Chemical Disposition & Toxicokinetics

Chemical disposition studies are performed to determine what happens to a substance in an organism after that organism is exposed. This process can be described by:

• Absorption: the body absorbs the substance
• Distribution: the substance is distributed into body blood and tissues
• Metabolism: the body breaks down or metabolizes the substance
• Excretion: the body excretes the substance

Based on these outcomes, chemical disposition studies are also called ADME studies. Toxicokinetics studies describe the rates at which a substance enters a biological system and what occurs to the substance once it is in the system.

Chemical Disposition Studies

Chemical disposition studies measure the absorption, distribution, metabolism, and excretion (ADME) of a chemical or substance. Studies are designed to determine the effect on each of these parameters of (1) species, (2) sex and age of animals, (3) dose level and frequency of dosing (e.g., single vs. repeated), and (4) route of exposure, and include an intravenous administration to determine the absorbed dose following dosing via the route of interest.

ADME studies use the radiolabeled chemical/substances (e.g., 14C or 2H), which enables easy detection of radiolabel equivalents in blood, tissues, and excreta. However, measuring radioactivity only monitors the total radiolabel and does not identify whether what is being measured is the parent chemical or one of its metabolites. Additional techniques such as chromatographic methods are used to differentiate between parent and metabolites.

Doses are typically set based on the reported LD50 values for a substance (e.g., 0.1, 0.01 and 0.001 of LD50). ADME studies are normally done very early in the sequence of studies used to fully characterize chemical toxicity, and their data are used to help set the dose and route of administration for subsequent toxicology studies.

Toxicokinetic Studies

Toxicokinetic (TK) studies follow the change in concentration of parent substances and/or metabolite(s) with time in blood/plasma or other tissues of interest. Like ADME studies, TK studies are designed to determine the effect of species, sex and age of animals, dose level and frequency of dosing (e.g., single vs. repeated), and route of exposure on TK parameters, and include an intravenous administration to determine the bioavailability following dosing via the route of interest.

TK studies are typically performed with unlabeled (no radioactivity) chemical/substance. Data from the ADME studies are generally used to determine the analytes to be monitored in TK studies using validated analytical methods such as gas chromatography- and liquid-chromatography-mass spectrometry. Doses are set based on the ADME and/or toxicology study doses. TK studies can be done early in the sequence of or in conjunction with studies used to fully characterize chemical toxicity. TK data are used to assist in the design and/or the interpretation of toxicology study data.

ADME and toxicokinetic studies are performed according to the NTP chemistry, ADME, and TK study specifications.