A Comprehensive Review on Lipid-Based and Targeted Drug Delivery Approaches for Improved Therapeutic Efficiency

The "bioavailability" refers to the rate and extent to which a drug's active ingredient is absorbed from a drug product and becomes available at the site of action. It is a significant pharmacological consideration that determines the effectiveness of drug therapy. A greater dose of a highly poorly bioavailable drug is required to establish the minimum effective level ^[1].

FACTORS AFFECTING BIOAVAILABILITY`

• Low water solubility: Drugs poorly absorbed in water are poorly soluble.
• Low rate of dissolving: Drugs might not be fully absorbed if they take a long time to dissolve.
• Physiological instability at pH: The drug might get degraded in the environment of the body.
• Ineffective penetration across biological membranes: The drug might fail to penetrate the body tissues. More amount of the drug can be metabolized by the liver before reaching the rest of the body as a result of extensive first-pass metabolism.
• Food: Food impairs the absorption of some drugs
• Variability in biology: Drug solubility can be influenced by differences in GI pH ^[2].

METHODS TO IMPROVE BIOAVAILABILITY

• Size reduction: Particles of the drug can dissolve more easily if they are small.
• Incorporation of ingredients that facilitate the dissolution of the drug is referred to as solubilizing excipients.
• Colloidal drug delivery systems: The drug is administered through microscopic particles.
• pH adjustment: Altering alkalinity or acidity to facilitate the dissolution of the drug.
• Solid dispersion: For improving dissolution, mix the drug with a carrier.
• Complexation: For increasing solubility, complex compounds are developed.
• Co-solvent: It is a combination of solvents.
• Micellar solubilization: Dissolving the drug with micelles.
• Hydrotropy: Utilizing hydrotropic agents.
• Nanotechnology: Utilization of magnetic nanoparticles and nanomaterials.
• Lipid-based formulations: Utilization of formulations based on lipids ^[3].

MAINLY FOCUSED ON TARGETED DRUG DELIVERY AND LIPID-BASED DRUG DELIVERY SYSTEM

TARGETED DRUG DELIVERY SYSTEM

The first to propose the concept of targeted agents was Ehrlichp in 1906. The drug carrier that selectively concentrates on the target site by local delivery or whole-body blood circulation is referred to as a targeted agent, also known as a targeted drug delivery system. Targeted agents fall into several types, and each has unique but connected purposes. There are many imaginable combinations due to this multiplicity of categories. Because fields like molecular biology, cell biology, and materials science have advanced much over time, targeted agents develop very fast; thus, research hotspots, in pharmaceutics and pharmacy worldwide, lay here. Western medicine has seriously researched and employed targeted agents within clinical settings ^[4].

The most commonly known type of targeting is active targeting, which involves imposing targeting features on the medicine. This method includes techniques for creating medications that target specific molecules in the body whose structure or function needs to be changed for therapeutic purposes. As a result, the drug's inherent signature is this targeting. However, most other active targeting strategies pertain to methods in which the drug is extrinsically endowed with the targeting property, for example, by conjugation to another entity that possesses targeting attributes. In this case, a drug can be linked to a part that does not show affinity or binding to a particular target but allows the drug's controlled release in response to particular environmental indicators of the disease site ^[5].

Preparation of products based on such a delivery system considers the unique characteristics of the target cells, the type of markers or transport carriers or vehicles that deliver the drug to certain receptors and ligands, and physically modified components. Drug delivery methods that are specifically targeted must be biochemically inert (non-toxic), non-immunogenic, physically and chemically stable both in vitro and in vivo, have uniform capillary distribution, and have limited drug distribution to target cells, tissues, or organs. The rate of drug release should be predictable and controlled, and the release of the drug shouldn't interfere with its function. It should release drugs at a therapeutic level with little to no leakage while in transit ^[6].

As far as the carriers are concerned, targeted drug delivery can be classified as liposomes, microparticles, nanoparticles, emulsions, etc. In this approach, conjugation of the medication with a carrier leads to the formation of a less toxic prodrug for a particular cellular target. There are two roles, which the carrier has. These two goals are enhancing the selectivity of the drug towards target cells and lowering the toxicity of the drug towards healthy cells. After the prodrug reaches the target cell, the drug is released from its relatively inactive or less toxic form to its active form to perform physiological function ^[4].

