Pharmacodynamics and Drug-Receptor Interactions

• These notes cover pharmacodynamics, drug-receptor interactions, adverse drug effects, physical dependence, allergic reactions, and drug interactions.

Pharmacodynamics

• Pharmacodynamics is the study of what drugs do to the body and how they do it.
• It involves the biochemical and physiological effects of drugs and their mechanisms of action.

Receptors

• Receptors are specialized target macromolecules on or within cells that drugs bind to in order to produce their effects.

Drug-Receptor Interactions

• A ligand is a substance that binds to a receptor.
• An agonist is a drug that binds to a receptor and activates it, producing a response; an example is morphine.
• A partial agonist is a drug that binds to a receptor and activates it, but produces a weaker response than a full agonist; an example is buprenorphine.
• An antagonist is a drug that binds to a receptor and blocks the action of an agonist or endogenous substance; an example is naloxone.
• An inverse agonist is a drug that binds to a receptor and causes an opposite effect to that of an agonist; an example is an antihistamine.

Adverse Drug Effects

• Adverse drug effects are unintended and undesirable reactions to a drug.

Type A Reactions

• Type A reactions are predictable and dose-dependent adverse effects that are related to the drug’s known pharmacology; an example is respiratory depression with opioids.

Type B Reactions

• Type B reactions are unpredictable and not dose-dependent adverse effects, often related to immune reactions or idiosyncratic responses; an example is anaphylaxis with penicillin.

Side Effects

• Side effects are predictable and often unavoidable secondary effects produced at a usual therapeutic dose.
• An example of a beneficial side effect is drowsiness from antihistamines being used to aid sleep.

Toxic Effects

• Toxic effects are adverse effects that occur due to excessive dosing or accumulation of a drug.
• It is vital not to exceed 4g of paracetamol in 24 hours due to the risk of liver damage.
• Early intervention in paracetamol overdose is important to prevent liver damage.
• Key principles of paracetamol overdose management include administering acetylcysteine to protect the liver.

Physical Dependence

• Physical dependence is a state in which the body has adapted to prolonged drug exposure, such that withdrawal symptoms occur if the drug is stopped.
• Withdrawal symptoms are signs that someone is physically dependent on the drug.
• Physical dependence differs from addiction as addiction involves compulsive drug-seeking behavior despite harm.

Allergic Reactions

• A defining criterion of an allergic drug reaction is that it involves an immune response.
• Clinical manifestations of allergic drug reactions include rash, hives, itching, swelling, and anaphylaxis.
• Nursing considerations for managing serious allergic drug reactions include stopping the drug, administering oxygen, epinephrine, antihistamines, and corticosteroids.

Drug-Drug Interactions

• Drug-drug interactions occur when one drug affects the way another drug acts on the body.
• Drug-drug interactions can occur due to changes in drug metabolism.

Cytochrome P450 Enzyme Inhibitors

• Taking a drug that inhibits cytochrome P450 enzymes in the liver increases the concentration of other drugs metabolized by those enzymes, examples of inhibitors are ketoconazole and grapefruit juice.
• The clinical implication is an increased risk of drug toxicity.

Cytochrome P450 Enzyme Inducers

• Taking a drug that induces cytochrome P450 enzymes in the liver decreases the concentration of other drugs metabolized by those enzymes, examples of inducers are rifampin and St. John's Wort.
• The clinical implication is a reduced therapeutic effect of the other drug.

Drug-Food Interactions

• The absorption of some drugs increases when taken with meals because certain foods can enhance the solubility or absorption of the drug; an example is griseofulvin.
• The absorption of some drugs decreases when taken with meals because food can bind to the drug, or delay gastric emptying; an example is tetracycline with dairy products.