Dose-Response Curve

Anatomy of a Dose-Response Curve

The classic dose-response curve is sigmoidal (S-shaped):

Response (%)
100 |           _______________
    |          /
 75 |         /
    |        /
 50 |-------/-------- EC50
    |      /
 25 |     /
    |    /
  0 |___/________________________
       Low          High
            Dose (log scale)

Key Components

Component	Description	What It Tells You
Baseline	Response at zero dose	Background effect level
Threshold	Minimum dose for effect	Where drug action begins
Linear region	Steep portion of curve	Dose-dependent response zone
EC50	Dose at 50% max effect	Drug potency
Emax	Maximum plateau	Intrinsic efficacy

Interpreting Curve Shape

Potency (Horizontal Position)

Curves shifted left indicate higher potency:

Response
    |    A    B    C
    |    /    /    /
    |   /    /    /
    |__/____/____/______
       Dose →

A = Most potent (lowest dose needed)
C = Least potent (highest dose needed)

Efficacy (Vertical Height)

Plateau height indicates maximum possible effect:

Response
High|    _____ Full agonist
    |   /
    |  /  ___ Partial agonist
    | / /
    |/_/_________
       Dose →

Slope (Steepness)

• Steep slope: Narrow dose range between minimal and maximal effect
• Shallow slope: Wide dose range, more dosing flexibility
• Hill coefficient: Mathematical measure of steepness

Types of Dose-Response Relationships

Graded Response

Measures effect intensity in a single system:

• Muscle contraction force
• Hormone secretion rate
• Receptor binding percentage

Quantal Response

Measures proportion of population responding:

• ED50: Dose effective in 50% of subjects
• TD50: Dose toxic to 50% of subjects
• LD50: Dose lethal to 50% of subjects (animal studies)

Practical Applications

Comparing Drugs

Drug	EC50	Emax	Interpretation
Drug A	1 nM	100%	Potent, full agonist
Drug B	10 nM	100%	Less potent, full agonist
Drug C	1 nM	50%	Potent, partial agonist

Therapeutic Window

Response
    |  Therapeutic  Toxic
    |     effect   effect
    |       /        /
    |      /   ___  /
    |     / __|   |/
    |____/__|_____|_____
          ↑___↑
       Therapeutic
         Window

A wide therapeutic window (large separation between therapeutic and toxic curves) indicates a safer drug.

Peptide-Specific Considerations

GLP-1 Agonists

Semaglutide dose-response for weight loss:

• Titration exploits the linear region
• Maximum doses approach Emax plateau
• Higher doses yield diminishing returns with more side effects

Growth Hormone Secretagogues

• Bell-shaped curves possible (inhibition at high doses)
• Receptor desensitization shifts curve right over time
• Pulsatile dosing maintains sensitivity

Frequently Asked Questions

Why are doses plotted on a log scale?

Drug effects typically span dose ranges of 100-fold or more. A logarithmic scale compresses this range, making the full curve visible and revealing the characteristic sigmoid shape that would appear as a sharp jump on a linear scale.

What does a flat dose-response curve mean?

A flat curve indicates no dose-dependent effect in that range. This could mean: doses are below threshold, doses are above Emax (plateau), the drug doesn’t affect that particular endpoint, or there’s a technical issue with the assay.

How do dose-response curves guide dosing decisions?

Optimal doses typically target the steep portion of the curve (responsive to dose adjustments) while avoiding the plateau (diminishing returns) and staying below toxic thresholds. Titration protocols are designed based on curve characteristics.