LIPOSOMES

Alec D. Bangham identified liposomes in the 1960s at the Babraham Institute of the University of Cambridge. It is a vesicle composed of one or more concentric lipid bilayers that enclose an aqueous compartment.  Initially made completely of natural lipids, the formulations can now be composed of surfactants and natural or synthetic lipids. They are capable of entrapment of hydrophilic compounds within the aqueous core and lipophilic compounds within the lipid membrane. These nearly spherical lipid vesicles can vary in size from a few nanometers to several micrometers. However, liposomes employed for drug delivery in medicine are 50–450 nm ^[7].

NANOPARTICLES

The term "nanotechnology" describes a new branch of research that deals with the creation of different nanomaterials.Objects with dimensions from 1 to 100 nm, and possibly differing from the bulk material in some aspects, are termed as nanoparticles.Among these, metallic nanostructures have been fabricated from copper, zinc, titanium, magnesium, gold, alginate, and silver. Nanoparticles have many applications, including medicinal treatments, industrial production in the sectors of solar and oxide fuel batteries for energy storage, and widespread integration into many cmon products such as clothing and cosmetics^[8].

LIPID-BASED DRUG DELIVERY

Solid self-emulsifying drug delivery - " Each of the formulation strategies has its benefits and drawbacks, and the most commonly used technique involves adding the active lipophilic ingredient to inactive lipid excipients, surfactant dispersions, self-emulsifying formulations, emulsions, and liposomes. Isotropic blends of natural or synthetic oils, solid or liquid surfactants, or one or more hydrophilic solvents and co-solvents/surfactants are called self-emulsifying oil formulations, or SEDDS. Due to their capability of solubilizing "lipophilic" drugs with poor water solubility and also solving the problem of bad drug absorption and bioavailability, lipidic excipients have increasingly attracted interest for being used in formulations and self-emulsifying lipid formulations (SELFs)^[9].

BIOLOGICAL  PROCESS IN TARGETED DRUG DELIVERY SYSTEM

The critical role of biological mechanisms in TDDS is in ensuring that medicines safely and effectively reach their targeted areas. Among the key biological mechanisms involved, the following stand out:

1. Ligand-Receptor Interaction: Target cells with overexpressed receptors are frequently targeted by TDDS using certain ligands that can identify and bind to them. The specificity and efficiency of the drug delivery process are increased by this interaction, which makes it easier for the target cells to selectively absorb the drug-loaded carrier by the target cells, the medication delivery mechanism becomes more efficient and selective.
2. Cellular Uptake and Endocytosis: Upon binding a ligand with a cell surface receptor, it often causes endocytosis, in which the drug carrier is engulfed by the cell membrane to permit access into the cell. This ensures the therapeutic drug reaches either close to the site of action of the cell or straight into the cytoplasm.
3. Release Mechanisms: Once it reaches the target cells, various mechanisms control the release of the drug from its carrier. Such mechanisms include pH-triggered release, enzymatic degradation of the carrier, or other stimuli-sensitive systems that only initiate drug release in specific conditions typical of the target tissue.
4. Biodistribution: Physiological factors, like blood flow and cellular density and composition of the extracellular matrix, impact drug carriers' biological uptake and biodistribution. Recognition and maximization of these. Several variables facilitate the buildup of drug carriers in the target location.
5. Immune Response: A key factor in TDDS is the immune system's reaction to drug carriers. Carriers can be altered to avoid immune detection, increase circulation duration, and decrease adverse effects, increasing the system's efficacy.
6. Biodegradability and removal: To prevent the components from building up in the body needlessly, which might result in toxicity, biological processes that control the drug carriers' breakdown and removal are essential. Much research is being done on biodegradable materials and polymers to provide safer and more efficient medication delivery methods. ^[10,11].

ADVANTAGES OF TARGETED DRUG DELIVERY SYSTEM

1. Improving Efficiency: Targeted drug delivery mechanisms enhance the effectiveness of drugs by sending them directly to the site of action, thus allowing for the use of lower concentrations of drugs. It may then lead to significant improvements in treatment, particularly in cancer treatment where conventional systemic treatments cannot adequately reach tumors.
2. Reduced Side Effects: Targeted drug delivery decreases the exposure of healthy tissues to strong medications and thus results in fewer and less severe side effects. This is particularly important with drugs that have narrow therapeutic windows, where small errors in dosing can mean toxicity.
3. Improved Drug Retention: Targeted drug delivery systems are designed to retain drugs at the site of action for longer periods, which is necessary for drugs that need to maintain prolonged effects. This enhances their therapeutic effectiveness.
4.  Higher Local Concentration: Targeted drug delivery can concentrate therapeutic agents much more effectively in tumor tissue than free drugs, primarily because of the enhanced permeability and retention (EPR) effect, which enables a greater accumulation of the nanoparticles in the tumor environment.
5. Enhanced Blood Residence; Time: Targeted systems prolong the time that drugs stay within the bloodstream, allowing for greater concentration at the desired target location. This, in turn, results in better treatment efficacy ^[12,13,14].

