Vol. 2, No. 1 2005
Drug Discovery Today: Technologies Editors-in-Chief Kelvin Lam – Pfizer, Inc., USA Henk Timmerman – Vrije Universiteit, The Netherlands DRUG DISCOVERY
TODAY
TECHNOLOGIES
Drug delivery/formulation and nanotechnology
Mucoadhesive systems in oral drug delivery Andreas Bernkop-Schnu¨rch Department of Pharmaceutical Technology, Institute of Pharmacy, Leopold-Franzens-University Innsbruck, Innrain 52, Josef Mo¨ller Haus, 6020 Innsbruck, Austria
Mucoadhesive systems in oral drug delivery promise the advantage of a prolonged gastric or small intestinal residence time, an intimate contact of the delivery system with the absorption membrane and the basis for interactions of multifunctional polymers with the
Section Editors: Daan J.A. Crommelin – Department of Pharmaceutical Science, Utrecht University, Utrecht, The Netherlands Gerrit Borchard – Enzon Pharmaceuticals, Piscataway, NJ, USA
mucosa such as permeation enhancement or enzyme inhibition. A promising new generation of mucoadhesive polymers are thiolated polymers – designated thiomers, which are supposed to provide sufficient
delivery, because the adhesion of drug delivery systems in the GI tract is in most cases insufficient to provide a prolonged residence time of delivery systems in the stomach or small intestine [6,7].
adhesion even on the gastrointestinal (GI)-mucosa. Micro- and nanoparticulate thiomer formulations
Mucoadhesive systems in oral drug delivery
should contribute to a further prolonged GI-residence
Mucoadhesion is provided by the formation of non-covalent bonds such as hydrogen bonds and ionic interactions or physical entanglements between the mucus gel layer and polymers. Mediated by mucoadhesive polymers, the residence time of dosage forms on the GI-mucosa should be prolonged, which allows a sustained drug release at a given target site to maximize the therapeutic effect. Furthermore, drug delivery systems can be localized on a certain surface area for the purpose of local therapy or of drug liberation at the ‘absorption window’ representing the GI-segment, where drug absorption takes place. For example, the absorption of riboflavin, which has its ‘absorption window’ in the stomach and upper segment of the small intestine could be strongly improved in human volunteers by oral administration of mucoadhesive microspheres versus non-adhesive microspheres [5]. In addition, mucoadhesive polymers can guarantee an intimate contact with the absorption membrane providing the basis for a high concentration gradient as driving force for a passive drug uptake, for the exclusion of a presystemic metabolism such as the degradation of orally
time.
Introduction In the early 1980s, academic research groups working in the ophthalmic field pioneered the concept of mucoadhesion as a new strategy to improve the efficacy of various drug delivery systems. Since then the potential of mucoadhesive polymers was shown in ocular [1], nasal [2], vaginal [3] and buccal drug delivery systems [4] leading to a significantly prolonged residence time of sustained release delivery systems on these mucosal membranes. In addition, the development of oral mucoadhesive delivery systems was always of great interest as delivery systems capable of adhering to certain gastrointestinal (GI) segments would offer various advantages. With few exceptions [5], however, mucoadhesive drug delivery systems have so far not reached their full potential in oral drug E-mail address: A. Bernkop-Schnu¨rch ([email protected]) 1740-6749/$ ß 2005 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddtec.2005.05.001
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given (poly)peptide drugs by luminally secreted intestinal enzymes, and for interactions of the polymer with the epithelium such as a permeation-enhancing effect [8] or the inhibition of brush border membrane bound enzymes [9]. Well-established mucoadhesive polymers are for instance poly(acrylates) and chitosans. Poly(acrylates) such as carbomers are believed to bind via hydrogen bonds [10], whereas chitosans seem to bind via ionic interactions between their primary amino substructures and sialic acid and sulfonic acid substructures of the mucus [11]. These polymers are able to provide therefore only a weak adhesive force being in many cases insufficient to guarantee the localization of the delivery system on the GI-mucosa for prolonged time periods. Recently, a novel promising strategy to improve mucoadhesion has been introduced into the pharmaceutical literature. The most commonly bridging structure in biological systems, the disulfide bond, is thereby utilized to improve adhesion of polymeric carrier systems to mucosal membranes. Thiolated polymers, designated as thiomers, are believed to interact with cysteine-rich subdomains of mucus glycoproteins forming disulfide bonds between the mucoadhesive polymer and the mucus layer. Evidence for the formation of disulfide bonds between thiomers and mucus glycoproteins has been provided by Leitner et al. [12] applying various analytical approaches. Owing to the immobilization of thiol groups on already well-established mucoadhesive polymers, their mucoadhesive properties are strongly enhanced. Examples for this improved mucoadhesion are given in Table 1. Covalent bonds are believed to be formed not only between thiomer and mucus, but also within the thiomer itself. This theory was confirmed by the decrease in free thiol groups within thiomers resulting in an increase in viscosity [13]. Inter- and intramolecular disulfide bonds improve the cohesive properties of the thiolated polymer compared to the unmodified polymer. In addition, the cohesive properties within the mucus gel layer are also enhanced. Because disulfide bonds are formed during and after thiomer mucus interpenetration, the mucus layer on which the delivery system is adhering becomes comparatively more stable so that the adhesive bond does not fail within the mucus gel
Vol. 2, No. 1 2005
layer itself. Owing to improved cohesive properties the disintegration time of poly(acrylic acid)–cysteine tablets, for instance, could be more than 70-fold prolonged compared to the unmodified polymer [14]. Although thiomers show strongly increased mucoadhesive properties, the adhesion of delivery systems being based on such polymers is nevertheless limited by the natural mucus turnover. The mucus turnover in the human intestine, for instance, was determined to be in the range of 12–24 h [15]. Time of adhesion is therefore limited by this permanent renewal process to several hours. Patents covering the thiomer technology are held by ThioMatrix GmbH (http:// www.thiomatrix.com/) and MucoBiomer GmbH (http:// www.mucobiomer.com/).
