Pharmacodynamics - Potency, Efficacy, and The Therapeutic Index

What is pharamacodynamics?

The study of how a drug changes the body. Specifically, it studies the relationship between drug plasma concentration and it's therapeutic effect (how much drug is needed for a drug to do its job) and the mechanisms of action of drugs.

Pharmacokinetics often influences pharmacodynamics

What is the quantal effect of a drug?

A yes/no response to whether a certain drug dosage produces a specific therapeutic effect.

Ex: Whether 20mg of a drug reduces systolic bp by 20mmHg

(note that the quantal effect does not specify magnitude of the medication)

What is the ED50 of the quantal effect of a drug?

Median effective dose.

The dosage of a drug that is required to produce a specific therapeutic response in 50% of patients.

Often referred to as the standard

What is the LD50 of a drug?

Median lethal dose

The dosage of a drug that is required to produce a lethal response in 50% of patients (kills 50% of pts)

What is the TD50 of a drug?

Median toxic dose

The dosage of a drug that is required to produce adverse effects in 50% of patients (overdose in 50% of pts)

What is the therapeutic index of a drug?

The ratio of a drug's toxic level (TD50) to its median effective dose (ED50).

A higher therapeutic index means that a drug has a higher therapeutic range and is generally more safer

A lower therapeutic index means that a drug has a lower therapeutic range and is generally less safe

What does a higher therapeutic index of a drug signify?

A higher therapeutic index means that a drug has a higher therapeutic range and is generally more safer

What does a lower therapeutic index of a drug signify?

A lower therapeutic index means that a drug has a lower therapeutic range and is generally less safe

What are dose-response curves?

Demonstrates the magnitude of biological response to a drug, which can derive a drug's therapeutic range (when a drug does it's intended job), the efficacy of a drug (how good a drug does it's job), and the potency of a drug (how much of a drug is needed compared to others to do it's job).

It is different from a quantal response as it observes a patient's response to different doses of a drug

What is phase 1 of the dose-response curve?

The lower end of the curve, represents the range where the drug dosage produces little to no effect on the patient

What is phase 2 of the dose-response curve?

Between phase 1 and phase 3, represents the range where there is a linear relationship between the amount of drug administered and the degree of the client's response.

The therapeutic range of a drug will generally be within this phase

What is phase 3 of the dose-response curve?

The higher end of the curve, represents the range where the drug produces the maximum amount of therapeutic effect possible, and changes to the drug dose does not increase it's effect and may lead to overdose

What is the potency of a drug?

Represents how much dose of a drug is needed to produce a specific therapeutic response in relation to other drugs

A higher potency means that a lower drug dose is necessary to elicit a therapeutic response

A lower potency means that a higher drug dose is necessary to elicit a therapeutic response

What does a higher potency of a drug represent?

A higher potency means that a lower drug dose is necessary to elicit a therapeutic response

What does a lower potency of a drug represent?

A lower potency means that a higher drug dose is necessary to elicit a therapeutic response

What is the efficacy of a drug?

The intensity of the maximum therapeutic effect of a drug.

A higher efficacy means that the drug elicits a higher maximum response.

A lower efficacy means that a drug elicits a lower maximum response.

What does a higher efficacy of a drug represent?

A higher efficacy means that the drug elicits a higher maximum response.

What does a lower efficacy of a drug represent?

A lower efficacy means that a drug elicits a lower maximum response.

What are quantal effect measurements of a drug?

Measurements that are derived from a yes/no response.

Includes the ED50 - median effective dose, and the therapeutic index

What are graded effect measurements of a drug?

Measurements that are obtained by observing and measuring a patient response to different doses of a drug, creating a dose-response relationship

Includes the therapeutic range of the drug, the efficacy of the drug, and the potency of the drug