Synthesis and in vitro/in vivo evaluation of 5-aminosalicyl-glycine as a colon-specific prodrug of 5-aminosalicylic acid

Oral Formulation of 5-Aminosalicylic Acid-Hemoglobin Bio-Adhesive Nanoparticles Enhance Therapeutic Efficiency in Ulcerative Colitis Mice: A Preclinical Evaluation

The oral formulation design for colon-specific drug delivery brings some therapeutic benefits in the ulcerative colitis treatment. We recently reported the specific delivery of hemoglobin nanoparticles-conjugating 5-aminosalicylic acid (5-ASA-HbNPs) to the inflamed site. In the current study, the therapeutic effect of the 5-ASA-HbNPs formulation was confirmed in vivo. This evaluation of 5-ASA-HbNPs not only shows longer colonic retention time due to adhesive properties, also provides full support for it as compared with free 5-ASA. It was considered as a suitable bio-adhesive nanoparticle with mucoadhesive property to pass through the mucus layer and accumulate into the mucosa. In UC model mice, a two-fold decrease in the disease activity indexes and colon weight/length ratios was significantly observed in the group treated with 5-ASA-HbNPs. This group received one percent of the standard dosage of 5-ASA (50 μg/kg), while, a similar result was observed for a significant amount of free 5-ASA (5 mg/kg). Furthermore, microscopic images of histological sections of the extracted colons demonstrated that the 5-ASA-HbNPs and 5-ASA groups displayed instances of inflammatory damage within the colon. However, in comparison to the colitis group, the extent of this damage was relatively moderate, suggesting 5-ASA-HbNPs improved therapeutic efficacy with the lower dosage form.

Hemoglobin bio-adhesive nanoparticles as a colon-specific delivery system for sustained release of 5-aminosalicylic acid in the effective treatment of inflammatory bowel disease

A colonic drug delivery system was developed to specifically deliver 5-aminosalicylic acid (5-ASA) to the inflamed site by conjugating with hemoglobin nanoparticles (HbNPs). The 5-ASA-HbNPs (eight 5-ASA molecules per Hb molecule) with the size of 220 nm and zeta potential of -14.6 mV is a tailored nanoparticle able to pass through the mucus layer. The 5-ASA-HbNPs do not undergo chemical and enzymatic hydrolysis in the simulated gastrointestinal fluids over 6 h. Significantly higher cellular uptakes and prolonged release was seen for the 5-ASA-HbNPs in Caco-2 cells, compared to free 5-ASA over 72 h. In addition, 5-ASA-HbNPs revealed similar therapeutic effectiveness with free 5-ASA against tumor necrosis factor and showed less inhibitory concentration (IC50) for myeloperoxidase enzyme activity. In vivo imaging of mouse demonstrated the localization of drug in the descending colon after oral administration and about 15% of the administered dose was recovered as 5-ASA from urine in 6 h. The use of these nanoparticles with the mucus adhesion properties and permeability to intestinal epithelial cells can be a good candidate with potential application in the colonic drug delivery field.

Transport Characteristics of 5-Aminosalicylic Acid Derivatives Conjugated with Amino Acids via Human H+-Coupled Oligopeptide Transporter PEPT1

5-Aminosalicylic acid (5-ASA) is used as first line therapy for symptom remission and maintenance of inflammatory bowel disease (IBD). Because 5-ASA is well absorbed from the small intestine when orally administered, several 5-ASA formulations for selective delivery to the colon have been developed and used in clinical practice. However, its delivery efficiency to local inflamed colonic sites remains low. Intestinal H⁺-coupled oligopeptide transporter 1 (PEPT1) expression in the colon is low, whereas its expression is induced in the colon under chronic inflammation conditions, such as IBD. Therefore, we considered that PEPT1 would be a target transporter to improve 5-ASA delivery efficiency to local colonic lesions. We evaluated the transport characteristics of dipeptide-like 5-ASA derivatives, which were coupling glycine (Gly), lysine, glutamic acid (Glu), valine (Val) and tyrosine to amino or carboxyl group of 5-ASA, in Caco-2 cells. [³H]Glycylsarcosine (Gly-Sar) uptake into Caco-2 cells was inhibited by all 5-ASA derivatives. In addition, 5-ASA derivatives (Gly-ASA, Glu-ASA and Val-ASA), which were coupled by glycine, glutamic acid and valine to amino group of 5-ASA, were taken up in a pH- and concentration-dependent manner and their uptake was inhibited by excess Gly-Sar. Two-electrode voltage-clamp experiment using human PEPT1 expressing Xenopus oocytes showed that Gly-ASA, Glu-ASA and Val-ASA induced marked currents at pH 6.0. Taken together, these results showed that these 5-ASA derivatives are transportable substrates for PEPT1.