DISADVANTAGES OF TARGETED DRUG DELIVERY SYSTEM

1. Biocompatibility Problems: While nanoparticles are meant to be biocompatible, their size, shape, and surface chemistry may still induce detrimental interactions or immune reactions.
2. Unpredictable Distribution: The effects and behavior of drug carriers might become unpredictable because of the lack of knowledge regarding how they distribute throughout the body ^[15].
3. Variability in Gastric Emptying: Inter-individual differences in food consumption and gastric emptying time may have a profound effect on when drugs are released.
4. Gastrointestinal Transit Irregularity: Contractions and peristalsis of the gastrointestinal tract may lead to fluctuations in the drug's transit, as it becomes increasingly harder to predict when it will be delivered to the colon.
5. Disorders Impacting Transit Time: Disorders such as ulcerative colitis, carcinoid syndrome, diarrhea, and inflammatory bowel disease (IBD) may accelerate transit in different regions of the colon, impacting the efficiency of the drug delivery system ^[16,17].

APPLICATIONS OF TARGETED DRUG DELIVERY SYSTEM

1. Improved Pharmacological Effects: By focusing drugs to the target area, TDDS enhances their therapeutic action, which is particularly beneficial in the treatment of localized diseases.
2. Reduction of Adverse Reaction: TDDS minimizes contact with normal tissues by depositing the drug locally at the lesion site, thus reducing adverse effects that are commonly associated with systemic drug application.
3. Use of Different Carriers: The various types of carriers, such as liposomes, nanoparticles, microspheres, and emulsions, that are used in TDDS have been discussed in the article. Each of these carriers has unique characteristics that can be applied for specific functions ^[4].
4. Targeted Delivery to Cancer Cells: CNTs can deliver anticancer drugs to tumor tissues specifically, with very little damage to normal tissues. The unique physicochemical properties of CNTs, which allow for effective drug loading and release at the cancer site, form the basis of this targeted approach.
5. Central Nervous System Delivery: Due to the blood-brain barrier, delivery of drugs to central nervous system tumors may be challenging. CNTs have been utilized in techniques to deliver therapeutic agents across this barrier, which could be beneficial in treating neurological disorders and brain cancers ^[18,19].

DIFFERENT TYPES OF DRUG TARGETING

1. Passive Targeting: Passive targeting relies on the spontaneous distribution of drug carriers to target tissues in the absence of ligands. The EPR effect facilitates nanoparticles, liposomes, and polymeric micelles to get deposited in tumor tissues because of their permeable vasculature and compromised lymphatic drainage. It is applied extensively in cancer therapy to promote the retention of drugs at the site of the tumor^[20].
2.  Active Targeting: Active targeting utilizes ligands like monoclonal antibodies, peptides, and small molecules that specifically bind to receptors overexpressed on pathological cells. Foliate receptor-targeted therapies and antibody-drug conjugates are some examples of this approach that provide specific delivery of drugs with less off-target effects^[21].
3. Physical Targeting: Physical targeting employs the application of external stimuli such as temperature, ultrasound, magnetic fields, or light to induce drug release at the target site. Thermosensitive liposomes, magnetically guided nanoparticles, and ultrasound-responsive microbubbles are examples of this method, which allows controlled and localized delivery of drugs ^[22].
4. Stimuli-Responsive Targeting: This method employs internal or external stimuli such as pH, redox potential, or enzyme activity to trigger the site-specific release of drugs. For instance, pH-sensitive nanoparticles release the drug payload in an acidic tumor microenvironment, whereas enzyme-responsive carriers selectively degrade in target tissues ^[23].
5. Ligand-Mediated Targeting: Ligand-mediated targeting increases specificity and uses biomolecules that target distinct cellular markers. Some examples are transferrin-modified nanoparticles for drug delivery across the blood-brain barrier and aptamer-functionalized carriers for cancer treatment. The approach enhances drug uptake and therapeutic efficacy ^[24].
6. Drug-Mediated Targeting: Drug carriers designed to interact with specific cellular receptors to allow for effective endocytosis are receptor-mediated targets.  This is a group that comprises extensively studied insulin receptor-mediated brain drug delivery and drugs targeting the epidermal growth factor receptor (EGFR) ^[25
7. Cellular Targeting: Cellular targeting systems employ altered cells, such as macrophages or stem cells, as drug carriers for therapeutic drug delivery. Cell-based approaches can be utilized to target diseased or inflammatory tissues and facilitate the transport of active medication across biological barriers ^[26].

