Ever since the invention by Bangham in 1966, liposomes has been evolving steadily not only as a nice tool to study the function of cell membranes but also as unique drug carriers with enhanced efficacy and reduced toxicity [Citation1,Citation2]. Currently, liposomes present as one of the best drug delivery systems, solely owing to their capacity to encapsulate various drug entities, excellent biocompatibility and easy interaction with biomembranes. Several decades of ordeal finally gave birth to the milestone product, long-circulating doxorubicin liposomes with the brand name Doxil®, together with several other liposomal products such as amphotericin B liposomes and paclitaxel liposomes. Although originally designed for parenteral delivery, liposomes were also attempted in oral delivery as early as in the 1970s for those entities with extremely low oral bioavailability, for example, insulin. Despite of the initial excitement in the observation of enhanced bioavailability of insulin, the interests in oral liposomes subsided due to the far-lower-than-appealing bioavailability, difficulties in formulation and uncertainties in in vivo fate. However, recently saw a resurgence of oral delivery of liposomes.
Challenges in oral liposome delivery
It is an odyssey from the ingestion of liposomes to the absorption of active ingredients embedded in liposomes. The first challenge emerges when liposomes are ingested and exposed to the harsh environment of the gastrointestinal (GI) tract. Conventional liposomes that composed of phospholipids and cholesterol are found to be highly susceptible to physiological factors such as gastric acid, bile salts and lipases [Citation3], whose destructive effects commonly result in losing of the liposomal integrity and thus leakage of the payload. Bile salts, surfactant secreted by liver, can disrupt the liposomal structure, whereas lipases are able to hydrolyze phospholipids and thereby bring about disruption of liposomes. It was observed that liposomes would lose integrity within 2 h in simulated intestinal fluid with significantly deformed structure [Citation4].
If liposomes are made survive the harsh GI environment, there follows the second challenge of poor permeability of liposomes across GI epithelia which presents as the main absorption barrier. Currently we know little about the exact mechanisms governing oral absorption of liposomes. There remains controversy over whether the active ingredients are released firsthand before being absorbed or whether the liposomes together with their payload can be absorbed intact by the GI epithelia. The mucus lining that coats and protects the GI epithelia sheds frequently, which as a result clears the substances that cannot penetrate the mucous layer quickly enough [Citation5]. Albeit there is chance for liposomes to penetrate the mucous layer, the GI epithelia blocks the entrance of intact liposomes into the circulation because they are commonly very large in size in comparison with easily absorbable small molecules, and the GI physiology has evolved to exclude exogenous particles. Although uptake by M cells located at the surface of follicle-associated epithelium was proposed as a possible mechanism for the internalization of intact liposomes, the limited population and variation in distribution significantly restricts its value as an absorption pathway. Till now, there is no persuasive evidence on the uptake of intact liposomes by the GI epithelia.
There is also a less noticeable yet important challenge during the development of liposomal formulations − the mass manufacturing challenge. Liposomes are soft self-assembled vesicles that endowed with instability during either formulation processing or storage. In spite of the successful marketing of doxorubicin liposomes product, there is not without problems in the manufacturing of liposomal formulations especially when scaling up. The batch size is still very small, which nevertheless sure will fail to meet the batch size requirement for oral administration. In order to ensure sufficient stability, solidification of the liposomes by various drying methods like freeze-drying and spray-drying has been attempted, but with little success because of the difficulties in reconstituting dried ‘liposomes’ into their original forms.
Modify to improve the stability & absorption of oral liposomes
In the past decade, there is an emerging trend in the field of enhancing oral delivery of liposomes by modification of either the liposomal surfaces or the liposomal compositions. The modifications aim to enhance both the stability and the absorption of the liposomes. In addition, enhancement in stability either in vitro or in vivo results in more liposomes surviving the harsh GI environment, and thereby further adds to enhancement in oral absorption of liposomes. Although the underlying mechanisms of oral absorption of liposomes are still unclear, some strategies are tested to be highly effective to enhance either the stability or oral permeation of liposomes.