A review on 5-aminosalicylic acid colon-targeted oral drug delivery systems

Site-specific colon drug delivery is a practical approach for the treatment of local diseases of the colon with several advantages such as rapid onset of action and reduction of the dosage of the drug as well as minimization of harmful side effects. 5-aminosalicylic acid (5-ASA) is a drug of choice in the treatment of inflammatory bowel disease and colitis. For the efficient delivery of this drug, it is vital to prevent 5-ASA release in the upper part of the gastrointestinal tract and to promote its release in the proximal colon. Different approaches including chemical manipulation of drug molecule for production of prodrugs or modification of drug delivery systems using pH-dependent, time-dependent and/or bacterially biodegradable materials have been tried to optimize 5-ASA delivery to the colon. In the current review, the different strategies utilized in the design and development of an oral colonic delivery dosage form of 5-ASA are presented and discussed.

Conjugation of metronidazole with dextran: a potential pharmaceutical strategy to control colonic distribution of the anti-amebic drug susceptible to metabolism by colonic microbes

Metronidazole (MTDZ), the drug of choice for the treatment of protozoal infections such as luminal amebiasis, is highly susceptible to colonic metabolism, which may hinder its conversion from a colon-specific prodrug to an effective anti-amebic agent targeting the entire large intestine. Thus, in an attempt to control the colonic distribution of the drug, a polymeric colon-specific prodrug, MTDZ conjugated to dextran via a succinate linker (Dex-SA-MTDZ), was designed. Upon treatment with dextranase for 8 h, the degree of Dex-SA-MTDZ depolymerization (%) with a degree of substitution (mg of MTDZ bound in 100 mg of Dex-SA-MTDZ) of 7, 17, and 30 was 72, 38, and 8, respectively, while that of dextran was 85. Depolymerization of Dex-SA-MTDZ was found to be necessary for the release of MTDZ, because dextranase pretreatment ensures that de-esterification occurs between MTDZ and the dextran backbone. In parallel, Dex-SA-MTDZ with a degree of substitution of 17 was found not to release MTDZ upon incubation with the contents of the small intestine and stomach of rats, but it released MTDZ when incubated with rat cecal contents (including microbial dextranases). Moreover, Dex-SA-MTDZ exhibited prolonged release of MTDZ, which contrasts with drug release by small molecular colon-specific prodrugs, MTDZ sulfate and N-nicotinoyl-2-{2-(2-methyl-5-nitroimidazol-1-yl)ethyloxy}-d,l-glycine. These prodrugs were eliminated very rapidly, and no MTDZ was detected in the cecal contents. Consistent with these in vitro results, we found that oral gavage of Dex-SA-MTDZ delivered MTDZ (as MTDZ conjugated to [depolymerized] dextran) to the distal colon. However, upon oral gavage of the small molecular prodrugs, no prodrugs were detected in the distal colon. Collectively, these data suggest that dextran conjugation is a potential pharmaceutical strategy to control the colonic distribution of drugs susceptible to colonic microbial metabolism.

5-Aminosalicylic Acid Azo-Linked to Procainamide Acts as an Anticolitic Mutual Prodrug via Additive Inhibition of Nuclear Factor kappaB

To improve the anticolitic efficacy of 5-aminosalicylic acid (5-ASA), a colon-specific mutual prodrug of 5-ASA was designed. 5-ASA was coupled to procainamide (PA), a local anesthetic, via an azo bond to prepare 5-(4-{[2-(diethylamino)ethyl]carbamoyl}phenylazo)salicylic acid (5-ASA-azo-PA). 5-ASA-azo-PA was cleaved to 5-ASA and PA up to about 76% at 10 h in the cecal contents while remaining stable in the small intestinal contents. Oral gavage of 5-ASA-azo-PA and sulfasalazine, a colon-specific prodrug currently used in clinic, to rats showed similar efficiency in delivery of 5-ASA to the large intestine, and PA was not detectable in the blood after 5-ASA-azo-PA administration. Oral gavage of 5-ASA-azo-PA alleviated 2,4,6-trinitrobenzenesulfonic acid-induced rat colitis. Moreover, combined intracolonic treatment with 5-ASA and PA elicited an additive ameliorative effect. Furthermore, combined treatment with 5-ASA and PA additively inhibited nuclear factor-kappaB (NFκB) activity in human colon carcinoma cells and inflamed colonic tissues. Finally, 5-ASA-azo-PA administered orally was able to reduce inflammatory mediators, NFκB target gene products, in the inflamed colon. 5-ASA-azo-PA may be a colon-specific mutual prodrug acting against colitis, and the mutual anticolitic effects occurred at least partly through the cooperative inhibition of NFκB activity.

