Pharmacokinetics studies the interaction of a drug compound with the body during the entire exposure. Pharmacokinetic assays are closely related to pharmacodynamic assessments. However, both parameters are unique. Pharmacodynamics is concerned with the effect of a drug product on the body. Pharmacokinetic assays are examined through absorption, distribution, metabolism, and excretion profiles. Understanding a better perspective of all these four parameters allows physicians to prescribe and administer drug products that elicit higher benefits with lower adverse events.
Drug bioavailability is also a critical aspect of ADME studies. Drug bioavailability refers to the concentration of active drug components available at the site of action. However, it differs from bioequivalence. The primary difference between bioavailability and bioequivalence is that bioequivalence is concerned with comparing equivalencies between two drug products. The current article discusses drug absorption and metabolism for pharmacokinetic CROs. Let us begin with drug absorption.
Drug absorption is a critical component of PK analysis in clinical trials. Drug absorption is concerned with transporting an unmetabolized drug product from the site of administration into circulation. Today there are several mechanisms for absorbing a drug product into the body, including carrier-mediated, membrane transport, passive diffusion, and other non-specific absorption modes.
Several factors can affect drug absorption, including both patient and drug-specific. Hence the extent of drug absorption varies among individuals. Researchers have different routes of administration, such as oral, intravenous, intramuscular, subcutaneous, and transdermal. However, most drug products are administered through the oral route. Having in-depth knowledge of the drug absorption process is vital for delivering better therapeutic effects and achieving reliable bioavailability.
Crossing the cell membrane is the primary issue of concern for a drug product. A drug must reach systemic circulation by crossing the cell membrane. Two primary ways to cross a cell membrane; carrier-mediated membrane transporters and passive diffusion.
Most drugs undergo modifications by bodily systems to generate components that can be excreted easily from the body. These biotransformations primarily occur in the liver. Understanding these biotransformation properties is vital for achieving optimal pharmacological effects. Biotransformation mechanisms are characterized into three phases; phase I transformation, phase II conjugation, and additional phase III modifications.
Phase I modifications generally employ oxidation, hydrolysis, reduction, and hydrogen removal to modify the chemical structure of a pharmaceutical drug. In some cases, phase I modifications may convert an inactive product into an active form.
Phase II modifications often conjugate a drug molecule with another molecule. Phase II modifications usually turn a compound inert and water soluble to excrete it from the body. Conjugation mechanisms can include methylation and glucuronidation. These modifications can happen in the liver, lungs, intestine, and kidneys. Phase III metabolism may also occur after phase II modifications. In this phase, cells excrete conjugates and drug metabolites.
Drug absorption and metabolism are critical aspects of pharmacokinetic studies. Only after an in-depth understanding of drug metabolism and absorption characteristics physicians and healthcare providers can accurately tailor drug doses to achieve desirable therapeutic effects without unwanted adverse reactions.