When a medicine enters the body, it doesn’t simply “work.” It interacts with specific proteins, triggers cellular signals, and produces measurable effects. Understanding how drugs act, why side effects occur, and how new medicines are developed is the heart of general pharmacology.
UNIT 2 focuses on pharmacodynamics, drug safety, interactions, and drug discovery, giving students a complete picture of how therapies are designed, evaluated, and approved.
Pharmacodynamics: What the Drug Does to the Body
Principles and Mechanisms of Drug Action
Pharmacodynamics studies the biological effects of drugs and their mechanisms. Most drugs act by binding to receptors or enzymes to alter normal physiological functions.
Mechanisms Include
- Receptor activation or blockade
- Enzyme inhibition
- Ion channel modulation
- Signal pathway alteration
These interactions determine therapeutic and toxic effects.
Receptor Theories and Classification
Receptor Concept
A receptor is a specialized protein that recognizes and binds a drug to produce a response.
Classification of Receptors
G-Protein–Coupled Receptors (GPCRs)
Respond through second messengers like cAMP.
Examples: adrenergic and muscarinic receptors.
Ion Channel Receptors
Open or close channels to allow ion flow.
Fast action, common in nervous system signaling.
Transmembrane Enzyme-Linked Receptors
Activate enzymes such as tyrosine kinase.
Important in growth factor signaling.
JAK-STAT Receptors
Transmit signals directly to nucleus through phosphorylation pathways.
Common in cytokine signaling.
Intracellular Receptors (Transcription Regulators)
Lipophilic drugs enter cells and regulate gene expression.
Examples include steroid hormones.
Understanding receptor types helps predict drug action speed and duration.
Drug–Receptor Interactions
Agonists and Antagonists
- Agonists activate receptors
- Antagonists block effects
Regulation of Receptors
Repeated drug use may cause:
- Up-regulation
- Down-regulation
- Desensitization
These changes affect responsiveness.
Dose–Response Relationship
Dose–response curves illustrate the relationship between dose and effect.
Important Terms
- Potency
- Efficacy
- EC50
- Maximum effect
Higher dose does not always mean better effect.
Therapeutic Index
Therapeutic index measures safety margin:
TI = Toxic dose / Effective dose
Higher TI indicates safer drugs.
Combined Effects of Drugs
When drugs are used together:
- Additive effect
- Synergism
- Antagonism
Understanding these effects helps optimize therapy.
Factors Modifying Drug Action
Drug response varies due to:
- Age
- Body weight
- Genetics
- Disease
- Diet
- Environment
Personalized medicine considers these factors.
Adverse Drug Reactions (ADRs)
ADRs are unwanted or harmful effects of drugs.
Types
- Type A (dose-dependent)
- Type B (allergic or idiosyncratic)
- Chronic effects
- Delayed effects
Monitoring ADRs ensures patient safety.
Drug Interactions
Pharmacokinetic Interactions
Affect absorption, distribution, metabolism, or excretion.
Examples:
- Enzyme induction
- Enzyme inhibition
- Protein binding displacement
Pharmacodynamic Interactions
Occur when drugs influence each other’s effects at receptors or systems.
May cause:
- Increased toxicity
- Reduced efficacy
Careful prescribing prevents harmful combinations.
Drug Discovery and Clinical Evaluation
Developing a new drug is a long and scientific process.
Drug Discovery Phase
Includes:
- Target identification
- Lead compound discovery
- Optimization
Computational chemistry accelerates this stage.
Preclinical Evaluation
Conducted on animals and cell cultures to test:
- Safety
- Toxicity
- Pharmacokinetics
- Pharmacodynamics
Only safe candidates move forward.
Clinical Trial Phases
Phase I
Safety testing in healthy volunteers.
Phase II
Efficacy testing in small patient groups.
Phase III
Large-scale confirmation of safety and efficacy.
Phase IV
Post-marketing surveillance.
Pharmacovigilance
Continuous monitoring of drugs after approval.
Helps detect rare side effects and ensures long-term safety.
Why These Concepts Matter in Pharmacy
Understanding pharmacodynamics and drug evaluation helps:
- Predict drug effects
- Avoid adverse reactions
- Improve safety
- Design better therapies
This knowledge is essential for rational drug use.