Bioadhesive N-(2-hydroxypropyl) methacrylamide copolymers for colon-specific drug delivery

Tumor Stimulus-Responsive Biodegradable Diblock Copolymer Conjugates as Efficient Anti-Cancer Nanomedicines

Biodegradable nanomedicines are widely studied as candidates for the effective treatment of various cancerous diseases. Here, we present the design, synthesis and evaluation of biodegradable polymer-based nanomedicines tailored for tumor-associated stimuli-sensitive drug release and polymer system degradation. Diblock polymer systems were developed, which enabled the release of the carrier drug, pirarubicin, via a pH-sensitive spacer allowing for the restoration of the drug cytotoxicity solely in the tumor tissue. Moreover, the tailored design enables the matrix-metalloproteinases- or reduction-driven degradation of the polymer system into the polymer chains excretable from the body by glomerular filtration. Diblock nanomedicines take advantage of an enhanced EPR effect during the initial phase of nanomedicine pharmacokinetics and should be easily removed from the body after tumor microenvironment-associated biodegradation after fulfilling their role as a drug carrier. In parallel with the similar release profiles of diblock nanomedicine to linear polymer conjugates, these diblock polymer conjugates showed a comparable in vitro cytotoxicity, intracellular uptake, and intratumor penetration properties. More importantly, the diblock nanomedicines showed a remarkable in vivo anti-tumor efficacy, which was far more superior than conventional linear polymer conjugates. These findings suggested the advanced potential of diblock polymer conjugates for anticancer polymer therapeutics.

In Vitro Methods to Study Colon Release: State of the Art and An Outlook on New Strategies for Better In-Vitro Biorelevant Release Media

The primary focus of this review is a discussion regarding in vitro media for colon release, but we also give a brief overview of colon delivery and the colon microbiota as a baseline for this discussion. The large intestine is colonized by a vast number of bacteria, approximately 1012 per gram of intestinal content. The microbial community in the colon is complex and there is still much that is unknown about its composition and the activity of the microbiome. However, it is evident that this complex microbiota will affect the release from oral formulations targeting the colon. This includes the release of active drug substances, food supplements, and live microorganisms, such as probiotic bacteria and bacteria used for microbiota transplantations. Currently, there are no standardized colon release media, but researchers employ in vitro models representing the colon ranging from reasonable simple systems with adjusted pH with or without key enzymes to the use of fecal samples. In this review, we present the pros and cons for different existing in vitro models. Furthermore, we summarize the current knowledge of the colonic microbiota composition which is of importance to the fermentation capacity of carbohydrates and suggest a strategy to choose bacteria for a new more standardized in vitro dissolution medium for the colon.

Polymer-drug conjugates in inflammation treatment

Inflammation is a vital defense mechanism of living organisms. However, persistent and chronic inflammation may lead to severe pathological processes and evolve into various chronic inflammatory diseases (CID), e.g. rheumatoid arthritis, multiple sclerosis, multiple sclerosis, systemic lupus erythematosus or inflammatory bowel diseases, or certain types of cancer. Their current treatment usually does not lead to complete remission. The application of nanotherapeutics may significantly improve CID treatment, since their accumulation in inflamed tissues has been described and is referred to as extravasation through leaky vasculature and subsequent inflammatory cell-mediated sequestration (ELVIS). Among nanotherapeutics, water-soluble polymer-drug conjugates may be highly advantageous in CID treatment due to the possibility of their passive and active targeting to the inflammation site and controlled release of active agents once there. The polymer-drug conjugate consists of a hydrophilic biocompatible polymer backbone along which the drug molecules are covalently attached via a biodegradable linker that enables controlled drug release. Their active targeting or bio-imaging can be achieved by introducing the cell-specific targeting moiety or imaging agents into the polymer conjugate. Here, we review the relationship between polymer conjugates and inflammation, including the benefits of the application of polymer conjugates in inflammation treatment, the anti-inflammatory activity of polymer drug conjugates and potential polymer-promoted inflammation and immunogenicity.

