Chinese Pharmaceutical Association

Institute of Materia Medica, Chinese Academy of Medical Sciences

Acta Pharmaceutica Sinica B

Acta Pharmaceutica Sinica B 2013;3(6):361–372

2211-3835 & 2013 InsElsevier B.V. All righhttp://dx.doi.org/10.10

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REVIEW

Oral lipid-based drug delivery systems – an overview

Sandeep Kalepun, Mohanvarma Manthina, Veerabhadhraswamy Padavala

Department of Pharmaceutical Technology, Shri Vishnu College of Pharmacy, Bhimavaram 534202, Andhra Pradesh, India

Received 2 August 2013; revised 10 September 2013; accepted 5 October 2013

KEY WORDS

Lipid-based formulation;Lipid excipient;Solubilization;Lymphatic absorption

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Abstract The formulation of drugs is carried out with the principle objective of enhancing theirbioavailability. Poorly water soluble drugs are challenging for the formulation scientists with regard tosolubility and bioavailability. Lipid-based drug delivery systems (LBDDS) are one of the emergingtechnologies designed to address such challenges. Encapsulating or solubilizing the drug in lipidexcipients can lead to increased solubilization and absorption, resulting in enhanced bioavailability.Recent advances in these formulation technologies have led to the successful commercialization of lipid-based formulations. This review provides a comprehensive summary and characterization of lipid-basedformulations, especially for oral delivery, from both physicochemical and biopharmaceutical perspectives.This review also focuses on the processing techniques necessary to obtain solid lipid-based formulationsfor oral delivery, along with a brief discussion of lipid excipients and their characterization.

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Figure 1 Schematic diagram of mechanisms of intestinal drugtransport from lipid-based formulations.

S. Kalepu et al.362

1. Introduction

Drawbacks to intravenous administration, including extravasationof drug or blood, catheter infections, and thrombosis can beprevented by administering the drug orally, making the oraldelivery the most popular route of administration1. Nonetheless,oral administration is limited by problems related to physico-chemical properties of the drug, including poor solubility, lowpermeability, instability, and rapid metabolism, all of whichdecrease oral bioavailability2.

With the advent of drug design, various molecules have beencreated that have a potential for therapeutic action. But most of thenewly discovered chemical entities are of high molecular weightand belong to biopharmaceutical classification system (BCS) – II,with poor aqueous solubility and high membrane permeability.Hence these two characteristics limit the bioavailability of orally-administered drugs3. These drugs have low solubility which leadsto low dissolution and limits absorption. This poor solubility notonly gives low oral bioavailability but also leads to high inter- andintra-subject variability and lack of dose proportionality4. Also,some of these drugs have enhanced bioavailability when co-administered along with food, e.g., halofantrine5 and danazol6. Inorder to formulate such drugs in a safe and efficacious form, abalance must be maintained between bioavailability, toxicity anddisposition within the body. Various techniques like microniza-tion, complexation with cyclodextrins, solid dispersions, permea-tion enhancers and surfactants have been reported to overcomesome solubility and permeability issues7.

In the last decade lipids have gained much interest as carriersfor the delivery of drugs with poor water solubility8. Theavailability of novel lipid excipients with acceptable regulatoryand safety profiles coupled with their ability to enhance oralbioavailability has helped in the development of lipid basedformulations as a means for drug delivery. Lipid-baseddrug delivery (LBDD) systems have gained much importance inthe recent years due to their ability to improve the solubility andbioavailability of drugs with poor water solubility9. The absorptionof drug from lipid based formulation depends on numerousfactors, including particle size, degree of emulsification, rate ofdispersion and precipitation of drug upon dispersion4,10. Lipid-based formulations may include oil solution or suspensions,emulsions, self-micro or self-nano emulsifying drug deliverysystems (SMEDDS/SNEDDS)10,11. Some of the drugs thatare successfully marketed as lipid based formulations includeefavirenz (Sustivas), saquinavir (Fortovases), ritonavir (Nor-virs), clofazamine (Lamprenes). An appropriate selection oflipid vehicles, formulation strategies and rational delivery systemdesign can lead to the success of lipid based drug deliverysystems12.

A water-insoluble drug can be formulated as a lipid-basedformulation when the drug itself is an oil-like substance (e.g., ethylicosapentate, tocopherol nicotinate, teprenone, indomethacin far-nesil and dronabinol), or when conventional formulationapproaches like granulation or soluble liquids in capsules do notenhance the oral bioavailability13. A variety of lipid-based systemscomposed of simple oil solutions to complex mixtures of oils, co-solvents, surfactants and co-surfactants can be obtained based onthe type of excipients and formulation variables14. Indeed, thesesystems can be converted to solid intermediates (powders, granulesand pellets) by various techniques and can be filled in hard gelatincapsules or can be compressed into tablets after blending withsuitable tableting excipients9.

2. Drug absorption

In practice, lipid formulations can be obtained as a result ofblending of excipients such as pure triglyceride oils, mixedglycerides, lipophilic surfactants, hydrophilic surfactants andwater-soluble co-solvents10. These systems increase absorptionfrom the gastrointestinal tract by accelerating the dissolutionprocess, facilitating the formation of solubilized phases by reduc-tion of particle size to the molecular level, yielding a solid-statesolution within the carrier4,15, changing drug uptake, efflux anddisposition by altering enterocyte-based transport16,17, and enhan-cing drug transport to the systemic circulation via intestinallymphatic system18–20.

