Self-Emulsifying Delivery Systems for Poorly Absorbed Drugs

Transcription

1 Jayvadan Patel and Anand Shah : Self-Emulsifying Systems for Poorly Absorbed Review Drugs Article 123 Self-Emulsifying Delivery Systems for Poorly Absorbed Drugs Jayvadan Patel* and Anand Shah Department of Industrial Pharmacy, S.K.Patel College of Pharmaceutical Education & Research, Ganpat University, Kherva ABSTRACT: Self-emulsifying drug delivery systems (SEDDS) are a type of emulsion that have received particular attention in recent years as a means of enhancing oral bioavailability of poorly absorbed drugs. These systems are ideally isotropic mixture of oil and surfactant (sometimes co-surfactant are added) that form emulsions on mixing with water with little or no energy input. Hydrophobic drugs are dissolved in SEDDS. After administering the drug by this system, emulsion is formed in the GIT by selfemulsification. Generally by this process bioavailability of the drug is increased and quantity required to exert desired effect is decreased. Oil surfactant ratio, amount of surfactant, type of surfactant, nature of oil affect the process of emulsification. In the emulsification process water penetrates into the oil water interface, leading to the formation of liquid crystalline phase, resulting in swelling at the interface. Generally, SEDDS are formed with triglyceride oils and nonionic surfactant. In the GIT, during the digestion of fat, drugs are getting released and lead to increase the absorption of drug by dissolution or by particle charges which increase the bioavailability. Some latest related to SEDDS is described in this review. KEY WORDS: Self-emulsifying drug delivery systems, bioavailability, poorly absorbed drugs. Introduction Emulsions are used as a vehicle for the administration of drug. It is this consideration that has received particular attention, especially due to the possibility of enhancing the oral bioavailability of poorly absorbed drugs (Attwood D et al., 1983). Drug delivery via lipid vehicles provides opportunities to enhance the absorption of many drugs. Although there are still numerous challenges associated with this delivery approach, an understanding of physical chemistry of surfactants and their mode of interaction with oils and water is essential. The use of high concentration of surfactants is a legitimate concern from a toxicological standpoint (Boyd J et al., 1972, Carey MC et al., 1983). Furthermore, there may be a suspicion that chemical stability of the drug will be compromised in an oily solution than in its crystalline form. Although there are a numbers of compounds for which SEDDS can be a suitable delivery system, the development of certain formulations by Sandoz, has conformed the potential of SEDDS to bring drugs especially, hydrophobic compounds to the market. (Charman SA, et al., 1992, Cheema et al., 1987) There are so many drugs which have poor bioavailability from marketed tablet formulations for which SEDDS can be a better choice (De Caro JD et al., 1986). These formulations are isotropic mixture of oils and surfactants which sometimes incorporates co-solvents that emulsify upon gentle agitation. * Corresponding author: Jayvadan Patel Ph: jayvadan04@yahoo.com Development of SEDDS The development of SEDDS may be traced back to the pesticide industry, where by many herbicides which are highly lipophilic and may not be prepared as concentrated aqueous solution were developed as SEDDS. Herbicide concentrates are necessary for easy of transportation. The herbicides which are dissolved in organic solvents containing surfactant could than be emulsified upon simple mixing with water. The concept of using such system for pharmaceutical purpose was initially developed by the Group of Groves (Dunkan QM et al., 2000, Fernando- Warnkulasuriya GLP et al., 1981). After their work on these lines, the issue relating to this technology has gained prominence. In these formulations, emulsification takes place in the gut and by this mechanism bioavailability enhancement takes place. The use of SEDDS may reduce required oral dose if there is an extensive improvement in oral bioavailability. The rate of gastric emptying of SEDDS is similar to solution so that they are particularly useful when repeat onset of action is necessary (Greenberger NJ et al., 1966). Mechanism of Self Emulsification In emulsification process the free energy ( G) associated is given by the equation: G = N i πr 2 i σ 1 In which N is Number of droplets with radius r and σ is interfacial energy (Groves MJ et al., 1976) It is apparent from equation that the spontaneous formation of the interface between the oil and water phases is energetically not favored. The system commonly classified as SEDDS have not yet been shown to emulsify spontaneously in the 123