Synthesis and Degradation Characteristics of Polyurethanes Containing AZO Derivatives of 5-Amino Salicylic Acid

New types of biodegradable and pH sensitive polyurethanes containing azo derivatives of 5-aminosalicylic acid (5-ASA) were synthesized by condensation of hexamethylene diisocyanate (HDI) with 4, 4-dihydroxy azobenzene-3, 3-dicarboxylic acid (AZO I) or 4, 4-dihydroxy 3-formyl azobenzene-3-carboxylic acid (AZO II). They contained both hydrolysable urethane bonds and enzymatically degradable azo-aromatic links. Degradation of polyurethanes (PU-AZO I and PU-AZO II) in phosphate buffer solutions was pH dependant. In basic conditions AZO I or AZO II was rapidly released over 48h. Incubation of PU-AZO I and PU-AZO II with rat cecal contents at 37°C gradually released 5-ASA, and the amount of drug released was 85% and 78% in 48h respectively. These polyurethane derivatives are being investigated for colon-specific drug delivery systems for treating inflammatory bowel disease based on the pH-dependant degradation of the polymer.

N-(2-Hydroxypropyl)methacrylamide Copolymer-9-Aminocamptothecin Conjugate: Colon-Specific Drug Delivery in Rats

An N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugate containing 9-aminocamptothecin (9-AC) boundvia a spacer containing an aromatic azo bond and leucylalanine (P-Azo-Leu-Ala-9-AC) was synthesized. The in vivo pharmacokinetic profile after oral administration was examined in rats and comparedto free 9-AC. The aromatic azo bond of P-Azo-Leu-Ala-9-AC was stable in stomach and small intestine; the delivery of a large amount of intact conjugate to the colon was achieved. In the colon, the azoreductase activity first cleaved the azo bond followed by peptidase catalyzed cleavage of the leucylalanyl drug derivative resulting in the release of free 9-AC. However, the release rate from the conjugate was not fast enough to achieve high colon concentrations of free 9-AC. The results of the study suggest design features for the second generation of conjugates, including the use of a side-chain with a higher cleavage rate in the colon, combined with the incorporation of bioadhesion technology, to increase colon transit time.

Antitumor efficacy of colon-specific HPMA copolymer/9-aminocamptothecin conjugates in mice bearing human-colon carcinoma xenografts

The antitumor activity of a colon-specific N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer - 9-aminocamptothecin (9-AC) conjugate (P-9-AC) was assessed in orthotopic and subcutaneous animal (HT29 xenograft) tumor models. P-9-AC treatment of mice bearing orthotopic colon tumors, with a dose of 3 mg/kg of 9-AC equivalent every other day for 6 weeks, resulted in regression of tumors in 9 of 10 mice. A lower dose of P-9-AC (1.25 mg/kg of 9-AC equivalent) every other day for 8 weeks inhibited subcutaneous tumor growth in all mice. No liver metastases were observed. Colon-specific release of 9-AC from polymer conjugates enhanced antitumor activity and minimized the systemic toxicity.

A novel approach for incorporation of mono-functional bioactive phenols into polyanhydrides

Antiseptics based on phenol and phenolic derivatives were chemically incorporated into polyanhydrides as pendant groups via ester linkages. Polyanhydrides with antiseptic loadings of 46-58 wt.-% were obtained with molecular weights ranging from 9 400-23 000. In general, polymers with the bulkier antiseptics were more difficult to polymerize and yielded lower molecular weights. All polyanhydrides were amorphous with glass transition temperatures ranging from 27-58 °C. Polymers were deemed noncytotoxic after culturing L929 mouse fibroblast cells in media containing the polymers at two concentrations (0.10 and 0.01 mg · mL(-1) ) over three days. In summary, mono-functional bioactives can be chemically incorporated into noncytotoxic polyanhydrides.

