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Pharmacokinetics Definition

The movement of drugs through the body's biological systems is called pharmacokinetics. These processes include absorption, distribution, bioavailability, metabolism, and excretion. You can examine the onset, duration, and severity of drug effects. It controls the movement of drugs in and out of the body.

Pharmacokinetics Definition

Pharmacokinetic applications

Pharmacokinetics uses mathematical formulas to describe what the body does to a drug or toxin during absorption, distribution, metabolism, and excretion. Pharmacokinetics can be used clinically to explore the relationship between drug dose, concentration, and resulting effects over time. Used in therapeutic drug monitoring to guide and optimize treatment by determining lozenges, dosing frequency, and intended effect while minimizing toxins. It can also identify medical relationships and fix problems that are not responding to treatment. Pharmacokinetics assesses drug or venom co-occurrence in cases of overdose/toxin by calculating the amount of drug or venom in the body and assisting sweating in eliminating chemicals involved in drug side effects To do.

Factors Influencing Drug Therapy

Pharmacokinetics vary from person to person and are influenced by factors such as age, sex, diet, environment, weight, pregnancy, patient pathophysiology, genetics, and drug or food interactions. Factors affecting pharmacokinetics and pharmacodynamics influence drug therapy. Total body water content, body fat percentage, muscle mass, organ size, blood volume and flow rate, and metabolic enzymes contribute to inter-individual heterogeneity in pharmacokinetics., which ultimately contributes to the therapeutic efficacy of drugs. Genetics can influence drug metabolism. Genetic differences can affect how drugs are metabolized in the body.

Age affects drug metabolism and excretion. For example, unlike adults, children may not have a well-developed liver function to digest drugs extensively. Some drugs clear faster in children. However, drug clearance is reduced in older adults with reduced renal, hepatic, and cardiac function. Disease, infection, and inflammation can decrease drug metabolism and increase half-life and duration of action. Drug-drug, food-drug, and drug-herb interactions can alter drug metabolism and the duration and potency of drug action.

Drug absorption

Drugs must enter the systemic circulation to reach their target tissues. These include direct delivery into the blood (intravenous or intraarterial administration), absorption through the skin, and passage through the digestive system. Blood flow, drug concentration at the delivered site, formulation, physicochemical qualities, and method of administration are all critical aspects of drug absorption. Local ph. and gastrointestinal contents can alter absorption when administered orally. The breakdown of solid drugs in solution also influences their absorption.

The membrane of the cell

Cell membranes serve as semipermeable walls that help prevent medicine motes from passing through. Cell membranes are composed of a phospholipid bilayer, with the hydrophilic heads of the phospholipid layers facing out and the lipophilic ends facing in. Glycoproteins are bedded throughout the bilayer and serve as ion channels, receptors, secondary couriers (G- proteins), or enzymes. For further specific purposes, some napkins have a kindly modified cell membrane.

As an illustration:

There are places in the capillary endothelium termed fenestrae where the outer and inner membranes are fused together with no intervening cytosol, making the endothelium relatively porous, especially to fluid. Gaps/clefts between cells in the glomerular endothelium allow bigger molecules to pass through. The blood-brain barrier (BBB) prevents certain chemicals (polar medicines) and proteins from crossing the two fluid compartments.

Drug transportation

Depending on the drug's physicochemical properties, the drug is absorbed from the administration site (such as the digestive system) by passive diffusion or active transport.
For example, small lipid- or water-soluble drugs can flow across the plasma membrane (via membrane bilayers or aqueous channels/pores, respectively) from areas of high concentration to areas of low concentration. This is called simple passive diffusion. Some drugs require the use of specific transmembrane carrier proteins to help cross-cell membranes. This is called auxiliary propagation. This process does not require energy, and the drug molecules do not move against the concentration gradient.

Some medications require a carrier protein to transport them across the cell membrane actively. This process involves energy consumption (often in the form of ATP) and can be carried against a concentration gradient, primarily in certain sections of the small intestine. This is known as active transportation. This is most common with medications structurally identical to endogenous substances, including vitamins, carbohydrates, and amino acids.

