The therapeutic index (TI; also referred to as therapeutic ratio) is a quantitative measurement of the relative safety of a drug. It is a comparison of the amount of a therapeutic agent that causes toxicity to the amount that causes the therapeutic effect.[1] The related terms therapeutic window or safety window refer to a range of doses optimized between efficacy and toxicity, achieving the greatest therapeutic benefit without resulting in unacceptable side-effects or toxicity.
Classically, for clinical indications of an approved drug, TI refers to the ratio of the dose of the drug that causes adverse effects at an incidence/severity not compatible with the targeted indication (e.g. toxic dose in 50% of subjects, TD50) to the dose that leads to the desired pharmacological effect (e.g. efficacious dose in 50% of subjects, ED50). In contrast, in a drug development setting TI is calculated based on plasma exposure levels.[2]
In the early days of pharmaceutical toxicology, TI was frequently determined in animals as lethal dose of a drug for 50% of the population (LD50) divided by the minimum effective dose for 50% of the population (ED50). In modern settings, more sophisticated toxicity endpoints are used.
For many drugs, severe toxicities in humans occur at sublethal doses, which limit their maximum dose. A higher safety-based therapeutic index is preferable instead of a lower one; an individual would have to take a much higher dose of a drug to reach the lethal threshold than the dose taken to induce the therapeutic effect of the drug. However, a lower efficacy-based therapeutic index is preferable instead of a higher one; an individual would have to take a higher dose of a drug to reach the toxic threshold than the dose taken to induce the therapeutic effect of the drug.
Generally, a drug or other therapeutic agent with a narrow therapeutic range (i.e. having little difference between toxic and therapeutic doses) may have its dosage adjusted according to measurements of its blood levels in the person taking it. This may be achieved through therapeutic drug monitoring (TDM) protocols. TDM is recommended for use in the treatment of psychiatric disorders with lithium due to its narrow therapeutic range.[3]
Term | Full form | Definition |
---|---|---|
ED | Effective Dose | the dose or concentration of a drug that produces a biological response.[4][5] |
TD | Toxic Dose | the dose at which toxicity occurs in 50% of cases. |
LD | Lethal Dose | the dose at which death occurs in 50% of cases.[6]: 73 [7][8] |
TI | Therapeutic Index | a quantitative measurement of the relative safety of a drug by comparison of the amount of a therapeutic agent that causes toxicity to the amount that causes the therapeutic effect[9] |
The therapeutic index is the ratio of the TD50 (or LD50) to the ED50, determined from quantal dose–response curves.
The therapeutic index is the ratio of the TD50 (or LD50) to the ED50, determined from quantal dose–response curves.