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Pharmacology > Drug Action > Flashcards

Drug Action Flashcards

1
Q

What is the definition of Pharmacodynamics?

A

What the drug does to the body.

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2
Q

What is the definition of Pharmacokinetics? What are the factors to consider when talking about Pharmacokinetics?

A

What the body does to the drug.

  1. Translocation (movement)
  2. Metabolism (chemical transformation)
  • absorption
  • distribution
  • metabolism
  • excretion
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3
Q

What is the definition of affinity?

A

The tendency of a ligand to bind to a receptor.

  • K(d) - concentration of a drug that binds to 50% of the receptors.
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4
Q

What is the definition of potency?

A

A measure of the concentration of the drug required to induce a given response.

  • EC50 - concentration of a drug that results in 50% of the maximum response.
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5
Q

What is the definition of efficacy?

A

The ability of a ligand, once bound, to elicit changes which lead to effects (maximum response).

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6
Q

What is the difference between an agonist and antagonist? Do they both have affinity and efficacy?

A
  1. Agonist - molecule which activates a receptor.
  2. Antagonist - molecule which blocks agonist action.
  • Agonists have affinity and efficacy
  • Antagonists only have affinity
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7
Q

What is the occupation theory?

A

The theory that drug effect is proportional to the number of receptors occupied.

  • Full occupancy = full effect
  • K(d) = EC50
  • However, this is not always true as some drugs have full effect at a relatively low concentration
  • K(d) not equal to EC50
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8
Q

What is a partial agonist?

A

A drug that induces a response but the response is non-maximal (low intrinsic activity).

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9
Q

Name some alterations in drug receptor activity. What are some possible explanations?

A
  • Desensitization/tachyphylaxis
  • Tolerance
  • Refractoriness
  • Resistance
  • Change in receptor
  • Loss of receptor
  • Exhaustion of mediator
  • Increased metabolic degradation
  • Physiological adaptation
  • Active extrusion of drug from cells
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10
Q

The chemical nature of a drug has some majory implications for ADME. Name some factors that affect the chemical nature of a drug.

A
  • Solubility - water, lipid?
  • Ionisation status (pKa) - weak acid/base
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11
Q

What are some considerations with regard to drug Distribution?

A
  • Translocation
  • pH and ion trapping
  • Plasma protein binding
  • Partition into body fat (and other tissue)
  • Body fluid compartments
  • Volume of Distribution
  • Blood Brain Barrier
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12
Q

Diffusion is important with regard to distribution of drugs around the body. What influences the diffusion of a drug?

A
  • The drug must be dissolved
  • It moves from an area of high concentration to an area of low concentration
  • It continues to move until equilibrium is reached
  • Only unionized drugs are diffusible, so the chemical nature of the drug and the environment in which it’s in influences its ionization status and therefore how it diffuses.
  • Weak acids are unionized in acidic solutions
  • Weak bases are unionized in basic solutions
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13
Q

Explain pH and ion trapping using a weak acid and weak base, and gastric juice and urine as a model.

A
  • Ion trapping is the containment of a drug (weak acid/base) in a compartment which ionizes that drug. Weak acids are ionized in basic environments so they become trapped/are unable to diffuse. Weak bases are ionized in acidic environments so they are trapped/unable to diffuse.
  • Remember, for a drug to diffuse out of a compartment, it must be unionized.
  1. Weak acid in gastric juice - mostly unionized, high rate of diffusion.
  2. Weak acid in urine - mostly ionized, low rate of diffustion.
  3. Weak base in gastric juice - mostly ionized, low rate of diffusion.
  4. Weak base in urine - mostly unionized, high rate of diffusion.
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14
Q

What are some consequences of ion trapping?

A
  • pH of urine can affect excretion.
  • pH of milk (acidic relative to plasma) can trap basic molecules and pass drugs to newborns; animal milk products for human consumption can contain unwanted drugs.
  • plasma pH can influence movement across the BBB
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15
Q

Plasma protein binding (albumin) binds many acidic drugs and some basic drugs. What are the implications in plasma protein binding?

