Wednesday, June 02, 2010
Featured Article
Check out our ADME Testing section for more information or to find companies that provide these services.
ADME, Defined
ADME stands for absorption, distribution, metabolism, and excretion. Absorption refers to the process by which the body (animal or human) takes up or absorbs the drug compound. The exact process of absorption really depends on how the drug is administered (e.g., oral route, respiratory/inhaled, intravenous, etc.). Once a drug or test compound is absorbed by the body, it is distributed to its target tissue via the bloodstream.
“Preclinical ADME testing is used to determine the bioavailability of a new drug, a compound, or a series of compounds,” says Chris Parker, chief commercial officer of Cellular Dynamics International (CDI), Madison, Wisc. “Bioavailability is a term that is used to describe whether a drug goes to the tissue of interest at the appropriate concentration and in the appropriate structure.”
“ADME is what the body does to a drug and impacts whether high enough blood levels of the drug will be achieved for a long enough time to elicit a desired pharmacological response,” says Philip Burton, PhD, director of ADME Services, CeeTox, Kalamazoo, Mich. ADME properties of a drug candidate have a profound impact on dose size and frequency of administration, in some cases requiring frequent, multiple dosing in order to maintain target blood concentrations.
Prior to reaching its target tissue, a drug may be metabolized into a more active compound by the liver. Metabolism occurs through a number of different enzymatic reactions catalyzed by a family of enzymes known as CYP450s. If a drug is not metabolized properly, the molecule may actually stay in the liver, where it could reach a harmful concentration, which ultimately leads to liver toxicity. Excretion refers to the process by which the body eliminates the drug, and this process occurs in the kidneys.
ADME: Tools and Technologies
In general, the tools and technologies employed in preclinical ADME testing include in vitro ELISA-based testing and mass spectrometry to analyze the substrates of drug metabolism and to determine the presence of specific metabolites. ELISA stands for enzyme-linked immunosorbent assay, which, in general, is a plate-based assay that is based on the qualitative and quantitative immune reaction between an antibody and its target antigen. Mass spectrometry is generally an instrumentation-based method that accurately and precisely measures the mass of a chemical entity. “Other in vitro assays such as genotyping or PCR-based analysis are also performed to determine the type of CYP450 in a given individual,” says Parker. Genotyping is a method of biochemically determining the exact sequence of a gene in question. PCR, which stands for polymerase chain reaction, is the process of biochemically amplifying a gene or part of a gene for quantitative or qualitative analysis.
Drug metabolism can be measured using whole cells or subcellular fractions such as S9 fractions or microsomes. Microsomes are vesicles of cellular endoplasmic reticulum, which is where most drugs are metabolized. “Almost all companies have some level of [ADME] testing done in-house. And smaller companies that have not made the commitment to set up these tests outsource them to CROs such as CeeTox,” says Burton. ADME testing also takes place in vivo, in animal models to look at how that molecule is being excreted.
CDI is developing and commercializing human cells derived from induced pluripotent stem cells (iPSCs) for use in drug discovery and toxicity testing. CDI is currently providing human iCell™ Cardiomyocytes in high purity, homogeneity, and quantities needed by its pharmaceutical customers. The company is also developing human iPSC-derived hepatocytes, neurons, and endothelial cells to add to its drug discovery and toxicity testing product line.
ADME: Part of the FDA Submission Process
ADME testing procedures are part of the New Drug Application (NDA). Historically, most ADME testing was performed in validated animal models relatively late in discovery programs, in multiple species. Similarly, the FDA requires a rodent and a non-rodent species, for example a rat and a monkey or a rat and a dog, for tox testing. “Over the years, the industry has tried to move this testing earlier and earlier in the discovery process,” says Burton, who uses the following example to illustrate the fail quickly, fail early approach. “Even if the drug is very well absorbed, if it is also very rapidly metabolized and eliminated, then it is going to be extremely difficult to maintain effective blood levels of that drug. So the industry has been trying to identify those kinds of compounds earlier and eliminate them from further consideration before they are subject to very expensive animal testing.”
A recent FDA requirement is to test for drug-drug interactions. In fact, many recent drug withdrawals were due to drug-drug interactions. A drug-drug interaction is basically an undesirable interaction of a new drug with an existing drug co-administered in patients. The most well recognized mechanism of drug-drug interactions involves metabolism. For example, when two drugs—an existing drug and a new drug—are co-administered in a patient, the new drug may decrease the metabolism of the existing drug, and when this occurs, the existing drug may accumulate to toxic levels. The converse interaction is also of concern, especially for drugs that have a narrow therapeutic index. Frequently, metabolism-mediated drug-drug interactions are accurately predicted with today’s in vitro methods and are typically part of the FDA review.
A new twist on the drug-drug interaction phenomenon involves drug transporter proteins. Drug transporters are proteins found in various cells and either serve to take up the drug or excrete it. “One of the phenomena that are tracked is a transport protein called P-glycoprotein, which, among other things, can have an impact on the ability of the drug to get into the brain, where it acts as part of the blood-brain barrier” says Burton. “For a compound that has CNS activity as its therapeutic indication, a drug that would not be a substrate of p-glycoprotein would be quite desirable.” P-glycoprotein can also play a major role in drug oral absorption and excretion. The FDA now requires information about whether a new drug is a substrate or inhibitor of P-glycoprotein as part of the NDA application for that compound. Earlier this year, a position paper, jointly authored by FDA, industry, and academic scientists, reviewed additional transport proteins that appear to play a major role in drug performance in vivo and will likely be subject to regulatory scrutiny in the future.
Check out our ADME Testing section for more information or to find companies that provide these services.