Therapeutic Drug Monitoring
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Therapeutic drug monitoring (TDM)
The measurement of drugs and/or their metabolites in body fluids (usually blood) to maintain therapeutic benefits.
Therapeutic range (TR)
Dose/concentration range of a drug. The drug produces the desired effect within that range
True/False Most drugs don’t require monitoring to achieve therapeutic levels
So true, my friend
When is TDM used?
When a drug has: - Narrow TR - Marked pharmacokinetic variabilities - Critical adverse effects
Main purposes of TDM
• Ensure correct drug dosages for TR • ID drug-drug interactions if multiple drugs are taken together
Factors influencing drug concentrations and efficacy
• Age • Gender • Genetics • Diet • Co-administered drugs • Naturopathic agents (natural)
Pharmacokinetics
Study of the movement of drugs in the body
Pharmacokinetics provides a time-course of drug concentrations in the body as a function of _____
• Absorption • Distribution • Metabolism • Excretion
Routes of administration
• Oral (most common/least invasive) • IV (most direct/effective) • IM • Subcutaneous • Aerosol • Transdermal patch • Rectal
Bioavailability
The fraction (%) of an administered drug that reaches the systemic circulation.
Oral drug absorption depends on these 3 factors
Efficiency of absorption from GI depends on 1) Dissociation from its administered form, 2) Solubility in GI fluids, and 3) Diffusion across GI membranes
Drugs most passively diffused from GI to bloodstream require what?
• Drug must be hydrophobic (nonionized) • Gastric acidity (weak acids best absorbed by stomach; weak bases best absorbed by intestine)
What affects drug absorption?
• Affected by changes in intestinal motility, pH, and inflammation • Administration with other substances (antacids, fiber, kaolin, sucralfate, cholestyramine, antiulcer medications)
Ability of drug to leave circulation depends on ____ of the drug.
Lipid solubility - Highly hydrophobic can easily cross cell membranes or lipid compartments - Polar/non-ionic can cross membranes but don’t enter lipid compartments - Ionized diffuse out of vasculature but at a slow rate
Volume of distribution equation
Volume of distribution = Dose of drug/Concentration of drug
Drug with large and small Vd: lipid properties
• Large Vd = Hydrophobic • Small Vd = Ionizable/protein-bound
True/False: Only bound drug fractions can interact with site of action and result in biological response (active fraction)
False Only the free/unbound fractions are the biologically active fractions
Major transporter of drugs and how it affects free vs bound drug concentrations
Albumin. Its concentration affects free vs bound drug
First pass effect
Phenomenon in which a drug is metabolized before reaching the circulatory system, which decreases its concentration before entering circulation
Biotransformation
Where a drug gets metabolized into a therapeutically active metabolite (usually in the liver)
Biochemical pathway responsible for large portion of drug metabolism. Basic function of pathway?
• Hepatic mixed-function oxidase (MFO) systems. • Function: converting hydrophobic substances to water-soluble products, which are then transported into bile or released into circulation for elimination by renal filtration
Phase 1 reactions in MFO system
Produce reactive intermediates
Phase 2 reactions in MFO system
Conjugate functional groups to reactive sites in water-soluble products
Specificity of MFO system
Non-specific. Allows many endo/exogenous substances to go through this pathway. E.g., grapefruit, alcohol, or caffeine