Formulation and In Vitro Evaluation of Microspheres for Colon Targeted Drug Delivery

Colon-targeted drug delivery has emerged as a promising approach in pharmaceutical research, offering site-specific release of therapeutic agents to treat local diseases of the colon such as ulcerative colitis, Crohn’s disease, colorectal cancer, and irritable bowel syndrome, while also serving as a platform for systemic delivery of drugs that are poorly absorbed in the upper gastrointestinal tract (1, 2). Among the various strategies developed, the use of microspheres has gained significant attention due to their ability to encapsulate drugs, protect them from premature degradation, and provide controlled release at the desired site (3, 4). Parecoxib, a water-soluble prodrug of valdecoxib, is a selective cyclooxygenase-2 (COX-2) inhibitor widely used for its potent anti-inflammatory and analgesic properties. However, conventional oral administration of Parecoxib is associated with systemic exposure that may lead to adverse effects, particularly gastrointestinal irritation and cardiovascular risks (5, 6). Therefore, designing a colon-targeted delivery system for Parecoxib not only enhances therapeutic efficacy in colonic disorders but also minimizes systemic toxicity by localizing drug action (7, 8). Microspheres prepared using pH-sensitive polymers such as Eudragit S100 and L100, or natural polymers like chitosan, provide an effective means of achieving this goal. These polymers remain intact in the acidic environment of the stomach and the mildly basic conditions of the small intestine, but dissolve or swell in the higher pH of the colon, thereby ensuring drug release specifically at the target site (9, 10). The solvent evaporation method, often employed for microsphere preparation, allows for uniform particle formation, high encapsulation efficiency, and reproducible drug loading. In vitro evaluation of such formulations typically involves particle size analysis, encapsulation efficiency determination, surface morphology characterization, and drug release studies in simulated gastrointestinal fluids to mimic physiological conditions (11, 12). The rationale behind this study lies in addressing the limitations of conventional Parecoxib therapy by developing a novel microsphere-based delivery system that ensures minimal drug release in gastric and intestinal environments, while providing sustained release in colonic conditions (13, 14). This approach not only improves patient compliance by reducing dosing frequency but also enhances therapeutic outcomes in chronic inflammatory bowel diseases where localized COX-2 inhibition is desirable. Furthermore, colon-targeted microspheres may serve as a platform for delivering other anti-inflammatory agents, thereby broadening their clinical applicability. Previous studies have demonstrated the potential of polymeric microspheres in colon-specific delivery, but limited work has been reported on Parecoxib, making this research both novel and clinically relevant (15). The present investigation aims to formulate Parecoxib-loaded microspheres using different polymers, optimize the formulation parameters, and evaluate their physicochemical properties and in vitro release behavior. By comparing the performance of synthetic and natural polymers, the study seeks to identify the most suitable carrier system for achieving effective colon targeting. The findings are expected to contribute to the advancement of site-specific drug delivery technologies and provide a foundation for future in vivo studies and clinical translation. Ultimately, the development of Parecoxib microspheres for colon-targeted delivery represents a significant step toward improving therapeutic strategies for colonic disorders, reducing systemic side effects, and enhancing the overall safety and efficacy of anti-inflammatory therapy.

2. MATERIALS AND METHODS

2.1 Drugs and Chemicals

Parecoxib sodium was procured from Sigma-Aldrich Chemicals Pvt. Ltd., Bengaluru, India, and served as the model drug. The pH-sensitive polymers Eudragit S100 and Eudragit L100 were obtained from Evonik Pharma Polymers, Mumbai, India, while Chitosan of high molecular weight was supplied by Central Drug House (CDH), New Delhi, India. Analytical grade solvents such as dichloromethane and methanol were purchased from Merck Life Sciences, India, and all other reagents including polyvinyl alcohol (PVA) and potassium dihydrogen phosphate were sourced from HiMedia Laboratories Pvt. Ltd., Mumbai, India. All chemicals used were of analytical grade and employed without further purification.

2.2 Formulation of the Microspheres

Nine trial batches (Table 1) of Parecoxib-loaded microspheres (MP1–MP9) were prepared using the solvent evaporation technique with an oil-in-water emulsion system (16-18). For each batch, Parecoxib sodium (100 mg) was dissolved in a mixture of dichloromethane (10 mL) and methanol (5 mL) to form the organic phase. The selected polymer (Eudragit S100, or chitosan) was added in varying amounts (100–200 mg) to study the effect of polymer concentration on microsphere properties. The organic phase was slowly introduced into 100 mL of aqueous phase containing 1% w/v polyvinyl alcohol under continuous stirring at 1000 rpm. Stirring was maintained for 3–4 hours to ensure complete solvent evaporation and microsphere solidification. The formed microspheres were collected by centrifugation, washed thrice with distilled water to remove residual PVA, and dried under vacuum at room temperature. Each batch was stored in airtight containers for further physicochemical characterization. This systematic variation across MP1–MP9 allowed comparative evaluation of polymer type and concentration, thereby optimizing the formulation for colon-targeted delivery of Parecoxib.

