Therapeutic index

The therapeutic index (TI) (also referred to as therapeutic ratio) is a comparison of the amount of a therapeutic agent that causes the therapeutic effect to the amount that causes toxicity. The related terms therapeutic window or safety window refer to a range of doses which optimize between efficacy and toxicity, achieving the greatest therapeutic benefit without resulting in unacceptable side-effects or toxicity.

Classically, in an established clinical indication setting of an approved drug, TI refers to the ratio of the dose of 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 levels.

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). Today, more sophisticated toxicity endpoints are used.

${\displaystyle {\mbox{Therapeutic Index}}={\frac {\mathrm {LD} _{50}}{\mathrm {ED} _{50}}}}$ in animal studies, or for humans, ${\displaystyle {\mbox{Therapeutic Index}}={\frac {\mathrm {TD} _{50}}{\mathrm {ED} _{50}}}}$

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