CARRIER TYPES IN TARGETED DRUG DELIVERY

Lipid-Based Carriers (Liposomes & Niosomes)

1. Liposomes: Lipid-based carriers, particularly liposomes, have garnered a lot of attention. Liposomes are composed of phospholipid bilayers and can contain both hydrophilic and hydrophobic drugs. Their surface chemical modification and biocompatibility facilitate targeted delivery. Early research has demonstrated that liposomes enhance drug accumulation in tumor tissues. Target ligands (like peptides or antibodies) and other surface modifications further improve cell-specific delivery. The value of \sup>2 For instance, It has been shown that anticancer drugs can be delivered to tumor cells that overexpress the folate receptor via folate-conjugated liposomes ^[27].
2. Niosomes: Niosomes are vesicles based on non-ionic surfactants that are more stable and less costly than liposomes.  They can be made for controlled release and can contain a variety of drugs. Niosomes have shown promise in transdermal drug delivery and targeted cancer treatment ^[28].

Polymer-Based Carriers (Polymeric Nanoparticles & Dendrimers)

1. Polymeric nanoparticles: Polymeric nanoparticles (PNPs) are multifunctional carriers composed of synthetic or natural polymers. To achieve specific drug delivery goals, they can have their size, shape, and surface properties changed. PNPs can encapsulate drugs in their matrix or adsorb them onto their surface.6 biodegradable polymers—like poly(lactic-co-glycolic acid) (PLGA)—are widely used because they enable controlled drug release and lessen long-term toxicity. PNPs have been used to deliver proteins, genes, and anticancer medications. The value of \sup>8 Using targeting ligands to modify the surface improves site-specific delivery and cellular uptake ^[29].
2. Dendrimers: Dendrimers, which are highly branched polymers with unique structures, provide unique advantages in drug delivery. They may have drugs encapsulated in their internal cavities affixed to their surface. Their uniform size and multivalency allow for precise control over the loading and release of medications. Dendrimers have been used to study the delivery of anticancer drugs, imaging agents, and genes ^[30].
3. Inorganic Nanoparticles (Gold & Silica): The distinct physicochemical characteristics of inorganic nanoparticles, like silica nanoparticles (SiNPs) and gold nanoparticles (AuNPs), make them appealing for TDDS. Because of their surface plasmon resonance, AuNPs can be utilized for photothermal therapy as well as medication administration. >12 For drug conjugation and selective distribution, they can be functionalized with a variety of ligands. 13 Research has demonstrated the effectiveness of AuNPs in the delivery of imaging agents and anticancer medications. >14
   Because of their large surface area and adjustable pore size, SiNPs—particularly mesoporous silica nanoparticles, or MSNs—allow for effective drug loading and regulated release. "15" For site-specific delivery, they can be altered with stimuli-responsive materials and targeting ligands. MSNs have been investigated for the delivery of proteins, genes, and anticancer medications. The value of \sup>16 ^[31].

TABLE 1: CARRIER SIZE AND APPLICATIONS ^{[32,33,34,35,36,37,38,39]}.

Lipid-Based Drug Delivery System

Lipid-based drug delivery systems (LBDDS) represent a class of formulation techniques employing synthetic or physiological lipids to enhance the therapeutic performance, solubility, and bioavailability of drugs, especially those characterized by poor water solubility (BCS Class II and IV). These approaches capitalize on lipids' ability to improve drug solubility, promote lymphatic transport and diminish first-pass metabolism. A wide variety of formulations fall under the umbrella of LBDDS, including but not limited to liposomes, solid lipid nanoparticles, emulsions, microemulsions, self-emulsifying drug delivery systems, simple lipid solutions and nanostructured lipid carriers ⁴⁰.

