Updated: 12/6/2019

Therapeutic Index

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Overview
  • Therapeutic index (TI) 
    • measurement of drug safety
    • refers to the relationship between toxic and therapeutic dosing
    • TI = TD50/ED50  
      • therapeutic index of a drug is the ratio of the dose that produces toxicity to the dose that produces a clinically desired or effective response
      • TD50 = the dose of drug that causes a toxic response in 50% of the population 
      • ED50 = the dose of drug that is therapeutically effective in 50% of the population
    • a safer drug has a higher therapeutic index
      • very large lethal dose, very small effective dose 
    • a more dangerous drug has a lower therapeutic index
      • may require regular monitoring of drug levels
      • examples include:
        • warfarin
        • lithium
        • digoxin
        • phenytoin
        • gentamicin
        • amphotericin
        • 5-fluoro-urocil
        • zidovudine
  • Therapeutic drug monitoring 
    • Therapeutic window (TW) is the range of drug concentrations in which a drug is effective
    • Therapeutic WindowTW concept is reflected in TI calculation
      • TW = MTC/MEC 
        • Minimum effective concentration (MEC) is the minimum concentration that is required for drug effect
        • Minimum toxic concentration (MTC) is the minimum concentration in which toxicity usually occurs
        • If you have a low dose for the drug to start working (MEC) and a high dose at which the drug is toxic (MTC) you have a wide window in which the drug can safely be used (TW)
 
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