The therapeutic index (TI) is the ratio of toxic dose to the therapeutic dose. A chemotherapeutic agent that is used for the treatment must have a toxic dose and a therapeutic dose.
The therapeutic dose is defined as the dose of a drug that is required for the clinical treatment of a particular infection. Whereas, toxic dose is defined as the dose of a drug at which it becomes toxic for the host.
A drug, used for the treatment, must have a toxic effect on organism but not on the host. The larger the TI, the safer the drug will be. A higher TI indicates a significant difference between toxic dose and therapeutic dose. With higher TI, a patient would have to take a much higher dose of the drug to reach the toxic level as compared to the dose required in eliciting the therapeutic effect. If the TI is small, it means the difference between the toxic dose and therapeutic dose is very small. In such cases, the drug needs to be monitored very carefully, as a minor increase in the dose of the drug can make it toxic from therapeutic.
For example, penicillin inhibits bacterial cell wall synthesis and as host cell lacks cell wall, so penicillin does not have an effect on host cell. This indicates that penicillin has high TI. But, if the drug inhibits the same process in the host cell also, it is considered to have low TI.
TI has certain limitations as it is not possible to measure the toxic dose of a drug in humans. Although animal studies help in measuring the toxic dose, it can’t be fully accepted for humans. Nevertheless, TI highlights the significance of the margin of safety, as distinct from the potency, in determining the usefulness of a drug.
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