Mechanisms of Drug Action

1. Receptor Binding

Agonists:

Drugs that bind to receptors and mimic the action of endogenous ligands, activating the receptor to produce a response.

Example: Morphine binds to opioid receptors as an agonist, providing pain relief by mimicking endorphins.

Antagonists:

Drugs that bind to receptors but do not activate them, blocking the action of endogenous ligands or agonists.

Example: Naloxone is an opioid antagonist that binds to opioid receptors, reversing opioid overdose by blocking receptor activation.

Partial Agonists:

Drugs that bind to and partially activate receptors, producing a response that is less than that of a full agonist.

Example: Buprenorphine is a partial agonist at opioid receptors, providing pain relief with a lower risk of respiratory depression.

2. Enzyme Inhibition

Competitive Inhibition:

Drugs compete with natural substrates for enzyme binding, inhibiting enzyme activity.

Example: Statins competitively inhibit HMG-CoA reductase, reducing cholesterol synthesis in the liver.

Non-Competitive Inhibition:

Drugs bind to a site other than the enzyme’s active site, altering enzyme structure and function.

Example: Aspirin irreversibly inhibits cyclooxygenase enzymes (COX-1 and COX-2), reducing inflammation and pain by blocking prostaglandin synthesis.

Prodrug Activation:

Some drugs are inactive in their administered form and need enzymatic conversion to become active.

Example: Enalapril is converted to its active form, enalaprilat, which inhibits the angiotensin-converting enzyme (ACE), lowering blood pressure.

3. Ion Channel Modulation

Ion Channel Blockers:

Drugs that block the passage of ions through channels, altering cell excitability and function.

Example: Lidocaine blocks sodium channels, preventing nerve impulse conduction and producing local anesthesia.

Ion Channel Openers:

Drugs that open ion channels, allowing ions to pass through and altering cellular activity.

Example: Minoxidil opens potassium channels, causing vasodilation and lowering blood pressure.

4. Interaction with Transport Systems

Transporter Inhibitors:

Drugs that block specific transport proteins, preventing the movement of substrates across cell membranes.

Example: SSRIs like fluoxetine inhibit the serotonin transporter (SERT), increasing serotonin levels in the brain to alleviate depression.

P-Glycoprotein Modulation:

Some drugs inhibit or stimulate P-glycoprotein (a drug efflux transporter), affecting drug absorption and distribution.

Example: Verapamil inhibits P-glycoprotein, potentially enhancing the bioavailability of co-administered drugs.

5. DNA Interaction

DNA Binding Agents:

Drugs that bind to DNA, inhibiting cell division or causing cell death, often used in cancer treatment.

Example: Cisplatin binds to DNA, interfering with DNA replication and transcription, leading to cancer cell death.

Antisense and siRNA Drugs:

Drugs that target specific RNA sequences to inhibit the synthesis of proteins related to disease.

Example: Fomivirsen is an antisense drug used to treat cytomegalovirus retinitis by binding viral mRNA.

6. Immune System Modulation

Immunosuppressants:

Drugs that inhibit the immune response, often used to prevent transplant rejection or treat autoimmune diseases.

Example: Cyclosporine inhibits calcineurin, reducing T-cell activation and immune response.

Immune Stimulants:

Drugs that enhance immune function, helping the body fight infections or cancer.

Example: Interferons boost the immune response in conditions like hepatitis and certain cancers.

7. Chemical Antagonism

Neutralization of a Substance:

Some drugs interact directly with other chemicals to neutralize or counteract their effects.

Example: Antacids neutralize stomach acid to relieve symptoms of acid reflux or indigestion.

8. Free Radical Scavenging

Antioxidants:

Some drugs act as antioxidants by scavenging free radicals, protecting cells from oxidative damage.

Example: Vitamin C and E are antioxidants that help prevent cellular damage by neutralizing free radicals.