Present state of the art The potential of thiomers to enhance and prolong the mucosal uptake of poorly absorbed drugs could meanwhile be shown for various challenging therapeutic agents such as hydrophilic macromolecular drugs in various in vivo studies. Kast et al. designed oral low molecular weight heparin (LMWH) delivery systems based on mucoadhesive polymers. The efficacy of unmodified polycarbophil and thiolated polycarbophil was thereby compared in vivo in rats. The oral administration of LMWH being embedded in thiolated polycarbophil resulted in a significantly increased absorption of LMWH compared to control tablets comprising unmodified polycarbophil or to an orally given aqueous heparin solution. An absolute bioavailability of 19.9 9.3% was obtained in case of the thiomer delivery system. Control tablets with heparin showed a slight increase in the bioavailability determined to be 5.8 1.4% compared to the oral heparin solution (2.3 2.8%). Furthermore, the thiomer delivery system displayed a prolonged efficacy of heparin compared to the other formulations [16]. In another study, pegylated insulin was incorporated in thiolated poly(acrylic acid) and orally administered to diabetic mice. Although pegylated peptide drugs are supposed to be absorbed from the GI-tract to a higher extent (http:// www.nobexcorp.com/science/technology.php), no thera-
Table 1. Thiolated polymers, which are interesting candidates as auxiliary agents Polymer
Additional information
Refs
Chitosan–iminothiolane
250-fold improved mucoadhesive properties
[13]
Poly(acrylic acid)–cysteine
100-fold improved mucoadhesive properties
[26]
Poly(acrylic acid)–homocysteine
Approximately 20-fold improved mucoadhesive properties
[27]
Chitosan–thioglycolic acid
Tenfold improved mucoadhesive properties
[28]
Chitosan–thioethylamidine
Ninefold improved mucoadhesive properties
[29]
Alginate–cysteine
Fourfold improved mucoadhesive properties
[30]
Poly(methacrylic acid)–cysteine
Improved cohesive and mucoadhesive properties
[31]
Sodium carboxymethylcellulose–cysteine
Improved mucoadhesive properties
[32]
84
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Vol. 2, No. 1 2005
Drug Discovery Today: Technologies | Drug delivery/formulation and nanotechnology
of the mucoadhesive calcitonin thiomer tablet was determined to be 1.35%, calculated on the basis of the area under the reduction in plasma calcium levels of the oral matrix tablets versus intravenous injection. Within this study it was furthermore shown that the mucoadhesive thiomer delivery system was even after 4 h still adhering in the stomach [18]. In another study performed in pigs, the oral bioavailability of antide – an antagonist of gonadotropin hormone-releasing hormone studied for the treatment of various disorders including hormone dependent prostate and breast cancer, endometriosis, benign prostate hyperplasia and uterine fibrosis – was significantly improved, when this peptide drug was incorporated in thiolated chitosan [19].
Prospects
Figure 1. Glycemic profiles in diabetic mice after single oral administration of pegylated insulin-loaded minitablets comprising thiolated poly(acrylic acid) (*) and of a pegylated insulin solution (&). Each point represents the mean SD of ten experiments (adapted from Caliceti et al. [17]). Within this figure the potential of thiomers to improve the therapeutic effect of orally administered peptides is demonstrated.
