Pharmacokinetic assays provide a quantitative description of what a body does to a drug. The four major components of the pharmacokinetics process are absorption, distribution, metabolism, and excretion commonly referred to as ADME. Absorption is the process of getting the drug from the site of delivery into the systemic circulation. For example, orally administered compounds must be soluble in the gut environment, then permeate the gastrointestinal tract and pass through the portal vein and the liver (known as the first pass) before reaching central circulation. Distribution describes the drug’s ability to enter and leave central circulation to reach tissues and organs throughout the body. Metabolism and excretion are the processes of eliminating drugs and xenobiotics from the body via metabolic processes or direct biliary or renal excretion of unchanged drug.