  • Don’t think of acidic vs basic drugs for the answer. It’s just a reminder of the affinity of albumin for drugs.
A
  • Increases water solubility for drugs
  • Decreases distribution of drugs out of plasma
  • A bound drug is an inactive drug
  • A bound drug is one that will not be metabolised as quickly (i.e. decreased metabolism)
  • Decreased excretion
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16
Q

The brain is considered a “protected” organ. Why? What are some other “protected” organs?

A

The BBB is effectively closed to most molecules because of the tight intercellular junctions (other areas of the body have “leaky” junctions). Molecules must have specific transporters or be able to diffuse through the cell.

  • Eye
  • Testis
  • Prostate
17
Q

What is the definition of volume of distribution? What are some factors affecting V(d)?

A
  • The fluid volume in which a drug seems to be dissolved.
  • V(d) = total amount of drug in the body / concentration of drug in the plasma
  • plasma volume: 0.05 L/kg
  • ECF: 0.2 L/kg
  • total body water: 0.55 L/kg
  • species
  • age
  • disease - fluid retention, obesity, dehydration, weight loss
18
Q

How do you calculate a loading dose?

A

Loading dose is equal to the volume of distribution (V(d)) multiplied by the effective concentration (C(eff) - measured).

  • if V(d) is 30 L, and C(eff) is 10mg/L then the LD is 300mg.
19
Q

What is the difference between Phase I and Phase II reactions in drug metabolism?

A
  1. Phase I metabolism involves a catabolic process (oxidation, reduction, or hydrolysis = introduction of a functional group). It usually results in a more active molecule.
  2. Phase II metabolism involves an anabolic process (conjugation). It usually results in a less active molecule.
  • Both processes decrease lipid solubility, resulting in increased renal excretion.
  • Metabolism mainly takes place in the liver, but also the plasma, kidney, lungs, or gut.
20
Q

What is First Pass Metabolism? What are some consequences of this?

A
  • Pre-systemic metabolism
  • The liver (or gut wall) metabolises the drug before it enters systemic circulation.
  • Poor bioavailability
  • Larger dose needed
  • Marked inter-individual variation - species, breed, individual
21
Q

What are some factors affecting metabolism?

A
  • previous drug therapy
  • age
  • disease
  • Genetics - species differences, inter-individual differences
22
Q

What are some methods of excretion?

A

Kidney and liver excretion

  1. Glomerular filtration
  2. Tubular secretion and reabsorption
  3. Tubular diffusion
  4. Finally, renal clearance
  • Biliary excretion and enterohepatic circulation
23
Q

Molecules of <20,000 MW diffuse across the glomerulus. Albumin is around 70,000 MW. Why is this important?

A

If a drug is bound to Alb, then it cannot cross the glomerulus when bound to Alb.

It therefore will be eliminated by tubular secretion (potentially the most effective mechanism of renal drug elimination).

However, most drugs are eliminated this way.

24
Q

What is tubular secretion and reabsorption?

A
  • The method of elimination which transports drugs into the renal tubules so they can be excreted in urine.
  • There are two carrier systems: one for acids, and one for bases.
  • Can reduce plasma concentrations to nearly zero, as they can work against an electrochemical gradient.
  • Can eliminate drugs bound to plasma proteins.
25
Q

What is tubular diffusion, and why is this important?

A
  • Water is reabsorbed (diffuses) from glomerular filtrate.
  • However, lipid soluble, unionized drugs are as well.
  • Ionized drugs are retained in the tubules, and excreted.
  • Ion-trapping is used to increase elimination (acidic urine used to excrete basic drugs…). = alkalinisation is used to excrete some acidic drugs.
26
Q

What is the definition of renal clearance?

A

The volume of plasma containing the amount of substance that is removed by the kidney in unit time.

27
Q

What is the half-life of a drug, and what determines this value?

A
  • It is the time taken for the plasma concentration to fall by 1/2.
  • It is determined by V(d) and clearance.
28
Q

How many half-lives does it take to achieve a steady state in normal kinetics?

A

3-5 plasma half-lives.

29
Q

What variables are involved in achieving a steady state?

A

Absorption time, Volume of distribution, and clearance rate.

Pharmacology (8 decks)

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