Among the various methods of drug delivery, lipid-based drug delivery systems have several advantages, especially in the case of poorly water-soluble drugs. Solubilization is improved and there is a possibility of lymphatic transport, besides eliminating first pass metabolism with LBDDS, whereas by conventional systems, these have relatively low dissolving rates, erratic absorption and poor bioavailability. With the help of formulations such as solid lipid nanoparticles, nano emulsions and self-emulsifying drug delivery systems (SEDDS), the added advantage could be improved stability, controlled release and targeted distribution. Lipid-based formulations or LBDDS would be the best alternative for almost all lipophilic, or BCS class II/IV drugs because they mimic the natural digestion and absorption pathways for lipids which result in higher and more constant plasma concentration, fewer doses and improved therapeutic effects ⁴¹.

Using formulations based on lipids and surfactants is one tactic that has been utilized to improve the oral absorption of medications that are not highly water-soluble. Nowadays, a lot of work has been done to make use of the potential of lipid-based methods for delivering drugs since they offer a suitable means of delivering bioactive agents and medications with different molecular weights, whether they are small or large, at particular times and locations. Lipids have gained a lot of significance in the past ten years as potential delivery routes for medications that are not highly soluble in water. Pharmaceuticals can be commercially formulated using these techniques for parenteral, pulmonary, oral, or topical administration. Over the past few decades, liposomes have become more and more popular in a variety of applications, such as gene transfer, cancer treatment and infectious diseases. Their capacity to store, shield and transport molecules with a variety of chemical and physical properties makes them one of the most adaptable carriers. Small, adaptable vesicles made of lipids are called liposomes. Because they are made of one or more lipid bilayers, they can be used to encapsulate hydrophilic, lipophilic, or amphiphilic substances. Lipid-based DDS contains polar lipids, such as phospholipids, triglycerides and fatty acids, which are biocompatible and biodegradable, making them perfect for use as drug delivery systems ⁴².

The future direction of lipid-based delivery systems would be to generate multifunctional carriers capable of targeted, stimuli-responsive and customizable delivery. Innovations in the sector would include surface-modified carriers for site-specific targeting, nanostructured lipids for better drug loading and hybrid lipid--polymer systems for improved stability. Precision would be achieved through this application with ligands, peptides, or antibodies and large-scale manufacturing techniques as well as environmentally friendly manufacturing, would make way for commercialization. Further, developments may involve future lipid systems by parenteral or oral delivery of gene treatments, biologics and small molecules with poor solubility. Advances would be powered by computer modeling and AI-reliant formulation design to ensure lipid-based therapies are more secure, effective and personalized for each patient ⁴³.

Some examples of challenges faced by lipid-based drug delivery systems are physical instability (phase separation and drug precipitation), chemical degradation of lipids and limited drug-excipient compatibility. Variability in bile salt secretion and fat breakdown, as well as high surfactant concentrations that may irritate the gastrointestinal tract, can create unpredictable bioavailability. Particle size and emulsification must be precisely controlled during manufacture and at scale-up to preserve performance. In vivo correlation and special characterization methods increase the regulatory burden. The high cost of pure excipients, storage stability and variability in lipid metabolism from patient to patient still stand as hurdles toward successful industrial translation of the product ⁴⁴.

Figure 3: Classification of LBDDS ⁴⁵_.

1. Emulsion-Based Methods

Emulsions are made when two insoluble fluids, such as oil and water, are mixed and maintained by emulsifying chemicals. Upon ingestion of the emulsion, the drug's absorption is facilitated by its dissolution in the oil phase. There are generally two kinds of emulsion systems, which are water and oil (o/w): oil droplets dispersed in water are a common way to provide drugs in oral and injectable form. granules of water suspended in oil are known as water dispersed in oil and are commonly used topically. they can improve bioavailability and solubilize hydrophobic (water-insoluble) drugs ⁴⁶.

2. Microemulsions

Isotropic water and oil dispersions known as microemulsions are stabilized by a surfactant interfacial coating and are typically mixed in a cosolvent, like medium-chain alcohol, a polyhydroxy molecule, or other surface-active agent. microemulsions are used extensively due to their various advantages, which include their scalability, ease of production, high solubilization power, thermodynamic stability and aesthetically pleasing look ⁴⁷.