Evaluation of glycine-bearing celecoxib derivatives as a colon-specific mutual prodrug acting on nuclear factor-κB, an anti-inflammatory target

In an inflammatory state where HOCl is generated, glycine readily reacts with HOCl to produce glycine chloramine, an anti-inflammatory oxidant. Colonic delivery of celecoxib elicits anticolitic effects in a trinitrobenzene sulfonic acid-induced rat colitis model. Glycine-bearing celecoxib derivatives were prepared and evaluated as a colon-specific mutual prodrug acting on nuclear factor-κB (NFκB), an anticolitic target. Glycylcelecoxib (GC), N-glycylaspart-1-ylcelecoxib (N-GA1C), and C-glycylaspart-1-ylcelecoxib (C-GA1C) were synthesized and their structures identified using infrared and proton nuclear magnetic resonance spectrometer. The celecoxib derivatives were chemically stable in pH 6.8 and 1.2 buffers. GC and C-GA1C were resistant to degradation in the small intestinal contents, while N-GA1C was substantially cleaved to release celecoxib. In contrast, all the celecoxib derivatives were degraded to liberate celecoxib in the cecal content. These results suggest that GC and C-GA1C could be delivered to and liberate celecoxib and glycine in the large intestine. In human colon carcinoma HCT116 and murine macrophage RAW264.7 cells, combined celecoxib–glycine chloramine treatment additively suppressed the production of proinflammatory NFκB target gene products. Collectively, our data suggest that C-GA1C is a potential colon-specific mutual prodrug acting against NFκB.

Optical and thermal properties of azo derivatives of salicylic acid thin films

N-acryloyl-4-aminosalicylic acid (4-AMSA), monomer (HL) and 5-(4'-alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HLn) are synthesized and characterized with various physico-chemical techniques. Thin films of 5-(4'-alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HLn) are prepared by spin coating technique. The X-ray diffraction (XRD) patterns of 4-aminosalicylic acid (4-ASA) and its derivatives are investigated in powder and thin film forms. Thermal properties of the compounds are investigated by thermogravemetric analysis (TGA). The optical energy gap and the type of optical transition are investigated in the wavelength range (200-2500 nm) for 4-ASA, HL and HLn. The values of fundamental energy gap (Eg) are in the range 3.60-3.69 eV for all compounds and the type of optical transition is found to be indirect allowed. The onset energy gap Eg(∗) appeared only for azodye compounds is found to be in the range 0.95-1.55 eV depending on the substituent function groups. The refractive index, n, shows a normal dispersion in the wavelength range 650-2500 nm, while shows anomalous dispersion in the wavelength rang 200-650 nm. The dispersion parameters ε∞, εL, Ed, Eo and N/m(∗) are calculated. The photoluminescence phenomena (PL) appear for thin films of 4-ASA and its derivatives show three main emission transitions.

Carboxymethylcellulose-tetrahydrocurcumin conjugates for colon-specific delivery of a novel anti-cancer agent, 4-amino tetrahydrocurcumin

Several curcumin derivatives are now becoming increasingly of interest because of their bioactive attributes, especially their action as antioxidants and anti-carcinogenic activities. Tetrahydrocurcumin (THC), an active metabolite of curcumin, was selected to be a proper starting material for the work presented here as it is stable in physiological pH and has the typical pharmacological properties of curcumin. We have now reported that novel synthesized water-soluble polymeric macromolecule prodrugs can specifically deliver the drug to the colon. To study the drug loading and drug release, THC was conjugated with a hydrophilic polymer, carboxymethylcellulose (CMC) with the degree of substitution (DS) values of 0.7 and 1.2. THC was also attached to two different spacers including p-aminobenzoic acid (PABA) and p-aminohippuric acid (PAH) via an azo bond that was cleaved by the azoreductase activities of colonic bacteria. The novel active molecule, 4-amino-THC, was readily released from the conjugates in the colon (>62% within 24h) with only very small amounts released in the upper GI tract (<12% over 12h). The polymer conjugates showed chemical stability at various pH values along the gastrointestinal tract and increased water solubility of up to 5mg/mL. 4-Amino-THC demonstrated cytotoxic ability against the human colon adenocarcinoma cell lines (HT-29) with an IC50 of 28.67 ± 1.01 μg/mL, and even greater selectivity (∼ 4 folds) to inhibit HT-29 cells than to normal human colon epithelial cell lines while curcumin was a non-selective agent against both cell lines. Our study has demonstrated that the use of THC-CMC conjugates may be a promising colon-specific drug delivery system with its sustained release in the colon to be an effective treatment for colonic cancer.