peutic effect at all could be observed in case of pegylated insulin being administered orally in aqueous solution. By contrast, when mice were dosed with tablets comprising the pegylated peptide drug and thiolated poly(acrylic acid), a pronounced decrease in the blood glucose level was achieved. This significant effect was maintained for even 24 h. The results of this study are shown in Fig. 1. The oral pharmacological efficacy of this mucoadhesive oral delivery system versus a subcutaneous injection was determined to be 7% [17]. Guggi et al. developed mucoadhesive tablets for oral administration of salmon calcitonin comprising chitosan– thiobutylamidine conjugate. Tested formulations were orally given to rats and the plasma calcium level was monitored as a function of time. Studies showed no statistically significant (p < 0.05) reduction of the plasma calcium level caused by salmon calcitonin, which was orally given in solution. Furthermore, no significant effect was observed after oral administration of tablets comprising the peptide drug and unmodified chitosan, although the native polymer is reported to be mucoadhesive. By contrast, in the presence of the mucoadhesive chitosan–thiobutylamidine conjugate a decrease of plasma calcium level of more than 5% for several hours was achieved. The pharmacological efficacy
The development of more novel mucoadhesive thiomers will certainly improve their in vivo performance further. Generally, the more thiol groups are immobilized on polymers, the higher are their mucoadhesive properties. This correlation could be demonstrated in various studies [20]. However, the maximum dotation in thiol has so far not been reached. In addition, different reactive thiol groups can be immobilized. An optimization of these parameters will contribute to the generation of more efficient thiomers. Apart from the development of novel thiomers also the type of delivery systems seems to have a great impact on the mucoadhesive properties of the delivery system in the GI-tract. Micro- and nanoparticles can also lead to a prolonged GI residence time. By diffusing into the mucus gel layer their transit time is often significantly reduced even without exhibiting any mucoadhesive properties. Coupe et al. [21], for instance, could show in human volunteers that 50% of orally administered non-adhesive particulate formulations are still present in the small intestine, whereas single unit dosage forms have already left this gut segment entirely. In addition to a prolonged GI-residence time, micro- and nanoparticles can improve solubility of poorly soluble drugs because of an extensively enlarged surface area between the therapeutic agent and the GI-fluid. Zili et al. [22], for instance, could strongly improve solubility of griseofulvin by incorporating the drug in nanoparticles. Furthermore, multiunit dosage forms are believed to guarantee a more accurate dosing than single unit dosage forms. The potential of micro- and nanoparticles for the oral administration of hydrophilic macromolecular drugs could already be demonstrated in various in vivo studies [23]. Companies such as Spherics (http://www.spherics.com/) or Debio (http://www.debio.com/) are specialized in the development of micro- and nanoparticular oral delivery systems. The combination of both the promising strategies, the use of thiomers on the one hand and micro- and nanoparticulate formulations on the other hand, should further improve the www.drugdiscoverytoday.com
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Table 2. Comparison summary Technology 1
Technology 2
Name of specific type of technology
Thiomer technology
Micro- and nanotechnology
Name of specific technologies with associated companies and company websites
Thiomer technology ThioMatrix GmbH Mitterweg 24, A-6020 Innsbruck, Austria
Macrospheres and nanospheres Spherics, Inc., 701 George Washington Highway, Lincoln, RI 02865, USA Debio Sphere Debiopharm Galenic Unit, 14, Rte des Avouillons, CH-1196 Gland, Switzerland
Pros
Relatively high mucoadhesive properties Can also be formulated to micro- and nanoparticles guaranteeing the advantages of both strategies
Well-established technology Safe toxicity profile
Cons
Full toxicity profile is so far missing Thiol/disulfide exchange reactions of thiol-bearing drugs with the thiomer cannot be excluded in all cases
Adhesion is primarily provided by the size of the formulation itself A controlled drug release is not always feasible
References
[16–18]
http://www.debio.com/e/debiorp/sphere_e.php [5,23,33,34]
mucoadhesive properties of according delivery systems. In first orientating studies the potential of this concept could already be verified in vitro. Microparticles based on a thiolated polymer were generated and their improved mucoadhesive properties were demonstrated on freshly excised intestinal mucosa [24]. In case of nanoparticles these properties should be even more improved. Thiomer micro- and nanoparticles can be stabilized by disulfide bonds within the particles. By contrast, on the surface as many free thiol groups as possible should be available to provide strong mucoadhesive properties. So far, thiomer microparticles were mainly prepared via emulsification solvent evaporation techniques [24]. Meanwhile, for preparation of thiomer nanoparticles various coacervation techniques are in use. Anionic and cationic thiomers form nanoparticles owing to the addition of multivalent cations such as Ca2+ and multivalent anions such as polyphosphates, respectively. These particles are then stabilized via disulfide bonds by an oxidation process and the multivalent counter ions – disturbing the mucoadhesive properties of these particles – are removed [25]. An overview Links About Debio’s oral nanoparticulate delivery systems: http://www.debio.com/e/debiorp/sphere_e.php About Spherics: http://www.spherics.com/ About ThioMatrix: http://www.thiomatrix.com/
Related articles Bernkop-Schnu¨ rch, A. et al. (2004) Thiomers: potential excipients for non-invasive peptide delivery systems. Eur. J. Pharm. Biopharm. 58, 253– 263 Bernkop-Schnu¨ rch, A. Thiomers: a new generation of mucoadhesive polymers. Adv. Drug Deliv. Rev. (in press)
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of the pros and cons of the thiomer technology and the micro- and nanotechnology is provided in Table 2.