3. EmulsionsNano

Nano emulsions are emulsions with droplets that are typically between 20 and 200 nm in size but less than 1μ. Because nano emulsions are easy to manufacture, biodegradable and biocompatible, they are utilized as carriers for lipophilic medications that are susceptible to hydrolysis. They function as a subcutaneous injection-based sustained release delivery system for depot formation. For some drugs, they improve stomach absorption and lessen intra- and inter-subject variability. Their incredibly wide interfacial area results in an amazing drug release profile. Nano emulsions for oral, via ingestion, ophthalmic, pulmonary and cutaneous delivery have been studied and developed ⁴⁸.

4. Liposomes

In the 1960s, Alec D. Bangham of the University of Cambridge's Babraham Institute discovered liposomes, which are made up of multiple concentric bilayers of lipids encasing an aqueous region. Phospholipids in an aqueous solution self-assemble to form these microscopic (unilamellar or multilamellar) vesicles, which are two-layered closed structures. Although liposomes are composed of a variety of chemicals, the two main ones are phospholipid and cholesterol. One type of phospholipid and sphingolipid is phosphoglycerides, which can also be hydrolyzed to create combination products. Based on their size and the quantity of bilayers they contain, liposomes can be divided into three fundamental kinds. Aqueous gaps divide the many lipid bilayers that make up multilamellar vesicles (MLVs). The diameter of these items often ranges from several hundred to thousands of nanometers; however, their sizes vary greatly. In contrast, the confined aqueous area of large unilamellar vesicles (LUVs) and small unilamellar vesicles (SUVs) is surrounded by a single bilayer. LUVs have a diameter of more than 100 nm, whereas SUVs are smaller. Liposomes can be administered orally, ophthalmically, pulmonary, or transdermally ⁴⁹.

5. Niosomes

Niosomes are compartments that form non-ionic bilayers mostly composed of surfactants. Despite having identical structural and physical properties, niosomes can profit from the use of synthetic surfactants, which are less expensive and more chemically stable than naturally occurring phospholipids. Niosomes can be created by hydrating artificial non-ionic surface active agents, either with or without the inclusion of lipids like cholesterol. Niosomes and liposomes function similarly, boosting the drug's absorption and lowering its excretion. Similar to liposomes, niosomes may be employed to transport medications to particular bodily regions. Like liposomes, niosomes' characteristics are determined by the composition of the bilayer and the manufacturing process. Additionally, niosomes have been employed to carry antigens and small molecules. However, niosomes as a delivery system can have significant disadvantages, including physical instability during storage caused by vesicles clumping together, fusing and leaking, which could result in encapsulated drug hydrolysis and reduce the shelf life of the dispersion ⁵⁰.

6. Transferosomes

The creation of transferosome technology aimed to provide a way to distribute bioactive substances over the epidermal barrier. The active ingredient may be found in the membrane of lipids or within the center, depending on how lipophilic it is. When applied topically, transferosomes may penetrate deeper, undamaged skin areas than liposomes, making them an efficient drug delivery method for transdermal applications. Because of this, they are able to offer higher concentrations of active substances. An antigen or drug can be put into these vesicles similarly to how liposomes are loaded. To use phospholipid vesicles as transdermal drug delivery vehicles, transferosomes were developed. Depending on how the drug is applied or administered, these self-optimized aggregates ultra-flexible membrane may transport it through or into the skin consistently and effectively. Both large and low molecular weight medications, including insulin, gap junction proteins, sex hormones, analgesics, anesthetics, corticosteroids and anticancer medicines, can be carried by them. Like liposomes, they are biodegradable and biocompatible due to their naturally occurring phospholipid composition. They have a trapping efficiency of up to 90% when it comes to lipophilic medicines. However, because of their propensity for oxidative disintegration, they have the drawback of being chemically fragile. Another obstacle that hinders the use of transfersomes as drug administration vehicles is the high formulation costs and the intrinsic purity of phospholipids ⁵¹.

