Drug Metabolism Studies
Because metabolites can have a major affect on how a drug or combination of drugs are metabolized and eliminated, the FDA has determined a drug assessment should include a description of its metabolism and how it may affect overall elimination. CellzDirect offers studies designed to determine metabolic stability and metabolite profiling for species comparison.
Metabolic Stability
Estimates of in vivo metabolic drug clearance can be determined from in vitro metabolism kinetic data. Metabolic stability studies are typically performed to estimate a drug candidate’s metabolic half-life and intrinsic clearance rates which can be major determinants for in vivo drug efficacy. Compounds with short half-lives may require multiple doses to maintain effective plasma levels, whereas compounds with longer half-lives and slower elimination kinetics may require complicated dosing regimens.
For typical studies, two concentrations of the test article are incubated with pooled microsomes and/or hepatocytes for 0, 10, 20, 30 and 60 minutes or 0, 30, 60, 120 and 240 minutes, respectively. Metabolic turnover is determined by loss of parent compound as measured by LC-MS/MS or HPLC and the half-life (t1/2) of the test article is estimated.
CLICK HERE TO READ MORE ABOUT METABOLIC STABILITY
Metabolic Profiling/Species Comparison
A description of all major circulating metabolites is necessary to assess the safety and effectiveness of a drug. Metabolic profiling aims to profile major metabolites of drug candidates that are formed in humans and compares the profiles for human and animal in vitro model systems. This information is then used to choose a rodent or non-rodent species for in vivo toxicity and pharmacokinetic studies.
The most complete picture of hepatic metabolism can be obtained with intact hepatocytes which provide cellular integrity with respect to enzyme architecture and phase II metabolism and potentially allow for any concentration gradients mediated by transporters that may affect exposure of drugs to enzymes. Other in vitro models typically utilized for metabolite profiling include subcellular fractions such as microsomes, S9, and cytosol.
Human and animal hepatocytes and/or microsomes are incubated with test article for multiple time points. The metabolism of the parent compound and appearance and characterization of metabolites are determined by HPLC and LC-MS/MS methods. Then, the metabolite profile for human is compared to animal species such as mouse, rat, dog and non-human primate.
Drug Metabolism Studies Based on the FDA Draft Guidance (2006)
"The desirable and undesirable effects of a drug arising from its concentrations at the sites of action are usually related either to the amount administered (dose) or to the resulting blood concentrations, which are affected by its absorption, distribution, metabolism, and/or excretion. Elimination of a drug or its metabolites occurs either by metabolism, usually by the liver or gut mucosa, or by excretion, usually by the kidneys and liver. In addition, protein therapeutics may be eliminated through a specific interaction with cell surface receptors, followed by internalization and lysosomal degradation within the target cell. Hepatic elimination occurs primarily by the cytochrome P450 family (CYP) of enzymes located in the hepatic endoplasmic reticulum, but may also occur by non-P450 enzyme systems, such as N-acetyl and glucuronosyl transferases. Many factors can alter hepatic and intestinal drug metabolism, including the presence or absence of disease and/or concomitant medications, or even some foods, such as grapefruit juice. While most of these factors are usually relatively stable over time, concomitant medications can alter metabolism abruptly and are of particular concern. The influence of concomitant medications on hepatic and intestinal metabolism becomes more complicated when a drug, including a prodrug, is metabolized to one or more active metabolites. In this case, the safety and efficacy of the drug/prodrug are determined not only by exposure to the parent drug but by exposure to the active metabolites, which in turn is related to their formation, distribution, and elimination. Therefore, adequate assessment of the safety and effectiveness of a drug includes a description of its metabolism and the contribution of metabolism to overall elimination. For this reason, the development of sensitive and specific assays for a drug and its important metabolites is critical to the study of metabolism and drug-drug interactions."