References 1 Hornof, M. et al. (2003) Mucoadhesive ocular insert based on thiolated poly(acrylic acid): development and in vivo evaluation in humans. J. Control. Release 89, 419–428 2 Tafaghodi, M. et al. (2004) Evaluation of the clearance characteristics of various microspheres in the human nose by gamma-scintigraphy. Int. J. Pharm. 280, 125–135 ¨ rch, A. et al. (2003) Intravaginal delivery: design, 3 Bernkop-Schnu challenges and solutions. Am. J. Drug Deliv. 1, 241–254 4 Korbonits, M. et al. (2004) A comparison of a novel testosterone bioadhesive buccal system, striant, with a testosterone adhesive patch in hypogonadal males. J. Clin. Endocrinol. Metab. 89, 2039–2043 5 Akiyama, Y. et al. (1999) Novel formulation approaches to oral mucoadhesive drug delivery systems. In Bioadhesive Drug Delivery Systems (Mathiowitz, E. et al., eds), E.Mathiowitzp. 177, Marcel Dekker 6 Khosla, L. et al. (1987) The effect of polycarbophil on the gastric emptying of pellets. J. Pharm. Pharmacol. 39, 47–53 7 Harris, D. et al. (1990) GI transit of potential bioadhesive formulations in man: A scintigraphic study. J. Control. Release 12, 45–49 8 Schipper, N.G.M. et al. (1997) Chitosans as absorption enhancers for poorly absorbable drugs. 2: Mechanism of absorption enhancement. Pharm. Res. 14, 923–929 9 Lueßen, H.L.V. et al. (1996) Mucoadhesive polymers in peroral peptide drug delivery. V. Effect of poly(acrylates) on the enzymatic degradation of peptide drugs by intestinal brush border membrane vesicles. Int. J. Pharm. 141, 39–47 ¨ rch, A. (2002) Mucoadhesive polymers. In Polymeric 10 Bernkop-Schnu Biomaterials (2nd edn) (Dumitriu, S., ed.), pp. 147–165, Marcel Dekker 11 Hassan, E.E. et al. (1990) A simple rheological method for the in vitro assessment of mucin–polymer bioadhesive bond strength. Pharm. Res. 7, 491–495 12 Leitner, V.M. et al. (2003) Thiolated polymers: evidence for the formation of disulphide bonds with mucus glycoproteins. Eur. J. Pharm. Biopharm. 56, 207–214 ¨ rch, A. et al. (2003) Thiolated polymers – thiomers: 13 Bernkop-Schnu synthesis and in vitro evaluation of chitosan–2-iminothiolane conjugates. Int. J. Pharm. 260, 229–237 ¨ rch, A. et al. (2000) Development of controlled drug release 14 Bernkop-Schnu systems based on polymer–cysteine conjugates. J. Control. Release 66, 39–48 15 Allen, A. et al. (1984) Mucus, mucosa. In Ciba Foundation Symposium 109 (Nugent, J. and O’Connor, M., eds)
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16
17 18
19 20
21 22
23 24
25
Kast, C.E. et al. (2003) Development and in vivo evaluation of an oral delivery system for low molecular weight heparin based on thiolated polycarbophil. Pharm. Res. 20, 931–936 Caliceti, P. et al. (2004) Development and in vivo evaluation of an oral insulin–PEG delivery system. Eur. J. Pharm. Sci. 22, 315–323 Guggi, D. et al. (2003) Systemic peptide delivery via the stomach: in vivo evaluation of an oral dosage form for salmon calcitonin. J. Control. Release 92, 125–135 ¨ rch, A. The use of thiolated polymers as carrier matrix in Bernkop-Schnu oral peptide delivery – proof of concept. J. Control. Release (in press) Albrecht, K. et al. In vitro evaluation of parameters influencing the mucoadhesive properties of thiomer particles. Proceedings of the Controlled Release Society Annual Meeting #688 Coupe, A.J. et al. (1991) Variation in gastrointestinal transit of pharmaceutical dosage forms in healthy subjects. Pharm. Res. 8, 360–364 Zili, Z. et al. (2005) Preparation and characterization of poly-varepsilon– caprolactone nanoparticles containing griseofulvin. Int. J. Pharm. 294, 261–267 Mathiowitz, E. et al. (1997) Biologically erodable microspheres as potential oral drug delivery systems. Nature 386, 410–414 ¨ rch, A. et al. (2003) Preparation and in vitro Bernkop-Schnu characterization of poly(acrylic acid)–cysteine microparticles. J. Control. Release 93, 29–38 ¨ rch, A. et al. Thiomers: preparation and in vitro evaluation Bernkop-Schnu of mucoadhesive particulate delivery systems. Proceedings of the Controlled Release Society Annual Meeting #357
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26
27
28
29 30
31
32 33 34