Drug delivery strategies in the therapy of inflammatory bowel disease

Inflammatory bowel disease (IBD) is a frequently occurring disease in young people, which is characterized by a chronic inflammation of the gastrointestinal tract. The therapy of IBD is dominated by the administration of anti-inflammatory and immunosuppressive drugs, which suppress the intestinal inflammatory burden and improve the disease-related symptoms. Established treatment strategies are characterized by a limited therapeutical efficacy and the occurrence of adverse drug reactions. Thus, the development of novel disease-targeted drug delivery strategies is intended for a more effective therapy and demonstrates the potential to address unmet medical needs. This review gives an overview about the established as well as future-oriented drug targeting strategies, including intestine targeting by conventional drug delivery systems (DDS), disease targeted drug delivery by synthetic DDS and disease targeted drug delivery by biological DDS. Furthermore, this review analyses the targeting mechanisms of the respective DDS and discusses the possible field of utilization in IBD.

Glycyrrhizin enhances therapeutic activity of a colon-specific methylprednisolone prodrug against experimental colitis

BACKGROUND

Co-administration of a reduction inhibitor and a colon-specific prodrug of a glucocorticoid susceptible to colonic reductive metabolism is suggested as a strategy to circumvent the therapeutic loss of the glucocorticoid delivered to and acting locally at the large intestine.

AIMS

We examined whether the strategy was feasible as a pharmacotherapy for treatment of inflammatory bowel disease.

METHODS

Glycyrrhizin (GCZ), a reduction inhibitor, was tested for its inhibition of the colonic metabolism of methylprednisolone (MP). Therapeutic activity against TNBS-induced rat colitis and adrenal suppression were compared after oral administration of methylprednisolone 21-sulfate sodium (MPS), a colon-specific prodrug of MP, or MPS/GCZ to colitic rats.

RESULTS

Upon incubation of MP with the cecal contents, MP disappeared, and this was delayed by addition of GCZ. In addition, more MP produced from MPS in the cecal contents accumulated in the presence of GCZ. Consistent with these results, upon oral administration of MPS/GCZ, MPS or MP, MP was detected at a greater level in the large intestine for MPS/GCZ. MPS/GCZ ameliorated TNBS-induced colitis of rats, and this therapeutic effect was superior to that of MPS and MP. Moreover, MPS/GCZ decreased the plasma levels of corticosterone and ACTH to a greater extent than MPS, but less than MP.

CONCLUSIONS

Co-administration of GCZ, a reduction inhibitor, may be a plausible strategy to reduce the therapeutic loss of MP produced from MPS in the large intestine, thus improving the therapeutic property of the prodrug against inflammatory bowel disease.

IN VITRO RELEASE OF HYDROCORTISONE BY GLYCINE-IMMOBILIZED EVAL MEMBRANE

A pH-sensitive membrane for colon-specific drug delivery was prepared by glycine-immobilization on poly (ethylene-co-vinyl alcohol) (Gly-EVAL) that can enhance the permeability of hydrocortisone at pH 7.4 and resist drug permeation at pH 2.0 or in gastric juice. As the results of drug releasing profile, it is proposed that the electrical repulsion occurring between adjacent carboxylate ions at pH 7.4 on Gly-EVAL causes the higher permeation rate of hydrocortisone. Consequently, the hydrocortisone coated by Gly-EVAL can escape from degradation in acid environment and release significantly in neutral or weak basic pH values, which is ideally suitable for local treatment of ulcerative colitis.

Synthesis and evaluation of N-nicotinoyl-2-{2-(2-methyl-5-nitroimidazol-1-yl)ethyloxy}-D,L-glycine as a colon-specific prodrug of metronidazole

Metronidazole (MTZ) is a drug of choice for protozoal infections such as luminal amoebiasis. We designed and synthesized N-nicotinoyl-2-{2-(2-methyl-5-nitroimidazol-1-yl)ethyloxy}-D,L-glycine (NMG) as a colon-specific prodrug of MTZ. The synthetic yield of NMG was about 34%. The apparent partition coefficient of MTZ was greatly reduced by the chemical modification. While (bio)chemically stable in the contents of the upper intestine, NMG was rapidly cleaved to liberate MTZ on incubation with the cecal contents of rats. MTZ metabolized quickly in the cecal contents at least partly by a microbial nitroreductase, suggesting that the metabolism of MTZ is relevant to its bioactivation leading to amoebicidal action. The systemic absorption, analyzed by the blood concentration and urinary recovery of NMG, was very low after oral administration of NMG. In parallel with this, whereas MTZ disappeared mostly during the transit of the proximal small intestine, a substantial amount of NMG remained in the small intestine moving down to the large intestine where it metabolized rapidly. Moreover, comparing systemic absorption of MTZ after oral administration of NMG or MTZ, NMG markedly reduced the systemic absorption. These results suggest that NMG is a potential colon-specific prodrug of MTZ which improves therapeutic and toxicological properties.

