Dose-Response Relationship

The dose-response relationship can also be called an exposure-response relationship; this study comes under the branch of pharmacology, specifically pharmacodynamics and pharmacokinetics. Pharmacodynamics is the study of effect and mechanism of the drug. Pharmacokinetics can be described as the study of drug action when administered into the body. Pharmacokinetics mainly deals with the absorption, bioavailability, distribution, metabolism, and excretion of the drug from the body. In this topic, we will mainly focus on understanding drugs and their mode of action which encompasses the dose-response relationship and the interpretation of the drug dose relationship curve.

What is the Dose?

The dose can be defined as the amount of drug administered into the body at a single time. For example, if 200mg of paracetamol (a common drug used for fever) is administered in a patient the dose is said to be 200mg. 

What is a Response?

The response can be defined as the expected action of the drug. For example, as mentioned if paracetamol is administered, since it is an antipyretic drug, the expected response is lowering the body temperature.

Mode of Action of a Drug

To understand the dose response relationship, it is important to understand the mechanism by which a drug functions. Any drug when administered to the body moves along with the bloodstream. When it reaches the target site, it binds specifically to the receptor present on the cells of the target site. The drug and receptor binding initiates a signaling cascade that ultimately generates the appropriate response. It can be represented as,

Drug + Receptor ---->  Drug - Receptor complex ------> Response.

[Image will be Uploaded Soon]

Activation of a signaling cascade is not the only method by which response is generated, the other method is chemical interaction. It can be defined as the method where the drug interacts with another biologically active molecule that can generate the response, or act as a secondary messenger for signaling pathways that in turn generate a response. Regardless of the mode of binding of the drug or the route of administration, it is the concentration of the drug at the target site that determines the response of the drug. 

Dose-Response Curve 

The dose-response relationship can be described as the systematic description of the magnitude of the effect of a drug as a function of dose. In simpler words, it is the relationship used for analyzing what kind of response will be generated at the administration of a specific dose of the drug. The cumulative dose response curve definition can be stated as the relationship of dose in response to the drug concentration. 

What is the Need for a Dose Response Curve?

It is Needed for Two Main Reasons, They are as Follows:

1. It is used to decide the dose of the drug.

2. It is used for the comparative study of the drug, that is the study of comparing dosage to the percentage of patients showing different effects.

3. It is used to determine drug efficacy, which is defined as the ability of the drug to elicit a response when bound to its receptor.

4. It is used to determine drug potency, which is defined as the amount of drug concentration needed to induce a response.

5. It is also used to determine drug safety.

The effect of the drug is a function of both dose and time, but the curve plotted is time-independent. The dose-response curve is plotted at maxima at the time of peak effect or under steady-state conditions, for example during continuous IV infusion. 

Construction of the Dose Response Curve

The dose response curve is a rectangular hyperbola, where the intensity of the response increases with the increases in the drug concentration. It can be used to plot the results of any kind of experiment. The X axis of the graph is used to plot the concentration of the drug and the Y axis is used for plotting the response. There are some examples of response such as a change in membrane potential, enzyme activity, secretion of secondary messenger, contraction, and relaxation of muscle, metabolic degradation of certain compounds and secretion of a certain hormone. 

Example of the Dose-Response Curve:

[image Will Be Uploaded Soon]

The Shape of the Curve

The sigmoidal shape of the dose response can be described by the application of the hill equation. The hill equation is a logarithmic function of the dose according to which the graph can be interpreted.  The dose response curve is a rectangular hyperbola because the drug receptor association obeys the “law of mass action”. 

E = Emax × [D] / kd + [D]

Where ,

E= observed dose response at a particular concentration

Emax = the maximum response

kd= the dissociation constant, it describes the strength of interaction of the drug to the receptor.

The Shape of a Standard Dose-Response Curve is Defined by Four Parameters as Follows.

1. The baseline response to the drug (at the bottom)

2. The maximum response to the drug (at the peak)

3. The slope

4. EC50, is the concentration of drug that induces a response halfway between the baseline and maximum response. 

Threshold and Interpretation of Threshold Limit

The threshold is an important feature of the graph. The threshold dose definition of pharmacology can be referred to as the concentration of the dose of the drug below which the patient does not have any harmful effect from the drug exposure. 

[Image will be Uploaded Soon]

The slope of the curve determines the threshold of the drug. A sharp increase in the slope of the graph shows the increased risk of toxicity with the increased dose concentration. Similarly, a flat slope defines the low risk of toxicity of the drug even when the concentration is increased.

Deviation from the standard curve leads to a steeper or shallower graph. The steepness of the graph is quantified by the slope factor also known as the Hill slope. A dose response curve that has a standard slope, has a hill slope of 1.0, as the steepness of the graph increases the slope factor of the graph increases.  The steepness of the curve is correlated with the potency of the drug. Steeper curves have maximal potency even at low drug concentration. 

Factors Affecting Drug Response

There are the Following Factors Affecting the Drug Response,

1. Age

2. Weight

3. Environment

4. Gender

5. Shock

6. Immune condition

The dose response curve is widely used in pharmacology, apart from studying the potency and risk associated with it is also used to study the following factors as it is used to study dose response relationship in epidemiology. Dose response relationship epidemiology refers to the study of at population level, the response to compromises of death, loss of consciousness, and other adverse effects of the drug in a population.