Strategies to improve outcome after islet transplantation using the GLP-1 receptor agonist, extendin-4

Abstract

Transplantation of pancreatic islets into the liver via the portal vein has emerged as a treatment option for patients with type I diabetes mellitus. However, loss of functional beta cell mass during isolation and following implantation is a major obstacle in obtaining good long-term results. Exendin-4, a glucagonlike peptide-1 (GLP-1) receptor agonist, improves glucose homeostasis in patients with diabetes. It also has anti-apoptotic and beta cell proliferative properties that could potentially improve islet graft survival and function. After clinical islet transplantation, recipients require life-long treatment with immunosuppressive drugs. These have multiple side effects and may also be detrimental for the transplanted islets. Encapsulization of the islet graft in an immunoprotective membrane is an attractive method to avoid use of long-term immunosuppression. However, poor survival of the encapsulated cells inside such devices is a major limiting factor and necessitates using a high number of islets inside large devices.

The aim of our first experimental study was to evaluate whether the actions of exendin-4 would improve the metabolic outcome after islet transplantation. A marginal number of rat islets were transplanted under the kidney capsule of diabetic athymic mice. We treated the recipient mice with exendin-4 i.p. (100 ng/mouse), once a day for 1 week. In two separate studies, evaluation was done on day 8 (short-term) and at 4 weeks (long-term). In the latter study, islets were cultured with exendin-4 (0.1 mnol/L) for 20 hours in addition to the recipient treatment. In the short-term study, 63% of exendin-4 treated mice achieved graft function compared to 21% of untreated mice (p = 0.03). In the long-term study, 88% of treated mice had functioning grafts compared with 22% of controls (p = 0.01). This study showed that islet pre-culture and a short course of therapy with exendin-4 improves metabolic control after islet transplantation.

One of the aims of our second study was to determine whether only pre-culturing islets or only giving recipient treatment with exendin-4 would be as effective as using a combination of these treatments. For this study, islets were transplanted under the kidney capsule and protocols similar to those of the first study were used. Islet pre-culture with exendin-4 alone or recipient treatment with exendin-4 did not improve graft outcome in comparison to controls. A combination of islet pre-culture and exendin-4 treatment of mice resulted in significantly better graft outcome when compared to controls and the preculture only group (73% vs. 26% and 28%; p = 0.03). We then evaluated the effects of using a combination of exendin4 pre-culture and recipient treatment on the outcome of a marginal number of rat islets placed inside TheraCyteTM device. Macroencapsulated islets were transplanted subcutaneously in diabetic athymic mice. The metabolic outcome of macroencapsulated islets was also improved by treating both islets and the recipients with exendin-4.

We conclude that a combination of islet pre-culture with exendin-4 for 20 hours followed by one-week recipient treatment with exendin-4 provides long-lasting beneficial effects on metabolic control after islet transplantation. Islet pre-culture or recipient treatment with exendin-4 alone did not improve islet graft outcome. In our model, the combined protocol also improved the outcome after transplanting macroencapsulated islets.