Site‐Specific Drug Delivery to the Middle Region of the Small Intestine by Application of Enteric Coating with Hypromellose Acetate Succinate (HPMCAS)

Enteric coatings that deliver drugs to specific regions of the small intestine were examined. Hypromellose acetate succinate (HPMCAS) with different values of succinoyl group contents was used. Decreasing the succinoyl group content resulted in an increase in the pH at which HPMCAS started to dissolve. Drug-containing granules with or without enteric coating were prepared and their in vitro dissolution in a simulated intestinal fluid of pH 6.8 was examined. Granules coated with HPMCAS having the succinoyl group content of 6.2% showed a lag time of about 30 min, although drug release from granules without coating was completed within 20 min. The time lag and dissolution rate were extended and reduced, respectively, as the succinoyl group content was decreased. Rat experiments indicated that enteric-coated granules disintegrated and the bulk of the drugs was immediately released when the granules reached a specific site of the small intestine where the pH corresponded to the pH at which the enteric coating agent started to dissolve. Similar results were observed in monkey experiments. It was suggested that HPMCAS with the succinoyl group content of about 5% was suitable as an enteric coating agent for delivering drugs to the middle-to-lower region of the small intestine.

Pharmacokinetic modeling of absorption behavior of 9-aminocamptothecin (9-AC) released from colon-specific HPMA copolymer-9-AC conjugate in rats

PURPOSE

To quantitate and predict colon-specific 9-aminocamptothecin (9-AC) release from the N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-9-AC conjugate and its absorption behavior after oral administration in rats.

METHODS

Drug distribution in the gastrointestinal (GI) tract and the plasma concentration-time profile of 9-AC released from the HPMA copolymer conjugate were predicted using the degradation, transit, and absorption rate constants in cecum. The fate of 9-AC in cecum and liver was measured by in-situ cecum absorption and liver perfusion.

RESULTS

Following oral administration of the conjugate, 9-AC was released rapidly in cecum. Based on the pharmacokinetic model, up to 60% of the dose was in the cecum at ~6 h, and 7% of the dose still remained there at 24 h. The predicted plasma concentration curve for released 9-AC after an oral dose of 3 mg/kg of 9-AC equivalent increased gradually and reached a peak of 98 nM at 7 h, then started decreasing slowly to 16 nM at 24 h. The bioavailability value was estimated as 0.31 after the first-pass elimination.

CONCLUSIONS

A pharmacokinetic model delineated the impact of GI transit, drug absorption rate, and first-pass metabolism on drug disposition following oral administration of HPMA copolymer-9-AC conjugate in rats.

Biodistribution and pharmacokinetics of colon-specific HPMA copolymer--9-aminocamptothecin conjugate in mice

A water soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer-9-aminocamptothecin (9-AC) conjugate was designed for oral colon-specific drug delivery in the treatment of colon cancer. Comparative studies between the polymer conjugate and free drug have been performed to assess their biodistribution and pharmacokinetics in mice. After oral administration of equal doses of the polymer conjugate or free 9-AC, the drug concentrations in major organs at fixed time points were determined using an HPLC-fluorescence assay. Only 2+/-1% of 9-AC released from the polymer conjugate was detected in the small intestine (SI), and the mean peak concentration of free 9-AC was 45-fold higher than that from released drug. Colon-specific release of 9-AC produced high local concentrations. The mean peak concentration of released 9-AC in cecal contents, feces, cecal tissue, and colon tissue were, respectively, 3.2-fold, 3.5-fold, 2.2-fold and 1.6-fold higher than that using free 9-AC. In plasma, the high and sharp drug concentration profile from free drug was in contrast to the relatively low and flat pharmacokinetic profile obtained from drug released from the HPMA copolymer. There was no significant difference between released and free drug for the area under the concentration-time curve (AUC) and bioavailability values. As a consequence of the colon-specific release of unmodified 9-AC from the polymer conjugate, antitumor efficacy can be anticipated to be enhanced due to prolonged colon tumor exposure to higher and more localized drug concentrations.