Larger drugs, mostly protein drugs, are transported by encapsulation of drug molecules across cell membranes to transport drugs within cells. This process is called endocytosis. The vesicles can be used intracellularly (such as iron), extruded into the cell (such as vitamin B12), or stored for later use (such as neurotransmitters). The transport of the medication across the cell membrane is involved in all pharmacokinetic processes.

Drug distribution

A drug is disseminated and removed after it enters the systemic circulation, typically at the same time. The body has multiple fluid compartments, and the medicine can be dispersed in any or all of them:

Organs with a good blood supply are located in the central compartment (brain, heart, kidneys, etc.).
Outer compartment: Tissues with low blood flow (such as fat and muscle tissue); for example, drugs found in fat take a long time to equilibrate, and tissues such as thiopental sodium may act as reservoirs. Plasma proteins can bind drugs. Only the free (unbound) medication can act on the target tissues' cell membranes. Protein binding must be considered when assessing a drug's concentration in the blood. This means that a high concentration does not always indicate that the drug is highly active because only a small fraction may be free while the rest is bound to protein. The unbound drug is transported to the tissues, whereas the bound drug remains in circulation. As the level of unbound drugs in circulation decreases, the bounded part of the medication is progressively released to meet the demand. As a result, the protein-bound portion of the medication remains in the blood as a reservoir.

Pharmacokinetic models

The body's handling of a drug can be quite complex, as numerous systems (such as absorption, distribution, metabolism, and elimination) work together to change drug concentrations in tissues and fluids. Biological systems must be simplified to forecast a drug's activity in the body. Applying mathematical principles to the various processes is one technique to achieve these simplifications.

To apply the mathematical principles, we need to select a body model. Compartment models are the basic type of model used in pharmacokinetics. The number of compartments required to describe the drug's activity in the body is used to classify the compartment model. Single, dual, and multi-compartment variants are available.

The compartments may reflect a group of comparable tissues or fluids rather than a specific tissue or fluid. These models can be used to forecast medication concentrations in the body over time.

Deterministic compartmental models are so named because the observed drug concentrations define the sort of compartmental model needed to describe the drug's pharmacokinetics. This concept will become clear as we look at one- and two-compartment models.

Protein binding hotspots

  • Protein-bound drugs are pharmacodynamically inactive as they are not digested or excreted.
  • Drugs reversibly bind to proteins through weak chemical interactions.
  • Drug toxicity may result from irreversible drug binding (e.g. irreversible binding of paracetamol metabolites to hepatocytes).
  • Most drugs bind to human serum albumin (HSA), the most abundant protein in plasma.
  • HSA contains four drug-binding sites.
  • Crowding out interactions can occur when two drugs compete for the same binding site on HSA. Displaced drugs may cause greater toxicity (e.g., co-administration of phenylbutazone and warfarin has a higher affinity for HSA, so phenylbutazone displaces warfarin from HSA and displaced warfarin may increase blood loss).
  • Some drugs (e.g., sodium salicylate, sodium benzoate, sulfonamides) have a higher affinity for HSA than bilirubin and displace bilirubin from protein binding sites. Free bilirubin crosses her BBB and causes brain damage (kernicterus).
  • Decreased protein binding of the drug may result in increased metabolism/excretion from the body.
  • Higher concentrations of free active substances in the body may lead to increased toxicity and therapeutic efficacy

Monitoring therapeutic drugs

The use of assay methods to determine plasma drug concentrations and interpret and apply the resulting concentration data to determine safe and effective dosing regimens is called therapeutic drug monitoring. Done properly, this approach can establish therapeutic drug concentrations more quickly and safely than empirical titration. Combined with the observation of drug clinical efficacy, it should be the safest method for optimal drug therapy.

The value of plasma drug concentration data stems from the idea that pharmacologic response is directly connected to drug concentration at the site of action.

For various drugs, patient studies have provided information about the safe and effective plasma concentration range, or therapeutic range, for treating specific diseases. Within this therapeutic range, the desired effects of the drug are noted. Below this, there is a high possibility that the therapeutic effect will not be obtained. This can have dangerous repercussions.

No definitive boundaries separate subtherapeutic, therapeutic, and toxic medication concentrations. Because of the heterogeneity in individual patient response, most medications have a grey area where these concentrations overlap. A drug's pharmacokinetic properties can cause variability in the plasma concentration obtained with a given dose when delivered to different persons.

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