New alginate microcapsule system for angiogenic protein delivery and immunoisolation of islets for transplantation in the rat omentum pouch

Severe hypoxia caused by a lack of vascular supply and an inability to retrieve encapsulated islets transplanted in the peritoneal cavity for biopsy and subsequent evaluation are obstacles to clinical application of encapsulation strategies for islet transplantation. We recently proposed an omentum pouch model as an alternative site of encapsulated islet transplantation and have also described a multi-layer microcapsule system suitable for coencapsulation of islets with angiogenic protein in which the latter could be encapsulated in an external layer to induce vascularization of the encapsulated islet graft. The purpose of the present study was to determine the angiogenic efficacy of fibroblast growth factor (FGF-1) released from the external layer of the new capsule system in the omentum pouch graft. We prepared 2 groups of alginate microspheres, each measuring ∼600 μm in diameter with a semipermeable poly-L-ornithine (PLO) membrane separating 2 alginate layers. While one group of microcapsules contained no protein (control), FGF-1 (1.794 μg/100 microcapsules) was encapsulated in the external layer of the other (test) group. From each of the 2 groups, 100 microcapsules were transplanted separately in an omentum pouch created in each normal Lewis rat and were retrieved after 14 days for analysis of vessel density using the technique of serial sample sections stained for CD31 with quantitative three-dimensional imaging. We found that FGF-1 released from the external layer of the test microcapsules induced a mean ± SD vessel density (mm(2)) of 198.8 ± 59.2 compared with a density of 128.9 ± 10.9 in pouches measured in control capsule implants (P = .03; n = 5 animals/group). We concluded that the external layer of our new alginate microcapsule system is an effective drug delivery device for enhancement of graft neovascularization in a retrievable omentum pouch.