Colon drug delivery

Oral drug delivery to the colon has attracted significant attention during the past 20 years. Colon targeting is recognised to have several therapeutic advantages, such as the oral delivery of drugs that are destroyed by the stomach acid and/or metabolised by pancreatic enzymes. Sustained colonic release of drugs can be useful in the treatment of nocturnal asthma, angina and arthritis. Local treatment of colonic pathologies, such as ulcerative colitis, colorectal cancer and Crohn's disease, is more effective with the delivery of drugs to the affected area. Likewise, colonic delivery of vermicides and colonic diagnostic agents requires smaller doses. This article aims to provide an insight into the design and manufacturing considerations, and an evaluation of colonic drug delivery systems in order to understand why there are still few delivery technologies that have reached the market, despite intensive research in this field. For this purpose, various approaches to colon-specific drug delivery are discussed.

Colon-specific 9-aminocamptothecin-HPMA copolymer conjugates containing a 1,6-elimination spacer

N-(2-Hydroxypropyl)methacrylamide (HPMA) copolymer-9-aminocamptothecin (9-AC) conjugate for oral colon-specific drug delivery was designed, synthesized, and characterized. The drug, 9-AC, was attached to the polymer carrier via a spacer containing a combination of an aromatic azo bond and a 4-aminobenzylcarbamate group. The design of the spacer ensured a fast and highly efficient release of unmodified 9-AC from the polymer in the colon by azo bond cleavage followed by a 1,6-elimination mechanism. An in vitro degradation study indicated that this conjugate was stable in simulated upper GI tract conditions, including small intestine (SI) contents, SI mucosa suspension, and in PBS (pH 1.5 and 7.4). A fast release of the unmodified drug (85+/-10% of 9-AC in 12 h) was detected in rat cecal contents. This drug delivery system has potential in the treatment of colon cancer.

Direct, Controlled Synthesis of the Nonimmunogenic, Hydrophilic Polymer, Poly(N-(2-hydroxypropyl)methacrylamide) via RAFT in Aqueous Media

Poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA) is a nonimmunogenic, neutral-hydrophilic polymer currently employed in the delivery of anticancer drugs. Herein, we report conditions that facilitate the direct, controlled RAFT polymerization of HPMA in aqueous media. We demonstrate that the use of 4-cyanopentanoic acid dithiobenzoate and 4,4'-azobis(4-cyanopentanoic acid) as the chain transfer agent (CTA) and initiating species, respectively, in the presence of an acetic acid buffer solution at 70 degrees C is a suitable condition leading to controlled polymerization. The "living" nature of these polymerizations is demonstrated via chain-extension of an HPMA macroCTA to yield the corresponding poly(HPMA-b-HPMA) "homopolymer".

Colonic drug delivery

Fifteen years of research in the area of colon-specific drug delivery has left us with a slim choice of viable techniques, not because of the lack of proofs of concept but because of the ambiguity regarding the therapeutic necessity of targeting the colon with drugs. Critical analysis of existing technologies as well as medically based novel ideas could lead to interesting prospects.:

Cytoplasmic delivery and nuclear targeting of synthetic macromolecules

Delivery of macromolecular drugs (e.g. antisense oligonucleotides, polymer-drug conjugates, etc.) designed to work in specific sites inside cells is complicated as macromolecules typically have access to fewer biological compartments than small molecules. To better understand the fate of macromolecules in cells and begin to alter that fate, we investigated the internalization and subcellular fate of N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers and HPMA copolymer-drug conjugates in Hep G2 and A2780 cells. The subcellular fate of fluorescently labeled polymers was monitored by confocal microscopy and subcellular fractionation. Initially, the HPMA copolymers and HPMA copolymer-drug conjugates were internalized by endocytosis and remained in endosomes/lysosomes. At longer incubation times (>8 h), small amounts of the HPMA copolymers were observed to enter the cytoplasm and accumulate in the nucleus of the cells. Nuclear accumulation was confirmed after cytoplasmic microinjection. Oligonucleotides conjugated via lysosomally degradable spacers entered into the cytoplasm and nucleus of the cells faster than the polymers. The effect of the subcellular location was correlated to the toxicity of the photosensitizer, mesochlorin e(6) (Mce(6))-HPMA copolymer conjugates. The plasma membrane and late endosomes were more sensitive to damage by Mce(6). Targeting the polymer conjugates to the nucleus with the nuclear localization sequence (NLS) as well as conjugating the Mce(6) via a degradable spacer increased cell adhesion and uptake, promoted their entry into the cytoplasm and nucleus of the cells, and increased their toxicity. To further promote entry of the polymers into the cytoplasm and nucleus of the cells, the protein transduction domain, Tat peptide, was conjugated to the HPMA copolymers. This resulted in high binding to the cell membrane, but also facilitated rapid (<5 min) entry of the macromolecules into the cytoplasm and nucleus of cells. These results will prove valuable in the future design of macromolecular therapeutics.