2.1. Lymphatic system

The lymphatic system plays an important role in the transport ofdrugs to the systemic circulation, given its extensive drainagenetwork throughout the body. Some of the advantages oflymphatic transport of drug are avoidance of first-pass metabolismand targeting of specific diseases which are known to spread vialymphatics, such as certain lymphomas and HIV. Possiblemechanisms by which lipids affect drug absorption, bioavailabilityand disposition after oral administration are summarized inFig. 121. The promising mechanisms include22: I facilitatingtranscellular absorption due to increased membrane fluidity; II,allowing paracellular transport by opening tight junctions; III,increased intracellular concentration and residence time by surfac-tants due to inhibition of P-gp and/or CYP450; IV, lipid stimula-tion of lipoprotein/chylomicron production15,18.

2.2. Digestion and solubilization

The balance between a drug's solubility in the aqueous environ-ment of the gastrointestinal lumen and its permeation acrossthe lipophilic membrane of enterocytes determines its rate andextent of absorption16. After oral administration of lipid-based

Figure 2 Lipid digestion and drug solubilization process in the small intestine.

Lipid-based drug delivery systems 363

formulations, gastric lipase initiates the digestion of exogenousdietary triglyceride (TG) and formulation TG. Simultaneously, themechanical mixing (propulsion, grinding and retropulsion) of thestomach facilitates formation of a crude emulsion (comprised ofaqueous gastric fluid and lipid digestion products). Later in thesmall intestine, TG is broken down to diglyceride, monoglycerideand fatty acids by pancreatic lipase together with its cofactor co-lipase203, acting primarily at the sn-1 and sn-3 positions of TG toproduce 2-monoglyceride and free fatty acid23.

Pancreatic phospholipase A2 digests the formulation-derived orbiliary-derived phospholipids (PL) by hydrolyzing at the sn-2position of PL to yield lysophosphatidylcholine and fatty acid24.The presence of exogenous lipids in the small intestine stimulatesthe secretion of endogenous biliary lipids from the gall bladder,including bile salt (BS), PL and cholesterol. Previously formedmonoglycerides, fatty acids, and lysophospholipid (products oflipid digestion) are subsequently incorporated into a series ofcolloidal structures, including micelles and unilamellar and multi-lamellar vesicles in the presence of bile salts. The solubilizationand absorptive capacity of the small intestine for lipid digestionproducts and drugs (D) is significantly enhanced due to theseformed lipid metabolites. In Fig. 2, the oil droplet in the intestine isrepresented in different colors to indicate undigested TG in thecore (orange) and digested products such as fatty acid (blue) andmonoglyceride (green) on the surface of the droplet.

2.3. The role of lipids in enhancement of bioavailability

The bioavailability of some of the drugs is increased when co-administered with food3,20,25. However, many drug moleculeshave negligible interaction with food. BCS class I drugs are notaffected by the presence or absence of food, but class II drugs havean altered absorption when co-administered with food. The reasonfor such enhanced bioavailability might be attributed to solubility,permeability and inhibition of efflux transporters in the presence offood26. Some of the drugs which show enhanced bioavailabilitywhen administered along with food are griseofulvin27, halofan-trine6, danazol5, troglitazone and atovaquone28. A guidancedocument entitled “Food-Effect Bioavailability and Fed Bioequi-valence” was issued by FDA in December 2002. The US FDArecommended high fat meals for food-effect studies because suchfatty meals (800–1000 cal, 50%–65% fat, 25%–30%

carbohydrates and 15%–20% proteins) affect GI physiology andmaximize drug transfer into the systemic circulation29.

In particular, it is the lipid component of the food that plays avital role in the absorption of lipophilic drugs,30 leading toenhanced oral bioavailability. This can be explained by the abilityof a high fat meal to stimulate biliary and pancreatic secretions, todecrease metabolism and efflux activity, to increase intestinal wallpermeability, and to a prolongation of gastrointestinal tract (GIT)residence time and transport via lymphatic system31. Triglyceridesand long chain fatty acids play a major role in prolonging the GITresidence time. Also, a high fat meal elevates the TG-richlipoproteins which react with drug molecules. This associationof lipoproteins with drug molecules enhances intestinal lymphatictransport and leads to changes in drug disposition and finallychanges the kinetics of the pharmacological actions of poorlysoluble drugs32. This food effect on drug absorption leads to aserious concern about the sub-therapeutic plasma drug concentra-tion when co-administered without food. Such food effect is also aserious problem for drugs with a narrow therapeutic index, whereincreased bioavailability may lead to serious untoward effects.Hence, control or/and monitoring of food intake is required whendosing such drugs.

However, food-dependent bioavailability can be significantlyreduced by formulating the drug as a lipid-based formulation,which can increase the solubility and dissolution of lipophilicdrugs and facilitate the formation of solubilized species, fromwhich absorption occurs. Hence, lipid-based formulations can beused to reduce the dose of drug while simultaneously enhancing itsoral bioavailability15.