¨tz, M.K. et al. (2002) Thiolated polymers: advance in Marschu mucoadhesion by use of in situ crosslinking poly(acrylic acid)–cysteine conjugates. Eur. J. Pharm. Sci. 15, 387–394 ¨ rch, A. et al. (2004) Synthesis and in vitro characterization Bernkop-Schnu of a poly(acrylic acid)–homocysteine conjugate. Drug Dev. Ind. Pharm. 30, 1–8 Kast, C.E. et al. (2001) Thiolated polymers – thiomers: development and in vitro evaluation of chitosan–thioglycolic acid conjugates. Biomaterials 22, 2345–2352 Kafedjiiski, K. et al. (2005) Synthesis and in vitro evaluation of a novel thiolated chitosan. Biomaterials 26, 819–826 ¨ rch, A. et al. (2001) Improvement in the mucoadhesive Bernkop-Schnu properties of alginate by the covalent attachment of cysteine. J. Control. Release 71, 277–285 ¨ rch, A. et al. (2004) Preparation and characterisation of Bernkop-Schnu thiolated poly(methacrylic acid)–starch compositions. Eur. J. Pharm. Biopharm. 57, 219–224 ¨ rch, A. et al. (2000) Synthesis and characterisation of Bernkop-Schnu mucoadhesive thiolated polymers. Int. J. Pharm. 194, 239–247 Carino, G.P. et al. (1999) Oral insulin delivery. Adv. Drug Deliv. Rev. 35, 249–257 Santos, C. et al. (2003) Evaluation of anhydride oligomers within polymer microsphere blends and their impact on bioadhesion and drug delivery in vitro. Biomaterials 24, 3571–3583
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Drug Discovery Today: Technologies Editors-in-Chief Kelvin Lam – Pfizer, Inc., USA Henk Timmerman – Vrije Universiteit, The Netherlands DRUG DISCOVERY
TODAY
TECHNOLOGIES
Drug delivery/formulation and nanotechnology
Mucoadhesive systems in oral drug delivery Andreas Bernkop-Schnu¨rch Department of Pharmaceutical Technology, Institute of Pharmacy, Leopold-Franzens-University Innsbruck, Innrain 52, Josef Mo¨ller Haus, 6020 Innsbruck, Austria
Mucoadhesive systems in oral drug delivery promise the advantage of a prolonged gastric or small intestinal residence time, an intimate contact of the delivery system with the absorption membrane and the basis for interactions of multifunctional polymers with the
Section Editors: Daan J.A. Crommelin – Department of Pharmaceutical Science, Utrecht University, Utrecht, The Netherlands Gerrit Borchard – Enzon Pharmaceuticals, Piscataway, NJ, USA
mucosa such as permeation enhancement or enzyme inhibition. A promising new generation of mucoadhesive polymers are thiolated polymers – designated thiomers, which are supposed to provide sufficient
delivery, because the adhesion of drug delivery systems in the GI tract is in most cases insufficient to provide a prolonged residence time of delivery systems in the stomach or small intestine [6,7].
adhesion even on the gastrointestinal (GI)-mucosa. Micro- and nanoparticulate thiomer formulations
Mucoadhesive systems in oral drug delivery
should contribute to a further prolonged GI-residence
Mucoadhesion is provided by the formation of non-covalent bonds such as hydrogen bonds and ionic interactions or physical entanglements between the mucus gel layer and polymers. Mediated by mucoadhesive polymers, the residence time of dosage forms on the GI-mucosa should be prolonged, which allows a sustained drug release at a given target site to maximize the therapeutic effect. Furthermore, drug delivery systems can be localized on a certain surface area for the purpose of local therapy or of drug liberation at the ‘absorption window’ representing the GI-segment, where drug absorption takes place. For example, the absorption of riboflavin, which has its ‘absorption window’ in the stomach and upper segment of the small intestine could be strongly improved in human volunteers by oral administration of mucoadhesive microspheres versus non-adhesive microspheres [5]. In addition, mucoadhesive polymers can guarantee an intimate contact with the absorption membrane providing the basis for a high concentration gradient as driving force for a passive drug uptake, for the exclusion of a presystemic metabolism such as the degradation of orally
time.
Introduction In the early 1980s, academic research groups working in the ophthalmic field pioneered the concept of mucoadhesion as a new strategy to improve the efficacy of various drug delivery systems. Since then the potential of mucoadhesive polymers was shown in ocular [1], nasal [2], vaginal [3] and buccal drug delivery systems [4] leading to a significantly prolonged residence time of sustained release delivery systems on these mucosal membranes. In addition, the development of oral mucoadhesive delivery systems was always of great interest as delivery systems capable of adhering to certain gastrointestinal (GI) segments would offer various advantages. With few exceptions [5], however, mucoadhesive drug delivery systems have so far not reached their full potential in oral drug E-mail address: A. Bernkop-Schnu¨rch ([email protected]) 1740-6749/$ ß 2005 Elsevier Ltd. All rights reserved.