Enhanced anaerobic degradation of polymeric azo compounds by Escherichia coli in the presence of low-molecular-weight redox mediators

The effects of the redox mediator lawsone (2-hydroxy-1,4-naphthoquinone) on the ability of Escherichia coli to reduce anaerobically polymeric azo compounds were analysed. Two types of polymeric azo compounds were tested, that have been proposed as putative tools for the site-specific targeting of drugs to the colon. The first group of polymers consisted basically of linear chains of polymethacrylic acid or polymethylmethacrylate which were interrupted by subunits of 4,4'-bis(methacryloylamino)azobenzene. These polymers differed significantly in their hydrophilicity according to the relative proportion of polymethacrylic acid used for the polymerization procedure. The second group of polymers consisted of almost water-insoluble poly(ether-ester)azo polymers that were composed of 4-(6-hydroxyhexyl)oxy-phenylazobenzoate and 16-hydroxyhexadecanoate. The addition of lawsone to the anaerobically incubated cultures of E. coli resulted in a pronounced increase in the reduction rates of the water-soluble poly(methacrylate-co-4,4'-bis(methacryloylamino)azobenzene) and in a much smaller, but significant, increase in the reduction rates of the hydrophobic poly(ether-ester)azo polymers. An increase in the amount of azo groups resulted, for the hydrophobic poly(ether-ester)azo polymers, in an increased reduction rate in the presence of the redox mediator lawsone.

Influence of the composition of in-vitro azo-reducing systems on the degradation kinetics of the model compound amaranth

The purpose of this study was to investigate the influence of the composition of in-vitro azo-reducing systems on the degradation kinetics of the model compound amaranth. The degradation kinetics of amaranth were determined under anaerobic conditions both in rat caecal content (ex-vivo) and in a variety of in-vitro degradation media derived from rat caecal content. It was observed that the reducing activity was highly dependent on the preparation method and composition of the degradation medium. In pure rat caecal content, the degradation of amaranth was apparent first order (k = 0.044 +/- 0.002 min(-1)), while dilution of the rat caecal content resulted in an apparent zero-order degradation. Both apparent zero- and first-order degradations were also observed in media made up of diluted rat caecal content to which cofactors such as NADP, D-glucose-6-phosphate, glucose-6-phosphate dehydrogenase and Bz were added. This study demonstrates that in-vitro azo-reducing kinetics are dependent on the composition and mode of preparation of the in-vitro media used. This has to be taken into account when evaluating the degradability of azo-aromatic drug delivery systems in-vitro.

Design of novel bioconjugates for targeted drug delivery

This paper summarizes recent work on the design and development of targeted polymeric bioconjugates based on N-(2-hydroxypropyl)methacrylamide (HPMA) copolymers. Polymerizable antibody Fab' fragment (MA-Fab') has been developed and used in the preparation of targeted HPMA copolymer-mesochlorin e6 conjugates for the treatment of human ovarian carcinomas. The reactivity of the MA-Fab' in copolymerization with HPMA depended on the length of the spacer between the monomer double bond and the antibody Fab' fragment. The biological activity of the antibody Fab' fragment was maintained after incorporation into the HPMA copolymer. Novel aromatic azo spacers were designed and incorporated into HPMA copolymer-drug (cyclosporin A, 9-aminocamptothecin) conjugates for the colon-specific drug delivery and for the treatment of colon diseases. The colon-specific drug release from the conjugates was controlled by the structures of both drug and spacers. Lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), were conjugated to the colon-specific polymer drug conjugates to enhance specific adhesion onto colon tissues.