3. Lipid excipients

A wide range of lipid excipients are available from excipientsuppliers. Since these lipids affect the absorption process, itis necessary to know the characteristics of various excipients14.The factors that determine the choice of excipients for lipid-basedformulations include miscibility; solvent capacity; self-dispersibilityand ability to promote self-dispersion of the formulation; digest-ibility and fate of digested products; regulatory issues – irritancy,toxicity, purity, chemical stability; capsule compatibility; meltingpoint, and cost.

For preparing lipid-based formulations, dietary oils composedof medium and long chain triglycerides, along with various

Table 1 Solubilizing excipients used in commercially available Lipid-based oral formulations36.

Water-insoluble excipients Triglycerides Surfactants

Bees wax, Long-chain triglycerides Polysorbate 20 (tween 20),Oleic acid, Hydrogenated soyabean oil, Polysorbate 80 (tween 80),Soy fatty acids, Hydrogenated vegetable oil, Sorbitanmonolaurate (Span20),D-α-Tocopherol (vitamin E), Corn oil, Olive oil, D-α -Tocopheryl PEG 1000 succinate (TPGS),Corn oil mono-di-triglycerides, Soyabean oil, Pea nut oil, Glycerylmonooleate,Medium chain (C8/C10) mono and diglycerides, Sesame oil. Polyoxyl 35 castor oil (cremophor EL),Propylene glycol esters of fatty acids. Medium-chain triglycerides Polyoxyl 40 hydrogenated castor oil (cremophor RH40),

Caprylic/capric Polyoxyl 60 hydrogenated castor oil (cremophor RH60),triglycerides derived from PEG 300 oleic glycerides (Labrafils M-1944CS),coconut oil or palm seed oil PEG 300 linoleic glycerides (Labrafils M-2125CS),

PEG 400 caprylic/capric Glycerides (Labrasols),PEG 1500 lauric glycerides (Gelucires 44/14).

Table 2 Composition of fatty acids found in lipid-basedexcipients36.

Fatty acid chain length(number of carbons)

Commonname

Meltingtemperature(1C)

8 Caprylic acid 16.510 Capric acid 31.612 Lauric acid 44.814 Myristic acid 54.416 Palmitic acid 62.918 Stearic acid 70.118 Oleic acid 16.018 Linoleic acid �5.018 γ-Linoleic acid �11.018 Ricinoleic acid 6.020 Arachidic acid 76.122 Behenic acid 80.0

S. Kalepu et al.364

solvents and surfactants are frequently chosen. Many lipids areamphiphilic in nature, having a lipophilic portion (fatty acid) and ahydrophilic portion33. The melting point increases as the fatty acidchain length increases, but it decreases with the increase in theunsaturation of the fatty acid and also increases the susceptibilityto oxidation9. A list of solubilizing agents used in lipid-basedformulations is mentioned in Table 1.

3.1. Classification of lipid excipients

3.1.1. TriglyceridesThe most common excipients used in lipid based drug delivery aretriglyceride vegetable oils. This is one class of lipid which doesnot present any safety issues, since they are fully digested andabsorbed14. Triglycerides can be further classified as long chaintriglycerides (LCT), medium chain triglycerides (MCT) and shortchain triglycerides (SCT). The capacity as a solvent for drugs ismainly decided by the effective concentration of ester groups34.MCT have a higher solvent capacity than LCT and are less proneto oxidation35. Oils from different vegetable sources have differentproportions of each fatty acid. The composition of fatty acids36

found in various lipid excipients are presented in Table 2. D-α-Tocopheryl polyethylene glycol 1000 succinate (Vitamin E TPGS)is derived from vegetable tocopherols. It is water soluble and acts

as absorption enhancer for poorly water-soluble drugs. Puretriglycerides are presented in refined vegetable oils37–39.

3.1.2. Mixed glycerides and polar oilsMixed glycerides are obtained by partial hydrolysis of vegetableoils. The starting material (triglyceride) and the extent of hydro-lysis determine the chemical composition of the mixed glyceridesproduced. Medium chain mixed glycerides are not susceptible tooxidation, have greater solvent capacity and promote emulsifica-tion. These polar oily excipients also improve solvent capacity andthe dispersibility of the formulation. Sorbitan trioleate (Span 85) isan example of polar oils. Apart from this, oleic acid is also used ina number of commercial products40,41.

3.1.3. CosolventsIn order to enhance the solubilization process, most marketed drugproducts use cosolvents40,41. The popular cosolvents used includeethanol, glycerol, propylene glycol and polyethylene glycols(PEG)-400. The reason for their use can be attributed to anincrease in the solvent capacity of the formulation for drugs and toaid the dispersion of systems which contain a high proportion ofwater soluble surfactants. However, there are several practicallimits related to these cosolvents, including precipitation of thesolubilized drug from the solvent due to loss of the solventcapacity following dilution42, immiscibility of some cosolventswith oils, and incompatibilities of low molecular weight solventswith capsule shells43.

