Cellulose-based azo conjugates release 5-ASA selectively in rat colonic contents

Researchers have synthesized a new family of polymer-based drug carriers designed to keep 5-aminosalicylic acid (5-ASA), a mainstay therapy for inflammatory bowel disease, inert until it reaches the colon. In a study published in Polymer Bulletin, Kashif-ur-Rehman Khan and Chitchamai Ovatlarnporn report the synthesis and laboratory evaluation of three hydroxypropyl cellulose (HPC) azo conjugates, labeled HPC-5-ASA-5-ASA 9, 10, and 11, in which the drug molecules are chemically tethered to the cellulose derivative through azo linkages. The strategy exploits a well-known feature of colonic biology: the resident bacterial enzyme azoreductase, which can cleave azo bonds and therefore act as a selective trigger for drug release at the site where chronic inflammatory bowel diseases primarily exert their effects.

The conjugates were prepared with good percentage yields and characterized by a standard panel of analytical techniques, including FT-IR and proton NMR spectroscopy to confirm the covalent attachment of 5-ASA, scanning electron microscopy to examine morphology, and thermogravimetric analysis and differential scanning calorimetry to assess thermal behavior. Stability testing covered the pH environments a dosage form would encounter on its journey through the gastrointestinal tract. All three conjugates proved stable at pH 1.2, 6.8, and 7.4. This pH stability matters because systemic treatment approaches may cause side effects, which colon-specific drug delivery systems are intended to manage by keeping the carrier intact outside the target region.

The release experiments used the intestinal and colonic contents of rats, incubated with the conjugates for 24 hours at 37 degrees Celsius. Under these conditions, all three conjugates liberated 5-ASA gradually across the full incubation period, consistent with enzymatic cleavage of the azo linkages by the microbial contents of the lower gut. Critically, no 5-ASA was detected when the conjugates were incubated with rat stomach homogenate, indicating that the carrier resists the gastric environment and does not shed its payload before reaching the target region. The maximum release observed was 38.8 percent of the attached 5-ASA, achieved in the colonic content, the highest among the compartments tested.

The authors interpret these results as evidence that HPC-5-ASA-5-ASA azo conjugates have potential for use as colon-specific delivery systems for 5-ASA. The macromolecular prodrug concept they build on is particularly promising for exploiting azoreductase, because the enzyme’s activity is concentrated in the colon rather than the stomach or small intestine, allowing the carrier to function as a chemical gatekeeper. Sustained, site-selective release could, in principle, concentrate the drug where chronic inflammatory bowel diseases mainly affect the lower gastrointestinal tract while limiting systemic exposure.

Several important caveats frame these findings. The study is a preclinical laboratory investigation: the release data come from rat gastrointestinal contents, not from human trials, and no efficacy against colonic inflammation was assessed. A maximum colonic release of 38.8 percent over 24 hours also leaves substantial room for optimization, and the degree to which rat gut microbiology predicts human azoreductase activity would need to be established in further work. The characterization and stability results nonetheless provide a proof of concept that hydroxypropyl cellulose can serve as an enzyme-responsive carrier for 5-ASA, and they define a benchmark, the 38.8 percent colonic release figure, against which future refinements of these dual-drug azo conjugates can be measured.

Subject of Research: Synthesis and evaluation of hydroxypropyl cellulose conjugates for colonic delivery of 5-aminosalicylic acid

Article Title: Synthesis and evaluation of hydroxypropyl cellulose conjugates for colonic delivery of 5-aminosalicylic acid

Article References: Khan, K.-U.-R., & Ovatlarnporn, C. (2026). Synthesis and evaluation of hydroxypropyl cellulose conjugates for colonic delivery of 5-aminosalicylic acid. Polymer Bulletin, 83(12), Article 694. https://doi.org/10.1007/s00289-026-06706-w

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DOI: 10.1007/s00289-026-06706-w

Keywords: Synthesis, evaluation, hydroxypropyl, cellulose, conjugates, colonic, delivery, aminosalicylic, acid, scientific research