Pharmacokinetics Study Notes

Introduction to Pharmacokinetics

  • Presented By: Sallianne Kavanagh, MPharm, MRPharmS, MSc, Cert Ipresc, MSc, FHEA

Aims of the Session

  • Introduce terminology specific to pharmacokinetics
  • Define pharmacokinetics
  • Identify individual needs for pharmacokinetic considerations
  • Introduce pharmacokinetic principles

Definition of Pharmacokinetics

  • Pharmacokinetics: The effect of the body on a drug.
    • Key Phrases:
    • "What the body does with the drug"
    • "What the body does to the drug"

Key Pharmacokinetic Processes

  • ADME:
    • Absorption: How a drug enters the bloodstream.
    • Distribution: How it spreads throughout the body.
    • Metabolism (Biotransformation): How the body chemically modifies the drug.
    • Excretion: How the drug is eliminated from the body.

Drug Formulation Factors

  • Drug formulation is influenced by:
    • Physical Properties:
    • Solubility, stability, and absorption characteristics.
    • Gastrointestinal Absorption:
    • Conditions that affect absorption like food.
    • Systemic Circulation Needs:
    • Direct access necessary for effectiveness in severe cases.

Routes of Drug Administration

  • Oral Administration (PO): Administered by mouth.
  • Sublingual: Absorbed under the tongue.
  • Buccal: Placed in the buccal cavity until it dissolves.
  • Intravenous (IV): Directly into the bloodstream.
  • Subcutaneous (SC) & Intramuscular (IM): Injected into tissue.
  • Other Routes:
    • Transdermal (skin patches), inhalation, rectal/vaginal, intrathecal, and topical applications.

Special Considerations for Drug Formulations

  • Some drugs must be given via injection (e.g., insulin).
  • Fast-acting formulations may be necessary (e.g., injectable morphine).
  • Longer action drugs can be delivered through sustained-release formulations (e.g., MST).
  • Transdermal Patches: Provide controlled drug releases, minimizing side effects.

Pharmacokinetics and Drug Management

  • Important for:
    • Drug interactions.
    • Disease state interactions.
  • Recommendations:
    • Dosage
    • Frequency
    • Administration route and formulation
  • Ensures safe and effective management of drug therapies.

Relationship Between Dose and Schedule

  • Sufficient drug quantity is necessary for therapeutic effect and is affected by:
    • Amount administered.
    • Concentration achieved.
    • Rate of ADME.

Therapeutic Window

  • Refers to the range of drug doses that can treat disease effectively while minimizing side effects.
  • Plasma drug concentration: Must reach the effective zone without causing toxicity.
  • Illustration:
    • Effective Dose vs. Not Effective Dose

Narrow Therapeutic Window

  • A small increase in the dose can lead to toxicity.
  • Examples of Drugs:
    • Phenytoin: Antiepileptic with variable dose response.
    • Lithium: Mood stabilizer with a narrow dosing range.
    • Cyclosporin: Used to prevent organ rejection in transplants.

Fate of a Drug in the Body

  • Process Flow:
    • Absorption → Distribution → Metabolism → Excretion
    • Involves liver (metabolism) and kidneys (excretion).
  • Outcome: Reaching the target site for pharmacological action while ensuring safe levels in the blood.

Next Steps in Pharmacokinetics

  1. Drug Administration (e.g., oral, parenteral).
  2. Drug absorption across GI mucosa or other sites.
  3. Drug enters systemic circulation.
  4. Distribution of drug to the site of action.
  5. Metabolism and excretion of the drug.