3.1.4. Water-insoluble surfactantsA group of lipid excipients with intermediate hydrophilic-lipophilic balance (HLB of 8–12) that adsorb at oil–waterinterfaces are available. Depending on the degree of ethoxylation,these have a finite solubility in water. They can form emulsions ifsubjected to shear and are sometimes referred as being ‘disper-sible’ in water. These substances can form micelles but are unableto self-emulsify due to their insufficiently hydrophilic nature.Oleate esters such as polyoxyethylene (20) sorbitan trioleate(Tween-85) and polyoxyethylene (20) glyceryl trioleate (Tagot-TO) are typical examples of water-insoluble surfactants whoseHLB values are between 11 and 11.5. However, a blend of Tween-80 and Span-80 with average HLB value of 11 is not similar toTween-85 in function. The former consists of both water-solubleand water-insoluble molecules, but the later consists of predomi-nantly of water-insoluble molecules10,44,45.

Lipid-based drug delivery systems 365

3.1.5. Water-soluble surfactantsThese are the most commonly used surfactants for the formulationof self-emulsifying drug delivery systems8,46. The materials withHLB value of approximately 12 or greater can form micellarsolutions at low concentrations by dissolving in pure water abovetheir critical micellar concentration47. These materials can besynthesized by mixing polyethylene glycols (PEG) with hydro-lyzed vegetable oils. Alternatively, alcohols can be made to reactwith ethyleneoxide to produce alkyl ether ethoxylate, which is acommonly used surfactant (e.g., cetostearyl alcohol ethoxylate‘cetomacrogol’). A reaction of sorbitan esters with ethylene oxideproduces polysorbates (predominantly ether ethoxylates)48.

Cremophor RH40 and RH60 (ethoxylated hydrogenated castoroil) are examples of this type which are obtained from hydro-genation of materials derived from vegetable oils. Cremophor EL(ethoxylated castor oil), which is not hydrogenated is also widelyused. Cremophor is known to enhance the absorption by inhibitingthe efflux pumps, but the mechanism of inhibition is not yetdetermined37,49,50. This might be attributed to a non-specificconformational change due to penetration of the surfactantmolecules into the membrane, adsorption on to the surface ofthe efflux pumps, or interaction of molecules with intracellulardomains of efflux pump51,52.

3.1.6. AdditivesIn order to protect the formulation from oxidation, various lipidsoluble anti-oxidants such as α-tocopherol, β-carotene, propylgallate, butylated hydroxyl toluene (BHT) or butylated hydro-xyanisole (BHA) can be used13.

3.2. Characterization of lipid systems

3.2.1. Physical analysisAnalysis of the thermal behavior of lipids during formulation is ofprimary importance, since lipid excipients have complex chemicalcompositions that lead to broad melting ranges. Various thermalproperties of lipids including crystallization temperature, meltingpoint, glass transition temperature and determination of solid fatcontent of the excipient versus temperature can be evaluated usingdifferential scanning calorimetry (DSC). The organization of thelipid during heating or cooling can be assessed by hot-stagemicroscopy. Crystallinity of a lipid excipient can be confirmed byX-ray diffraction (XRD)9.

3.2.2. Chemical analysisHigh performance liquid chromatography (HPLC) and gas chro-matography (GC) can be used to determine the exact compositionof ethers, esters and fatty acid distribution. Other chemical indicesare also available: the molecular weight of fatty acids can bedetermined from their saponification value; the saturation ofhydrocarbon chains can be measured using an iodine-based assay;oxidative changes can be determined by measuring peroxides; freefatty acids can be measured from acid content; and free hydroxylgroups can be determined by measuring hydroxyl group content9.

3.2.3. Dissolution in biorelevant media and dispersion testingThe FDA requires conventional USP dissolution testing as aquality control tool for the formulations, but this dissolution doesnot correlate to the in vivo behavior of lipid-based formulations. Inthe GIT lipids are subjected to digestion processes in the presenceof gastric and pancreatic lipases19,53. These lipases also affect the

emulsification and dispersion properties of the lipid excipients,leading to altered solubilization capacity in vivo. Hence, thedigestibility of the lipid excipients must be considered whenselecting lipid-based formulations10,54,55. To assess such effectsand predict in vivo behavior, dissolution testing in biorelevantmedia can be helpful. The effectiveness of self-emulsifyingformulations can be determined by dispersion testing (emulsifica-tion capacity and particle size). Photon correlation spectroscopy(PCS) or laser light diffraction can be used to measure the particlesize, and visual observation can be helpful to predict emulsifica-tion capacity56.

3.2.4. Analysis of physiological effects of excipientsLipid-based excipients are known to enhance the oral absorptionof drugs by affecting various physiological processes. Theseinclude stimulating bile flow and pancreatic juice secretion57,prolonging gastric emptying58, increasing the membrane fluidity,opening of tight junctions19,59, promoting lymphatic transport ofdrugs, thus avoiding first pass metabolism, and inhibiting effluxtransporters. In order to assess these effects various in vitro modelsare available, including intestinal microsomes, Caco-2 cells,everted gut sac, Using chamber, and in situ perfusion assays60.

3.3. Regulatory status of lipid excipients

Not all excipients are inert substances, and some may be toxic atincreased concentrations61. In the Code of Federal Regulations, theFDA has published a list of substances that are generallyrecognized as safe (GRAS). Apart from this, it also maintains alist of inactive ingredients for excipients entitled Inactive Ingre-dient Guide (IIG) that are approved and can be incorporated inmarketed products62. This guide provides the list of maximumamount allowed for excipients, which can be used for a specificroute of administration. Once an inactive ingredient has beenapproved for a product through a particular route of administra-tion, it can be used in any new drug formulation and does notrequire extensive review. The formulator can take the informationfrom both GRAS and IIG when developing a new formulation.Currently, the FDA does not have any process or mechanism toevaluate the safety of excipients individually. Instead, the exci-pients are reviewed and approved as ‘components’ of the drug orbiological product in the application. Since excipients play anintegral part in the formulation and cannot be reviewed separatelyfrom the drug formulation, the regulatory process is appropriatefrom a scientific standpoint.