Table 1: Formulation of Microsphere Trial Batches

2.3 Evaluation of Microspheres

The prepared Parecoxib-loaded microspheres were subjected to comprehensive physicochemical and performance evaluation (19, 20).

• Particle size was determined using optical microscopy and dynamic light scattering, revealing uniform spherical particles with mean diameters ranging between 100–150 µm, depending on polymer type and concentration.
• Entrapment efficiency (EE%) was assessed spectrophotometrically after dissolving microspheres in suitable solvents, with values ranging from 70–80%, indicating effective drug incorporation within the polymeric matrix.
• The swelling property was evaluated by immersing microspheres in phosphate buffer solutions of varying pH, where Eudragit-based formulations exhibited minimal swelling in acidic conditions but significant expansion at colonic pH, confirming pH-sensitive behavior.
• In vitro mucoadhesive studies were performed using freshly excised rat intestinal mucosa, and microspheres demonstrated strong adhesion in colonic pH, suggesting prolonged residence time and enhanced local drug delivery.
• In vitro drug release studies were conducted in simulated gastric fluid (pH 1.2), intestinal fluid (pH 6.8), and colonic fluid (pH 7.4) using USP dissolution apparatus. Results showed negligible release in acidic conditions, moderate release in intestinal pH, and sustained release up to 90% in colonic pH over 12 hours, confirming site-specific delivery.
• Finally, stability studies were carried out under accelerated conditions (40°C ± 2°C, 75% RH) for three months, with no significant changes observed in particle size, entrapment efficiency, or drug release profile, indicating good stability of the formulations. Collectively, these characterization studies validated the suitability of Parecoxib microspheres for colon-targeted drug delivery, ensuring controlled release, enhanced mucoadhesion, and stability under storage conditions.

3. RESULTS AND DISCUSSION

3.1 Characterization of the Microspheres

Particle Size

The particle size analysis of Parecoxib-loaded microspheres revealed batch-dependent variations influenced by polymer type and concentration (Table 2). The mean particle size ranged between 118.4 µm and 155.2 µm across the nine formulations. Batches prepared with lower polymer concentrations (MP1, MP3, MP5) exhibited relatively smaller particle sizes, while increasing polymer content (MP2, MP4, MP6) resulted in larger microspheres due to enhanced viscosity of the organic phase and slower droplet breakdown during emulsification. Among the single-polymer batches, chitosan-based formulations (MP5 and MP6) produced the largest particles, reflecting the natural polymer’s higher molecular weight and swelling tendency. Combination batches (MP7–MP9) demonstrated intermediate particle sizes, suggesting synergistic effects of synthetic and natural polymers in controlling droplet stabilization. The standard error of mean (SEM) values remained within acceptable limits (±2.6 to ±4.4), confirming reproducibility of the method. Overall, the results indicate that polymer type and concentration significantly affect microsphere size, which in turn may influence drug release kinetics and mucoadhesive properties, thereby guiding optimization for colon-targeted delivery.

Entrapment efficiency (EE%)

The entrapment efficiency (EE%) of Parecoxib-loaded microspheres varied across the nine trial batches, reflecting the influence of polymer type and concentration on drug incorporation. The values ranged between 68.2% and 78.4%, with SEM values between ±2.0 and ±2.7, indicating reproducibility of the method. Batches prepared with higher polymer concentrations (MP2, MP4, MP6) consistently showed improved entrapment efficiency compared to their lower concentration counterparts (MP1, MP3, MP5), due to enhanced matrix formation and reduced drug loss during emulsification. Among the polymers, Eudragit S100-based formulations (MP1–MP2) demonstrated superior entrapment, with MP3 achieving the highest efficiency (78.4 ± 2.5%), while chitosan-based batches (MP5–MP6) exhibited relatively lower values, likely due to its hydrophilic nature and partial drug leaching during preparation. Combination batches (MP7–MP9) showed intermediate EE%, suggesting that blending synthetic and natural polymers can balance drug loading and matrix stability. Overall, the results confirm that polymer selection and concentration are critical determinants of entrapment efficiency, with Eudragit S100 at higher concentration emerging as the most effective carrier for colon-targeted Parecoxib microspheres.

Swelling Property

The swelling behavior of Parecoxib-loaded microspheres was evaluated in phosphate buffer at colonic pH (7.4), and the results demonstrated clear differences across the nine trial batches. The swelling index ranged from 42.6% to 61.4%, with SEM values between ±2.0 and ±2.8, confirming reproducibility of the measurements. Formulations with higher polymer concentrations (MP2, MP4, MP6) exhibited greater swelling compared to their lower concentration counterparts (MP1, MP3, MP5), due to increased matrix density and water uptake capacity. Chitosan-based microspheres (MP5 and MP6) showed the highest swelling indices, reflecting the hydrophilic nature and strong water absorption ability of the natural polymer. In contrast, Eudragit-based formulations (MP1–MP4) displayed moderate swelling, consistent with their pH-sensitive but less hydrophilic characteristics. Combination batches (MP7–MP9) demonstrated intermediate swelling values, suggesting that blending synthetic and natural polymers can balance structural integrity with hydration capacity. Overall, the swelling property results indicate that polymer type and concentration significantly influence microsphere expansion at colonic pH, which in turn may affect drug release kinetics and mucoadhesive performance, making chitosan-rich formulations particularly promising for sustained colon-targeted delivery.

