In vitro metabolism
Our first instalment of ADME-Tox e-book series focuses on in vitro metabolism beginning with an introduction chapter explaining the importance of ADME-Tox studies in drug discovery and development. We then move on to the actual studies starting with metabolic stability covering basic assay conditions and explaining concepts, such as in vitro clearance (CLinc), in vitro half-life (t1/2), unbound fraction (Fub) and how to use this data to predict in vivo kinetics (e.g, total hepatic clearance, CLH, and hepatic extraction ratio, Eh).
The next part of the book explains the importance of metabolite profiling and identification and how to set up incubation and analysis conditions for such assays. The incubation conditions cover areas such as different enzyme types for the study (CYP, FMO, UGT, SULT) and choosing the correct enzyme source (microsomes, hepatocytes, S9 fraction). Analytical methods such as LC/MS, LC/RAD, LC/PDA are discussed in some detail. The last part of our first book is about characterization of chemically reactive metabolites and how to apply assays using trapping agents such as GSH and KCN, in avoiding unexpected drug-induced toxicities in later drug development phases.
The second book of our e-book series introduces us to the inevitable by-product of polypharmacy; drug-drug interactions, (DDI) and more precisely studying its pharmacokinetic side. Since the emphasis on current regulatory guidance is on metabolic interactions, the book focuses on the most relevant DDI enzyme groups, such as cytochrome P450 (CYP) and in their inhibition and induction in vitro. Enzyme inhibition chapter covers basic enzyme kinetics and most common enzyme sources and their advantages and disadvantages in order to help planning inhibition studies, e.g., a CYP inhibition study.
CYP induction is the most important reason for DDIs caused by elevated drug metabolism rates, and therefore is discussed in the book in some detail. CYP induction is usually studied by measuring the increase of mRNA levels of the metabolic enzyme encoding gene, which is an indicator of increased enzyme activity. This chapter also discusses another approach for studying CYP induction; nuclear receptor activation, a screening assay which can be used in the early phase of drug discovery and development. Finally, the book explains how to make in vivo extrapolations from in vitro CYP inhibition and induction data and the differences between current models for assessing for actual DDI risks.