The evolution of drug delivery science has moved steadily toward precision, safety, and improved therapeutic outcomes. Traditional drug delivery systems often expose the entire body to a drug, leading to unwanted side effects and reduced efficiency. Targeted drug delivery systems address these limitations by directing drugs specifically to diseased tissues or cells. Unit 4 focuses on the principles, approaches, advantages, and limitations of targeted drug delivery, along with an introduction to advanced carrier systems such as liposomes, niosomes, nanoparticles, and monoclonal antibodies.
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Concept of Targeted Drug Delivery
Targeted drug delivery refers to the selective delivery of a drug to a specific site of action while minimizing exposure to healthy tissues. The primary goal is to achieve maximum therapeutic effect with minimal systemic toxicity. This approach is especially valuable in the treatment of cancer, autoimmune disorders, infectious diseases, and chronic conditions requiring long-term therapy.
Unlike conventional dosage forms, targeted systems rely on biological, chemical, or physical mechanisms to localize the drug at the desired site. These systems often exploit differences between normal and diseased tissues, such as receptor expression, pH, enzymatic activity, or blood supply.
Approaches to Targeted Drug Delivery
Passive Targeting
Passive targeting depends on the natural distribution of drug carriers within the body. A classic example is the enhanced permeability and retention (EPR) effect observed in tumor tissues, where leaky blood vessels allow macromolecules and nanoparticles to accumulate selectively. This approach does not require ligand–receptor interaction but relies on physiological differences.
Active Targeting
Active targeting involves attaching specific ligands, such as antibodies, peptides, or sugars, to the drug carrier. These ligands bind selectively to receptors overexpressed on target cells. This method enhances cellular uptake and specificity, making it highly effective in cancer therapy and targeted immunotherapy.
Physical and Chemical Targeting
Some systems utilize external stimuli such as magnetic fields, ultrasound, temperature, or pH changes to control drug localization and release. These strategies allow precise spatial and temporal control over drug delivery.
Advantages and Disadvantages of Targeted Drug Delivery
Advantages
Targeted drug delivery offers several clinical and pharmaceutical benefits. It reduces drug dose requirements, minimizes adverse effects, improves patient compliance, and enhances therapeutic index. By concentrating drugs at the disease site, it allows the use of potent drugs that would otherwise be too toxic for systemic administration.
Disadvantages
Despite its advantages, targeted delivery systems face challenges such as complex formulation design, high production costs, stability issues, and regulatory hurdles. Additionally, variability in biological responses among patients can affect targeting efficiency.
Liposomes: Vesicular Carriers with Proven Clinical Success
Introduction and Structure
Liposomes are spherical vesicles composed of one or more phospholipid bilayers enclosing an aqueous core. They can encapsulate both hydrophilic and lipophilic drugs, making them versatile carriers.
Applications
Liposomes enhance drug stability, reduce toxicity, and improve pharmacokinetics. Liposomal formulations are widely used in cancer therapy, antifungal treatment, and vaccine delivery. Their biocompatibility and ability to be surface-modified for active targeting make them a cornerstone of targeted drug delivery research.
Niosomes: Stable and Cost-Effective Alternatives
Concept and Advantages
Niosomes are non-ionic surfactant-based vesicles similar to liposomes but offer greater chemical stability and lower cost. They are less susceptible to oxidation and hydrolysis compared to phospholipid-based systems.
Therapeutic Use
Niosomes are used for targeted delivery of anticancer agents, anti-inflammatory drugs, and vaccines. Their ability to enhance drug penetration and prolong circulation time makes them suitable for controlled and targeted delivery applications.
Nanoparticles: Nanoscale Precision in Drug Targeting
Introduction to Nanoparticles
Nanoparticles are solid colloidal systems ranging from 10 to 1000 nanometers in size. Drugs can be dissolved, entrapped, or adsorbed onto their surface.
Applications in Targeted Therapy
Nanoparticles offer controlled drug release, improved bioavailability, and site-specific delivery. They are extensively used in cancer therapy, gene delivery, and diagnostic imaging. Surface functionalization with ligands further enhances active targeting capabilities.
Monoclonal Antibodies: Biological Precision Tools
Concept and Mechanism
Monoclonal antibodies are highly specific proteins designed to bind to a single antigen or receptor. In targeted drug delivery, they may be used alone as therapeutic agents or conjugated with drugs, toxins, or radioactive substances.
Clinical Applications
Monoclonal antibodies are widely used in oncology, autoimmune disorders, and infectious diseases. Antibody–drug conjugates combine the specificity of antibodies with the potency of cytotoxic drugs, delivering them directly to diseased cells while sparing healthy tissue.
Applications of Targeted Drug Delivery Systems
Targeted drug delivery systems have transformed modern pharmacotherapy. They are extensively applied in cancer treatment, targeted antibiotic therapy, gene therapy, vaccine delivery, and management of chronic diseases. These systems also play a crucial role in personalized medicine by tailoring therapy based on patient-specific biological markers.