4. Lipid-based formulations

4.1. Lipid formulation classification system

A working model of a lipid formulation classification system(LFCS) was introduced in 200010 and an extra type of formulationwas added in 20068. The in vivo behavior of the formulation canbe interpreted easily by LFCS. With reference to the physico-chemical properties of specific drugs, the most suitable formula-tion can be identified through LFCS. Table 3 shows the variousclasses of LFCS. Most of the marketed products are Type IIIsystems, which are diverse with a wide range of oil- and water-soluble substances. Hence, this group has been further divided intoType III A (oils) and Type III B (water-soluble) based on theproportion of oils and water-soluble substances8,10.

Table 3 The lipid formulation classification system14.

Type I Type II Type IIIA (fine emulsion) Type IIIB (microemulsion) Type IV

Oils withoutsurfactants

Oils and waterinsolublesurfactants

Oils, surfactants, cosolvents(both water-insoluble and water-soluble excipients)

Oils, surfactants, cosolvents (bothwater-insoluble and water-soluble excipients)

Water-solublesurfactants and cosolvents(no oils)

Non-dispersing Emulsion (SEDDS) SEDDS/SMEDDS formedwith water-soluble components

SEDDS/SMEDDS formed withwater-soluble components andlow oil content

Disperses typicallyto form a micellarsolution

Requires digestion Will be digested Digestion may not be necessary Digestion may not be necessary Limited digestion

Table 4 Commercially available Lipid-based products for oral administration41.

Trade name Molecule Therapeutic use Company

Agenerases Amprenavir HIV antiviral Glaxo SmithKlineRocaltrols Calcitriol Calcium regulator RocheCipros Ciprofloxacin Antibiotic BayerNeorals Cyclosporin A/I Immuno-suppressant NovartisGengrafs Cyclosporin A/III Immuno-suppressant AbottAccutanes Isotretinoin Anti-comedogenic RocheKaletras Lopinavir and Ritonavir HIV antiviral AbottNorvirs Ritonavir HIV antiviral AbottLamprenes Clofazamine Treatment of Leprosy Alliance laboratoriesSustivas Efavirenz HIV antiviral Bristol-MeyersFenogals Finofibrate Anti hyperlipproteinomic GenusRestandols Testosterone undecanoate Hormone replacement therapy Organon laboratoriesConvulexs Valproic acid Anti epileptic PharmaciaJuvelas Tocopherolnicotinate Hypertension, hyperlipidemic Eisai Co.

S. Kalepu et al.366

4.2. Formulation approaches

Lipid-based drug delivery systems can be developed successfullyby careful consideration of the formulation objectives. Table 4indicates the list of commercially available lipid-based products41.The systematic approach includes pre-selection of excipients basedon their melting point, fatty acid composition, HLB value,digestibility and disposability; screening of selected excipientsfor solubility, dissolution/dispersion properties, stability and com-patibility; identification of a formulation technique which issuitable for the intended dosage form; design of appropriateanimal models to predict the in vivo performance of the chosenformulation; and optimization of the formulation considering thedrug loading and dissolution profile.

4.2.1. Oily liquidsSome drugs are highly lipophilic and have solubility in oils only,e.g. steroids, and are solubilized in triacyglycerols only. Suchdrugs have to be formulated as oily liquids by solubilizing the drugin oil. But the quantity of oil required to dissolve a unit dose ofdrug is very high, which restricts or limits the usage of drug in oilformulations. An oily solution of bupivacaine free base wasformulated, using a mixture of fractionated coconut oil (Visco-leos) and castor oil by Larsen et al.63. The formulated oily liquidshowed a prolonged local analgesic activity and reduced systemictoxicity when administered subcutaneously to male Wistar rats.

4.2.2. Mixed micellesThese systems consist of more than one molecular species. Thesemicelles represent a disc like structure and resemble a lipid bilayer.In detergent-lipid mixed micelles, the detergent molecules shieldthe lipid molecules against water at the edges. Increased activityagainst taxol-resistant and sensitive lung cancer cell lines wasobserved when paclitaxel and parthenolide were co-encapsulatedin mixed micelles of PEG 2000 – distearoyl phosphatidylethano-lamine (DSPE) and Vitamin E TPGS64. Chen et al.65 reportedenhanced anti-tumor activity of methotrexate against multidrugresistant tumors when conjugated with polymeric mixed micelles(composed of Pluronic F127 and P105).

