Pharmacokinetics

Pharmacokinetics & Pharmacodynamics

  • Presented by Michael Fraser, PA-C

Definitions

  • Pharmacology: Study of substances interacting with living systems.

  • Toxicology: Study of undesirable chemical effects on living things.

  • Pharmacy: Practice of preparation and dispensing medications.

Drug Classifications

Brand Name

  • Medication name owned by a manufacturer (e.g., Prozac).

Generic

  • Comparable to brand in terms of strength, form, and intended use.

  • Marketed under its chemical name (e.g., fluoxetine).

  • Cost-effective, can reduce costs up to 85% when competition arises.

Formulary

  • Preferred list of brand and generic medications covered by insurance.

  • Co-pays determined by tier levels; some require prior approval.

Pharmacodynamics

  • Definition: Study of drug interactions with living tissues.

  • Focus: Effects the drug has on the body over time.

    • What the drug does to the body and how it works.

Pharmacokinetics

  • Definition: Study of drug movement through the body.

  • Key Processes: Absorption, Distribution, Metabolism, Excretion.

    • What the body does to the drug.

Factors Determining Drug Action

  • Amount reaching the site of action.

  • Route of administration.

  • Activity at the site.

  • Tissue concentration.

  • Possible drug-food interactions.

Absorption

  • Definition: Passage of the drug across cell membranes into the blood.

  • Factors: Dosage form, solubility, pH, blood flow to the site, contact time.

Types of Drug Transport

  • Diffusion: Acid/base transport.

    • Acidic drugs absorbed in the stomach; basic drugs absorbed in the small intestine.

Factors Affecting GI Absorption

  • GI motility, pH, surface area, blood flow, food presence, drug composition.

Routes of Administration

  • Enteral: Orally (PO), Sublingual (SL), Rectally (PR).

  • Parenteral: Intravenously (IV), Intramuscular (IM), Subcutaneous (SQ or SC), Topical, Transdermal.

Route Considerations

  • Route based on patient conditions; IV and IM often used for rapid action.

  • SL and Transdermal are viable for those unable to swallow pills.

  • Rectal options assist with nausea/vomiting scenarios.

Advantages/Disadvantages of PO Administration

  • Advantages: Cost-effective, easy, self-administered.

  • Disadvantages: Possible destruction by stomach acid, first-pass effect, not tolerated by nauseated patients.

Examples of PO Administered Drugs

  • Birth control pills, antibiotics.

Advantages/Disadvantages of SL Administration

  • Advantages: Rapid absorption, bypassing of the stomach, no first-pass effect.

  • Disadvantages: Must be lipid soluble.

Examples of SL Administered Drugs

  • Nitroglycerin, Buprenorphine, Vitamin B12.

Advantages/Disadvantages of PR Administration

  • Advantages: Rapid absorption, good in nausea/vomiting cases, no first-pass effect.

  • Disadvantages: Unpleasant for patients, must be lipid soluble.

Example of PR Drug

  • Compazine suppositories.

IV Administration

  • Advantages: Very fast, 100% bioavailability, precise control.

  • Disadvantages: Difficult overdose management, pain at the site, required skilled care.

Example of IV Drugs

  • Heparin, antibiotics.

IM and SQ Administration

IM

  • Advantages: Fast absorption, sustained release possible.

  • Disadvantages: Slow absorption if improperly injected.

  • Examples: Vaccines, Toradol.

SQ

  • Advantages: Fast absorption, sustained release.

  • Disadvantages: Pain at injection site, risk of lipodystrophy (dents in tissue).

Examples of SQ Drugs

  • Insulin, B12.

Topical and Transdermal Administration

Topical

  • Advantages: Direct delivery, minimizes side effects.

  • Disadvantages: Local irritation, potential difficulty in application.

Transdermal

  • Advantages: Systematic application, useful for NPO patients.

  • Disadvantages: Requires lipid-soluble drugs, variability in absorption due to biological factors.

Pharmacokinetics Parameters

  • Clearance: Rate of drug removal from the body (mL/min).

  • Volume of Distribution: Hypothetical estimate of drug distribution.

  • Half-life: Time for 50% of a drug to be eliminated; influences side effects.

Calculation of Volume of Distribution

  • Dependent on drug route, blood flow, permeability, and protein binding.

Loading Dose

  • Purpose: Achieve therapeutic levels quickly.

  • High initial dose followed by maintenance dose.

Drug Metabolism

  • Definition: Chemical alteration of drugs mainly in the liver and small intestine.

First Pass Effect

  • Extensive liver metabolism affecting bioavailability; critical in oral drugs.

  • Consideration for elderly, GI issues, or liver disease.

Cytochrome P450

  • Importance in drug metabolism; interactions with foods and other drugs can modify efficacy.

Excretion

  • Pathways: Via urine, lungs, feces, saliva; mainly through kidneys.

Creatinine Clearance

  • Indicator of kidney function; crucial for drug dosing adjustments.

Drug Reactions

  • Types: Predictable side effects, toxic effects, and allergic reactions.

  • Secondary Effects on other systems may require medication changes.

Clinical Trials Phases

  • Phase I: Safety evaluation on small group.

  • Phase II: Expanded study for effectiveness.

  • Phase III: Larger studies for comparison; if satisfactory, drug approval.

Pharmacoeconomics

  • Evaluation of medication cost versus clinical efficacy; focus on generics to manage escalating costs.

Prescribing Regulations

  • PAs can prescribe in 50 states, with specific legislation in New York on controlled substances.

Controlled Substance Categories

  • Schedule I: High abuse potential, no medical use.

  • Schedule II: High abuse potential, secure storage required.

  • Schedule III-V: Gradual decrease in abuse potential.

Questions?

  • Focus on the problem but remain open to potential solutions in pharmacology.