Synthesis and characterization of methacrylic derivatives of 5-amino salicylic acid with pH-sensitive swelling properties

The purpose of this study is to develop novel colon-specific drug delivery systems with pH-sensitive swelling and drug release properties. Methacrylic-type polymeric prodrugs with different content levels of 5-amino salicylic acid (5-ASA) were synthesized by free radical copolymerization of metacrylic acid (MAA), polyethylene glycol monomethacrylate (PEGMA), and a methacrylic derivative of 5-ASA (methacryloyloxyethyl 5-amino salicylate [MOES]). The copolymers were characterized, and the drug content of the copolymers was determined. The effect of copolymer composition on the swelling behavior and hydrolytic degradation was studied in simulated gastric fluid (SGF, pH 1.2) and simulated intestinal fluid (SIF, pH 7.2). The swelling and hydrolytic behavior of the copolymers was dependent on the content of MAA groups and caused a decrease in gel swelling in SGF or an increase in gel swelling in SIF. Drug release studies showed that increasing content of MAA in the copolymer enhances the hydrolysis in SIF but has no effect in SGF. The results suggest that hydrogen-bonded complexes are formed between MAA and PEG pendant groups and that these pH-sensitive systems could be useful for preparation of a controlled-release formulation of 5-ASA.

Biorecognizable HPMA copolymer-drug conjugates for colon-specific delivery of 9-aminocamptothecin

N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer conjugates for colon-specific delivery of 9-aminocamptothecin (9-AC) were designed. They hold 9-AC bound via spacers containing amino acid residues and aromatic azo bonds. In vitro release profiles of 9-AC from HPMA copolymer conjugates were evaluated under artificial conditions that simulated large intestinal azoreductase and peptidase activities. The studies indicated that the azo bond was reduced first, followed by the release of unmodified 9-AC from the 9-AC containing fragment by peptidases. Release profiles depended on the chemical structure of the peptide part of the spacer. Conjugates containing leucylalanine showed high colon-specific release of 9-AC when compared to alanine containing conjugates. It appears that the studied conjugates are suitable as colon-specific drug delivery systems.

Synthesis of bioadhesive lectin-HPMA copolymer-cyclosporin conjugates

An amino group containing cyclosporin A (CsA) derivative has been synthesized and conjugated to N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer via an aromatic azo bond, which can be specifically cleaved by azoreductase activity in colon to release the drug for the treatment of colon diseases. Lectins, peanut (Arachis hypogea) agglutinin (PNA) and wheat germ agglutinin (WGA), have been conjugated to HPMA copolymer-CsA derivative conjugates (PCsA), respectively, to give bioadhesive conjugates. The PNA and WGA are the targeting proteins that can bind to diseased colon tissue and healthy tissue, respectively. There were on average four P(CsA) copolymer chains attached on one WGA molecule with a drug content of 16.0 wt % and five P(CsA) copolymer chains attached on one PNA molecule with a drug content of 11.5 wt %. The incubation of a P(CsA) copolymer with the rat cecal contents resulted in the cleavage of the azo bond and release of the cyclosporin derivative. The biological evaluation of the conjugates is under way.

Synthesis of polystyrene nanospheres having lactose-conjugated hydrophilic polymers on their surfaces and carbohydrate recognition by proteins

We prepared a poly(vinylamine)-grafted polystyrene nanosphere by the free radical polymerization of poly(N-vinylacetamide) macromonomer and styrene with AIBN in ethanol and subsequent acid hydrolysis. We conjugated lactose on the nanosphere by an amide linkage and found that the lactose was recognized by a galactose-specific lectin RCA120, which was analyzed by enzyme-linked lectin assay (ELLA). The binding ability of the lectin to lactose on the nanosphere was at least 10(2) times larger than that to a monomeric lactose. Binding ability was clearly affected by the lactose density on the nanosphere and showed a maximum value at the suitable surface density of lactose. We found that the carbohydrate-conjugated nanosphere would be a useful biomaterial for a site specific drug carrier.