4.2.3. Self-emulsifying systemsAs the name suggests, these systems have the ability to emulsifydue to the presence of one or more surfactants in addition to theoily phase. The lipophilic drug is solubilized in the oily vehicle.The surfactant helps in dispersing the oily vehicle in the GI fluid,which leads to the formation of a micro-emulsion. Dependingupon the size of the emulsion particles, these systems can befurther classified as self-micro emulsifying drug delivery systems(SMEDDS) or self-nano emulsifying drug delivery systems(SNEDDS). The formulation generally consists of drug, oilyvehicle, surfactant, co-surfactant and even co-solvents. PuerarinSMEDDS sustained release pellets for oral delivery were devel-oped using castor oil as the oil phase, Cremophors EL as theemulsifier, and 1,2-propanediol as the co-emulsifier by Zhang

Lipid-based drug delivery systems 367

et al.66. The formulated self-emulsifying pellets provided asustained drug release while simultaneously improving the oralbioavailability of puerarin.

4.2.4. LipososmesLiposomes have spherical bilayer structures which resemble thecell membrane in their arrangement. The lipids mainly used arephospholipids, which are amphiphilic in nature, having a hydro-philic head and hydrophobic tail (fatty acid). These phospholipidswhen hydrated in water form spherical bilayer structures, orientingwith their hydrophobic part facing each other (toward inside) andhydrophilic part facing outwards. The advantage of these systemsis that hydrophilic substances can be embedded in the aqueousinternal spaces of the globules, while hydrophobic drugs can beembedded within the inner fatty acid layers. Propylene glycolliposomes loaded with epirubicin have been reported to overcomemulti-drug resistance in breast cancer. These liposomes showed agood permeability to both the cell membrane and nuclearmembrane of the tumor cell67.

4.2.5. Solid lipid nanoparticlesRecently, solid lipid nanoparticles (SLN) have gained muchinterest due to their ability to enhance bioavailability along withcontrolled and site-specific drug delivery. Hence they are exploitedas probable possibilities as carriers for oral intestinal lymphaticdelivery. SLNs are typically spherical particles (size range 10–1000 nm) with a solid lipid core matrix (stabilized by surfactants)that can solubilize lipophilic molecules. Lipids mainly usedinclude monoglycerides (e.g. glycerol monostearate), diglycerides(e.g. glycerol behenate), triglycerides (e.g. tristearin), fatty acids(e.g. stearic acid), steroids (e.g. cholesterol), and waxes (e.g. cetylpalmitate). As reported by Venishetty et al.68, the oral bioavail-ability of carvedilol could be improved by formulating a polymer(N-carboxymethyl chitosan) that coated carvedilol solid lipidnanoparticles. Monoglyceride was used as the lipid along withsoya lecithin and poloxamer 188 as surfactants for the formulationof carvedilol-loaded SLN.

4.3. Conversion of liquid–lipid formulations to solidintermediates

Oral delivery is the most preferred and popular route of drugadministration. Liquids and solids can be given through oral route.With regard to the physical and chemical stability of liquids andsemisolids, these can be transformed into solid particles (powdersor granules), which can be filled into capsules or alternativelycompressed into tablets by selecting suitable tableting excipients.

4.3.1. Spray congealingThis is also referred to as spray cooling. In this method, moltenlipid is sprayed into a cooling chamber and, on contact with thecool air, congeals into spherical solid particles. The solid particlesare collected from the bottom of the chamber, which can be filledinto hard gelatin capsules or compressed into tablets. Ultrasonicatomizers are frequently used to generate solid particles in thisspray cooling process. The parameters to be considered are themelting point of the excipient, the viscosity of the formulation andthe cooling air temperature inside the chamber to allow instantsolidification of the droplets. Praziquantel granules were preparedby melt granulation using PEG 4000 or Poloxamer 188 as ameltable binder and lactose monohydrate as filler69. In another

study by Cavallari et al.70, microparticles with narrow sizedistribution were obtained when stearoyl polyoxylglycerides(Gelucires 50/13) were used as an excipient and significantlyenhanced the drug release of poorly water soluble drugs likediclofenac.

4.3.2. Spray dryingThis method is somewhat similar to previous one, but differs in thetemperature of the air inside the atomizing chamber. In thismethod, the drug solution (drug in organic solution/water) issprayed into a hot air chamber, where the organic solvent or waterevaporates giving rise to solid microparticles of drug. During thisprocess, along with the lipid excipients, solid carriers like silicondioxide can be used. Gelucire™ (lipid excipient), enhances thedrug release process by forming hydrogen bonds with the activesubstance, leading to the formation of stable solids of amorphousdrug in microparticles71,72. Solid dispersions of glibenclamidewere prepared using a spray drying technique by Chauhan et al.72.In this method, glibenclamide was dissolved in sufficient solventto yield a clear solution. Silicon dioxide was added and theresulting suspension was spray-dried using a spray dryer underfixed conditions of inlet and outlet temperature, feed rate andatomization air pressure.

4.3.3. Adsorption onto solid carrierThis is a simple process and economical (in the context ofequipment investment). In this method, a liquid–lipid formulationis adsorbed onto solid carrier like silicon dioxide, calcium silicateor magnesium alumino-metasilicate. The liquid–lipid formulationis added to the carrier by mixing in a blender. The carrier must beselected such that it must have greater ability to adsorb the liquidformulation and must have good flow property after adsorption.Gentamicin and erythropoetin with caprylocapryoyl polyoxylgly-cerides (Labrasols) formulations were successfully converted intosolid intermediates whose bioavailability was maintained evenafter adsorption on carriers. Advantages of this method includegood content uniformity and high lipid exposure73–75. Ito andco-workers75 have developed a solid formulation of gentamicinusing emulsifier and adsorbent. Using solid adsorbents likecalcium silicate, magnesium alumino-metasilicate and silicondioxide, the liquid mixture (drug and emulsifier like Labrasols)was converted to a solid by a kneading process.

