Several factors need to be taken into account when selecting the appropriate study design for assessing the in vivo performance of a lipid-based drug formulation. These include animal species, use of anesthesia, route of dosing (oral vs. intraduodenal), dosing volumes (absolute volume and volume per kg of body weight), ability to administer a human-size clinical dosage unit, the type and number of samples required (e.g., lymph, blood, bile, urine, multiple vs. single samples), study cost and relative simplicity. The importance of each of these factors is likely to vary depending on the stage of development and whether the purpose of the study is to simply demonstrate initial proof of concept or rather to provide a more structured evaluation of potential clinical formulations. When deciding on the particular animal model or study design, a clear understanding of the questions that need to be answered is paramount. A discussion of the specific advantages and disadvantages of different animal models is given below and is structured in the form of questions that those models might be used to answer.
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