N,N'-Bis(5-aminosalicyl)-L-cystine is a potential colon-specific 5-aminosalicylic acid prodrug with dual therapeutic effects in experimental colitis

To evaluate N,N'-bis(5-aminosalicyl)-L-cystine (5-ASA-Cys) as a potential colon-specific 5-aminosalicylic acid prodrug with dual therapeutic effects in experimental colitis, the pharmacokinetics and therapeutic activity were investigated after oral administration of 5-ASA-Cys and amelioration of experimental colitis was compared after rectal administration of 5-aminosalicylic acid (5-ASA) and/or cysteine. In addition, the gluthathione (GSH) level in the inflamed colonic tissue was examined after administration of cysteine or 5-ASA-Cys. Oral administration of 5-ASA-Cys delivered much greater amount of 5-ASA to the large intestine and excreted lower amount of 5-ASA via urine than that of free 5-ASA. Oral administration of 5-ASA-Cys ameliorated experimental colitis of rats induced by TNBS, which was more effective than that of sulfasalazine. Although cysteine administered rectally was not significantly effective, intracolonic treatment with both 5-ASA and cysteine showed a synergic effect in alleviating the rat colitis. Furthermore, not only 5-ASA-Cys administered orally but also cysteine administered rectally increased the glutathione level in the inflamed colonic tissue. Taken together, these results suggest that 5-ASA-Cys is a potential colon specific 5-ASA prodrug with dual therapeutic effects on experimental colitis and cysteine modulation of the glutathione level may be relevant to the dual effects of the prodrug.

Poly(ethylene glycol)-mesalazine conjugate for colon specific delivery

Chronic inflammatory bowel diseases (IBDs) are still waiting for improved and innovative therapeutic treatments, which can overcome the limits of the current approaches. Since IBDs affect mainly the lower tract of the intestine, a localized therapy in the colon tract will avoid most of the problems caused by systemic or poor selective therapies. Particularly promising are the advance drug delivery systems that can reach specific colon delivery, thus guaranteeing active agent release only at the site of action. This approach can meet two aims at the same time, first of all the drug will not affect healthy tissue and second a lower drug dose may be used because all the administered active agent will reach the target. To obtain a specific colon delivery we exploited the azoreductase enzymes, selectively present only in colon, by inserting an azo linker between a selected drug and a macromolecular carrier. The drug employed is mesalazine, a well know and used agent against IBDs. Poly(ethylene glycol) (PEG), of different molecular weights and structures, was used as carrier. Three different conjugates were synthesized and characterized, and the most promising one, with highest drug loading thanks to the use of diamino PEG of 4 kDa, was further investigated in vitro on mouse colonic epithelial cells (CMT-9) and in vivo on model mice with induced colitis. The data presented here demonstrate that PEG conjugation of mesalazine prevents drug release and absorption in upper intestine, after oral administration of the conjugates, and that the azo linker ensures a good drug release in the colon tract. The results in vivo take into consideration mice bodyweight gain, tissue histology and interleukin-2 beta as an index of inflammation. These parameters, all together, demonstrated the conjugate effectiveness against the controls.

Synthesis andin vitroevaluation ofN-nicotinoylglycyl-2-(5-fluorouracil-1-yl)-d,l-glycine as a colon-specific prodrug of 5-fluorouracil

N-Nicotinoylglycyl-2-(5-fluorouracil-1-yl)-D,L-glycine (NGFG) was synthesized as a colon-specific prodrug of 5-fluorouracil (5-FU) expecting that hydrolysis of nicotinoyl and glycyl moieties by microbial enzymes in the colon will give 2-(5-fluorouracil-1-yl)-D,L-glycine, which releases 5-FU spontaneously. To in vitro-evaluate colon targetability of NGFG, apparent partition coefficient and chemical/biochemical stability of NGFG in the contents or/and tissue of the various segments of the gastrointestinal tract were determined. Low partition coefficient and stability of NGFG in the upper intestinal condition suggested its delivery to the colon in intact form after oral administration. Incubation with rat cecal contents produced 5-FU and its metabolite about 16%. Structural modification to enhance amide hydrolysis, the rate determining step in NGFG bioactivation, is suggested.

