Design, Formulation, And Pharmacological Evaluation of Novel Lipid-Based Nanocarriers for Targeted Delivery of Synthesized Anti-Inflammatory Agents

Novel drug delivery systems (NDDS) have opened new avenues for the development of nano- and micro-formulations, offering solutions to the challenges posed by poorly soluble and poorly permeable drugs, particularly those classified under Biopharmaceutical Classification System (BCS) Classes II and IV. These advanced systems aim to deliver drugs directly to the target site in low concentrations, thereby enhancing therapeutic efficiency. NDDS encompasses a wide variety of formulations, including microparticles, nanoparticles, and lipid-based carriers such as liposomes, niosomes, phytosomes, micelles, hydrogels, quantum dots, nanotubes, and dendrimers. Nanoparticulate systems, typically ranging from 1 to 100 nm in size, facilitate improved drug movement across biological barriers due to their nanoscale properties. This advancement has led to a broad spectrum of applications in both treatment and diagnostics. Lipid-based drug delivery systems, including liposomes, niosomes, and micelles, have gained considerable popularity and are FDA-approved. These lipid-based systems have proven to be effective for the delivery of natural phytoconstituents and inorganic particles like gold. One of the major advantages of lipid-based NDDS is their compatibility with a wide range of drugs, improving their solubility, permeability, and bioavailability.

Reasons for the Use of NDDS for BCS Class II and IV Drugs:

1. Poor Solubility and Permeability: Drugs in BCS Classes II and IV typically exhibit poor solubility and permeability, which hinder their therapeutic effectiveness.
2. Increased Surface Area: Reducing particle size increases the surface area, thereby improving the dissolution rate of poorly soluble drugs.
3. Unique Properties of Nanomaterials: Nanomaterials exhibit distinct optical, electrical, chemical, and physical properties, making them versatile for use in medical and biological applications.
4. Enhanced Bioavailability: The mobility of nanoparticles helps improve the bioavailability of drugs.
5. Targeted and Controlled Delivery: Nanomaterials are ideal for the targeted and controlled delivery of biopharmaceuticals.
6. Improved Membrane Penetration: Due to their nanosized structure, these materials can easily cross mucosal membranes, while microsystems can traverse epithelial linings.
7. Increased Efficacy and Reduced Side Effects: The ability to target specific sites enhances therapeutic efficacy while minimizing side effects.
8. Protection from First-Pass Metabolism: Nanosized formulations protect drugs from enzymatic degradation and first-pass metabolism.

Solubility and Permeability Considerations

Solubility is a crucial factor that directly influences drug activity and bioavailability. Several factors affect the solubility of drugs, such as the drug’s pKa, the pH of the gastrointestinal tract (GIT), and the luminal pH. Physiological and physicochemical properties of the drug also play an important role in solubility. According to the United States Pharmacopeia (USP 38) and the European Pharmacopoeia, solubility is categorized into seven distinct groups. The Biopharmaceutics Classification System (BCS), introduced by Amidon et al. in 1995, classifies drugs based on their solubility and permeability characteristics. This system is widely used for the development of immediate-release oral dosage forms. BCS Class II and IV drugs, which face significant challenges related to solubility and permeability, require specialized formulation strategies to enhance their therapeutic potential.

The development of effective drug formulations is often hindered by challenges such as low aqueous solubility, limited permeability, poor absorption, significant first-pass metabolism, systemic degradation, and the activity of efflux transporters like P-glycoprotein. These factors greatly impact the clinical performance of many therapeutic agents. To address these limitations, researchers have explored various advanced drug delivery approaches, including lipid-based systems, polymer-based carriers, nanocarriers, nanocrystals, liquisolid technology, and solid dispersions (Figure 1). Among these, lipid-based nanoparticulate formulations have shown significant promise due to their ability to enhance solubility, improve bioavailability, and bypass metabolic barriers.

Liposomes

Liposomes are spherical vesicular structures composed of amphiphilic phospholipids, which have the unique ability to encapsulate both hydrophilic and hydrophobic drug molecules. Due to their amphiphilic nature, these lipids can self-assemble into bilayered vesicles, making liposomes highly versatile carriers for various therapeutic agents.

Mechanism of Liposome Formation

Liposome formation begins by introducing lipid components into an aqueous environment. Through hydrophobic and hydrophilic interactions—either between lipid molecules themselves or between lipids and water—bilayer structures are formed (Figure 2). These bilayers can then organize into vesicles under the influence of external energy sources such as sonication, homogenization, heating, or freeze-thaw cycles. This energy input helps in shaping and stabilizing the vesicular structures for effective drug delivery.

A. Classification of Liposomes Based on Size and Lamellarity

Liposomes can be categorized based on their size, number of lipid bilayers, and structural arrangement. One major classification divides them into multilamellar and unilamellar vesicles:

• Multilamellar Vesicles (MLVs): These are typically larger than 0.5 µm and consist of multiple concentric lipid bilayers. When the number of bilayers exceeds five, the vesicles are referred to as multilamellar.
• Oligolamellar Vesicles: These are similar in structure to multilamellar vesicles but contain fewer bilayers—generally between 2 and 5.
• Unilamellar Vesicles (ULVs): These liposomes consist of a single lipid bilayer surrounding the aqueous core. They are further divided based on size:
  • Small Unilamellar Vesicles (SUVs): Generally less than 100 nm in diameter.
  • Large Unilamellar Vesicles (LUVs): Larger than SUVs, typically above 100 nm.