2 124 International Journal of Pharmaceutical Sciences and Nanotechnology Volume 1 Issue 2 July - September 2008 thermodynamic sense. The process of self-emulsification was observed using light microscopy. Groves and Mustafa (Embleton JK et al., 1997) developed a method of quantitatively assessing the ease of emulsification by monitoring the turbidity of the oil-surfactant in a water stream using phosphated nonylphenoloxylate (PNE) and phosphated fatty alcohol ethoxlate (PFE) in n-hexane. Pouton (De Caro JD et al., 1986) has argued that the emulsification properties of the surfactant may be related to phase inversion behavior of the system. For example, on increase the temperature of an oil in water system stabilized using nonionic surfactant, the cloud point of the surfactant will be reached followed by phase inversion (Groves MJ et al., 1974). The surfactant is highly mobile at the phase inversion temperature; hence the o/w interfacial energy is minimized leading to a reduction in energy required to cause emulsification (Groves MJ et al., 1974). The specificity of surfactant combination required to allow spontaneous emulsification may be associated with a minimization of the phase inversion temperature, there by increasing the ease of emulsion (Humberstone AJ et al., 1997). Phase studies are also necessary for liquid crystal formation in self-emulsification. These indicate that good formulations are usually operating close to a phase inversion region and in a region of enhanced close to a phase inversion region and in a region of enhanced aqueous solubilization. In the phase diagram of the system (30 % w/w tween and 85/70 % w/w MCT oil) for dilution in water over a range of temperature shows that the phase inversion region is at approximately 40 C and the system works well at ambient temperature up to 60 C above which water in oil emulsion tend to form (Kovarik KN et al., 1994). The emulsification process may be associated with the ease with which water penetrates the oil-water interface with the formation of liquid crystalline phases resulting in swelling at the interface thereby resulting in greater ease of emulsification. However, for system containing cosurfactant, significant partitioning of components between the oil and aqueous phases may take place leading to a mechanism described as diffusion and stranding, where by the oil is solubilized, leading to migration in to the aqueous phase. Formulation of SEDDS The present article focuses on isotropic mixture which emulsifies to produce fine oil in water emulsion under condition of gentle agitation. To incorporate the drug into a suitable oil-surfactant mixture, the solution or suspension may then be presented as a dosage form such as a liquidfilled soft or hard gelatin capsule. The earliest reports of self-emulsifying system using pharmaceutical materials were of pastes, based on waxy alcohol ethoxate (De Caro JD et al., 1986). If the formulation includes a substantial proportion of hydrophobic compound the undiluted formulation may well be a microemulsion itself and could be used to solubilize hydrophilic drugs (Kovaric JM et al., 1994, Lin JH et al., 1991) Oil surfactant ratio is one of the most important consideration relating to the formulation. The emulsification properties are highly sensitive to relatively small changes in this ratio. For Example, Pouton (Attwood D et al., 1983) showed that the time taken for emulsification of tween 85/miglyol-840 mixes was highly dependent on the oil/surfactant ratio. The most rapid emulsification occurred at an optimum surfactant content of 35% w/w though it was concluded that all system containing % w/w tween 85 emulsified vary rapidly. If the surfactant content was more than 50 % the formation of viscous gels appeared to retard self-emulsification, though these systems produce very fine dispersion if more energy for dispersion was provided by homogenization. Wakerly et al., (Lin TJ et al., 1977) have studied the particle size of emulsions produced using Tagat To and Miglyol-812 finding a minimum in size at approximately 50% w/w surfactant. And also found an increase in particle size of Tagat To / Miglyol-812 emulsion on addition of probucol with decrease in proportion of surfactant required to produce a minimum size. Type of surfactants is another parameter for SEDDS. The toxicity of the surfactant is also to be considered. For toxicity reason, today work has focused on the use of nonionic surfactant particularly tween and spans. Surfactant may inhibit the digestion of oils, so selection of surfactant is an important criteria. For example, studies using cremophor RH40, an ethoxylated triglyceride over a 90-min period, while inclusion of liphophilic surfactant caused partial recovery of lipolysis (Linthcrst JN et al., 1977). Soloman et al., (Liu H et al., 1991, MacGregor KJ et al., 1997), examined a wide range of nonylphenylethoxylates, allowing examination of the relationship between HLB and lipolysis inhibition. Lipolysis is inhibited to greater extent by surfactants with greater HLB value of 12 and most effective lipolysis inhibition between HLB 13 to 17. Nature of oil is very important in formation of SEDDS. Chemical structure of oil components and interaction of those components with the various enzymes, surfactants and proteins associated with digestion and absorption process for example, fatty acid chain length is important factor for chylomicron. Short and medium chain acids are predominantly absorbed by portal blood system while longer chain fatty acids may be re-esterified in the cells lining the small intestine and absorbed via the lymphatics (Ockner RK et al., 1974, Ockner RK et al., 1974). The absorption enhancement is greater when using unsaturated fatty acids. Cheema et al., (Pouton CW 1985), reported that a