7. Phytosomes / Herbosomes

Any herbal remedy's ability to work depends on how well the therapeutically effective ingredient is dispersed. They have very little absorption when used orally or topically. Phytosomes are a new class of herbal compounds that are more easily absorbed than extracts. "Some" indicates something that looks like a cell, while "phyto" indicates a plant. Through the addition of phospholipids to standardized herbal preparations, phytosome technology has enhanced the ingestion and bioavailability of specific plant constituents. Because emulsified formulations are amphiphilic, they readily penetrate and cross lipid-rich biomembranes, increasing the bioavailability of active phytochemical components. Many well-known herbal extracts, including ginseng, green tea, kushenin, marsupsin, curcumin, olive fruits and leaves grape seed, hawthorn and Ginkgo biloba, have been extracted using the phytosome process. Phytosomes have a higher bioavailability than uncomplexed plant extracts, according to research on pharmacokinetics and activity in both humans and animals ⁵².

Lipid Particulate System

8. Lipospheres

Initially, lipospheres were defined as a solid lipophilic core composed of triglycerides or fatty acid analogs, stabilized by a phospholipid monolayer and a fine dispersion of firm spherical particles with a diameter of 0.2 to 100 µm. Lipospheres have been used in the delivery of anti inflammatory medications, local anesthetics, antibiotics, anticancer drugs, insect repellents, vaccines, proteins and peptides. Unlike microdroplets, vesicles, or liposomes, the lipospheres contain a rigid inner core at room temperature. Because the lipospheres include at least two phospholipid layers, they differ from microspheres of evenly distributed particles in homogeneous polymers. Comparing lipospheres to emulsion-based systems like vesicles and liposomes, the former are more stable and disperse more efficiently than most suspension based systems. Because the vehicle in lipospheres is solid, there is less chance of a contact between the delivered material and the vehicle than in emulsion systems. Furthermore, by altering the solid core or the phospholipid coating, the substance's dispersion rate from lipospheres can be changed. In addition to being easier to make than vesicles like liposomes, they have more intrinsic stability ⁵³.

1. Nanostructured Lipid Carriers

Solid Lipid Nanocarriers (SLNs) were introduced in the early 1990s by Professors R.H. Müller and M. Gasco. NLCs are colloidal carriers with a mean particle size in the nanometer range and a lipid-based center composed of a combination of liquid and solid lipids. The second generation of lipid nanoparticles, known as nanostructured lipid carriers (NLC), provide a more effective substitute for liposomes and other particulate delivery methods frequently seen in medications. Generally recognized as safe (GRAS) materials are used to generate nanostructured lipid carriers (NLCs). They are especially successful in encapsulating lipophilic and weakly water-soluble pharmaceuticals and provide the benefit of being easily scalable for large-scale manufacture. The high-water content of SLN dispersions, drug ejection during storage and the poor drug-loading capacity for certain compounds are some of the constraints of SLNs that the NLC system solves. Lipid carriers with a nanostructure can be divided into three categories: many, amorphous and imperfect ⁵⁴

Mechanism of Drug Release in Lipid-Based System

10. Digestion and Solubilization

The primary component of the formulation is lipids, which are metabolites of fatty acids; however, in order to increase solubilization and dispersion properties, a few or more surface active agents and maybe a hydrophilic cosolvent may be required ⁵⁵.

Based on their HLB value, surfactants are categorized; stronger hydrophilicity is indicated by a larger HLB value (≥10), while higher lipophilicity is indicated by a lower HLB value (≤10) ⁴³. The equilibrium within a medicament's dissolvability in the gastrointestinal lumen's watery environment and its capacity to penetrate enterocytes' lipophilic membrane determines both the pace and degree of absorption. Gastric lipase starts breaking down dietary triglycerides (TG) and formulation triglycerides following oral ingestion of lipid compositions. At the same time, the stomach's mechanical processes of propulsion, grinding and retropulsion aid in combining the aqueous gastric fluid with the results of lipid breakdown to create a crude emulsion. Diglycerides, 2 monoglycerides and free fatty acids are produced in the small intestine by the further breakdown of triglycerides by pancreatic lipase and its cofactor, co-lipase. This process primarily occurs at the sn-1 and sn-3 regions of the triglycerides. Pancreatic phospholipase A2 hydrolyzes the sn-2 position in formulation-derived or biliary phospholipids to create lysophosphatidylcholine and fatty acids. When extraneous lipids are found in the small intestine, the gallbladder releases natural hepatic lipids, including cholesterol, phospholipids and bile salts. Bile salts combine with monoglycerides, fatty acids and lysophospholipids when lipids are broken down to form a variety of dispersion configurations, including micelles and single-layer or multi-layer vesicles. The solubilization and absorption of lipid digestion products and medications in the small intestine are significantly improved by these lipid metabolites ⁵⁶.