By optimization of the lipid compositions, liposomes can be made more stable. It was reported that substituting common lipids with hydrogenated phospholipid of higher phase transition temperatures significantly strengthens the lipid bilayers, which serves as the basis for the Doxil® formulation. Paradoxically, incorporation of bile salts in the lipid bilayers was found to be able to stabilize the liposomes and protect the payload, rather than destabilize it [Citation6]. Another even more effective approach to stabilizing liposomes is coating with various polymers. The coating with long-chain polyethylene glycols, similar approach employed by injectable long-circulating liposomes, was found to be able to help liposomes withstand the erosive effect of bile salts and protect the payload [Citation7]. Other reported coating materials include chitosan, polysaccharides, proteins and their derivatives [Citation8,Citation9].
Although the mechanisms governing oral absorption of liposomes are still not elucidated yet, various strategies were proved to be effective, for example, incorporation of enhancers, improvement of mucoadhesiveness, polymer coating and ligand-mediated targeting to epithelia [Citation10–15]. It should be noted that some efficacy should be ascribed to enhanced stability rather than improvement in transmembrane absorption. Yet there are some approaches that do function to facilitate absorption of liposomes. For instance, besides improved stability bile salts-enriched liposomes (bilosomes) showed significantly enhanced oral bioavailability for a series of active ingredients and elicited significant immunological responses for orally vaccines [Citation16]. Coating with polymers such as polysaccharides and chitosan reinforces the adhesion with the epithelial mucus lining and increases the opportunities of absorption of the payloads. Evidence also showed that coating with chitosan could interfere with the tight junctions and facilitate absorption of the drug, and even the liposomes as a whole. Modification with tocopheryl polyethylene glycol 1000 succinate (TPGS) and pluronics could enhance absorption through well-accepted mechanisms of inhibition of pumping out by P-glycoproteins. However, it is paradoxical that the coating will camouflage the liposomes and thereby presents the behavior of the particles of the coating materials, whereas the efficacy always depends on the recognition of naked liposomes per se. It seems that shedding mechanisms should be taken into account when designing polymer-coated liposomal delivery systems. Active targeting to the intestinal epithelia by modification with ligands is an emerging trend to increase the oral uptake of nanoparticles [Citation16]. A majority of studies in this field employ the M cell pathway through receptor-induced phagocytosis, taking advantage of the expression of a variety of receptors on the surface of M-cells, for example, intercellular adhesion molecule-1, l-fucose and β1-integrin [Citation17]. Lectins, a class of glycoproteins of botanical origin that can reversibly bind to specific carbohydrate residues of the surface proteins or lipids, has been observed to be able to enhance the oral absorption of liposome-bound entities [Citation18]. However, owing to the limited population of M cells along the whole intestinal wall lining, the potential of transenterocytic phagocytic pathway has also been explored, although it is just at its beginning, aiming at various receptors for nutrients such as vitamins and carbohydrate, and proteins and peptides [Citation19,Citation20].
Conclusion & future perspective
In conclusion, oral delivery of liposomes sheds light on enhanced absorption of entities of extremely low bioavailability, but only with limited progress due to the various challenges as described in this article. In general, if liposomes are intended to deliver poorly water-soluble small molecular drugs, a pathway of transition into mixed micelles before absorption by passive diffusion may be functional. However, for labile entities such as biomacromolecules, it is of utmost importance to maintain the liposomal integrity in the intestinal lumen and achieve significant uptake of the liposomes by the GI epithelia. Although there is speculation that liposomes may be absorbed intact, no direct evidence has ever been reported. To pave the way for efficient oral absorption of liposomes, leading strategies should be to stabilize liposomes both in vitro and in vivo, and enhance the uptake efficiency of liposomes by intestinal epithelia. To battle the latter, it is urgent to decipher the underlying mechanisms of oral absorption of liposomes, and thereby stimulate innovative designing of novel liposomal delivery systems with improved bioavailability.
Financial & competing interests disclosure
W Wu would like to thank Shanghai Commission of Science and Technology for financial support (14JC1490300). The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.
