with IVIVE understanding has been shown to be successful in the prediction of human hepatic clearance especially when incorporating a regression offset. However, when metabolism is no longer rate limiting, as in the case with uptake transporter substrates, other approaches are needed to understand the mechanism of clearance. To address this, Soars et al., (2007) published a method known as the ‘media loss’ assay. This assay demonstrates certain advantages over the traditional oil spin method for measuring hepatic uptake in that it is a simple procedure which facilitates the use of non-radiolabelled compound and is more amenable to use in drug discovery.
Practically, the media loss assay is similar in format to the conventional hepatocyte suspension method. However, to define an uptake substrate, rapid sampling is required within the first two minutes of the assay. Furthermore, the aliquots are centrifuged immediately, before sampling and analysing the supernatant. This method ensures that only disappearance of parent compound is measured from the media rather than from the media and the cells. By using this approach, the assay takes into account the hepatic uptake which can be the rate-limiting step in the hepatic clearance.
It is important to note that hepatic uptake transporters have also been associated with drug toxicity which may arise from drug-drug interactions or increased intracellular concentration of parent or metabolite in certain tissues. One of the most well documented examples is related to patients with genetic polymorphisms associated with low OATP1B1 activity. This polymorphism contributes to an increased risk of statin-induced myopathy. Therefore, understanding if a drug undergoes hepatic uptake and the identity of the transporter involved can also prove useful in predicting potential drug-induced toxicity.
Through its parent company, Evotec, Cyprotex are able to now assess hepatic uptake using the media loss assay.
More information on the assay can be found on the Cyprotex website. Learn more