Biopharmaceutics is a vital discipline that bridges pharmaceutical sciences and clinical therapeutics. It explains how the physicochemical properties of drugs, the dosage form, and the route of administration influence the rate and extent of drug absorption and distribution in the body. Unit 1 introduces the foundational concepts of biopharmaceutics, focusing on drug absorption and distribution—two key determinants of therapeutic success. This article presents these principles in a clear, news-style educational format for pharmacy students.
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What Is Biopharmaceutics?
Concept and Importance
Biopharmaceutics is the study of the relationship between the drug’s formulation and its biological availability. It helps explain why the same drug may produce different therapeutic outcomes when administered in different dosage forms or via different routes.
Understanding biopharmaceutics enables pharmacists to design effective formulations, predict drug behavior in the body, and ensure consistent therapeutic outcomes.
Drug Absorption: Entry of Drugs into Systemic Circulation
Definition of Absorption
Drug absorption is the process by which a drug moves from the site of administration into the systemic circulation. For most orally administered drugs, absorption occurs primarily in the gastrointestinal tract (GIT), although non-peroral routes also play an important role.
Mechanisms of Drug Absorption Through the GIT
Passive Diffusion
Passive diffusion is the most common mechanism of drug absorption. Drugs move across biological membranes from a region of higher concentration to lower concentration without energy expenditure. Lipid-soluble, non-ionized drugs cross membranes more easily, making this mechanism highly dependent on lipophilicity and ionization state.
Facilitated Diffusion
Facilitated diffusion involves carrier proteins but does not require energy. The process is saturable and selective. Certain drugs structurally similar to endogenous substances utilize this mechanism for absorption.
Active Transport
Active transport requires energy and specific carrier proteins. Drugs absorbed by this mechanism can move against a concentration gradient. This pathway is especially important for drugs resembling natural nutrients, such as amino acids or vitamins.
Endocytosis and Pinocytosis
Large or highly polar molecules, including some peptides and proteins, may be absorbed via endocytosis. Although limited in extent, this mechanism is clinically significant for certain specialized drugs.
Factors Influencing Drug Absorption Through the GIT
Physicochemical Factors
Drug solubility, particle size, ionization (pKa), and lipid solubility strongly influence absorption. Weakly acidic drugs are better absorbed in the stomach, while weak bases are absorbed more efficiently in the intestine.
Physiological Factors
Gastric emptying time, intestinal motility, pH variation, blood flow, and surface area significantly affect absorption. The large surface area of the small intestine makes it the primary site for drug absorption.
Dosage Form and Formulation Factors
The formulation type—tablet, capsule, suspension, or solution—affects dissolution rate and availability. Modified-release formulations are designed to control absorption rate for prolonged therapeutic action.
Absorption from Non-Peroral Extra-Vascular Routes
Parenteral Routes
Intramuscular and subcutaneous routes bypass the GIT and first-pass metabolism. Drug absorption from these routes depends on blood flow, muscle mass, and formulation properties.
Transdermal and Topical Routes
Transdermal delivery provides controlled drug release through the skin, avoiding first-pass metabolism. Lipophilic drugs with low molecular weight are ideal candidates for this route.
Pulmonary and Nasal Routes
Drugs administered via inhalation or nasal sprays are rapidly absorbed due to large surface area and rich blood supply, making these routes useful for quick therapeutic action.
Drug Distribution: Transport of Drugs Throughout the Body
Definition of Distribution
Distribution refers to the reversible transfer of drugs from systemic circulation to tissues and organs. It determines the concentration of drug at the site of action, influencing therapeutic and toxic effects.
Tissue Permeability of Drugs
Role of Biological Barriers
Drug distribution is influenced by membrane permeability and specialized barriers such as the blood–brain barrier and placental barrier. Lipid-soluble and unionized drugs cross these barriers more easily than polar compounds.
Binding of Drugs in the Body
Plasma Protein Binding
Many drugs bind reversibly to plasma proteins such as albumin and α-1 acid glycoprotein. Only the free (unbound) fraction is pharmacologically active and capable of crossing membranes.
Tissue Protein Binding
Drugs may also bind to tissue proteins, affecting storage and duration of action. Strong tissue binding can prolong drug effect and delay elimination.
Apparent Volume of Drug Distribution (Vd)
Concept and Clinical Relevance
The apparent volume of distribution is a theoretical value representing the extent of drug distribution in the body. A high Vd indicates extensive tissue distribution, while a low Vd suggests confinement to plasma.
Factors Affecting Protein–Drug Binding
Drug-Related Factors
Drug concentration, affinity for proteins, and chemical structure influence binding extent.
Patient-Related Factors
Age, disease states (liver or kidney disease), nutritional status, and concurrent drug therapy alter protein binding and free drug concentration.
Kinetics and Clinical Significance of Protein Binding
Binding Kinetics
Protein binding is usually reversible and follows the law of mass action. Competition between drugs for binding sites can increase free drug levels, leading to toxicity.
Clinical Significance
Protein binding affects drug distribution, elimination, drug interactions, and dosage adjustments. Highly protein-bound drugs require careful monitoring, especially in patients with altered plasma protein levels.