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Evolution of pharmacokinetic (PK) Studies in Drug Discovery and Development

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Clinical pharmacokinetics is an integral part of drug discovery and development due to its ability to assess the biological properties of drug compounds. Understanding the biological properties of a drug product empower scientists to design appropriate preclinical and clinical trials that have high chances of approval.

Understanding the Role of Clinical Pharmacokinetics

PK studies assess the movement of a drug compound in the human body, which helps in evaluating the efficacy and safety of the intended use of the drug. This is achieved by monitoring the absorption, distribution, metabolism, and excretion activities related to the drug. Once ADME studies are completed, the concentration of drug compound at the site of action is evaluated along with its toxicity. Below, we have explained the fundamental parameters that are assessed through clinical pharmacokinetics.

Absorption

Absorption occurs when the drug product enters the bloodstream. The drug substance can be absorbed in various ways, such as through lungs, skin, or gastrointestinal tract. The gastrointestinal tract is the most important absorption site as it impacts the concentration level of the drug at the site of action.

Distribution

Distribution is the method of drug compound transportation in the body. In this process, the drug transfer from the site of absorption to the site of action is observed. This is influenced by various factors such as plasma concentration, lipid solubility, transport proteins, and plasma protein binding.

Metabolism

Metabolism of the drug compounds includes conversion of the consumed compound into metabolites. This usually occurs in the liver, where it is converted through two phases. In the first phase, liver enzymes catalyse microsome reactions in the liver. The second phase leads to the conjugation of metabolites. Both of these phases function in a manner to make drug soluble, which makes excretion easy. Drug metabolism is impacted by various factors such as genetics, dose, disease state, and administration of the drug.

Excretion

Excretion is the process of eliminating the drug compound from the body, which usually occurs through urine.

Pharmacokinetic Parameters

The PK study corresponding to a particular drug product aims at assessing various pharmacokinetic parameters. These parameters reveal the biological features of the compounds in the drug.

Bioavailability

Bioavailability is the concentration of drug that reaches the systemic circulation or the site of action, unchanged. Many factors influence the concentration of the drug at the site of action, such as drug decomposition, physiochemical properties, absorption time and surface, small intestine perfusion and pH, etc. The need of assessing bioavailability is to find out the correct dose of the drug product.

Dose-Response

The concentration of the PK sample found at the site of action impacts the response of the body towards the drug. The clinical pharmacokinetics allow scientists to find out the therapeutic index of the drug by assessing bioavailability. The therapeutic index is a measure of safety of a drug or the minimum toxic dosage of the drug. Knowing the therapeutic index helps in evaluating the optimal dose of a drug compound. The pharmacokinetic studies are necessary for knowing the life-cycle of the drug compound once it is consumed by the human body. Tracing the overall path of the compound till it reaches the site of action helps in finding the optimal dose of the drug. It is known that a drug compound may be therapeutic when taken in low dosage but fatal when taken in high dosages. Hence, knowing the optimal dose via therapeutic index before the release of the drug is important.
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