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Pharmacokinetics is the study of how a drug moves throughout the body and provides important information on systemic exposure to the drug over time. In order to collect relevant PK, clinical studies must be conducted that help determine the onset, magnitude, and duration of a drug’s effect. Together, PK and PD data can be used to describe the safety and efficacy of a drug by establishing dose-exposure-response relationships.

the movement of a drug from its application to its absorption and distribution throughout the body. Pharmacokinetics encompasses capillarization, absorption, transport and distribution. Pharmacokinetic properties include peak plasma levels and time to reach maximum plasma concentration (Tmax); half-life; clearance (CL); the volume of distribution at steady state; bioavailability in terms of area under the curve (S/Z ratio) and the renal clearance (CLRT) for drugs that undergo renal excretion after reaching maximum plasma concentrations (i.e., < 100ng/mL).

The pharmacokinetics definition of a drug are the processes that take place in the body after it is administered and before it reaches its bloodstream. These processes can be described as absorption, distribution, metabolism, and excretion. Therefore, designing an optimal dosing regimen based on PK and PD data is important to ensure that the majority of patients will attain the proper therapeutic exposure range that does not cause intolerable side effects. This can be accomplished by examining a drug’s PK through the processes of absorption, distribution, metabolism, and excretion.

The Pharmacokinetics Process: ADME

Absorption, distribution, metabolism, and excretion (ADME) are the internal processes that describe how a drug moves throughout and is processed by the body. Understanding the ADME properties of any drug discovery provides essential information for developing safe and effective pharmacotherapy.

Absorption

Drug absorption is the process in which drugs travel from the site of administration to systemic circulation. The extent of absorption into the bloodstream can be described by bioavailability, which is defined as the fraction of the drug that reaches the site of action. As a result, absorption is not measured for intravenously administered drugs

Distribution

When a drug is taken, it is absorbed into the body. Once in the blood, it circulates through the body. This process depends on many factors, including the kidneys and liver, and is measured by the volume of distribution which describes how much drug is in flesh versus blood.

Pharmacokinetics is the study of how a drug moves through the body. It includes all the factors that affect the absorption, distribution, and metabolism of a drug. The main pharmacokinetic factors include the route of administration, the dosage form and its bioavailability.

Metabolism

Drug metabolism is the process of chemically altering the molecules of a drug, which creates new compounds. The compounds created from this process are known as metabolites. Metabolism may occur in many areas of the body including the gastrointestinal tract, kidneys, and plasma, but the majority of metabolism occurs in the body’s largest internal organ, the liver.

In the liver, there are many enzymes that can process drugs via Phase 1 and Phase 2 metabolic pathways. Metabolites formed from Phase 1 reactions are more likely to be pharmacologically active. In contrast, Phase 2 reactions generally render a compound more water soluble and pharmacologically inactive.

Excretion

When a drug is primarily excreted via the kidneys, impaired kidney function can significantly affect a drug’s PK. One type of change in the PK of a drug that can occur as a result of impaired renal function is a decrease in the excretion of the drug or a decrease in the excretion of metabolites of the drug.

Why Pharmacokinetic Studies Are Important?

Pharmacokinetics (PK) studies allow the characterization of ADME properties of a drug discovery early in development. They also provide critical information including the impact of food interactions (in orally administered drugs), drug-drug interactions, and organ impairment on the disposition of a drug. The foundational information gained from PK studies helps guide what additional studies are needed, as certain types of clinical studies may not be helpful or relevant for all drugs.

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