day when the measurement of radioactivity in excreta is1% of administered
radioactivity.
18.2.3 Dose Selection, Formulation, and Administration
A test article (radiolabeled and nonlabeled) should have a certificate of
analysis with a current use date (expiration date) for the animals in the mass
balance studies. For selection of dose, in general, the toxicity data is used.
The dose with the minimum toxicity (1/7th–1/10th of STD 10 , i.e., the single
dose that is severely toxic to 10% of the animals) should be used. If it is a
high dose drug, which is nontoxic to animals, a well tolerated mid-dose may
be chosen and consideration should be given to PK and bioavailability of the
drug in the species. A dose that gives sufficient circulating radioactivity and
sufficient amounts of metabolites in excreta for metabolite identification is
preferred.
A dose vehicle used in toxicity studies is preferred in the mass balance
studies. Using polyethylene glycol (PEG) should be avoided if possible, since it
suppress ionization of compounds during metabolite identification by mass
spectrometry (MS). Dose formulation is usually prepared on the day of dosing.
If the drug is stable, dose solution may be prepared a day before dosing and
stored at 20 C. Target dose level (mg/kg) and volume (mL/kg) and drug
concentration in dose solution (mg/mL) should be defined in the study
protocol. Concentration of drug (mCi/mg) in formulation should be verified by
analyzing triplicate aliquots ( 100 mL; diluted if necessary) from top, middle,
and bottom of the dose solution by liquid scintillation counter (LSC) at
predose and postdose. The specifications (including identification, manufac-
turer, physical description, lot number, specific activity for radiolabeled
compounds, expiration date, storage temperature) of components of the dose
solution should be documented in the study file. The route of administration in
animals is usually the one proposed for clinical use (IV or oral). IV dosing is
performed by bolus injection or by slow injection or infusion depending on the
toxicity of drug. For oral dosing by gavages, toxicity studies are used as a
reference. Single doses are normally used for ADME studies although repeated
dose studies may be required in special cases.
At predose, if the vehicle contains an ingredient such as cremophor and
Tween-80 that causes allergic reaction in animals, a pretreatment dosing
regimen may be given to animals to minimize the chances for this reaction
occur. At postdose, in the event that an adverse reaction is to occur, additional
treatment may be followed as judged to be appropriate.
The volume of the radiolabeled dose formulation to be administered to each
animal is calculated based on the body weights taken on the day of dosing. If a
syringe is used for dosing, the actual amount of administered dose to each
animal is determined by weighing the dosing syringe before and after dose
administration. The dose apparatus is flushed with an appropriate amount of
dose vehicle. If dose vehicle contains an ingredient that is toxic to animal at
578 ADME STUDIES IN ANIMALS AND HUMANS