Therapeutic Drug Monitoring

Therapeutic Drug Monitoring (TDM)

Definition of TDM

  • TDM is the analysis, assessment, and evaluation of the circulating concentration of drugs in serum, plasma, and whole blood.

  • Objectives of TDM:

    • Ensure a given dose produces:

    • Maximum therapeutic benefit.

    • Minimal toxic side effects.

Basis of TDM

  • Key Factors Influencing TDM:

    1. Route of administration.

    2. Rate of absorption.

    3. Distribution of the drug within the body.

    4. Rate of elimination.

  • Complexities:

    • Many factors influence the four key aspects of TDM, making the process of achieving therapeutic drug concentrations complicated.

Figure: Overview of Drug Concentration Influencing Factors

  • Processes Include:

    • Absorption from the gastrointestinal tract.

    • Distribution throughout the body.

    • Metabolism and elimination (renal excretion).

    • Protein binding.

  • Impact on:

    • Biological response and metabolite formation.

Mechanism of Therapeutic Drugs

  • Receptor Interaction:

    • Drugs mediate actions by interacting with receptors such as enzymes, structural proteins, transport proteins, and macromolecules.

  • Potential Actions:

    • Compete with endogenous substances for binding sites.

    • Block formation, release, uptake, or transport of essential substances.

    • Modify cellular functions through changes in membrane permeability or DNA transcription.

Dosage Regimens

  • General Rule:

    • TDM is rarely needed for well-established dosage regimens unless there are significant concerns.

  • Process When TDM Required:

    1. Establish baseline concentration that is therapeutic.

    2. Initiate therapy.

    3. Individualized monitoring for each patient to maintain regimen effectiveness.

Indications for TDM

  • TDM is indicated when:

    • There are small gaps between therapeutic and toxic doses.

    • Serious consequences arise from overdosing or under-dosing.

    • Poor dose-concentration correlation exists, but a correlation between drug concentration and effects is strong.

    • Physiological changes occur in the patient that may alter drug concentration.

    • Drug interactions are suspected.

    • Patient compliance monitoring is needed.

Factors Leading to Adoption of TDM

  • Key Advancements:

    • Expanded range of drugs with therapeutic potential but narrow therapeutic ranges.

    • Development of analytical methods (accurate, precise, sensitive, and specific) for TDM.

    • Use of computers for pharmacokinetic data analysis to predict dosing.

Pharmacokinetics Overview

  • Defined as the activity of a drug in the body, emphasizing absorption, distribution, metabolism, and excretion.

  • Application of Pharmacokinetics:

    • Involves mathematical modeling of drug concentration.

    • Supports establishment or modification of dosage regimens.

  • Bioavailability:

    • Represents the fraction of an administered dose that reaches systemic circulation in an unchanged form.

Stages of Pharmacokinetics

  • Absorption Process:

    • Effective concentration must be achieved at the drug's site of action.

    • Chronic drug administration typically occurs extravascularly before entering the bloodstream.

  • Routes of Administration:

    • IV (Intravenous): Direct and effective.

    • IM (Intramuscular) and SC (Subcutaneous): Through muscle tissue.

    • Inhalation and transdermal delivery (patches).

    • Rectal administration via suppository.

    • Oral: Most common route.

Factors Affecting Drug Absorption

  • Critical Elements:

    • Drug nature: Dissociation from the dosing form is essential.

    • Solubility in gastrointestinal fluids; weak acids absorbed better in the stomach, weak bases in the intestine.

    • Diffusion across GI membranes, influenced by abnormal GI motility, pH changes, inflammation/disease, age, pregnancy, and food interactions.

Drug Metabolism

  • Hepatic Portal System:

    • All substances from the intestine enter the hepatic portal system first before general circulation.

    • First-pass metabolism can significantly affect bioavailability.

  • First-Pass Effect:

    • Refers to significant hepatic uptake/metabolism leading to reduced bioavailability.

    • Varies among patients influenced by genetics (pharmacogenomics).

Drug Biotransformation

  • Defined as the conversion of a drug to its metabolites in biological systems.

  • Clinical Consideration:

    • Patients with impaired liver function may require reduced drug dosages due to a decreased rate of metabolism and elimination.

Protein Binding Dynamics

  • General Properties:

    • Drugs enter circulation mainly bound to proteins (e.g., acidic drugs to albumin, basic drugs to globulin).

  • Active Fraction Impact:

    • Only the free fraction can interact with therapeutic sites.

    • Influenced by serum protein content, competition for binding sites from other substances, and stress factors.

  • Toxicity Risks:

    • Increased free fraction can lead to significant toxicity.

Distribution of Drugs

  • Drugs circulate in the bloodstream, interact with blood constituents, and are carried to different body organs.

  • The Process:

    • Free drug crosses various membranes to interact with receptors and elicit biological responses.

Factors Influencing Distribution

  • Include:

    • Drug binding to circulating blood components.

    • Binding to fixed receptors.

    • Membrane permeability.

    • Solubility in lipids (easier for hydrophobic drugs).

Drug Metabolism and Elimination

  • Excretion:

    • Occurs through biliary, intestinal, pulmonary, or renal routes.

  • Key Metabolic Routes:

    • Hepatic metabolism leading to bile or bloodstream, followed by renal excretion.

  • Organ Function Impact:

    • Decreased liver function raises serum drug concentrations and pharmacological response.

Hepatic Drug Metabolism Overview

  • Xenobiotics:

    • Substances not inherently human and that can enter pathways intended for endogenous substances.

  • Mixed Function Oxidase (MFO) System:

    • Major metabolic process converting non-polar drugs to polar, water-soluble substances through enzymatic reactions (including oxidation, reduction, hydrolysis, deamination, conjugation).

  • Divided into:

    • Phase 1: Produces reactive intermediates through functional group changes.

    • Phase 2: Conjugates those intermediates to form water-soluble products.

Rate of Metabolism Variability

  • Varies based on:

    • Individual patient differences.

    • Changes in hepatic status, age, weight, genetics.

    • Environmental exposures.

    • Drug-food interactions, co-administered drugs, or alcohol.

Drug Metabolites

  • Some drug biotransformation produces active metabolites needing monitoring.

Renal Excretion Details

  • Plasma free fraction of drugs and their metabolites undergo glomerular filtration and tubular secretion.

  • Creatinine Clearance:

    • Evaluates kidney function reliably and correlates effective elimination rates.

Concentration of Circulating Drugs

  • Pharmacokinetics Study Use:

    • Establish needed dosage regimens based on expected drug levels in blood.

    • Observations:

    • If absorption rate exceeds distribution and elimination, serum concentrations rise.

    • If absorption is less, concentrations fall.

Peak and Trough Understanding

  • Drugs are not typically administered as a single large dose but scheduled at intervals, leading to varying concentrations known as:

    • Maximum: peak level.

    • Minimum: trough level.

  • Goals:

    • Achieve therapeutic peak and trough without approaching toxicity.

    • Steady-state evaluations require about 5-7 doses for accurate assessment.