No writing assistance was utilized in the production of this manuscript.
References
- Fan Y Zhang Q . Development of liposomal formulations: from concept to clinical investigations. Asian J. Pharm. Sci.8 (2), 81–87 (2013).
- Torchilin VP . Recent advances with liposomes as pharmaceutical carriers. Nat. Rev. Drug Discov.4 (2), 145–160 (2005).
- Rowland RN Woodley JF . The stability of liposomes in vitro to pH, bile salts and pancreatic lipase. Biochim. Biophys. Acta620 (3), 400–409 (1980).
- Liu W Ye A Liu W et al. Behaviour of liposomes loaded with bovine serum albumin during in vitro digestion. Food Chem.175, 16–24 (2015).
- Ensign LM Cone R Hanes J . Oral drug delivery with polymeric nanoparticles: the gastrointestinal mucus barriers. Adv. Drug Deliv. Rev.64 (6), 557–570 (2012).
- Hu S Niu M Hu F et al. Integrity and stability of oral liposomes containing bile salts studied in simulated and ex vivo gastrointestinal media. Int. J. Pharm.441 (1–2), 693–700 (2013).
- Li H Song JH Park JS et al. Polyethylene glycol-coated liposomes for oral delivery of recombinant human epidermal growth factor. Int. J. Pharm.258 (1–2), 11–19 (2003).
- Manconi M Nacher A Merino V et al. Improving oral bioavailability and pharmacokinetics of liposomal metformin by glycerolphosphate-chitosan microcomplexation. AAPS PharmSciTech14 (2), 485–496 (2013).
- Smistad G B⊘yum S Alund SJ et al. The potential of pectin as a stabilizer for liposomal drug delivery systems. Carbohydr. Polym.90 (3), 1337–1344 (2012).
- Klemetsrud T Jonassen H Hiorth M et al. Studies on pectin-coated liposomes and their interaction with mucin. Colloids Surf. B Biointerfaces103, 158–165 (2013).
- Sugihara H Yamamoto H Kawashima Y et al. Effectiveness of submicronized chitosan-coated liposomes in oral absorption of indomethacin. J. Liposome Res.22 (1), 72–79 (2012).
- Parmentier J Hofhaus G Thomas S et al. Improved oral bioavailability of human growth hormone by a combination of liposomes containing bio-enhancers and tetraether lipids and omeprazole. J. Pharm. Sci.103 (12), 3985–3993 (2014).
- Huang YB Tsai MJ Wu PC et al. Elastic liposomes as carriers for oral delivery and the brain distribution of (+)-catechin. J. Drug Target19 (8), 709–718 (2011).
- Gradauer K Barthelmes J Vonach C et al. Liposomes coated with thiolated chitosan enhance oral peptide delivery to rats. J. Control. Release172 (3), 872–878 (2013).
- Chen D Xia D Li X et al. Comparative study of Pluronic® F127-modified liposomes and chitosan-modified liposomes for mucus penetration and oral absorption of cyclosporine A in rats. Int. J. Pharm.449 (1–2), 1–9 (2013).
- Zhang X Wu W . Ligand-mediated active targeting for enhanced oral absorption. Drug Discov. Today19 (7), 898–904 (2014).
- des Rieux A Pourcelle V Cani PD et al. Targeted nanoparticles with novel non-peptidic ligands for oral delivery. Adv. Drug Deliv. Rev.65 (6), 833–844 (2013).
- Makhlof A Fujimoto S Tozuka Y et al. In vitro and in vivo evaluation of WGA-carbopol modified liposomes as carriers for oral peptide delivery. Eur. J. Pharm. Biopharm.77 (2), 216–224 (2011).
- Zhang X Qi J Lu Y et al. Biotinylated liposomes as potential carriers for the oral delivery of insulin. Nanomedicine10 (1), 167–176 (2014).
- Agrawal AK Harde H Thanki K et al. Improved stability and antidiabetic potential of insulin containing folic acid functionalized polymer stabilized multilayered liposomes following oral administration. Biomacromolecules15 (1), 350–360 (2014).