4.3.4. Melt granulationThis is also referred as pelletization, which transforms a powder mix(with drug) into granules or pellets76–78. In this method a meltablebinder (molten state) is sprayed onto the powder mix in presence ofhigh-shear mixing. This process can be referred to as a ‘pump on’technique. Alternatively, the meltable binder is blended with powdermix and due to the friction of particles (solid/semisolid) during thehigh-shear mixing, the binder melts. The melted binder forms liquidbridges between powder particles and forms small granules whichtransform into spheronized pellets under controlled conditions. Depend-ing on the fineness of the powder, 15%–25% of the lipid-based bindercan be used. The parameters to be considered during the process arebinder particle size, mixing time, impellar speed and viscosity of thebinder on melting79. The dissolution rate of diazepam was improved byformulating melt agglomerates containing solid dispersions of diaze-pam by Seo et al.80. Lactose monohydrate was melt-agglomerated witha meltable binder like PEG 3000 of Gelucires 50/13 in a high shearmixer. Polyoxylglycerides, partial glycerides or polysorbates, and

S. Kalepu et al.368

lecithins are some of the lipid excipients used in the melt granulationtechnique to form self-micro-emulsifying systems80,81.

4.3.5. Supercritical fluid-based methodThis method uses lipids for coating drug particles to produce soliddispersions. In this method, the drug and lipid-based excipients aredissolved in an organic solvent and then in supercritical fluid(carbon dioxide)82,83, by elevating the temperature and pressure.The coating process is facilitated by a gradual reduction inpressure and temperature in order to reduce the solubility of thecoating material in the fluid and hence precipitate onto the drugparticles to form a coating84,85. The solubility of the formulationcomponents in the supercritical fluid and stability of the substanceduring the process are important considerations of this method.Glyceryl trimyristate (Dynasan™ 114) and steoaroyl polyoxylgly-cerides (Gelucires 50/02) have been used in this process for theircontrolled-release applications82,83. Sethia and squillante84–86 haveformulated solid dispersions of carbamazepine using supercriticalcarbon dioxide. Vitamin E TPGS and Gelucires 44/14 weresuccessfully used with this technique (supercritical fluid proces-sing) for enhancement of the bioavailability of carbamazepine. Thesolid dispersions formulated with TPGS showed enhanced bioa-vailability of drug when compared to formulations prepared withGelucires 44/14 as lipid excipient.

4.4. Characterization of lipid-based formulations

4.4.1. In vitro studiesA preliminary guideline for formulation development and assess-ment of drug release can be obtained from in vitro studies. Apartfrom assessing batch-to-batch consistency, these studies alsodepict the in vivo evaluation of the lipid-based formulation.

In vitro evaluation of lipid-based drug delivery systems can bedone with the use of lipid digestion models. In order to assess theperformance of an excipient during formulation development and topredict in vivo performance, it is necessary to design an in vitrodissolution testing method. This can be termed as “simulated lipolysisrelease testing”, which is described in the literature19. The instrumentused for studying enzymatic hydrolysis of lipids is a pH-stat titrationsystem, which is depicted in Fig. 3. The basic principle on which thissystem works requires maintaining a constant pH during a reactionwhich releases or consumes hydrogen ions. If any deviation is found,it is compensated by the reagent addition.

Figure 3 pH-Stat titration system.

The model consists of a temperature-controlled vessel (37 1C),which contains a model intestinal fluid, composed of digestionbuffer, bile salt (BS) and phospholipid (PL). Into this model a fluidlipid-based formulation is added and to initiate the digestionprocess pancreatic lipase and co-lipase were added. As thedigestion process starts it results in the liberation of fatty acids,causing a transient drop in pH. This drop in pH is quantified by apH electrode. The pH electrode is coupled to a pH-stat metercontroller and autoburette. An equimolar quantity of sodiumhydroxide is added to titrate the liberated fatty acids by theautoburette, so as to prevent a change in pH of the digestionmedium from a pre-set pH value. By quantifying the rate ofsodium hydroxide addition and considering the stoichoimetricrelationship between fatty acids and sodium hydroxide, the extentof digestion can be quantified. During the digestion process,samples can be withdrawn and separated into a poorly dispersedoil phase, highly dispersed aqueous phase and precipitated pelletphase by centrifugation, as shown in Fig. 4. Quantification of drugin the highly dispersed aqueous phase indicates that drug has notprecipitated, from which an assumption can be made with respectto in vivo performance of the lipid-based formulation.

The effect of the concentration of bile salts, calcium and lipaseactivity on the digestion process was investigated87,88. Resultsshowed that all three parameters had an impact on the initial rate ofhydrolysis, whereas the subsequent stages were affected bycalcium concentration and lipase activity. To study effect of foodfor poorly water-soluble drugs, an in vitro lipid digestion modelwas developed by Christensen and co-workers89. In this model, thetransfer of lipophilic drugs from fractionated coconut oil andsesame oil to the aqueous phase was studied. Dahan and Hoff-man90 have presented the importance of in vitro lipolysis modelfor optimizing the oral lipid-based formulations with regard to pre-systemic metabolism in the gut.