Design and evaluation of colon specific drug delivery system containing flurbiprofen microsponges

The purpose of this study was to design novel colon specific drug delivery system containing flurbiprofen (FLB) microsponges. Microsponges containing FLB and Eudragit RS 100 were prepared by quasi-emulsion solvent diffusion method. Additionally, FLB was entrapped into a commercial Microsponge 5640 system using entrapment method. Afterwards, the effects of drug:polymer ratio, inner phase solvent amount, stirring time and speed and stirrer type on the physical characteristics of microsponges were investigated. The thermal behaviour, surface morphology, particle size and pore structure of microsponges were examined. The colon specific formulations were prepared by compression coating and also pore plugging of microsponges with pectin:hydroxypropylmethyl cellulose (HPMC) mixture followed by tabletting. In vitro dissolution studies were done on all formulations and the results were kinetically and statistically evaluated. The microsponges were spherical in shape, between 30.7 and 94.5microm in diameter and showed high porosity values (61-72%). The pore shapes of microsponges prepared by quasi-emulsion solvent diffusion method and entrapment method were found as spherical and cylindrical holes, respectively. Mechanically strong tablets prepared for colon specific drug delivery were obtained owing to the plastic deformation of sponge-like structure of microsponges. In vitro studies exhibited that compression coated colon specific tablet formulations started to release the drug at the 8th hour corresponding to the proximal colon arrival time due to the addition of enzyme, following a modified release pattern while the drug release from the colon specific formulations prepared by pore plugging the microsponges showed an increase at the 8th hour which was the time point that the enzyme addition made. This study presents a new approach based on microsponges for colon specific drug delivery.

Evaluation of 5-aminosalicyltaurine as a colon-specific prodrug of 5-aminosalicylic acid for treatment of experimental colitis

We previously reported that 5-aminosalicyltaurine (taurine-conjugated 5-ASA, 5-ASA-Tau) showed a potential as a colon-specific prodrug of 5-aminosalicylic acid (5-ASA) by in vitro evaluation. In this report, we in vivo-evaluated 5-ASA-Tau as a colon-specific prodrug for treatment of experimental colitis. Taurine conjugation of 5-ASA greatly reduced absorption of 5-ASA from the intestine. Oral administration of taurine-conjugated 5-ASA not only increased the colonic delivery efficiency of 5-ASA but also decreased the systemic absorption of free 5-ASA as compared with that of 5-ASA and, moreover, taurine is similarly effective to known colon-specific carriers for 5-ASA, glycine and aspartic acid, suggesting that taurine conjugation is an efficient way to increase the therapeutic effect and to reduce the adverse effects of 5-ASA. Intracolonic treatment with combined 5-ASA/taurine additively ameliorated TNBS-induced colitis rats indicating that taurine acted as not only a promoiety but also a therapeutically active agent. Furthermore, 5-ASA-Tau is slightly more effective than sulfasalazine in alleviating the colonic inflammation induced by TNBS. Taken together, our data suggest that 5-ASA-Tau is a potential colon-specific prodrug of 5-aminosalicylic acid with improved therapeutic activity against inflammatory bowel disease.

Late metal salicylaldimine complexes derived from 5-aminosalicylic acid — Molecular structure of a zwitterionic mono Schiff base zinc complex

Condensation of salicylaldehyde (2-HOC6H4C(O)H) with 5-aminosalicylic acid (5-H2NC6H3-2-(OH)-CO2H) afforded the Schiff base 2-HOC6H4C(H)=NC6H3-2-(OH)-5-CO2H (a). Similar reactivity with 5-bromosalicylaldehyde was also observed to give 5-Br-2-HOC6H3C(H)=NC6H3-2-(OH)-5-CO2H (b). Reaction of these salicylaldehydes with Pd(II), Cu(II), and Zn(II) salts gave the corresponding bis(N-arylsalicylaldiminato)metal complexes (M = Pd (1), Cu (2), Zn (3)). The molecular structure of the Schiff base compound a has been confirmed by an X-ray diffraction study. Crystals of a were monoclinic, space group P2(1)/c, a = 7.0164(7) Å, b = 11.0088(11) Å, c = 14.8980(15) Å, β = 102.917(2)°, Z = 4. The molecular structure of a novel zwitterionic conformer of 3a was also characterized by an X-ray diffraction study. Crystals of 4 were monoclinic, space group P2(1)/c, a = 9.5284(5) Å, b = 19.5335(11) Å, c = 8.6508(5) Å, β = 90.596(1)°, Z = 4. All new compounds have been tested for their antifungal activity against Aspergillus niger and Aspergillus flavus. Key words: 5-aminosalicylic acid (5-ASA), antifungal, copper, palladium, salicylaldimines, Schiff base, zinc.

Synthesis and anti-inflammatory testing of some new compounds incorporating 5-aminosalicylic acid (5-ASA) as potential prodrugs

This work includes the synthesis of 15 final compounds (6a-h and 7b-h) as prodrugs of 5-ASA in the form of the acid itself, esters and amides linked by an amide linkage through a spacer to the endocyclic ring N of nicotinamide. Also, 15 new intermediate compounds were prepared. The target compounds (6b, 6f, 7b, and 7e-h) revealed potent analgesic and anti-inflammatory activities in comparison to sulfasalazine and 5-ASA. In addition, ulcerogenicity, LD50, in vivo and in vitro metabolism of compound 7f were determined.

