Development of enteric-coated calcium pectinate microspheres intended for colonic drug delivery

Enteric-coated calcium pectinate microspheres (MS) aimed for colon drug delivery have been developed, by using theophylline as a model drug. The influence of pectin type (amidated or non-amidated) and MS preparation conditions (CaCl2 concentration and crosslinking time) was investigated upon the drug entrapment efficiency and its release behaviour. Drug stability and drug–polymer interactions were studied by Differential Scanning Calorimetry, thermogravimetry, X-ray diffractometry and FTIR spectroscopy. Enteric coating with Eudragit S100 enabled maintenance of MS integrity until its expected arrival to colon. The coating was also useful to improve the stability of MS during storage, avoiding morphologic changes observed for uncoated MS stored under ambient conditions. Entrapment efficiency increased by reducing cross-linking time, and (only in the case of non-amidated pectin) by increasing CaCl2 concentration. On the other hand, release tests performed simulating the gastro-intestinal pH variation evidenced an inverse relationship between CaCl2 concentration and drug release rate, whereas no influence of both pectin type and cross-linking time was found. Unexpectedly, addition of pectinolytic enzymes to the colonic medium did not give rise to selective enzymatic degradation of MS. Notwithstanding this unforeseen result, coated MS prepared at 2.5% w/v CaCl2 concentration were able to adequately modulate drug release through a mixed approach of pH and transit time control, avoiding drug release in the gastric ambient, and reaching the colonic targeting where 100% release was achieved within less than 24 h.