Azo Polymeric-drug As Colonic Specific Drug Delivery System: Synthesis, Biodegradation And Drug Delivery Properties

Inflammatory bowel disease (IBD) is often localized to specific sites in the gastrointestinal tract (GIT). As a result, this disease can be treated with oral site-specific (targeted) drug delivery systems. Targeted delivery systems for treatment of IBD are designed to increase local tissue concentrations of antiinflammatory drugs from lower doses compared with systemic administration. While excluding some approaches, it is convenient to categorize targeted delivery systems into one of four categories: (1) the passage of time (temporal control of delivery), (2) pH-based (triggered by a change in local pH as the formulation passes down the GIT), (3) enzyme-based (the enzymes found locally in a region of the gut breakdown a prodrug or a formulation to release drug), and (4) pressure-based systems.In this paper, by combining the unique biocompatibility and hydrophilicity of poly (ethylene oxide) (PEG) with the biodegradability and low toxicity of 5, 5'-azodisalicylic acid (Olsalazine, OLZ), a linear azo polymer was constructed. PEG-OLZ copolymer is reductively cleaved by colonic bacteria to the therapeutically active mesalamine (5-aminosalicylic acid, 5-ASA) component through ester bond hydrolysis and azo bond reduction. The resultant azo-containing copolymer with azo groups bound in the main chain can be used as a polymeric prodrug for 5-ASA or as a colon-specific drug-delivery carrier for other bioactive substances such as protein and peptide. Main research contents and results are shown as follows:1. OLZ, the prodrug of 5-ASA was synthesized by diazotization and couple with methyl salicylate. The product was characterized by IR, ¹H NMR, ¹³C NMR and the melting point.2. Di-(2-hydroxyethyl) 5,5'-azodisalicylate was synthesized by reacting OLZ with glycol. Dicarboxylated PEG was prepared following the procedures of reacting PEG with succinic anhydride. By ester coupling reaction with dicyclohexyl carbodiimide / N, N'-dimethyl amino pyridine as the active carboxyl group reagents, a linear PEG-OLZ copolymer was constructed. By changing the chain length of the PEGsegments, the resultant PEG-OLZ copolymers can be modulated to have unique physical and physiological properties, such as hydrophilicity and biodegradability. The degree of polymerization of the azo polymers depends significantly on the molecular weight of PEG, where the increase of the molecular weight of PEG decreases the DPs of the copolymers. The solubility in common organic solvent and the low melting points (less than 40 °C) make PEG-OLZ copolymers excellent carrier for drug delivery. By introducing the PEG segments with different molecular weights to the PEG-OLZ copolymers, the hydration values of the copolymers can be adjusted over a wide range, for example, from 8.9% to 87.5% in 37°C/82% relative humidity desiccators.3. In phosphate buffer saline (PBS) solution, PEG-OLZ copolymers can release OLZ through the ester bond hydrolysis. The OLZ release rate of PEGioooo-OLZ copolymer is faster than PEG400-OLZ copolymer. The rate of ester bond cleavage of PEG4000-OLZ copolymer in pH 7.4 PBS is faster than pH 6.8 PBS.4. OLZ and PEG-OLZ copolymers were incubated with various segments of gastrointestinal (GI) tracts of rats at 37 °C, and the diminishing rates of azo bond were determined. Little reductive percent of azo bonds were investigated in the stomach and small contents media. However, the rate of cleavage of azo bond in the cecum contents medium was faster than other GI mediums. The results suggested that PEG-OLZ copolymers can be able to specifically deliver 5-ASA in the colon and be promising for the treatment of colon cancer.OLZ and the copolymers of PEG4000-OLZ and PEGioooo-OLZ were incubated in the whole-cell bacterial extract isolated from rat cecum and the biodegradation of the substances with pre-incubated for 14 hours was evaluated by monitoring the diminishing of azo-bond and the molecular weight changes. It is found that the accessibility of reduction sites is hindered for polymeric substrates compared to OLZ. In vitro biodegradation of the PEG-OLZ copolymers monitored by GPC measurement shows that the molecular weight of PEG-OLZ copolymers decreases continuously within 24 hours, and the degradation rate of the azo copolymers is much faster in the biological medium with cecum contents than that without cecum contents, suggestingthat the azo reduction should take place prior to the hydrolysis of the ester linkage. And 5-ASA is sustained-released from the aqueous PEG-OLZ copolymer solution with cecum contents in the duration of more than 32 hours detected by HPLC method.5. In vivo degradation of PEG-OLZ copolymers as colon-specific polymeric prodrug of 5-ASA was evaluated using Sprague-Dawley male rats as the test animals. After oral administration, the concentrations of 5-ASA and its metabolism N-acetyl-5-aminolicylic acid (ace-5-ASA) in the biologic samples were measured by HPLC. The HPLC condition of the biodegraded samples was that the mobile phase consisted of a mixture of 0.1 M acetic acid and 0.4% (v/v) triethylamine (pH 4.3) with acetonitrile, in the ratio of 89.5:10.5 (v/v) and the elution was monitored by UV absorption at 254 nm. But the quanta of 5-ASA can not be detected because of other compounds in the plasma, urine or faeces samples having the same retention time as 5-ASA in this condition. So the quanta of 5-ASA were determined by converting 5-ASA into ace-5-ASA.After oral administration of OLZ and PEG-OLZ copolymers, no 5-ASA and ace-5-ASA in the plasma above the limit of quantification can be measured at any sampling point by HPLC. As a result of reductive and hydrolysis splitting of PEG-OLZ copolymers in the intestinal lumen, 5-ASA as well as its main metabolite ace-5-ASA may be poorly absorbed in the intestine of rats. Therefore, the low plasma concentration of ace-5-ASA results in the incapability of detection.After oral administration of OLZ and PEG-OLZ copolymers to rats, ace-5-ASA can be detected after 8 h leg time. So it can be presumed that PEG-OLZ copolymers can transit the stomach and small intestine safely, and 5-ASA can be released in the colon. The peak concentrations of 5-ASA and ace-5-ASA of the OLZ group in the urine or feces were greater than that in the PEG-OLZ copolymer groups, but the time of the maximum concentration of 5-ASA and ace-5-ASA in the OLZ group was shorter than that in the PEG-OLZ copolymer groups. The significant sustained release profile of 5-ASA from PEG-OLZ copolymers in contrast to OLZ in the rats suggests that PEG-OLZ copolymer is a promising colon-specific polymeric prodrug of 5-ASA. In company with the membrane forming property, the azo copolymer can also be used ascolon-specific carrier for other bioactive substances.