Novel drug delivery strategies for the treatment of inflammatory bowel disease

Inflammatory bowel disease (IBD) encompasses two idiopathic inflammatory diseases of the intestinal tract: Crohn's disease and ulcerative colitis. Existing therapy for IBD consists mainly of orally or rectally administered small drug molecules, such as 5-aminosalicylates and corticosteroids, or potent systemic immune suppressants. IBD presents a challenging target for drug delivery, particularly by the oral route, as, contrary to most therapeutic regimens, minimal systemic absorption and maximal intestinal wall drug levels are desired. Several delivery strategies are employed to achieve this goal, including the chemical modification of the drug molecules, the use of controlled- and delayed-release formulations and the use of bioadhesive particles. The goal of this review is to summarise existing IBD therapy and examine novel approaches in intestinal drug delivery.

Prednisolone 21-sulfate sodium: a colon-specific pro-drug of prednisolone

Prednisolone 21-sulfate sodium (PDS) was synthesized as a colon-specific pro-drug of prednisolone with the expectation that it would be stable and non-absorbable in the upper intestine and release prednisolone by the action of sulfatase once it was delivered to the colon. In-vitro/in-vivo properties were investigated using rats as test animals. PDS was chemically stable at pH 1.2, 4.5, 6.8 and 8.0, and the apparent partition coefficient was 0.11 in 1-octanol/pH 6.8 buffer solution at 37 degrees C. PDS was stable on incubation with the contents of the stomach or small intestine. When PDS (0.1 mg equiv. of prednisolone) was incubated with the caecal contents (0.05 g), prednisolone was produced to a maximum 54% of the dose in 6 h and decreased thereafter, which suggested that reduction of the A ring took place in addition to the hydrolysis by sulfatase. After oral administration of PDS, a small portion of prednisolone was recovered from the cecal contents but not from the small intestine. Neither PDS nor prednisolone was detected in the plasma, suggesting that absorption of PDS is limited. The data demonstrate that the sulfate ester can serve as a novel colon-specific pro-moiety by limiting the absorption of the pro-drug in the upper intestine and releasing the active compound by the action of microbial sulfatase in the colon.

Synthesis and properties of 5-aminosalicyl-taurine as a colon-specific prodrug of 5-aminosalicylic acid

5-Aminosalicylic acid (5-ASA) is an active ingredient of therapeutic agents used for Crohn's disease and ulcerative colitis. Because it is absorbed rapidly and extensively in the upper intestine, delivery of the agent specifically to the colon is necessary. We selected taurine as a colon-specific promoiety and designed 5-aminosalicyltaurine (5-ASA-Tau) as a new colon-specific prodrug of 5-aminosalicylic acid (5-ASA). It was expected that introduction of taurine would restrict the absorption of the prodrug and show additive effect to the anti-inflammatory action of 5-ASA after hydrolysis. 5-ASA-Tau was prepared in good yield by a simple synthetic route. The apparent partition coefficient of 5-ASA-Tau in 1-octanol/pH 6.8 phosphate buffer or CHCl3/pH 6.8 phosphate buffer was 0.10 or 0.18, respectively, at 37 degrees C. To determine the chemical and biochemical stability in the upper intestinal environment, 5-ASA-Tau was incubated in pH 1.2 and 6.8 buffer solutions, and with the homogenates of tissue and contents of stomach or small intestine of rats at 37 degrees C. 5-ASA was not detected from any of the incubation medium with no change in the concentration of 5-ASA-Tau. On incubation of 5-ASA-Tau with the cecal and colonic contents of rats, the fraction of the dose released as 5-ASA was 45% and 20%, respectively, in 8 h. Considering low partition coefficient and stability in the upper intestine, 5-ASA-Tau might be nonabsorbable and stable in the upper intestine. After oral administration, it would be delivered to the colon in intact form and release 5-ASA and taurine. These results suggested 5-ASA-Tau as a promising colon-specific prodrug of 5-ASA.

Synthesis and in vitro properties of prednisolone 21-sulfate sodium as a colon-specific prodrug of prednisolone

Colon-specific delivery of glucocorticoids is highly desirable for the efficient treatment of inflammatory bowel disease. We synthesized prednisolone 21-sulfate sodium (PDS) as a colon-specific prodrug of prednisolone (PD) and investigated its properties using rats as test animals. We expected that introduction of sulfate ester as a sodium salt might increase the hydrophilicity and restrict the absorption in the GI tract. If PDS is stable and nonabsorbable in the upper intestine, it will be delivered to the colon as an intact form, where it hydrolyze by the sulfatase to release PD. Compared with PD, the solubility of PDS increased and the apparent partition coefficient decreased greatly. PDS was stable on incubation with pH 1.2 and 6.8 buffer solutions and with the contents of the stomach and small intestine. On incubation with the cecal contents, PDS decreased to 9.6% of the dose in 10 h producing PD. The amount of PD increased to give a maximum 54% of the dose and decreased. As a control, when PD was incubated with the cecal contents, it decreased to 29% of the dose in 8 h, which implied that reduction of PD proceeded under such conditions. These results suggested that hydrolysis of PDS took place to produce and accumulate PD, which decreased by reduction as the incubation period extended. Our results suggested that PDS can be a promising colon-specific prodrug of PD, and sulfate ester group might serve as a potential colon-specific promoiety, especially for the drugs which are resistant to reduction in the colon.