3 Jayvadan Patel and Anand Shah : Self-Emulsifying Systems for Poorly Absorbed Drugs 125 greater degree of unsaturation led to a more rapid on set of lipoprotein synthesis as a result of faster absorption or greater affinity of fatty acid binding protein for unsaturated fatty acids have lower melting points as compared to saturated and increased fluidity. Liquid crystal formation from oil is depends on oil polarity which would influence the emulsification process. Very polar or nonpolar oils tend to form poor emulsion. Miglyol-812 and 840 with intermediate polarity have shown favorable emulsification properties with tween 85. Solubility of drug in the oil-surfactant mixture is very important, when solubility of drug in vegetable oil is not a problem, the simplest formulation and most desirable may well be a simple oil solution which it self be emulsified in the gut during digestion (Pouton CW 1997). Absorption of Drug Most of the dietary lipids are triglycerides which are fatty acids ester of glycerol. Generally lipase digests the fat. On ingestion of the triglycerides a course emulsion is believed to form in stomach with dietary phospholipids, proteins and polysaccharides are believed to be potent emulsifiers, forming a monolayer around the triglyceride droplets (Pouton CW 2000, Pouton CW 1982). Around 10 to 40% of normal fat digestion takes place in the stomach, involving hydrolysis to diglycerides and fatty acids. This process is being done by human gastric lipase (HGL). Short chain fatty acids may dissolve into the aqueous phase followed by absorption across the stomach mucosa, while longer chain acids may remain incorporated in the emulsion droplet core (Ptachcinsky RJ et al., 1986, Shah NH et al., 1994). The emulsion passes to the upper section of the large intestine where particle size reduction of the droplets takes place due to the presence of a range of emulsifying agent including bile salts, monoglycerides, cholesterol, lecithin and lysolecithin, yielding an approximate size range of 0.5 to 1 µm (Shinoda K et al., 1986). The mechanism responsible for such efficient emulsification of ingested triglycerides are not yet clear, although in vitro studies have shown that mono olein, oleic acid and monomeric bile salts may at intestinal ph significantantly lower the interfical tension of triolein droplets (Solomon LJ et al., 1996) there by allowing emulsification to take place under condition of comparatively low shear. Then lypolysis takes place via triacylglycerol acyl hydrolase, typically referred to as pancreatic lipase. This enzyme act specifically at the surface of emulsion droplets and causes hydrolysis of triglyceride at the 1 st and 3 rd position to produce fatty acids, diglycerides and monoglycerides (Staggers JE et al., 1981). Fig. 1 Processing of lipids and co-administered drug

4 126 International Journal of Pharmaceutical Sciences and Nanotechnology Volume 1 Issue 2 July - September 2008 Fig. 2 Intestinal pre-absorptive processes. Fig. 3 Distribution of drug solubilized in an emulsion.

