Introduction to Pharmacology and Pharmacokinetics

TL;DR

Pharmacology is all about how drugs interact with your body. Pharmacokinetics (PK) describes what your body does to the drug, covering its journey from entry to exit. Understanding PK helps us predict how much drug is needed and how often to give it for the best effect.

1. The Mental Model

Think of a drug as a guest arriving at a party (your body). Pharmacology is the study of how that guest interacts with other guests and the venue. Pharmacokinetics specifically tracks the guest's entire journey: how they get in, move around, get changed, and finally leave.

2. The Core Material

Pharmacology is the broad study of drugs, including their origin, chemistry, effects, and uses. Within pharmacology, we often divide drug action into two main areas:

• Pharmacodynamics (PD): What the drug does to the body. This involves how drugs interact with specific targets (like receptors) to produce a therapeutic effect or side effects. We'll cover this more in depth later.
• Pharmacokinetics (PK): What the body does to the drug. This is the journey a drug takes through your body. It's often summarized by the acronym ADME: Absorption, Distribution, Metabolism, and Excretion.

Let's break down each part of ADME:

2.1 Absorption

This is how a drug gets from where it's administered (e.g., swallowed, injected) into the bloodstream.

• Oral (PO) drugs: Need to dissolve in the GI tract, then pass through the intestinal wall and liver before reaching general circulation. This process can be slow and incomplete.
• Intravenous (IV) drugs: Go directly into the bloodstream, so absorption is 100% and immediate.
• Other routes: Injections (intramuscular, subcutaneous), transdermal patches, inhaled medications – each has its own absorption characteristics.
• Bioavailability: This is the fraction of the administered drug that actually reaches the systemic circulation unchanged. For IV drugs, bioavailability is 100% (or 1). For oral drugs, it's often much less due to incomplete absorption and "first-pass metabolism."

2.2 Distribution

Once in the bloodstream, the drug needs to travel to its site of action.

• Blood flow: Organs with high blood flow (heart, liver, kidneys, brain) receive drugs more quickly.
• Tissue binding: Drugs can bind to proteins in the blood (like albumin) or accumulate in certain tissues (like fat). Only unbound (free) drug can exert an effect or be metabolized/excreted.
• Barriers: Some areas, like the brain (blood-brain barrier) and placenta, have barriers that restrict drug entry, protecting them but also making treatment harder for some conditions.
• Volume of Distribution (Vd): This is a hypothetical volume that describes how widely a drug distributes in the body. A high Vd means the drug spreads out into tissues, while a low Vd means it stays mainly in the blood. It helps determine the loading dose.

2.3 Metabolism (Biotransformation)

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This is the process of chemically altering a drug, primarily in the liver, to make it easier to excrete.

• Main goal: Convert lipid-soluble drugs (which are hard to excrete) into more water-soluble compounds.
• Phases:
  • Phase I reactions: Introduce or unmask polar groups (e.g., oxidation, reduction, hydrolysis). Often involves the cytochrome P450 (CYP450) enzyme system. These can activate or inactivate drugs.
  • Phase II reactions: Attach polar molecules (conjugation) to the drug or its Phase I metabolite, making it even more water-soluble.
• First-pass metabolism: For orally administered drugs, a significant portion can be metabolized by liver enzymes before reaching general circulation. This reduces bioavailability.

2.4 Excretion

This is the removal of the drug and its metabolites from the body.

• Kidneys (renal excretion): The most common route. Drugs are filtered by the glomeruli, secreted by tubules, and some are reabsorbed. Kidney function is crucial here.
• Liver/Bile (biliary excretion): Some drugs are excreted into bile, then pass into the feces.
• Other routes: Lungs (volatile anesthetics), sweat, tears, breast milk.
• Clearance (CL): A measure of the body's ability to eliminate a drug. It's the volume of blood cleared of drug per unit of time (e.g., mL/min).
• Half-life (t½): The time it takes for the concentration of a drug in the plasma to reduce by half. This helps determine dosing intervals.

Here's a visual summary of the ADME process:

graph TD
    A["Drug Administration (Oral, IV, etc.)"] --> B{{"Absorption"}}
    B --> C["Systemic Circulation (Bloodstream)"]
    C --> D{{"Distribution"}}
    D --> E["Target Tissues (Effect)"]
    D --> F{{"Metabolism (Liver, etc.)"}}
    F --> G["Metabolites"]
    C --> G
    G --> H{{"Excretion (Kidneys, Bile, etc.)"}}
    H --> I["Drug Eliminated from Body"]

3. Worked Example

Let's consider a patient receiving a new oral medication.

Scenario: A 60-year-old patient is prescribed Drug X, an oral medication for hypertension. The drug has a known bioavailability of 50%, a half-life of 8 hours, and is primarily metabolized by the liver.

Problem: How does the patient's body handle this drug, and what does the bioavailability mean?

Explanation:

1. Administration & Absorption: The patient takes Drug X orally. It dissolves in their stomach and intestines. Let's say 70% of the drug is absorbed from the gut into the portal vein.
2. First-Pass Metabolism: Before this absorbed drug reaches the general circulation, it travels directly to the liver via the portal vein. The liver metabolizes a significant portion of it. If 30% of the absorbed drug is metabolized in the first pass, then only 70% of that 70% (0.70 * 0.70 = 0.49 or 49%) makes it into the systemic circulation. This aligns with the stated 50% bioavailability.
3. Distribution: Once in the bloodstream, Drug X distributes throughout the body, including to the blood vessels where it exerts its blood-pressure-lowering effect. Some might bind to plasma proteins.
4. Metabolism (Continued): The liver continues to metabolize the drug circulating in the blood, converting it into inactive (or sometimes active) metabolites.
5. Excretion: The kidneys then filter these metabolites (and any remaining unchanged drug) out of the blood, and they are excreted in urine.
6. Half-life Impact: With an 8-hour half-life, if the patient has a peak plasma concentration of 100 ng/mL, after 8 hours, it will be ~50 ng/mL. After another 8 hours (total 16 hours), it will be ~25 ng/mL, and so on. This helps determine that a once or twice daily dosing schedule might be appropriate to maintain therapeutic levels.

4. Key Takeaways

• Pharmacology is the study of how drugs interact with living systems.
• Pharmacokinetics (PK) describes the journey of a drug in the body: Absorption, Distribution, Metabolism, and Excretion (ADME).
• Bioavailability is the fraction of drug reaching systemic circulation, which is 100% for IV drugs but often lower for oral drugs due to absorption and first-pass metabolism.
• The liver is the primary site of drug metabolism, converting drugs into more water-soluble forms for excretion.
• The kidneys are the main route of drug excretion, removing drugs and their metabolites from the body.
• Half-life (t½) tells you how long it takes for drug concentration to halve, influencing how often a drug needs to be taken.

Common Mistakes to Avoid:
- Confusing pharmacokinetics (what the body does to the drug) with pharmacodynamics (what the drug does to the body).
- Assuming all drugs are 100% absorbed and reach their target equally well, especially for oral medications.
- Forgetting the significant impact of first-pass metabolism on oral drug bioavailability.
- Underestimating the role of liver and kidney function; impaired function can drastically alter drug levels and effects.

5. Now Try It

Imagine you're reviewing a new drug for a patient with kidney disease. This drug is primarily renally excreted and has a half-life of 12 hours in someone with normal kidney function. What might you predict about the drug's half-life and concentration levels in your patient, and why? What's one practical implication for their dosing?

What success looks like: You should be able to explain that the half-life would likely be longer and drug concentrations higher in the patient with kidney disease, leading to a need for a reduced dose or less frequent dosing to prevent accumulation and toxicity.