DOI: 10.1016/j.ddtec.2005.05.001
www.drugdiscoverytoday.com
83
Drug Discovery Today: Technologies | Drug delivery/formulation and nanotechnology
given (poly)peptide drugs by luminally secreted intestinal enzymes, and for interactions of the polymer with the epithelium such as a permeation-enhancing effect [8] or the inhibition of brush border membrane bound enzymes [9]. Well-established mucoadhesive polymers are for instance poly(acrylates) and chitosans. Poly(acrylates) such as carbomers are believed to bind via hydrogen bonds [10], whereas chitosans seem to bind via ionic interactions between their primary amino substructures and sialic acid and sulfonic acid substructures of the mucus [11]. These polymers are able to provide therefore only a weak adhesive force being in many cases insufficient to guarantee the localization of the delivery system on the GI-mucosa for prolonged time periods. Recently, a novel promising strategy to improve mucoadhesion has been introduced into the pharmaceutical literature. The most commonly bridging structure in biological systems, the disulfide bond, is thereby utilized to improve adhesion of polymeric carrier systems to mucosal membranes. Thiolated polymers, designated as thiomers, are believed to interact with cysteine-rich subdomains of mucus glycoproteins forming disulfide bonds between the mucoadhesive polymer and the mucus layer. Evidence for the formation of disulfide bonds between thiomers and mucus glycoproteins has been provided by Leitner et al. [12] applying various analytical approaches. Owing to the immobilization of thiol groups on already well-established mucoadhesive polymers, their mucoadhesive properties are strongly enhanced. Examples for this improved mucoadhesion are given in Table 1. Covalent bonds are believed to be formed not only between thiomer and mucus, but also within the thiomer itself. This theory was confirmed by the decrease in free thiol groups within thiomers resulting in an increase in viscosity [13]. Inter- and intramolecular disulfide bonds improve the cohesive properties of the thiolated polymer compared to the unmodified polymer. In addition, the cohesive properties within the mucus gel layer are also enhanced. Because disulfide bonds are formed during and after thiomer mucus interpenetration, the mucus layer on which the delivery system is adhering becomes comparatively more stable so that the adhesive bond does not fail within the mucus gel
Vol. 2, No. 1 2005
layer itself. Owing to improved cohesive properties the disintegration time of poly(acrylic acid)–cysteine tablets, for instance, could be more than 70-fold prolonged compared to the unmodified polymer [14]. Although thiomers show strongly increased mucoadhesive properties, the adhesion of delivery systems being based on such polymers is nevertheless limited by the natural mucus turnover. The mucus turnover in the human intestine, for instance, was determined to be in the range of 12–24 h [15]. Time of adhesion is therefore limited by this permanent renewal process to several hours. Patents covering the thiomer technology are held by ThioMatrix GmbH (http:// www.thiomatrix.com/) and MucoBiomer GmbH (http:// www.mucobiomer.com/).
Present state of the art The potential of thiomers to enhance and prolong the mucosal uptake of poorly absorbed drugs could meanwhile be shown for various challenging therapeutic agents such as hydrophilic macromolecular drugs in various in vivo studies. Kast et al. designed oral low molecular weight heparin (LMWH) delivery systems based on mucoadhesive polymers. The efficacy of unmodified polycarbophil and thiolated polycarbophil was thereby compared in vivo in rats. The oral administration of LMWH being embedded in thiolated polycarbophil resulted in a significantly increased absorption of LMWH compared to control tablets comprising unmodified polycarbophil or to an orally given aqueous heparin solution. An absolute bioavailability of 19.9 9.3% was obtained in case of the thiomer delivery system. Control tablets with heparin showed a slight increase in the bioavailability determined to be 5.8 1.4% compared to the oral heparin solution (2.3 2.8%). Furthermore, the thiomer delivery system displayed a prolonged efficacy of heparin compared to the other formulations [16]. In another study, pegylated insulin was incorporated in thiolated poly(acrylic acid) and orally administered to diabetic mice. Although pegylated peptide drugs are supposed to be absorbed from the GI-tract to a higher extent (http:// www.nobexcorp.com/science/technology.php), no thera-
Table 1. Thiolated polymers, which are interesting candidates as auxiliary agents Polymer
Additional information
Refs
Chitosan–iminothiolane
250-fold improved mucoadhesive properties
[13]
Poly(acrylic acid)–cysteine
100-fold improved mucoadhesive properties
[26]
Poly(acrylic acid)–homocysteine
Approximately 20-fold improved mucoadhesive properties
[27]
Chitosan–thioglycolic acid
Tenfold improved mucoadhesive properties
[28]
Chitosan–thioethylamidine
Ninefold improved mucoadhesive properties
[29]
Alginate–cysteine
Fourfold improved mucoadhesive properties
[30]
Poly(methacrylic acid)–cysteine
Improved cohesive and mucoadhesive properties
[31]
Sodium carboxymethylcellulose–cysteine
Improved mucoadhesive properties
[32]
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Drug Discovery Today: Technologies | Drug delivery/formulation and nanotechnology
of the mucoadhesive calcitonin thiomer tablet was determined to be 1.35%, calculated on the basis of the area under the reduction in plasma calcium levels of the oral matrix tablets versus intravenous injection. Within this study it was furthermore shown that the mucoadhesive thiomer delivery system was even after 4 h still adhering in the stomach [18]. In another study performed in pigs, the oral bioavailability of antide – an antagonist of gonadotropin hormone-releasing hormone studied for the treatment of various disorders including hormone dependent prostate and breast cancer, endometriosis, benign prostate hyperplasia and uterine fibrosis – was significantly improved, when this peptide drug was incorporated in thiolated chitosan [19].
