Pharmacokinetics - Drug Absorption: Download PDF, Notes, and PPT
Delve into the fundamental principles of Pharmacokinetics with a focus on Drug Absorption. This downloadable PDF resource provides a comprehensive overview of how drugs enter the systemic circulation from the site of administration. Access detailed notes and related PowerPoint presentations (PPTs) to understand the various mechanisms of absorption, factors influencing it, and its clinical significance. This material is essential for students of pharmacology, medicine, and pharmacy aiming to grasp the ADME (Absorption, Distribution, Metabolism, Excretion) processes.
Our PDF covers topics such as routes of administration, membrane transport mechanisms, bioavailability, and the first-pass effect. Download now to enhance your understanding of this critical aspect of how the body handles drugs, ensuring effective and safe pharmacotherapy.
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Pharmacokinetics: The Journey of a Drug - Focus on Absorption
Pharmacokinetics is the study of "what the body does to a drug," encompassing the processes of Absorption, Distribution, Metabolism, and Excretion (often abbreviated as ADME). Among these, absorption is the initial critical step that determines the onset, intensity, and duration of a drug's action, particularly for drugs administered extravascularly (e.g., oral, intramuscular, subcutaneous). This PDF, titled "Pharmacokinetics-absorption1-1," likely provides a foundational understanding of this complex process.
What is Drug Absorption?
Drug absorption is defined as the process by which a drug moves from its site of administration into the systemic circulation (bloodstream). For a drug to exert its therapeutic effect systemically, it must first be absorbed. The only route of administration that bypasses absorption is the intravenous (IV) route, where the drug is directly introduced into the bloodstream, resulting in 100% bioavailability instantaneously.
Mechanisms of Drug Transport Across Membranes
For absorption to occur, drugs must cross biological membranes, which are primarily lipid bilayers. Several mechanisms facilitate this transport:
- Passive Diffusion: This is the most common mechanism for drug absorption. It involves the movement of a drug across a membrane from an area of high concentration to an area of low concentration, down its concentration gradient. This process does not require energy and is not saturable. Lipid-soluble (lipophilic) and un-ionized drugs readily cross membranes by passive diffusion. The rate of diffusion is governed by Fick's Law, which depends on the concentration gradient, surface area of the membrane, lipid solubility of the drug, and thickness of the membrane.
- Facilitated Diffusion: This process also moves drugs down a concentration gradient but requires the assistance of a carrier protein embedded in the membrane. It does not require energy but is saturable and can be subject to competition by other substances that use the same carrier. Glucose and amino acids are examples of substances transported this way.
- Active Transport: This mechanism involves carrier proteins and requires energy (ATP) to move drugs against their concentration gradient (from low to high concentration). It is saturable, specific, and subject to competition. Many drugs, particularly those structurally similar to endogenous substances, utilize active transport (e.g., L-dopa, iron). P-glycoprotein (P-gp) is an important efflux transporter that actively pumps drugs out of cells, reducing their absorption.
- Pinocytosis and Phagocytosis (Endocytosis/Exocytosis): These processes involve the engulfment of large molecules or particles by the cell membrane. The cell membrane invaginates around the substance, forming a vesicle that is then internalized (endocytosis) or externalized (exocytosis). This is important for the absorption of some large protein drugs or particulate drug delivery systems.
Factors Influencing Drug Absorption
Numerous factors can affect the rate and extent of drug absorption:
- Drug-related factors:
- Physicochemical properties: Lipid solubility, molecular size, degree of ionization (pKa of the drug and pH of the environment), particle size, and formulation (e.g., tablets, capsules, solutions, suspensions). Solutions are generally absorbed faster than suspensions or solid dosage forms.
- Concentration: Higher concentrations generally lead to faster absorption by passive diffusion.
- Patient-related factors (Physiological factors):
- Route of Administration: The chosen route significantly impacts absorption (e.g., oral, topical, intramuscular, subcutaneous, inhalation).
- Absorbing Surface Area and Blood Flow: Larger surface areas (like the small intestine) and higher blood flow at the site of administration enhance absorption. Shock or conditions reducing blood flow can impair absorption.
- Gastrointestinal (GI) Factors (for oral administration):
- pH of GI fluids: Affects drug ionization. Acidic drugs are better absorbed in the stomach (acidic pH), while basic drugs are better absorbed in the intestine (alkaline pH). However, the intestine's vast surface area often makes it the primary site for most drug absorption regardless of pH.
- Gastric emptying time: The rate at which stomach contents move to the intestine. Delayed gastric emptying can slow the absorption of drugs primarily absorbed in the intestine.
- Intestinal motility: Very rapid motility (e.g., diarrhea) can reduce contact time and thus absorption.
- Presence of food: Food can affect absorption variably – it may decrease, delay, increase, or have no effect on the absorption of different drugs.
- GI diseases: Conditions like malabsorption syndromes or inflammatory bowel disease can alter drug absorption.
- First-Pass Metabolism (Presystemic Metabolism): For orally administered drugs, after absorption from the GI tract, they pass through the portal vein to the liver before reaching systemic circulation. Some drugs are extensively metabolized in the intestinal wall or liver during this first pass, significantly reducing the amount of active drug that reaches the systemic circulation. This is known as the first-pass effect.
- Drug Interactions: Co-administration of other drugs can affect absorption (e.g., antacids can alter gastric pH, some drugs can bind to others preventing absorption).
Bioavailability (F)
Bioavailability refers to the fraction or percentage of an administered drug dose that reaches the systemic circulation in an unchanged (active) form. It is denoted by 'F'. For IV administration, F=1 (or 100%). For other routes, F is usually less than 1 due to incomplete absorption and/or first-pass metabolism. Understanding bioavailability is crucial for dose calculations and when switching between different routes of administration or drug formulations.
The study of drug absorption is fundamental to designing effective drug therapies. By understanding the mechanisms and influencing factors, pharmaceutical scientists can develop formulations that optimize drug delivery, and clinicians can make informed decisions to ensure patients receive the intended therapeutic benefits. This PDF on "Pharmacokinetics-absorption1-1" is a valuable resource for building this foundational knowledge.
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