Epigallocatechin gallate (EGCG) is well known for its benefits such as the antioxidant, antibacterial , cancer chemopreventive , and anti-inflammatory properties . Two identified issues – high instability and low bioavailability, however, limit its utilization in food and drug industries. This work aims to address these two issues via the approach of solid dispersion. Polymers were employed in solid dispersions to separate, stabilize, and protect the EGCG molecules, and reduce its release to slow down the dissolution of EGCG, which potentially improves its bioavailability.
The solid dispersions were generated by lyophilization of EGCG-polymer mixtures in water-dioxane solvent system. Nine commonly used pharmaceutical polymers were screened based on their solubility in the two solvents, and their compatibility with EGCG. The generated solid dispersions were characterized by x-ray powder diffraction, scanning electron microscopy, and modulated differential scanning calorimetry. The physicochemical stability of the solid dispersions and pure amorphous EGCG were evaluated under elevated temperature/relative humidity (40 ℃/75% RH), and in simulated gastric and intestinal fluids. The drug release was evaluated in PBS (pH 7.4) at 37 ℃ with a UV-Vis dip probe. The release profiles were analyzed with pseudo second order and first order kinetics models.
Four polymers were found to be suitable to generate amorphous EGCG dispersions:
Table 1. Mixtures proportions of the dispersions
Based on the mDSC thermograms of the four dispersions, the HPMCAS and Soluplus® dispersions show a single Tg, indicating good miscibility of EGCG and the two polymers. No clear Tg is noticed for the HPMCP dispersion, while for the cellulose acetate dispersion, the glass transition region seems spanning a very wide region 59-119 ℃. These results may indicate phase separations between HPMCP, cellulose acetate and EGCG.
Based on the assessments on crystallinity, the dispersions significantly improved EGCG physical stability under the stress condition of 40 °C/75% RH. Increased chemical stability was also observed in the dispersion of EGCG-Soluplus® based on coloration of the post-stress materials. Stress study was also performed in simulated gastrointestinal tract: Based on observation under polarized microscope, birefringence/extinction was observed for amorphous EGCG in SGF and SIF within 15 min, indicating occurrence of crystallization. All the four dispersions remained amorphous after 24 h in both SGF and SIF.
The pure EGCGs, and the dispersions of HPMCAS, HPMCP, cellulose acetate show fast releases that most EGCG was released within 20 min. The release profiles were well fitted with pseudo-second-order model (data not shown here). For the Soluplus® dispersion, the release is much prolonged that only half of EGCG is released in the first 20 min and the dissolution continues up to 24 h. Soluplus® tend to form micelles, with a critical micelle concentration of 7.6 mg/L. In this study the Soluplus® concentration in the dissolution media is much above this value (7353 mg/L). The formed Soluplus® micelles wrap the EGCG slowing down the release of EGCG. A biphasic model, combining pseudo-second-order and first-order kinetics (PSO-FO), is proposed to describe the dissolution behavior of Soluplus® dispersion:
This model is in great consistency with the release profile of Soluplus®, suggesting that there may be two mechanisms responsible for the release behavior.
The solid dispersion approach is shown to significantly improve the physicochemical stability of EGCG. This enhanced stability is of great importance in processing, transportation, and storage in food and drug industries. In addition, the dispersion appears to largely extend the EGCG residence time in intestine, which opens up new possibilities to improve its bioavailability.
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(Originally presented by Yizheng Cao, Jing Teng, Jon Selbo at the 2017 AAPS Annual Meeting and Exposition in San Diego, CA)
Appendix 1: mDSC results of the four dispersions and amorphous EGCG
Appendix 2: XRPD patterns of the dispersions and aEGCG after stressed at 40°C/75%RH for 11 days
Appendix 3: EGCG release profiles for cEGCG, aEGCG, and the dispersions in pH 7.4 PBS medium at 37 °C
Appendix 4: EGCG release in the first 20 min
Appendix 5: The release profile of Soluplus®-EGCG dispersion