Choose the true statement(s) about therapeutic index (TI). Select all that apply.
a. A drug with a high therapeutic index would be effective above the dose at which it is potentially toxic.
b. A narrow TI is preferable.
c. A drug for which the maximum safe dose is close to the minimum effective dose would have a high TI.
d. It is one measure of a drug’s general safety.
e. A drug that is not selectively toxic would most likely have a high TI.

Verified step by step guidance

1. Step 1: Understand the definition of Therapeutic Index (TI). TI is a ratio that compares the toxic dose of a drug to its effective dose, usually expressed as \(\text{TI} = \frac{\text{TD}_{50}}{\text{ED}_{50}}\), where \(\text{TD}_{50}\) is the dose that causes toxicity in 50% of the population, and \(\text{ED}_{50}\) is the dose that produces the desired effect in 50% of the population.
2. Step 2: Analyze statement (a): "A drug with a high therapeutic index would be effective above the dose at which it is potentially toxic." Since a high TI means the toxic dose is much higher than the effective dose, the drug is effective at doses well below toxic levels, so this statement is false.
3. Step 3: Analyze statement (b): "A narrow TI is preferable." A narrow TI means the effective dose and toxic dose are close, which increases risk. Therefore, a narrow TI is generally not preferable, making this statement false.
4. Step 4: Analyze statement (c): "A drug for which the maximum safe dose is close to the minimum effective dose would have a high TI." If the safe dose is close to the effective dose, the TI is low, not high. So this statement is false.
5. Step 5: Analyze statements (d) and (e): Statement (d) says TI is one measure of a drug’s general safety, which is true because TI indicates the margin between effective and toxic doses. Statement (e) says a drug that is not selectively toxic would most likely have a high TI; however, lack of selective toxicity usually means higher toxicity to host cells, implying a low TI. So (d) is true and (e) is false.

Key Concepts

Therapeutic Index (TI)

The therapeutic index is a ratio that compares the toxic dose of a drug to its effective dose, indicating the drug's safety margin. A high TI means there is a large gap between effective and toxic doses, making the drug safer to use. Conversely, a low or narrow TI indicates a small margin, requiring careful dosing.

Selective Toxicity

Selective toxicity refers to a drug's ability to target harmful organisms or cells without damaging the host. Drugs with high selective toxicity tend to have a higher therapeutic index because they affect pathogens more than human cells, reducing side effects and toxicity.

Interpretation of TI Values in Drug Safety

Understanding TI values helps determine drug safety: a narrow TI means the effective and toxic doses are close, increasing risk, while a high TI indicates safer dosing. Therefore, drugs with a narrow TI require close monitoring, and a high TI is generally preferable for therapeutic use.