Although unilamellar vesicles share the same structural design, they differ primarily in their size, which influences drug loading and release characteristics.

Classification of Liposomes

Liposomes can be classified based on size and lamellarity, lipid composition, and preparation methods. Each classification provides insight into their structure, behavior, and potential therapeutic applications.

A. Based on Size and Lamellarity

Liposomes vary in size and the number of lipid bilayers they contain. These characteristics influence drug loading, release profiles, and cellular uptake.

The composition of the liposome determines its functionality, stability, and suitability for specific drug delivery applications.

Several techniques have been developed to prepare liposomes, each affecting the vesicle size, lamellarity, and entrapment efficiency.

Niosomes are vesicular drug delivery systems formed from non-ionic surfactants, often considered as alternatives to liposomes. These vesicles are composed primarily of surfactants like fatty alcohols, esters, or block copolymers, along with cholesterol to stabilize the structure.44,45 The surfactants used in niosomal formulations have both hydrophilic and hydrophobic regions, enabling them to self-assemble into bilayered vesicles. Based on the nature of their head groups, surfactants can be classified as anionic, cationic, amphoteric, or non-ionic. Among these, non-ionic surfactants are preferred due to their low toxicity, greater stability, and enhanced biocompatibility.46

• Enhance bioavailability of poorly absorbed drugs.
• Improve solubility and permeability, especially via M-cell mediated transcytosis in Peyer’s patches of the intestine.
• Enable controlled and sustained drug release.
• Can be easily modified due to the presence of both hydrophilic and lipophilic regions.

Limitations of Niosomes

• Prone to physical instability (e.g., fusion or aggregation).
• Potential hydrolysis of the encapsulated drug.
• Leakage or leaching of the encapsulated contents over time.

Solid Lipid Nanoparticles (SLNs)

Solid Lipid Nanoparticles (SLNs) are developed to address limitations of traditional colloidal drug delivery systems such as liposomes, emulsions, and polymeric nanoparticles. SLNs are composed of physiological lipids—including triglycerides and glycerides of fatty acids—which provide excellent biodegradability and biocompatibility.51

These systems offer several advantages over earlier nanoparticle technologies, such as:

• Simplified manufacturing processes
• Higher entrapment efficiency
• Improved physical stability
• Easier scalability for industrial production

Key Ingredients Used in SLN Formulation

• Lipids: Triglycerides, partial glycerides
• Fatty acids
• Steroids
• Waxes

(Figure 4: Illustration of Different Methods for SLN Preparation)

Various preparation techniques for SLNs include:

• High-pressure homogenization
• Microemulsion method
• Solvent emulsification-evaporation
• Ultrasonication
• Double emulsion method
• Spray drying and lyophilization

Solid Lipid Nanoparticles (SLNs)

Solid Lipid Nanoparticles (SLNs) are emerging as a promising drug delivery system to overcome the limitations of conventional colloidal carriers like emulsions, polymeric nanoparticles, and liposomes. They are formulated using physiological lipids (e.g., glycerides, fatty acids), ensuring biocompatibility, biodegradability, and reduced toxicity. SLNs provide enhanced drug entrapment efficiency, physical stability, and easier scalability in pharmaceutical manufacturing.51

A. High-Pressure Homogenization (HPH)

High-pressure homogenization is a widely used industrial method for SLN production. It involves forcing the lipid phase through a narrow gap at high pressure, which leads to particle size reduction. The method is divided into:

I. Hot Homogenization Method

In this approach, the lipid is melted above its melting point, and the drug is dissolved in the molten lipid. The lipid phase is then mixed with a hot aqueous phase containing surfactants, forming a pre-emulsion. The mixture is then homogenized at high pressure to form SLNs.53

II. Cold Homogenization Method

For heat-sensitive drugs, cold homogenization is preferred. Here, the drug-lipid mixture is first solidified using liquid nitrogen or dry ice, then ground into a fine powder and dispersed in a cold aqueous surfactant solution. This dispersion is homogenized at or below room temperature.53

In this method, lipophilic materials are dissolved in an organic solvent and emulsified in an aqueous phase to form an oil-in-water (o/w) emulsion. Mechanical stirring facilitates solvent evaporation, leading to lipid precipitation as solid nanoparticles. The polarity of the organic and aqueous phases must be opposite for successful emulsion formation.58,59

• Requires high concentrations of surfactants for small particle formation.
• Time and energy-intensive process.
• Use of non-biocompatible solvents may necessitate further purification.

C. Solvent Emulsification Diffusion Technique

This method involves forming a suspension from a partially water-miscible emulsion, followed by diffusion of the solvent into the aqueous phase, leading to precipitation of lipid nanoparticles. Solvents commonly used include ethyl acetate, butyl lactate, benzyl alcohol, isopropyl acetate, etc. The miscibility of the solvent in water is a critical factor in this method.

Table 9: SLNs Prepared by Solvent Emulsification Diffusion

The process begins with the addition of the organic phase into the aqueous phase, resulting in the formation of an oil-in-water (o/w) emulsion. This emulsion is then diluted with water. Under continuous stirring by a mechanical agitator, the drug dissolved in the organic solvent rapidly solidifies as the organic solvent diffuses from the dispersed droplets into the surrounding aqueous phase. This solvent diffusion leads to the formation of hollow spherical nanoparticles (illustrated in Figures 7 and 8).