Abstract by Maja Nørgaard Kristensen

Ulcerative colitis (UC) is an idiopathic disease that causes inflammation in the rectum, spreading towards the ascending colon to varying degrees. In UC patients with active mild to moderate disease, oral and rectal drug formulations are often used in combination to ensure high luminal drug concentrations in the entire colon. However, high luminal drug concentrations are often associated with side effects. Thus, the present PhD thesis aimed to improve the UC treatment by applying cylindrical micrometer-sized containers, or microcontainers (MCs), for oral drug delivery of UC-specific drugs targeting the inflamed colon.

Firstly, the gastrointestinal (GI) transit time and potential adhesion of MCs to the mucosa were investigated in healthy rats. The MCs were filled with the contrast agent barium sulfate (BaSO₄) and coated with poly(lactic-co-glycolic acid) (PLGA) to seal the contrast agent inside the containers. Around 500 MCs were administered to a rat in a size-9 gelatin capsule via oral gavage (ejected by air from the gavage tube). The rats were killed at different time points ranging from 0.5 to 10 h for in situ planar x-ray imaging to count the MCs in the harvested GI tracts. The MCs were primarily found in the stomach at 0.5 and 1 h and partly at 2 h. At 2-3 h post-administration, MCs were spread out in the small intestine and started to enter the cecum after 3 h. The number of MCs in the cecum increased from 3-6 h, and colonic entry of MCs appeared after 5 h. Due to a relatively long gastric residence time of MCs, a study was initiated to investigate the impact of three commonly used oral gavage techniques (air-, water-, and piston dosing using flexible gavage tubes) on GI transit. Paracetamol and BaSO₄ were filled into the MCs and coated with Eudragit S 100 (E-S100) to investigate the drug absorption and GI transit simultaneously. The absorption of paracetamol correlated well with the MCs reaching the distal jejunum and ileum but was dependent on the oral gavage technique applied. The air-dosed MCs were retained in the stomach for up to 0.5 h, while water dosing inappropriately resulted in MCs being found in the stomach for up to 3 h, causing non-quantifiable plasma paracetamol concentrations during the study. Since air dosing was associated with distress in rats upon capsule administration due to the introduced air, this technique limited its relevance from an animal welfare perspective. Piston-dosed MCs were in the stomach for up to 1 h, but a more pronounced interindividual rat variability was observed for this technique. Upon inspection of the luminal MC distribution, they were primarily located in the content without proximity to the mucosa, regardless of the applied gavage technique. In conclusion, the air and piston dosing techniques did not appear to affect GI transit of MCs substantially, in contrast to co-administration with water.

Secondly, 5% (w/v) dextran sulfate sodium (DSS) was administered to rats in the drinking water for 7 days to chemically induce UC-like inflammation in the colon to investigate the GI transit of MCs and concurrent drug absorption of paracetamol. The onset and progression of colitis were evaluated by the body weight change, stool consistency, and the presence of blood. All parameters were significantly affected compared to healthy rats, resulting in a disease activity index (DAI) score of ~5 on day 7. Post-mortem, significant colonic shortening (55.7 ± 1.0 cm/kg vs. 64.9 ± 1.5 cm/kg) was evident in DSS-induced colitis rats (DSS rats) compared to healthy rats, whereas the spleen weight was unaffected. Plasma C-reactive protein (CRP) and lipocalin-2 concentrations were not significantly upregulated compared to healthy rats, suggesting a local inflammatory response, as histological assessment confirmed mucosal damage and inflammation in the colon. The relevance of the model concerning the testing of UC-specific drug formulations was investigated by measuring pH through the GI tract of anesthetized rats. The gastric and small intestinal pH were similar between healthy and DSS rats and comparable to values reported in UC patients. Yet, the duodenal pH values appeared slightly higher in DSS rats (pH 7.1 ± 0.1) than in UC patients (pH ~6.4). In the proximal colon, the pH was significantly lower in DSS rats (pH 6.9 ± 0.2) compared to healthy rats (pH 7.9 ± 0.1), while being similar in the distal colon (pH 7.6 ± 0.1 vs. 7.8 ± 0.1). The colonic pH of DSS rats was similar to UC patients at stages before needing a colectomy. Thus, the DSS model was considered appropriate for investigating pH-dependent drug formulations.

Lastly, MCs and microparticles (MPs) containing paracetamol and BaSO₄ were compared in piston-dosed DSS rats. MCs and MPs with E-S100 were included for GI transit comparisons to the previous study in healthy rats. Generally, DSS and healthy rats showed similar small intestinal transit times. The pH-independent polymer, Eudragit RS PO (E-RS), was applied on MCs and MPs to allow tracking through the GI tract, but a clustering tendency of E-RS MCs and MPs affected the GI transit compared to the two other types of coatings and challenged the purpose of this coating. Colonic drug targeting was achieved for MPs by double coating with the two pH-dependent polymers, Eudragit E PO as the inner layer and E-S100 as the outer layer. Double-coated MCs failed to release paracetamol in the colon, possibly due to a weak point of the double coating at the base of the MCs, resulting in premature release in the upper GI tract. Overall, MCs and MPs with similar coatings showed equivalent GI transit times and luminal distributions over time but with different absorption profiles due to dissimilar release rates and challenges with double-coated MCs. However, neither MCs nor MPs appeared to adhere or accumulate in any GI tract regions of the DSS rat. In contrast, MCs and MPs traveled in the luminal content without mucosal proximity, similarly to the transit in healthy rats. The DSS rat model may have limitations for studying size-mediated targeting and accumulation of MCs and MPs. Yet, the results suggest that MCs may have limited potential in treating UC due to the lack of mucosal proximity.