What are Lipid-Based Drug Delivery Systems (LBDDS)?

Lipid-based drug delivery systems are a focal point in pharmaceutical science due to their ability to enhance the solubility, stability, and bioavailability of poorly soluble drugs. These systems offer a promising approach to amplify the therapeutic impact of drugs while minimizing their adverse effects. Lipid-based drug delivery systems encompass a range of formulations utilizing lipids as carriers for drug molecules. These systems can be categorized based on their composition and structure, including liposomes, lipid nanoparticles, solid lipid nanoparticles, nanostructured lipid carriers, and lipid micelles. They offer distinct advantages in terms of drug loading capacity, stability, and release kinetics. Currently, lipid-based drug delivery systems are being extensively investigated for broad applications in drug delivery and therapy. These systems exhibit promising potential in delivering anticancer drugs, anti-inflammatory drugs, antibacterial drugs, and vaccines. Notably, the use of lipid nanoparticles for targeted drug delivery to specific tissues or cells, such as tumors, inflammatory tissues, or infected cells, is an area of active research.

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What is Lipid Based Formulation Classification?

The routes available for administering lipid-based drug delivery systems (LBDDS) include oral, gastrointestinal, ocular, nasal, skin/transdermal, and vaginal routes. Among these, the oral route is the most favored due to its non-invasiveness, cost-effectiveness, and lower likelihood of side effects compared to routes like injections. It is also considered the simplest and most convenient method for chronic treatment. However, during the initial stages of development, formulation strategies based on rational and systematic approaches are crucial to prevent instability and poor correlation between in vitro and in vivo results, thereby enhancing the success rate of formulation development.

Factors Affecting Lipid-based Delivery System

Solubility

Although lipids (fatty acid derivatives) are the primary components of the formulation, one or more surfactants and potentially hydrophilic co-solvents may be necessary to aid in dissolution and enhance dispersion properties. Surfactants are categorized based on their hydrophilic-lipophilic balance (HLB) values. Lower values (≤10) indicate higher lipophilicity, while higher values (≥10) indicate higher hydrophilicity. To guide the formulation design process, most lipids used in these oral formulations have a specified 'required HLB' value, which corresponds to the optimal HLB of the surfactant blend needed to emulsify the oil in water. Various emulsifiers can be employed for different formulations, depending on their HLB values, as outlined in Table 2.

Table 2. Emulsifiers used in lipid-based formulations.

Dispersion

The examination of emulsification and dispersion properties of formulations containing candidate drugs with adequate solubility in aqueous carriers is essential. An initial assessment can be conducted by observing preparations mixed with water under a microscope. Effective emulsification is indicated by vigorous mixing at the water/formulation interface, involving diffusion and stranding mechanisms. Additionally, it is crucial that no drug precipitation occurs once the preparation is fully mixed with the aqueous medium. Utilizing techniques such as laser scattering to measure the particle size of emulsion droplets can be highly beneficial in identifying promising formulations.

Fig. 1. Ternary phase diagram for lipid formulations (J Pharm (Cairo). 2014, 2014: 801820).

The ternary phase diagram is commonly used to identify the emulsification structural type and study the behavior of the preparation during dilution (see Fig. 1). The line connecting points A and B illustrates the dilution process of a mixture initially containing 35% surfactant and 65% oil. This path traverses through the water-in-oil microemulsion and lamellar liquid crystal phases before reaching a stable bicontinuous oil-in-water microemulsion upon further dilution. While constructing the entire phase diagram may not always be necessary, understanding the structural evolution along the dilution path is crucial for ensuring the formation of a stable dispersed structure post-dilution. Combining low and high HLB surfactants typically leads to smaller emulsion droplet sizes compared to using a single surfactant.

Digestion

The impact of intestinal lipases on the behavior of lipid preparations in the gastrointestinal tract is significant and must be considered in the design process. It has been well established that lipids that are non-dispersible but digestible, such as triglycerides, can undergo metabolism by lipases to form mono/diglycerides and fatty acids, which helps emulsify any remaining oil. Therefore, to ensure the necessary small particle size and large surface area for effective drug release, a high quantity of surfactants may not be necessary. In 2000, Pouton introduced a classification system for lipid formulations based on the formulation's composition and the role of digestion in promoting dispersion, as outlined in Table 3.

Table 3. Pouton's classification of lipid-based delivery systems.

Absorption

The ultimate objective of any oral lipid preparation is to ensure the effective absorption of drugs by intestinal mucosal cells. In Fig. 2, the process of lipid-based pharmaceutical preparations in the intestinal environment is illustrated. Initially, the components are dispersed to create lipid droplets (for type I preparations) or emulsion droplets (for type II-III). Subsequently, the digestion products undergo lipolysis and are dissolved by bile acids to form colloidal mixed micelles. It is hypothesized that the drug is then extracted from the mixed micelles of emulsion oil droplets and bile salts and taken up by the mucosal cells of the intestinal wall.

Fig. 2. Intestinal absorption of lipid-based pharmaceutical formulations (J Pharm (Cairo). 2014, 2014: 801820).

Benefits of Lipid-based Delivery Systems

• Drugs are released in a controlled and targeted manner.
• Pharmaceutical stability.
• High and enhanced drug content (compared to other carriers).
• Feasibility of carrying lipophilic and hydrophilic drugs.
• It is biodegradable and biocompatible.
• Auxiliary materials are widely used.
• The versatility of the recipe.
• Low risk status.
• The passive, non-invasive vesicle formation system can be immediately commercialized.

Design Guidelines of Lipid-based Delivery System

Lipid-based formulations will remain crucial for preparing poorly soluble drugs, although designing these formulations can pose challenges. Currently, the seven guidelines for lipid preparation design can be summarized as follows:

1. Maintain drug solubility in the formulation, post-dispersion, and post-digestion.
2. Characteristics of colloidal substances post-processing in the gastrointestinal environment may enhance absorption more than the preparation's characteristics.
3. Higher lipid (> 60%) and lower surfactant (< 30%) and cosolvent (< 10%) proportions generally improve drug dissolution after dilution.
4. Medium-chain triglycerides enhance drug solubility and stability, while long-chain triglycerides are better at forming bile salt lipid colloids, leading to increased bioavailability.
5. IIIB SMEDDS formulation yields smaller droplet sizes post-dispersion, but efficacy is more reliant on surfactant characteristics, with indigestible surfactants often offering higher bioavailability.
6. Type IV formulations (surfactants/co-solvents) may benefit from using two surfactants instead of one for more effective dispersion.
7. Type IV preparations can increase drug solubility but require careful design to prevent drug precipitation post-dispersion.

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Reference

1. Shrestha, H. et al. Lipid-Based Drug Delivery Systems. J Pharm (Cairo). 2014, 2014: 801820.