Prospects
Figure 1. Glycemic profiles in diabetic mice after single oral administration of pegylated insulin-loaded minitablets comprising thiolated poly(acrylic acid) (*) and of a pegylated insulin solution (&). Each point represents the mean SD of ten experiments (adapted from Caliceti et al. [17]). Within this figure the potential of thiomers to improve the therapeutic effect of orally administered peptides is demonstrated.
peutic effect at all could be observed in case of pegylated insulin being administered orally in aqueous solution. By contrast, when mice were dosed with tablets comprising the pegylated peptide drug and thiolated poly(acrylic acid), a pronounced decrease in the blood glucose level was achieved. This significant effect was maintained for even 24 h. The results of this study are shown in Fig. 1. The oral pharmacological efficacy of this mucoadhesive oral delivery system versus a subcutaneous injection was determined to be 7% [17]. Guggi et al. developed mucoadhesive tablets for oral administration of salmon calcitonin comprising chitosan– thiobutylamidine conjugate. Tested formulations were orally given to rats and the plasma calcium level was monitored as a function of time. Studies showed no statistically significant (p < 0.05) reduction of the plasma calcium level caused by salmon calcitonin, which was orally given in solution. Furthermore, no significant effect was observed after oral administration of tablets comprising the peptide drug and unmodified chitosan, although the native polymer is reported to be mucoadhesive. By contrast, in the presence of the mucoadhesive chitosan–thiobutylamidine conjugate a decrease of plasma calcium level of more than 5% for several hours was achieved. The pharmacological efficacy
The development of more novel mucoadhesive thiomers will certainly improve their in vivo performance further. Generally, the more thiol groups are immobilized on polymers, the higher are their mucoadhesive properties. This correlation could be demonstrated in various studies [20]. However, the maximum dotation in thiol has so far not been reached. In addition, different reactive thiol groups can be immobilized. An optimization of these parameters will contribute to the generation of more efficient thiomers. Apart from the development of novel thiomers also the type of delivery systems seems to have a great impact on the mucoadhesive properties of the delivery system in the GI-tract. Micro- and nanoparticles can also lead to a prolonged GI residence time. By diffusing into the mucus gel layer their transit time is often significantly reduced even without exhibiting any mucoadhesive properties. Coupe et al. [21], for instance, could show in human volunteers that 50% of orally administered non-adhesive particulate formulations are still present in the small intestine, whereas single unit dosage forms have already left this gut segment entirely. In addition to a prolonged GI-residence time, micro- and nanoparticles can improve solubility of poorly soluble drugs because of an extensively enlarged surface area between the therapeutic agent and the GI-fluid. Zili et al. [22], for instance, could strongly improve solubility of griseofulvin by incorporating the drug in nanoparticles. Furthermore, multiunit dosage forms are believed to guarantee a more accurate dosing than single unit dosage forms. The potential of micro- and nanoparticles for the oral administration of hydrophilic macromolecular drugs could already be demonstrated in various in vivo studies [23]. Companies such as Spherics (http://www.spherics.com/) or Debio (http://www.debio.com/) are specialized in the development of micro- and nanoparticular oral delivery systems. The combination of both the promising strategies, the use of thiomers on the one hand and micro- and nanoparticulate formulations on the other hand, should further improve the www.drugdiscoverytoday.com
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Table 2. Comparison summary Technology 1
Technology 2
Name of specific type of technology
Thiomer technology
Micro- and nanotechnology
Name of specific technologies with associated companies and company websites
Thiomer technology ThioMatrix GmbH Mitterweg 24, A-6020 Innsbruck, Austria
Macrospheres and nanospheres Spherics, Inc., 701 George Washington Highway, Lincoln, RI 02865, USA Debio Sphere Debiopharm Galenic Unit, 14, Rte des Avouillons, CH-1196 Gland, Switzerland
Pros
Relatively high mucoadhesive properties Can also be formulated to micro- and nanoparticles guaranteeing the advantages of both strategies
Well-established technology Safe toxicity profile
