Lecture 2: Drug-Receptor I

Flashcards covering drug-receptor interaction concepts including affinity, efficacy, receptor occupancy, radioligand binding assay parameters, and the Langmuir/Scatchard equation.

How is a ligand defined in the context of drug-receptor interactions?

A molecule that binds to a specific receptor site on another molecule, which may or may not trigger a biological response.

What property governs the tendency for a drug to bind to a receptor?

Affinity.

What describes the tendency for a drug, once bound, to activate a receptor?

Efficacy.

How do drugs of high potency generally relate to receptor affinity?

They generally have a high affinity for their receptors.

What is the key difference between agonists and antagonists regarding receptor interaction?

Both bind to receptors, but only agonists 'activate' the receptors to induce signaling and a response.

In the reversible interaction of two molecules, why is the forward rate described as a second-order reaction?

Because it depends on the concentration of two types of molecules: $k_{+1}[A][R]$.

What is the mathematical definition of the dissociation constant $K_D$ at equilibrium?

$$K_D = \frac{k_{-1}}{k_{+1}} = \frac{[A_{eq}][R_{eq}]}{[A_{eq}R_{eq}]}$$

How is receptor occupancy defined mathematically?

$$\text{Occupancy} = \frac{\text{number of receptors occupied}}{\text{total number of receptors}}$$

What is the range of values for receptor occupancy?

It varies between $0$ (no drug present) and $1$ (all receptors occupied).

Which experimental approach measures changes in the refractive index of a surface when molecules bind to it?

Surface Plasmon Resonance (SPR).

What is the principle behind Fluorescence Polarization Assays (FPAs)?

A fluorescently labeled drug is used; when it binds to a receptor, its rotational freedom decreases, leading to increased polarization of emitted fluorescence.

What does Isothermal Titration Calorimetry (ITC) measure in a binding interaction?

The heat released or absorbed during the interaction.

Why are additives like protease inhibitors or antioxidants like ascorbic acid used in radioligand binding assays?

To protect the integrity of the tissue/receptors or the ligand if they are susceptible to degradation or oxidation.

What are the specific activity and half-life characteristics of the radio-label $^3H$?

Specific activities $> 80\text{ Ci } mmol^{-1}$ and a long half-life of approximately $12.5\text{ years}$.

What is a disadvantage of using $^{125}I$ as a radio-label, despite and its high specific activity?

It has a short half-life of $67\text{ days}$, is more readily degraded, and can reduce the biological activity of the ligand.

Why is the speed of separation of bound and free ligands critical in an assay?

It must be compatible with the rate of dissociation; lower affinity (higher $K_D$) requires faster or more efficient separation.

How is non-specific binding typically determined in a radioligand assay?

By adding an excess of non-radioactive drug to displace the specific radioactive bound drug.

What is the formula to calculate specific binding in data analysis?

$$\text{Specific binding} = \text{Total bound} - \text{nonspecific binding}$$

According to the Langmuir/Scatchard equation, what is the formula for calculating specific drug bound?

$$\text{Specific Bound} = \frac{B_{max}X_a}{X_a + K_D}$$

What does the parameter $B_{max}$ represent in receptor binding data?

The binding capacity, often expressed per mg protein.

How is $K_D$ defined in terms of occupancy on a binding curve?

$K_D$ is the concentration of ligand required to occupy $50\text{\%}$ of receptors at equilibrium.

How does affinity relate to the numerical value of $K_D$?

A low $K_D$ indicates high affinity, while a high $K_D$ indicates low affinity.

In competition assays, how is the inhibition constant denoted and what are its units?

It is denoted as $K_i$, with units in Molar.

What was the unlabelled ligand used to displace $^3H$-DAMGO in the mu-opioid receptor competition assay example?

Naloxone.

To which overarching law does the binding of drugs to receptors adhere?

The Law of Mass Action.