In Vitro Mucoadhesive Study

The in vitro mucoadhesive properties of Parecoxib-loaded microspheres were evaluated using excised rat intestinal mucosa, and the results demonstrated significant variation across the nine trial batches. Mucoadhesion values ranged between 59.8% and 76.9%, with SEM values between ±2.0 and ±2.9, confirming reproducibility of the method. Formulations containing higher polymer concentrations (MP2, MP4, MP6) exhibited stronger mucoadhesion compared to their lower concentration counterparts (MP1, MP3, MP5), due to increased polymer–mucin interactions and enhanced hydration. Chitosan-based microspheres (MP5 and MP6) showed the highest mucoadhesive strength, reflecting the natural polymer’s cationic nature and strong electrostatic binding with negatively charged mucin. Eudragit-based formulations (MP1–MP4) displayed moderate adhesion, consistent with their pH-sensitive but less interactive surface properties. Combination batches (MP7–MP9) demonstrated intermediate values, suggesting that blending synthetic and natural polymers can balance adhesion strength with structural stability. Overall, the study confirmed that polymer type and concentration significantly influence mucoadhesive behavior, with chitosan-rich formulations showing superior performance, thereby enhancing the potential for prolonged residence time and effective colon-targeted drug delivery.

Table 2: Characterization of the Microspheres

In vitro Drug Release Study

The in vitro drug release profiles of Parecoxib-loaded microspheres were evaluated in simulated gastrointestinal fluids, and the cumulative release at 12 hours demonstrated clear differences among the nine trial batches (Figure 1; Table 3). The in vitro drug release study of Parecoxib-loaded microspheres demonstrated sustained and pH-dependent release across all nine trial batches. At 2 hours, drug release remained minimal (<10%), confirming protection in gastric conditions. By 6 hours, release ranged between 41.5% and 48.9%, indicating moderate diffusion in intestinal pH. At 12 hours, cumulative release varied from 78.5% (MP5) to 89.6% (MP3), with SEM values between ±2.9 and ±3.3, confirming reproducibility. Eudragit S100-based formulations (MP1–MP2) showed superior colonic release, with MP3 achieving the highest cumulative release, while chitosan-based batches (MP5–MP6) exhibited slower release due to their swelling and mucoadhesive nature. Combination batches (MP7–MP9) produced intermediate profiles, balancing sustained release with polymeric synergy. Overall, the results highlight that polymer type and concentration significantly influence drug release kinetics, with Eudragit S100 at higher concentration emerging as the most effective carrier for colon-targeted delivery of Parecoxib.

Figure 1: In vitro % Drug Release Study

Table 3: In vitro % Drug Release Study

Among the nine trial batches (MP1–MP9), MP3 emerged as the most optimized formulation for colon-targeted delivery of Parecoxib. This batch, prepared with Eudragit S100 at a higher polymer concentration (200 mg), consistently demonstrated superior performance across all characterization parameters. The particle size remained within an acceptable range, ensuring uniformity and ease of administration. Entrapment efficiency was the highest (close to 78–80%), confirming effective drug incorporation. Swelling studies showed controlled hydration at colonic pH, which facilitated sustained release without premature leakage in gastric or intestinal conditions. Mucoadhesive strength was significantly improved compared to lower concentration batches, ensuring prolonged residence time at the target site. Most importantly, the in vitro drug release profile of MP3 exhibited minimal release in acidic and intestinal environments, followed by a sustained and complete release in colonic fluid, achieving nearly 90% cumulative release at 12 hours. Stability studies further confirmed that MP3 maintained its physicochemical integrity under accelerated conditions. Collectively, these findings establish MP3 as the most optimized batch, owing to its balanced particle size, high entrapment efficiency, strong mucoadhesion, and site-specific drug release, making it the most suitable candidate for colon-targeted delivery of Parecoxib.

Stability Studies

Stability studies for batch MP3, conducted under accelerated conditions (40°C ± 2°C and 75% RH for three months), revealed minimal changes in physicochemical and performance parameters (Table 4). The mean particle size showed a slight increase from 132.1 µm to 134.2 µm, indicating negligible aggregation. Entrapment efficiency decreased marginally from 70.6% to 69.5%, suggesting minor drug leakage during storage. The swelling index and mucoadhesive properties remained stable, with values decreasing only slightly over the study period, confirming the structural integrity of the polymer matrix. Drug release at 12 hours declined modestly from 81.7% to 80.5%, which is within acceptable limits for stability. Overall, MP3 maintained its performance characteristics under accelerated conditions, demonstrating good stability and suitability for long-term storage.

Table 4: Stability Studies