4.4.2. In vivo studiesThe impact of excipients on the bioavailability and pharmacoki-netic profile of drugs can be estimated by designing appropriatein vivo studies. A detailed study of intestinal lymphatic absorption

Figure 4 Phase separation in the lipolysis sample after ultracen-trifugation.

Lipid-based drug delivery systems 369

is required, since lipid-based formulations enhance bioavailabilityby improving the intestinal uptake of drug. Due to insufficientclinical data and differences in methods and animal models used,studies related to the drug transport by lymphatic system havebecome difficult91. Hence further work has to be carried out toestablish an in vivo method and model to predict lymphatic drugtransport. A lipid-based formulation of saquinavir mesylate (For-tovases) enhanced the bioavailability of the drug up to three-foldwhen compared to Invirases (saquinavir in hard gelatin cap-sules)92. A study in a mesenteric lymph duct-cannulated rat modelwas conducted to understand the mechanism for the improvedbioavailability with this formulation. The results showed thatenhanced solubilization and permeability of the drug in thelipid-rich pre-absorptive intestinal environment was the mainreason for the increase bioavailability from Fortovases39.

In lymphatic absorption the important step is the association ofdrug with chylomicrons in the enterocyte. In order to study this, anexperimental rat model with blocked chylomicron flow to eluci-date the lymphatic transport of Vitamin D3 was conducted. Uponcomparison of these results with the mesenteric lymph duct-cannulated rat model, it showed that the association of drug withchylomicrons led to 75% of the Vitamin D3 being absorbedthrough lymphatic uptake93–95. The effect of food (high fatty meal)on the bioavailability of cyclosporine was studied by formulatingthe drug as Sandimmune Neorals (lipid-based formulation-microemulsion of a surfactant, lipophilic and hydrophilic solvents,and ethanol), and simple emulsion in soft gelatin capsules, and wascarried out in healthy human volunteers. The results confirmed thatthe effect of food was lessened with the lipid-based formulation(Sandimmune Neorals), when compared to a significant effect offood (37% increase in area under the curve) from the simpleemulsion. This allowed more flexibility of the lipid-based for-mulation with regards to their dietary schedule94.

4.4.3. In vitro–in vivo correlation (IVIVC)The in vivo oral bioavailability of two model drugs, griseofulvinand dexamethasone, was predicted by designing an in vitrolipolysis and ex vivo intestinal permeability model. Both drugshad good correlation with their in vitro and in vivo data in regardsto lipolysis and absorption. But, the actual in vivo data failed tocorrelate with the ex vivo permeation studies. Griseofulvin, withlimited solubility (5 mg/mL), is influenced by the lipid excipients,the presence of bile salts, phospholipids and lipolytic products inthe digestive media95. A sound in vitro–in vivo correlation willhelp to maximize the development potential and commercializa-tion of lipid-based formulations. A shortened drug developmentperiod and improved product quality could be achieved bydeveloping a model that correlates the in vitro and in vivo data.Determining the solubility, dissolution, lipolysis of the lipidexcipient, intestinal membrane techniques (isolated animal tissueand cell culture models) are various in vitro techniques that can beused to assess lipid-based formulations96. Such techniques provideinformation about specific aspects of the formulation only. But, itis important to know the in vivo interaction and performance ofthese systems.

Similar to that of in vivo enterocytes, Caco-2 cells produce andsecrete chylomicrons on exposure to lipids. More study has to becarried out on the choice of the most suitable in vivo model forassessing the lipid-based formulations.

For initial screening and identifying a prototype formulation,the rat is a reliable model. However, the different anatomy and

physiology the rat, such as the lack of a gall bladder and differentexpression and pattern levels for intestinal enzymes makescorrelation of results to human beings challenging. Several lipid-based formulations have been assessed using a dog model97, but apoor correlation was observed between dog and human due todifferences in GIT physiology like gastric pH and enzymaticprofile of enterocytes. In terms of close relation between theanatomy and physiology of the GIT, the pig can be considered as amost suitable non-primate animal model98. This model permits theoral administration of full-sized human dose, and even fed versusfasting studies can be done. The impact of intestinal absorptionand first-pass metabolism of drugs can be studied using the pigmodel99. Hence, in order to establish reliable in vitro–in vivocorrelations in lipid-based formulations, selection of the mostappropriate in vitro and in vivo models is of primary importance.

5. Conclusions

As many drugs are successfully marketed as lipid-based formula-tions, the lipid-based drug delivery system (LBDDS) has a widescope in terms of solubility and bioavailability enhancement. Thisreview focused on the current trends in the field of LBDDS withrespect to formulation approaches and their characterization.However, a few limitations of the technology such as the stabilityof lipid-based formulations, manufacturing methods, the lack of adatabase considering the solubility of drugs in lipids, indicate thatdevelopment of proper regulatory guidelines for lipid-basedformulations still need to be addressed in depth to advance thetechnology. Further research has to be carried out in this fieldregarding the design of a proper in vivo model to correlate the dataobtained in vitro studies to the actual in vivo experience. Thisreview provides a summary of lipid-based formulations which maybe helpful for the advancement of this technology to obtain a safer,more stable and efficacious drug products.

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