Drug Dose Response Relationships: Quantifying Drug Response

Question and Answer flashcards reviewing concepts of dose-response relationships, dissociation constants, fractional occupancy, allosteric modulation, and isobolograms.

What general pathway describes the relationship between drug concentration at the receptor site and biological effect?

$\text{Concentration} \rightarrow \text{Occupancy} \rightarrow \text{Effect}$, where receptor occupancy is directly proportional to drug concentration.

Based on experimental data where a drug concentration of $10\,M$ produces a $36\%$ effect and $20\,M$ produces a $64\%$ effect, what is the approximate $EC_{50}$?

Approximately $15\,M$.

What formula defines the dissociation constant $K_d$ in terms of ligand and receptor concentrations as well as rate constants?

$$K_d = \frac{[L][R]}{[LR]} = \frac{k_{-1}}{k_{+1}}$$

What is the fractional occupancy when the drug concentration equals $K_D$ in this graph?

The fractional occupancy is $0.5$ (or $50\%$).

Why can a full biological response be achieved at less than $100\%$ receptor occupancy?

Due to downstream amplification, which causes the $EC_{50}$ to shift left of $K_D$ and results in spare receptors.

Do positive allosteric modulators activate receptors in the absence of an agonist?

No, allosteric modulators do not activate receptors in the absence of an agonist (only allosteric agonists do).

In the provided graphs showing allosteric modulation, how do the blue and red modulators differ in their effect on the agonist (green curve)?

The blue modulator increases maximum efficacy without changing potency, while the red modulator increases potency without changing maximum efficacy.

How are additive effects, synergism, and negative synergism defined in a drug-drug interaction context?

Additive effects occur when both drugs act independently ($2 + 2 = 4$); synergism occurs when the combination is more effective than the sum ($2 + 2 = 5$); negative synergism (antagonism) occurs when the combination is less effective ($2 + 2 = 3$).

On an isobologram, what region represents positive synergism (superadditive effect)?

The region below the straight additive line, where lower concentrations of both drugs are required to achieve a $50\%$ response.

Based on the provided concentration data for Drug X and Drug Y to produce $ED_{50}$, what type of drug interaction is present?

Antagonistic (negative synergism or subadditive effect).