Dendrimers conjugates for colonic delivery of 5-aminosalicylic acid

The water-soluble polyamidoamine (PAMAM) dendrimer conjugates for colonic delivery of 5-aminosalicylic acid (5-ASA) were designed. The drug was bound to the dendrimer using two different spacers containing azo-bond, p-aminobenzoic acid (PABA) and p-aminohippuric acid (PAH). Incubation of PAMAM dendrimer conjugates containing PABA and PAH spacers with rat cecal contents at 37 degrees C gradually released 5-ASA with time and the amount of drug released was 45.6 and 57.0% of the dose in 24 h, respectively. The release of the drug from the commercial prodrug, sulfasalazine was significantly faster than both conjugates (80.2% of the dose in 6 h). No 5-ASA was detected from the incubation of dendrimer conjugates with the homogenate of the stomach or phosphate buffer, pH 1.2 and 6.8. Only a small amount of 5-ASA was found after incubation of both conjugates with the homogenate of the small intestine for 12 h. It appears that this PAMAM dendrimer can be developed for use as a carrier for colon-specific drug delivery.

Colon-specific drug delivery: new approaches and in vitro/in vivo evaluation

The necessity and advantages of colon-specific drug delivery systems have been well recognized and documented. In the past, the primary approaches to obtain colon-specific delivery achieved limited success and included prodrugs, pH- and time-dependent systems, and microflora-activated systems. Precise colon drug delivery requires that the triggering mechanism in the delivery system only respond to the physiological conditions particular to the colon. Hence, continuous efforts have been focused on designing colon-specific delivery systems with improved site specificity and versatile drug release kinetics to accommodate different therapeutic needs. Among the systems developed most recently for colon-specific delivery, four systems were unique in terms of achieving in vivo site specificity, design rationale, and feasibility of the manufacturing process (pressure-controlled colon delivery capsules (PCDCs), CODES, colonic drug delivery system based on pectin and galactomannan coating, and Azo hydrogels). The focus of this review is to provide detailed descriptions of the four systems, in particular, and in vitro/in vivo evaluation of colon-specific drug delivery systems, in general.

Colon-specific prodrugs of 5-aminosalicylic acid: synthesis and in vitro/in vivo properties of acidic amino acid derivatives of 5-aminosalicylic acid

5-aminosalicyl-L-aspartic acid (5-ASA-Asp) and 5-aminosalicyl-L-glutamic acid (5-ASA-Glu) were synthesized and their properties as colon-specific prodrugs of 5-aminosalicylic acid (5-ASA) were investigated employing rats as test animals. Incubation of 5-ASA-Asp and 5-ASA-Glu with the homogenates of tissue and contents of stomach or small intestine released no 5-ASA, indicating that they were stable in this condition. Incubation of 5-ASA-Asp with the cecal contents released 5-ASA 37%, whereas 5-ASA-Glu released only 8% of the dose in 16 h. Plasma concentration of 5-ASA-Asp after intravenous administration decreased rapidly and became undetectable in 60 min. No 5-ASA was detected in the blood, which indicated 5-ASA-Asp was stable in the plasma. After oral administration of 5-ASA-Asp, concentration of 5-ASA, its metabolite N-acetyl-5-ASA, and 5-ASA-Asp in the plasma, feces, and urine was determined. In the plasma, 5-ASA-Asp was not detected and the concentration of 5-ASA or N-acetyl-5-ASA was very low. About 33% of the administered dose was recovered as 5-ASA and N-acetyl-5-ASA and 43% as 5-ASA-Asp from feces, and 20% as 5-ASA and N-acetyl-5-ASA and 1% as 5-ASA-Asp from urine in 24 h. These results suggested that most of 5-ASA-Asp was delivered to the large intestine and about half of the administered dose was activated to liberate 5-ASA. After oral administration of free 5-ASA, fecal recovery was only 7% of the dose in 24 h and more than 80% was recovered from urine. Comparing 5-ASA-Asp and free 5-ASA, the amount of 5-ASA available in the large intestine was much larger, while the amount of 5-ASA in urine, which might be related to the systemic toxicity of 5-ASA, was much lower by the administration of 5-ASA-Asp than free 5-ASA.