Colonic drug targeting

Specific targeting of drugs to the colon is recognized to have several therapeutic advantages. Drugs which are destroyed by the stomach acid and/or metabolized by pancreatic enzymes are slightly affected in the colon, and sustained colonic release of drugs can be useful in the treatment of nocturnal asthma, angina and arthritis. Treatment of colonic diseases such as ulcerative colitis, colorectal cancer and Crohn's disease is more effective with direct delivery of drugs to the affected area. Likewise, colonic delivery of vermicides and colonic diagnostic agents require smaller doses. This article is aimed at providing insight into the design considerations and evaluation of colonic drug delivery systems. For this purpose, the anatomy and physiology of the lower gastrointestinal tract are surveyed. Furthermore, the biopharmaceutical aspects are considered in relation to drug absorption in the colon and hence various approaches to colon-specific drug delivery are discussed.

Design of microencapsulated chitosan microspheres for colonic drug delivery

Among the different approaches to achieve colon-selective drug delivery, the use of polymers, specifically biodegraded by colonic bacteria, holds great promise. In this work a new system which combines specific biodegradability and pH-dependent release is presented. The system consists of chitosan (CS) microcores entrapped within acrylic microspheres. Sodium diclofenac (SD), used as a model drug, was efficiently entrapped within CS microcores using spray-drying and then microencapsulated into Eudragit L-100 and Eudragit S-100 using an oil-in-oil solvent evaporation method. The size of the CS microcores was small (1.8-2.9 microns) and they were encapsulated within Eudragit microspheres (size between 152 and 233 microns) forming a multireservoir system. Even though CS dissolves very fast in acidic media, at pH 7.4, SD release from CS microcores was delayed, the release rate being adjustable (50% dissolved within 30-120 min) by changing the CS molecular weight (MW) or the type of CS salt. Furthermore, by coating the CS microcores with Eudragit, perfect pH-dependent release profiles were attained. No release was observed at acidic pHs, however, when reaching the Eudragit pH solubility, a continuous release for a variable time (8-12 h) was achieved. A combined mechanism of release is proposed, which considers the dissolution of the Eudragit coating, the swelling of the CS microcores and the dissolution of SD and its further diffusion through the CS gel cores. In addition, infrared (IR) spectra revealed that there was an ionic interaction between the amine groups of CS and the carboxyl groups of Eudragit, which provided the system with a new element for controlling the release. In conclusion, this work presents new approaches for the modification of CS as well as a new system with a great potential for colonic drug delivery.

6A-O-[(4-biphenylyl)acetyl]-alpha-, -beta-, and -gamma-cyclodextrins and 6A-deoxy-6A-[[(4-biphenylyl)acetyl]amino]-alpha-, -beta-, and -gamma-cyclodextrins: potential prodrugs for colon-specific delivery

Cyclodextrins (CyDs) are known to be fermented to small saccharides by colonic microflora, whereas they are only slightly hydrolyzable and thus are not easily absorbed in the stomach and small intestine. This property of CyDs is particularly useful for colon-specific delivery of drugs. In this study, an antiinflammatory 4-biphenylylacetic acid (BPAA) was selectively conjugated onto one of the primary hydroxyl groups of alpha-, beta-, and gamma-CyDs through an ester or amide linkage, 6A-O-[(4-biphenylyl)acetyl[-alpha-, -beta-, and -gamma-CyDs (1-3) and 6A-deoxy-6A-[[(4-biphenylyl)acetyl]amino]-alpha-, -beta-, and -gamma-CyDs (4-6). In rat cecal and colonic contents (10%, w/v), 1 and 3 released more than 95% of BPAA within 1-2 h, and 2 released about 50% of the drug within 12 h. The amide prodrugs, 4-6, did not release BPAA in the cecal contents, but gave BPAA/maltose or BPAA/triose conjugates linked through an amide bond. On the other hand, these prodrugs were found to be stable in the contents of rat stomachs and small intestines, in intestinal or liver homogenates, and in rat blood. The serum levels of BPAA increased about 3 h after oral administration of 1 and 3 to rats, accompanying a marked increase in the serum levels, whereas 2 and 4-6 resulted in little increase of the serum levels. These facts suggest that BPAA is released after the ring opening of CyDs followed by the ester hydrolysis, and the BPAA activation takes place site-specifically in the cecum and colon. Therefore, the present CyD prodrug approach provides a versatile means of constructing a novel colon-specific drug delivery system.