5 Jayvadan Patel and Anand Shah : Self-Emulsifying Systems for Poorly Absorbed Drugs 127 The bioavailability enhancement of a drug is generally depends on its dissolution behavior or particle charges of the drug molecule. The drug L-365, 260 was prepared as labrafil M2125 and tween 80 SEDDS preparation (Swenson ES et al., 1994) which showed a seven to eight folds in increase in C max compared to a tablet or suspension formulation. The absorption profile for SEDDS were similar to the profile observed after administration of a PEG-600 solution, but the rate limiting step to absorption may be dissolution of the drug in GIT fluid rather than lipid digestion. Shah et al., (Wakerley MG et al., 1987), examined the bioavailability of Ro , a highly lipophillic naphthalene derivative, demonstrating a four folds greater bioavailability for SEDD formulation than PEG-400 solution and demonstrative a 20 folds greater bioavailability than from a standard tablet formulation. Here the aspect of drug absorption from SEDD formulation is due to the particle charge. Conclusion This article has out lined the delivery of drugs to GI tract by SEDDS. From the formulation point of view, it is necessary to consider the emulsification properties of lipid vehicle and the solubility of drug in an oil or oil-surfactant mixture, both in terms of determining whether the drug will be in suspension or in solution form to solubilize adequate quantities of drug in lipid vehicle (Ptachcinsky RJ et al., 1986). The digestion of the oil having a principle role in the absorption process. For such drug SEDDS is more conventional. So, in future SEDDS is likely to play an important role in the formulation of poorly soluble drugs. References Attwood D and Florence AT. Surfactant Systems: their chemistry, Pharmacy and biology. Champman and Hall. London.74: (1983). Boyd J, Parkinson C and Sherman P. Factors affecting emulsion stability and the HLB concept. J. Colloid Interface Sci. 41: (1972). Carey MC, Small DM and Bliss CM. Lipid digestion and adsorption. Annu Rev Physiol 45: (1983). Charman SA, Charman WN, Rogge MC, Wilson TD, Dutko FJ and Pouton CW. Self-emulsifying systems: formulation and biological evaluation of an investigative lipophilic compound. Pharm. Res. 9: 87-94(1992). Cheema M, Palin K and Davis SS. Lipid vehicles for intestinal lymphtic drug transport. J Pharm Pharmacol 39: 55-56(1987). De Caro JD, Rouimi P and Rovery M. Hydrolysis of p- nitrophenyl acetate by the peptide chain fragment ( ) of porcine pancreatic lipase. Eur J Biochem 158: (1986). Dunkan QM, Craig M, Patel J and Ashford M. Pharmaceutical Emulsion and Suspension.VOL-105, Marcel Dekker, Inc., New York, 2000 Embleton JK and Pouton CW. Structure and function of gastrointestinal lipases. Adv Drug Del Rev 25: 15-32(1997). Fernando-Warnkulasuriya GLP, Stagers JE, Frost SC and Wells MA. Studies on fat digestion, absorption and transport in the suckling rat. 1. Fatty acid composition and concentration of major lipid components. J Lipid Res 22: (1981). Greenberger NJ, Rodgers JB and Isselbache K Jr. Absorption of medium and long chain triglycerides:factors influencing their hydrolysis and transport. J Clin Invest 45: (1966). Groves MJ and de Galindez DA. The self-emulsifying action of mixed surfactants in oil. Acta Pharm. Suec. 13: (1976). Groves MJ and Mustafa RMA. Measurement of the spontaneity of self-emulsifiable oils. J Pharm Pharmacol 26: (1974). Groves MJ, Mustafa RMA and Carless JE. Phase studies of mixed phosphated surfactants n-hexane and water. J Pharm Pharmcol 26: (1974). Humberstone AJ and Charman WN. Lipid-based vehicles for the oral delivery of poorly water soluble drugs. Adv Drug Deliv Rev 25: (1997). Kovarik JM, Mueller EA, Van Bree JB, Tetzloff W and Kutz K. Reduced inter and intraindividual variability in cyclosporine pharmacokinetics from a microemulsion formulation. J. Pharm Sci. 83: (1994). Lin JH, Chen I and Lievens H. The effect of dosage forms on the oral absorption of L , a potent CCK B receptor antagonist in dogs. Pharm Res 8: S-272(1991). Lin TJ, Kurihara H and Ohta H. Prediction of optimum o/w emulsification via solubilization measurement.j.soc.cosmet.chem. 28: (1977). Linthcrst JM, Clark SB and Holt PR. Triglyceride emulsification by amhipaths in the intestinal lumen during digestion of fat. J Colloid Interface Sci. 60: 1-10(1977). Liu H, Adachi I, Horkoshi I and Ueno M. Promotion of intestinal drug absorption by milk fat globule membrance. Yakugaku Zasshi 111: (1991). MacGregor KJ, Embleton JK, Lacy JE, Perry EA, Solomon LJ, Seager H and Pouton CW. Influence of lipolysis on drug absorption from the gastrointestinal tract. Adv Drug Deliv Rev 25: 33-46(1997).

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