A majority of pharmaceutical compounds are delivered orally, as the ease of use of an oral medication makes it more accessible to patients. It is estimated that around three in five commercially available small-molecule drugs are administered orally and oral formulations are assessed to be approximately 90% of the global market share. The use of the oral route as a primary means of delivery requires that a compound be able to get through the beginning stages of the digestive system (mouth, stomach, and small intestine) and then the liver before entering the circulatory system. This route leads to a great reduction in the concentration of a drug once it reaches the circulatory system due to absorption losses and losses related to metabolism and the formation of metabolites. Compounds must remain active, or become active, after being metabolized by the body or a therapeutic effect will not be achieved.
The first place that metabolism occurs for oral delivery is in the small intestine, where a compound will pass through the cell layer in order to enter portal circulation, then the liver and eventually the circulatory system. The liver is where the bulk of metabolism takes place, and there are many enzymes involved in this process. The cytochrome P450 enzymes, in particular, are major players in this process and can take hydrophobic compounds and convert them into hydrophilic substances, which are then able to be transported and excreted by the body. Interestingly, scientists have been able harness these metabolic processes to create pro-drugs which can become active after they enter the body.
Phase contrast and bile canaliculi staining of HUREL human hepatocyte pooled lot on day 9 post-seeding. CYP3A activity was assessed using the substrate midazolam on customer days 1, 4, and 9, showing consistent levels of activity over the course of nine days.
The HUREL Model
Understanding the metabolic clearance of a compound is important for determining a suitable dosing regimen for the use of the drug in a clinical setting. Additionally, understanding how a drug is metabolized and what kinds of metabolites are created can help in assessing off-target effects of a drug. Visikol offers several assay systems which can be used to evaluate a compound’s metabolic activity. The HUREL model in particular has been highly validated with regard to evaluating drug clearance. All pooled primary hepatocyte lots are quality control tested for their ability to metabolize key compounds which correspond to different CYP450 enzymes, including CYP3A and CYP2C9 which are two of the most common CYP450 enzymes present in the human liver. Not only does the HUREL model offer a human primary hepatocyte model, but also offers several zoological models (e.g. mouse, rat, dog, primate, etc.) which can be useful for in vivo animal study comparisons.
Metabolism studies are not limited to the HUREL model, they can also be evaluated in 3D and ex vivo models (such as the 3D primary human hepatocyte spheroid model, and precision cut liver slice (PCLS) models). Additionally, cell-free assays are available to evaluate the enzyme kinetics associated with a compound of interest. These cell/tissue-based assays can also be utilized to evaluate liver toxicity in addition to or separate from metabolism end points. If you are interested in learning about our liver models, please reach out!