In vitro degradation of pH-sensitive hydrogels containing aromatic azo bonds

Biodegradable and pH-sensitive hydrogels containing azoaromatic moieties were synthesized from the same polymeric precursors by two synthetic methods, namely a polymer-polymer reaction and cross-linking of polymeric precursors. The effect of the synthetic route employed and the detailed network structure on in vitro degradation of hydrogels was studied. Regardless of the synthetic method used, two patterns of degradation were observed. Hydrogels with lower cross-linking density underwent a surface erosion process and degraded at a faster rate. Hydrogels with higher cross-linking densities degraded at a slower rate by a process where a colourless degradation front moved inward to the yellow core. It appears that hydrogels synthesized by a polymer-polymer reaction degraded at a slightly faster rate than their analogues synthesized by cross-linking of polymeric precursors. The degradation rate of a hydrogel was compared with those of a linear azopolymer and a low-molecular-weight azosubstrate (methyl orange) respectively. The degradation rates were in the order of hydrogel < linear azopolymer < low-molecular-weight azosubstrate.

Degradation of insulin and calcitonin and their protection by various protease inhibitors in rat caecal contents: implications in peptide delivery to the colon

The objective of this study was to examine the metabolism of insulin and calcitonin, and their protection by various protease inhibitors, in the large intestine. Fresh caecal contents were prepared from non-fasted rats and the degradation of insulin and calcitonin was studied in a suspension of rat caecal contents, as a model of the content of the large intestine. Both insulin and calcitonin were metabolized in suspensions of rat caecal contents, but the degradation of calcitonin was much faster than that of insulin. The degradation of insulin was fastest at pH 6.8. Protease inhibitors such as camostat and aprotinin inhibited the degradation of insulin and calcitonin in rat caecal contents, which was consistent with the high chymotrypsin activity of these contents. These findings suggest that care should be taken when administering peptide drugs to the large intestine for colon-specific drug delivery because they can be degraded in rat caecal contents. Protease inhibitors might be useful for increasing the stability of these peptides in the large intestine, thereby improving their large-intestinal absorption to the systemic circulation.

Conjugates of insulin with copolymers of N-(2-hydroxypropyl) methacrylamide: effects on smooth muscle cell proliferation

The hypothesis that an elevated plasma insulin level contributes to an increase in coronary heart disease has led to studies of the mitogenic effect of native insulin and its conjugates on smooth muscle cells (SMC). In this study, insulin was covalently attached to two water-soluble polymers containing N-(2-hydroxypropyl)methacrylamide using the mixed anhydride method. The first polymer was a copolymer of N-(2-hydroxypropyl)methacrylamide and N-methacryloyldiglycine. The second one was a terpolymer of two of the above-given monomers and R-(-)-1-methyl-2-methacryloylamidoethyl 2-acetamido-2-deoxy-beta-D-glucopyranoside. Insulin conjugates were isolated and characterized, and the mitogenic effect on SMC was investigated. The results showed that only conjugates of insulin and terpolymers bearing pendant N-acetyl-glucosamine groups do not have a mitogenic effect on SMC while maintaining the hypoglycemic activity of insulin. This finding suggests that some inter- or intramolecular interactions of coupled insulin with the sugar moiety(ies) attached to the polymer backbone contribute to the observed effects.