Cons
Full toxicity profile is so far missing Thiol/disulfide exchange reactions of thiol-bearing drugs with the thiomer cannot be excluded in all cases
Adhesion is primarily provided by the size of the formulation itself A controlled drug release is not always feasible
References
[16–18]
http://www.debio.com/e/debiorp/sphere_e.php [5,23,33,34]
mucoadhesive properties of according delivery systems. In first orientating studies the potential of this concept could already be verified in vitro. Microparticles based on a thiolated polymer were generated and their improved mucoadhesive properties were demonstrated on freshly excised intestinal mucosa [24]. In case of nanoparticles these properties should be even more improved. Thiomer micro- and nanoparticles can be stabilized by disulfide bonds within the particles. By contrast, on the surface as many free thiol groups as possible should be available to provide strong mucoadhesive properties. So far, thiomer microparticles were mainly prepared via emulsification solvent evaporation techniques [24]. Meanwhile, for preparation of thiomer nanoparticles various coacervation techniques are in use. Anionic and cationic thiomers form nanoparticles owing to the addition of multivalent cations such as Ca2+ and multivalent anions such as polyphosphates, respectively. These particles are then stabilized via disulfide bonds by an oxidation process and the multivalent counter ions – disturbing the mucoadhesive properties of these particles – are removed [25]. An overview Links About Debio’s oral nanoparticulate delivery systems: http://www.debio.com/e/debiorp/sphere_e.php About Spherics: http://www.spherics.com/ About ThioMatrix: http://www.thiomatrix.com/
Related articles Bernkop-Schnu¨ rch, A. et al. (2004) Thiomers: potential excipients for non-invasive peptide delivery systems. Eur. J. Pharm. Biopharm. 58, 253– 263 Bernkop-Schnu¨ rch, A. Thiomers: a new generation of mucoadhesive polymers. Adv. Drug Deliv. Rev. (in press)
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of the pros and cons of the thiomer technology and the micro- and nanotechnology is provided in Table 2.
References 1 Hornof, M. et al. (2003) Mucoadhesive ocular insert based on thiolated poly(acrylic acid): development and in vivo evaluation in humans. J. Control. Release 89, 419–428 2 Tafaghodi, M. et al. (2004) Evaluation of the clearance characteristics of various microspheres in the human nose by gamma-scintigraphy. Int. J. Pharm. 280, 125–135 ¨ rch, A. et al. (2003) Intravaginal delivery: design, 3 Bernkop-Schnu challenges and solutions. Am. J. Drug Deliv. 1, 241–254 4 Korbonits, M. et al. (2004) A comparison of a novel testosterone bioadhesive buccal system, striant, with a testosterone adhesive patch in hypogonadal males. J. Clin. Endocrinol. Metab. 89, 2039–2043 5 Akiyama, Y. et al. (1999) Novel formulation approaches to oral mucoadhesive drug delivery systems. In Bioadhesive Drug Delivery Systems (Mathiowitz, E. et al., eds), E.Mathiowitzp. 177, Marcel Dekker 6 Khosla, L. et al. (1987) The effect of polycarbophil on the gastric emptying of pellets. J. Pharm. Pharmacol. 39, 47–53 7 Harris, D. et al. (1990) GI transit of potential bioadhesive formulations in man: A scintigraphic study. J. Control. Release 12, 45–49 8 Schipper, N.G.M. et al. (1997) Chitosans as absorption enhancers for poorly absorbable drugs. 2: Mechanism of absorption enhancement. Pharm. Res. 14, 923–929 9 Lueßen, H.L.V. et al. (1996) Mucoadhesive polymers in peroral peptide drug delivery. V. Effect of poly(acrylates) on the enzymatic degradation of peptide drugs by intestinal brush border membrane vesicles. Int. J. Pharm. 141, 39–47 ¨ rch, A. (2002) Mucoadhesive polymers. In Polymeric 10 Bernkop-Schnu Biomaterials (2nd edn) (Dumitriu, S., ed.), pp. 147–165, Marcel Dekker 11 Hassan, E.E. et al. (1990) A simple rheological method for the in vitro assessment of mucin–polymer bioadhesive bond strength. Pharm. Res. 7, 491–495 12 Leitner, V.M. et al. (2003) Thiolated polymers: evidence for the formation of disulphide bonds with mucus glycoproteins. Eur. J. Pharm. Biopharm. 56, 207–214 ¨ rch, A. et al. (2003) Thiolated polymers – thiomers: 13 Bernkop-Schnu synthesis and in vitro evaluation of chitosan–2-iminothiolane conjugates. Int. J. Pharm. 260, 229–237 ¨ rch, A. et al. (2000) Development of controlled drug release 14 Bernkop-Schnu systems based on polymer–cysteine conjugates. J. Control. Release 66, 39–48 15 Allen, A. et al. (1984) Mucus, mucosa. In Ciba Foundation Symposium 109 (Nugent, J. and O’Connor, M., eds)
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