Advantages of Lipid-Based Drug Delivery System

• For stability purposes during gastrointestinal transit, lipid-based drug delivery systems solubilize poorly soluble drugs as they are incorporated into their lipid phase.
• Such approaches increase the concentration gradient of the drug, help maintain solubility and favor enhanced absorption, which adds to increased oral bioavailability.
• The functions of these systems allow movement of a medicine in a lymphatic transport system to evade metabolism and promote increased availability to the systemic circulation by first-pass hepatic elimination.
• They minimize the effects of the gastrointestinal environment on drug dissolution; hence, they reduce the variability of drug absorption among different patients.
• Lipid-based system, such as might prolong an effect for which a drug should be administered infrequently; thus, it can provide sustained or controlled release.
• Such drug matrices can protect against chemical and enzymatic disintegration in the gastrointestinal tract before absorption and are consequently stabilized.
• They support high drug-loading of amphiphilic and hydrophobic kinds of molecules, thus making them versatile for different forms of therapeutic agents.
• Lipid formulations can be quite versatile, and lipids have been produced for various purposes. Triglycerides and phospholipids have been used for this purpose. They are chosen according to the targets for drug delivery.
• The working mechanism of these agents to enhance permeability across biological membranes entails interaction with the lipid bilayer and disruption of tight junctions to facilitate drug transport.
• They help by improving osmotic absorption of drugs, reducing frequency of dosing, and consequently the therapeutic outcome of drugs in all contributing to achieving better patient compliance and patient satisfaction ^57–59.

Disadvantages Of Lipid-Based Drug Delivery Systems

• The physical instability exhibited by lipid drug delivery systems may lead to problems such as phase separation, creaming, or precipitation of the drug during storage.
• For hydrophilic molecules, dossier requirements for their approval quickly become restrictive.
• Certain machines might be needed for few more processes in making certain kind of items.
• Regulatory concerns about the safety of lipid excipients and particularly those lipids used in novel drug delivery systems have in certain cases hindered their cyclic use.
• The seasonal variations contribute to differences in light intensity, which is responsible for shadow zones; consider this when setting up experiments.
• Risk of lipid oxidation can lead to degradation of both excipients and the drug, impacting efficacy.
• It can be quite demanding when it comes to scaling up from laboratory to industrial production due to the complexities and complications involved in comes formulations.
• Surfactants in self-emulsifying formulations could be responsible for intestinal discomfort in some individuals with hypersensitivity.
• Batch-to-batch variability in natural lipid sources can affect the consistency and reproducibility of the formulation.
• Lipid-based systems occasionally encounter compatibility issues with some packaging materials, so careful selection of the containers is necessary ^60,61.

Self-Microemulsifying Drug Delivery System

SMEDDS are isotropic blends of hydrophobic surfactants, oils and occasionally co-solvents that quickly form fine o/w emulsions when gently stirred or when the digestive tract experiences motility. Self-nanoemulsifying drug delivery systems (SNEDDS) often produce emulsions with a droplet size of less than 100 nm, while SMEDDS typically produce emulsions with a droplet size between 100 and 250 nm ⁶².

Oil, co-solvents and hydrophilic surfactants that quickly create an o/w microemulsion when gently stirred and then diluted in an aqueous medium that will be encountered in the gastrointestinal tract ⁵¹. It is important to note that the low-energy emulsification approach for creating o/w nanoemulsions is the same as this technique for creating a fine o/w emulsion utilizing SMEEDS/SNEDDS. Diffusion from oil globule to dissolving media is the rate-limiting stage for drug material dissolution from SMEDDS and it is impacted by the system's surfactant and co-surfactant concentrations. SMEDDS and SEDDS vary primarily in that SMEDDS create clear microemulsions with droplet sizes smaller than 250 nm, whereas SEDDS usually create opaque emulsions with droplet sizes larger than 300 nm (Pouton 2000). Furthermore, SMEDDS have a lower oil content than 20%, whereas SEDDS have an oil content of 40–80%. HLB512 surfactants are typically used to prepare SEDDS, whereas HLB412 surfactants are used to prepare SMEDDS and SNEDDS. When compared to the same drug supplied as a manufactured microemulsion, the microemulsion globule generated by SMEDDS/SNEDDS tended to be larger ⁶³.

Figure 4: Preparation of a solid self-emulsifying Drug Delivery system