Insulin-loaded alginate microspheres for oral delivery – Effect of polysaccharide reinforcement on physicochemical properties and release profile Susana Martins, Bruno Sarmento * , Eliana B. Souto, Domingos C. Ferreira Department of Pharmaceutical Technology, Faculty of Pharmacy, University of Porto, Porto, Portugal Received 27 April 2006; received in revised form 11 January 2007; accepted 12 February 2007 Available online 20 February 2007 Abstract Oral administration of insulin requires protein protection from degradation in the gastric environment and its absorption improve- ment in the intestinal tract. To achieve this objective several types of microspheres composed of alginate, chitosan and dextran sulphate have been prepared by ionotropic gelation. Parameters such as the mean particle size, swelling behaviour, insulin encapsulation effi- ciency, loading capacity and release profiles in simulated gastric and intestinal fluids have been compared for the systems developed. In this study, attempts have been made to increase insulin protection and to improve its release from microspheres by reinforcing the alginate matrix with chitosan and/or dextran sulphate. Dextran sulphate was able to avoid insulin release at pH 1.2, protecting the pro- tein from the acidic environment and reducing the total insulin released at pH 6.8. This effect was explained by an interaction between the permanent negatively charged groups of dextran sulphate and insulin molecules. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Alginate; Chitosan; Dextran sulphate; Insulin; Microspheres; Gastric retention 1. Introduction The development of improved delivery systems for oral administration of insulin is a field of great interest in the treatment of diabetes mellitus to overcome the problem of daily subcutaneous injections. Oral administration is a more convenient approach considering the pain and possi- ble infections caused by injections, thus leading to a higher patient compliance. This administration route prevents the occasional hyperinsulinaemia observed by subcutaneous administration, since the principal organ in glucose homeo- stasis is the liver and this should obviously be the prime tar- get for intervention (Arbit, 2004). A successful oral drug delivery system needs to be developed on the basis of increased resistance of the drug carrier against gastrointes- tinal enzymes and pH gradients (i.e. from 1to 3 in the stom- ach to 6 to7 in the intestine). In addition, time-controlled release is also a pre-requisite for such oral drug delivery systems (Janes & Alonso, 2003). It is known that microencapsulation of labile proteins improves their protection against gastric pH and enzymatic attack, providing a controlled release profile of the entrapped molecules (Onal & Zihnioglu, 2002), and further enhancement of their intestinal absorption. Microencapsu- lation of proteins can be performed using alginate, chitosan and dextran sulphate. Being biodegradable, biocompatible, non-toxic and mucoadhesive macromolecules these natural polysaccharides have been widely used in the formulation of several drug delivery systems (Lin, Yu, & Chien, 2005; Onal & Zihnioglu, 2002; Yu et al., 2004). Alginate is an anionic polysaccharide consisting of var- ious ratios of guluronic and mannuronic acid units linked by glycosidic bonds. Gelation can be induced by cross-link- ing the guluronic acid units with di- or polyvalent cations such as calcium. Major disadvantages of alginate beads 0144-8617/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2007.02.012 * Corresponding author. Tel.: +351 222078949; fax: +351 222003977. E-mail address: bruno.sarmento@ff.up.pt (B. Sarmento). www.elsevier.com/locate/carbpol Carbohydrate Polymers 69 (2007) 725–731